MicroBiology MCQs with Answer and Explnation | Chapter 27

Answer: Change in single base pair

A point mutation refers to a specific type of mutation affecting a single nucleotide base pair in DNA. This can involve substituting one base pair for another, deleting a base pair, or inserting a new base pair. Among the choices given, only a change in a single base pair qualifies as a point mutation.

The other options are incorrect:

• Deletion: This involves the removal of one or more base pairs from DNA, not just a single one. While it can have significant consequences, it’s not considered a point mutation.
• Insertion: This involves adding one or more base pairs to DNA, again exceeding the scope of a single base pair change characteristic of point mutations.
• Duplication: This involves copying a section of DNA, potentially including multiple base pairs, not just a single one. Although it can affect DNA structure, it’s not classified as a point mutation.

Answer: Ovary

While the other options all harbor diverse communities of microorganisms, the ovaries, located deep within the body and protected by several layers of tissue, are generally considered sterile and devoid of any resident microbiota.

The other options are incorrect:

• Upper respiratory tract: This area, including the mouth, nose, and throat, hosts a rich and diverse population of bacteria, fungi, and viruses, many of which are beneficial for maintaining oral and respiratory health.
• External genitalia: Both male and female external genitalia harbor bacterial communities, with differences in composition due to hormonal and physical factors. These communities play a role in immune function and protection against pathogens.
• Skin: The skin is the largest human organ and harbors a vast and diverse ecosystem of microorganisms, including bacteria, fungi, and even mites. This microbiota plays a crucial role in skin health, barrier function, and immune defense.

Answer: Produce a hypertonic environment

Salt and sugar, when added to food, create a hypertonic environment outside the microbial cells. This means the concentration of solutes outside the cell is higher than inside. To balance this pressure difference, water flows out of the microbial cells through a process called osmosis. This dehydration stresses the microbes, inhibiting their growth and spoilage.

The other options are incorrect:

• Make them acid: While some acidic preservatives exist, salt and sugar themselves don’t significantly alter the pH of food. Microbes can tolerate a wide range of pH values.
• Produce a hypotonic environment: A hypotonic environment has a lower concentration of solutes outside the cell, attracting water into the microbes and favoring their growth. This is the opposite of the desired effect for preservation.
• Deplete nutrients: While salt and sugar can affect the availability of certain nutrients for microbes, it’s not their primary mode of preservation. The water loss caused by hypertonicity is more effective in inhibiting microbial growth.

Answer: Streptococcus agalactiae

The CAMP (Christie, Atkins, Munch-Petersen) test is a diagnostic test used to identify Group B Streptococcus (GBS), and Streptococcus agalactiae is a type of Group B Streptococcus. The test involves streaking a culture of the bacteria on a blood agar plate, and the positive CAMP test is characterized by enhanced hemolysis at the intersection of the test organism and a beta-hemolytic Staphylococcus aureus streak.

The other options are incorrect:

• Staphylococcus aureus: Staphylococcus aureus is not the causative agent indicated by a positive CAMP test for neonatal meningitis. The CAMP test is specific for identifying Group B Streptococcus, not Staphylococcus aureus.
• E. coli: Escherichia coli (E. coli) is a common cause of neonatal meningitis, but it is not associated with a positive CAMP test. The CAMP test is specific for Streptococcus agalactiae.
• Haemophilus: While Haemophilus influenzae can cause meningitis, it is not identified through the CAMP test. The CAMP test is designed for detecting Group B Streptococcus, as mentioned earlier.

Answer: Specificity and Memory

The adaptive immune system is distinguished by its ability to target specific pathogens and develop long-lasting memory of past encounters. These two features, specificity and memory, set it apart from the innate immune system, which offers a broad, non-specific response.

The other options are incorrect:

• Immediate and Broad: While the innate immune system acts quickly against a wide range of threats, the adaptive response takes time to tailor its attack to a specific pathogen.
• Innate and Short: This option combines characteristics of both systems, but inaccurately portrays the adaptive immune system. It is not innate (developed over time) and its memory can be long-lasting.
• Non-specific and Fast: These attributes describe the innate immune system, not the adaptive one. Although the adaptive response can be rapid upon recognizing a previously encountered pathogen, its initial stage typically requires time for specific recognition and activation.

Answer: Nonsense

A nonsense mutation introduces a stop codon within a coding sequence, prematurely terminating protein translation. This typically results in a non-functional or truncated protein, potentially leading to significant disruptions in cellular processes and impacting the organism’s health.

The other options are incorrect:

• Silent: A silent mutation alters a codon such that it codes for the same amino acid, hence having minimal to no impact on protein function.
• Missense: This type of mutation changes a single codon, leading to the incorporation of a different amino acid into the protein. While this can alter protein function, the severity depends on the specific amino acid substitution and its role in the protein structure and activity.
• Inversion: An inversion involves a reversal of a DNA segment within a chromosome. Depending on the location and size of the inversion, it can disrupt gene expression or alter protein coding sequences, but the impact can vary from minimal to severe.

Answer: Base substitution

X-rays are a form of ionizing radiation that can damage DNA by ejecting electrons from atoms, creating free radicals and reactive ions. These can directly break or chemically modify DNA bases, leading to various types of mutations. Among the choices provided, base substitution is the most accurate description of how X-rays typically cause mutations.

The other options are incorrect:

• Deletion: While X-rays can also cause deletions by breaking DNA strands, this doesn’t always involve base substitution. It can result in the loss of multiple bases consecutively.
• Transition: This refers to a specific type of base substitution where a purine (A or G) is replaced by another purine, or a pyrimidine (C or T) by another pyrimidine. While X-rays can cause transitions, they can also cause transversions, another type of substitution where a purine is replaced by a pyrimidine or vice versa.
• Transversion: As explained above, transversions are possible mutations caused by X-rays, but it’s not as broad as “base substitution” which encompasses both transitions and transversions.

Answer: Endotoxin

All Gram-negative bacilli possess endotoxin, also known as lipopolysaccharide (LPS). Endotoxin is a component of the outer membrane of Gram-negative bacteria and is released when the bacterial cells are disrupted or die. Endotoxin can trigger inflammatory responses in the host, leading to various effects such as fever and activation of the immune system. It plays a significant role in the pathogenesis of infections caused by Gram-negative bacteria.

The other options are incorrect:

• Capsule: Not all Gram-negative bacilli have a capsule. Capsules are structures that some bacteria use for protection against host immune responses, but the presence of a capsule is not a universal characteristic of Gram-negative bacilli.
• Exotoxin: Exotoxins are toxic substances produced and released by certain bacteria. The presence of exotoxins is not a universal characteristic of all Gram-negative bacilli; it varies among bacterial species.
• Coagulase: Coagulase is an enzyme produced by some bacteria, particularly Staphylococcus species. It is not a characteristic of Gram-negative bacilli; instead, it is more associated with certain Gram-positive bacteria, such as Staphylococcus aureus.

Answer: Reverse transcriptase

HIV-1, like all retroviruses, possesses a unique enzyme called reverse transcriptase that allows it to convert its single-stranded RNA genome into double-stranded DNA. This DNA is then integrated into the host cell’s genome, enabling the virus to hijack the cell’s machinery for replication.

The other options are incorrect:

• Protease: This enzyme is vital for HIV-1 maturation, cleaving viral polyproteins into functional proteins needed for virion assembly and infection. However, it does not directly participate in RNA-to-DNA conversion.
• Integrase: This enzyme integrates the viral DNA into the host cell’s genome, a crucial step in the viral life cycle. However, it requires the pre-existence of double-stranded viral DNA created by reverse transcriptase.
• RNase: While HIV-1 encodes an RNase H domain within reverse transcriptase, its primary function is to degrade the RNA template strand during DNA synthesis, not to independently convert RNA to DNA.

Answer: Contact with urine from infected animal

The primary mode of transmission of Leptospirosis from animals to humans is through contact with urine from infected animals. Leptospira bacteria are shed in the urine of infected animals, most commonly rodents, and can survive in soil and water for weeks or even months. Humans can become infected when this contaminated water or soil comes into contact with broken skin, mucous membranes (eyes, nose, mouth), or even through cuts or abrasions.

The other options are incorrect:

• Animal scratch: While bites and scratches from infected animals can transmit Leptospirosis, it’s a less common route compared to exposure to urine. The bacteria need to enter the body through open wounds or mucous membranes.
• Animal bite: Similar to scratches, bites are a potential but less common transmission route. The key factor is contact of infected animal fluids with open wounds or mucous membranes.
• Arthropod vector: Some Leptospira species can be transmitted by certain rodents and other mammals acting as reservoirs, but this type of transmission is far less prevalent than direct contact with contaminated urine.

Answer: Staphylococcus aureus

Food intoxication refers to a type of foodborne illness caused by the ingestion of preformed toxins present in contaminated food. Staphylococcus aureus is known for producing heat-stable toxins that can withstand cooking temperatures. When contaminated food is consumed, the toxins can cause rapid onset of symptoms, such as nausea, vomiting, abdominal cramps, and diarrhea.

The other options are incorrect:

• Clostridium tetani: Clostridium tetani is the bacterium responsible for tetanus, a disease that is not associated with food intoxication. Tetanus is typically contracted through wounds or cuts contaminated with the bacterium.
• Streptococcus pyogenes: Streptococcus pyogenes is associated with various infections, such as strep throat and skin infections, but it is not known for causing food intoxication. Food intoxication is more commonly linked to the ingestion of toxins produced by certain bacteria.
• Salmonella: Salmonella is a bacterium that can cause foodborne infections, leading to a condition known as salmonellosis. Unlike food intoxication, which involves the ingestion of preformed toxins, salmonellosis results from the ingestion of live bacteria present in contaminated food.

Answer: Ticks

Crimean-Congo hemorrhagic fever (CCHF) is a viral disease that is primarily transmitted to humans through the bite of infected ticks, particularly those of the genus Hyalomma. Ticks serve as both the reservoir and vector for the CCHF virus. Humans can become infected when bitten by an infected tick or through direct contact with blood or tissues from infected animals.

The other options are incorrect:

• Mosquitoes: Mosquitoes are not the vectors for Crimean-Congo hemorrhagic fever. While mosquitoes are known to transmit various diseases, CCHF is primarily associated with tick bites.
• Fleas: Fleas are not the vectors for CCHF. The main vector for this viral disease is ticks, specifically those belonging to the genus Hyalomma.
• Sand-fly: Sand-flies are associated with the transmission of diseases like Leishmaniasis, but they are not the vectors for Crimean-Congo hemorrhagic fever. Ticks play a crucial role in the transmission of CCHF.

Answer: It has only trophozoite stage.

Giardia lamblia has both trophozoite and cyst stages, not just trophozoite. The trophozoite is the active, dividing stage that lives in the small intestine, while the cyst is the inactive, infectious stage that is shed in feces and can survive in the environment for months.

The other options are incorrect:

• It is transmitted by the fecal-oral route: This is correct. Giardia spreads through contaminated water, food, or surfaces that have been touched by someone infected with the parasite.
• It can be diagnosed by the string test: This is correct. The string test involves swallowing a long, gelatin capsule attached to a string. The capsule dissolves in the small intestine, and the string is then pulled back up. If Giardia is present, it will stick to the string and can be seen under a microscope.
• It can divide by binary fission: This is correct. Binary fission is how Giardia reproduces. One cell simply divides into two identical cells.

Answer: Autoclaving

Autoclaving is the most widely used and effective method for sterilizing culture media due to its ability to destroy all forms of microbial life, including spores, without significantly harming the media itself. It achieves this by using saturated steam under pressure (typically 121°C for 15-20 minutes).

The other options are incorrect:

• β-radiation: While effective against some microbes, its limited penetration depth makes it unsuitable for large volumes of media. Additionally, radiation can damage the nutrients in the media and requires specialized equipment and safety precautions.
• Hot air oven: Requires prolonged exposure at high temperatures (160°C for 2 hours) which can damage heat-sensitive components of the media and is not reliable for killing all spores.
• Tyndallization: Effective for eliminating spores but is time-consuming and impractical for large volumes of media. It involves repeated cycles of heating and cooling, taking advantage of the fact that germinated spores are more susceptible to heat.

Answer: Be secreted into mucus

IgG (Immunoglobulin G) is a class of antibodies that performs various functions in the immune system. However, IgG is not typically secreted into mucus. IgA is the predominant immunoglobulin found in mucosal secretions, including mucus.

The other options are incorrect:

• Opsonize bacteria: Correct. IgG is involved in opsonization, which is the process of marking pathogens, particularly bacteria, for phagocytosis by immune cells.
• Activate complement: Correct. IgG can activate the complement system, which is a series of proteins that enhance the immune response, leading to the lysis of target cells.
• Cross the placenta: Correct. IgG is the only antibody class that can cross the placenta from the mother to the fetus, providing passive immunity to the newborn.

Answer: Spontaneous mutations

Mutations that arise without any known external or environmental trigger are called spontaneous mutations. These mutations often occur due to natural errors during DNA replication, repair, or recombination.

The other options are incorrect:

• Induced mutations: These are mutations caused by known mutagens, like UV radiation, chemicals, or certain viruses.
• Fused mutations: This term isn’t a standard type of mutation. It might refer to the joining of two separate mutations within the same gene, but wouldn’t apply to the origin of the mutations themselves.
• None of the above: Since spontaneous mutations are a well-established concept in genetics, this wouldn’t be an accurate answer.

Answer: Replication

DNA copying happens during a process called replication. During replication, the double helix unwinds, and each strand serves as a template for the creation of a new complementary strand. This ensures that each daughter cell receives an exact copy of the original DNA.

The other options are incorrect:

• Transformation: This refers to the uptake of foreign DNA by a cell, usually from the environment. While it involves DNA, it doesn’t involve the copying of the cell’s own DNA.
• Translation: This process involves reading the instructions encoded in mRNA and using them to build proteins. It doesn’t involve copying DNA itself.
• Transcription: This process involves copying DNA into mRNA, which is then used for translation. While it

Answer: All of these

Nonspecific chemical barriers to infection are our body’s first line of defense against harmful microorganisms. These barriers don’t target specific pathogens but create a hostile environment for any potential invader. Here’s why each of the options is a great example:

• Unbroken skin: Our skin acts as a physical and chemical barrier. Its outer layer, the stratum corneum, is composed of dead cells and keratin which create a tough, waterproof barrier. Additionally, the skin produces sebum, an oily substance with antimicrobial properties that further inhibits the growth of microorganisms.
• Cilia in respiratory tract: The lining of our respiratory tract is covered in tiny hair-like structures called cilia. These cilia beat in a wave-like motion, trapping and moving mucus and any foreign particles, including bacteria and viruses, towards the throat where they can be swallowed or coughed up.
• Lysozyme in saliva: Saliva contains an enzyme called lysozyme, which breaks down the cell walls of certain bacteria, rendering them harmless.

Answer: Ciliary Lining

The ciliary lining is the most relevant nonspecific host defense mechanism related to the trachea. The trachea, also known as the windpipe, is lined with epithelial cells that bear numerous hair-like projections called cilia. These cilia work in coordinated waves to move mucus and trapped particles, including dust, bacteria, and viruses, upwards towards the pharynx to be swallowed or expelled through coughing. This continuous clearance action provides essential defense against inhaled pathogens before they can establish an infection.

The other options are incorrect:

• Lacrimation: This refers to the production of tears, which primarily protects the eyes. While tears can indirectly affect the upper respiratory tract by draining pathogens away from the eyelids, their main function is not associated with the trachea specifically.
• Desquamation: This describes the shedding of dead skin cells, which contributes to nonspecific defense on various surfaces like the skin and the inside of the mouth. However, in the trachea, ciliated epithelial cells are not continuously being shed like skin cells; they remain functional and play a more active role in defense through their movement.
• Lactic acid: While lactic acid can contribute to defense in some environments, like the vaginal canal and on the skin, it’s not a major or specific defense mechanism in the trachea.

Answer: Steam at 121°C for 30 minutes

Autoclaving uses saturated steam under pressure, not dry air, to sterilize materials. The combination of high temperature and pressure effectively kills all microorganisms, including bacteria, viruses, and spores, which are the most resistant form of microbial life.

The other options are incorrect:

• Dry air at 121°C and 15 lbs pressure: Dry air alone, even at high temperatures, is not effective for sterilization. It lacks the penetrating power and moisture needed to destroy microorganisms, especially spores.
• Steam at 100°C for 30 minutes: While steam at 100°C can kill some bacteria and viruses, it is not sufficient for complete sterilization. The higher temperature and pressure used in autoclaving are necessary to ensure the destruction of all types of microorganisms, including spores.
• Dry air at 160°C for 30 minutes: Similar to the first option, dry air alone, even at a higher temperature, is not effective for sterilization. It can also damage heat-sensitive materials.

Answer: Autoclaving

Autoclaving is the most common and effective method for sterilizing culture media. It uses saturated steam under pressure (typically 121°C for 15-20 minutes) to destroy all forms of microbial life, including spores, which are highly resistant to other methods.

The other options are incorrect:

• β-radiation: While effective against some microbes, its limited penetration depth makes it unsuitable for large batches of media. Additionally, radiation can damage the nutrients in the media and requires specialized equipment and safety precautions.
• Hot air oven: Requires prolonged exposure at high temperatures (160°C for 2 hours) which can damage heat-sensitive components of the media and is not reliable for killing all spores.
• Tyndallization: Effective for eliminating spores but is time-consuming and impractical for large volumes of media. It involves repeated cycles of heating and cooling, taking advantage of the fact that germinated spores are more susceptible to heat.

Answer: 4-6

Sabouraud dextrose agar is typically adjusted to a pH of 5.6, which falls within the range of 4-6. This slightly acidic pH is optimal for the growth of fungi, especially dermatophytes, while also inhibiting the growth of most bacteria.

The other options are incorrect:

• 1-2: This is too acidic and would inhibit the growth of most fungi, including dermatophytes.
• 6-8: While some fungi can tolerate this range, it’s not ideal for most dermatophytes and might allow some bacteria to grow.
• 8-10: This is too alkaline and would significantly inhibit the growth of most fungi, including dermatophytes.

Answer: agranular, macrophages

Monocytes are a type of agranular leukocyte, meaning they lack the visible cytoplasmic granules characteristic of neutrophils and eosinophils. They develop into macrophages, large phagocytic cells capable of engulfing and digesting debris, pathogens, and worn-out cells.

The other options are incorrect:

• Granular, phagocytes: While monocytes are indeed phagocytic, they are specifically agranular.
• Agranular, mast cells: Mast cells are another type of agranular leukocyte, but they have distinct functions and origins different from monocytes.
• Agranular, T cells: T cells are also agranular but belong to the lymphocyte category, distinct from monocytes in terms of development and function.
• Granular, B cells: B cells are lymphocytes characterized by cytoplasmic granules. Monocytes, again, are a separate lineage with different roles.

Answer: Dry heat

While all the listed options can have some antimicrobial properties, only dry heat is a true sterilizing agent capable of destroying all forms of microbial life, including bacteria, viruses, and spores.

The other options are incorrect:

• Ether: While it can kill some bacteria and viruses, it’s not effective against all microorganisms and is not considered a reliable sterilizing agent. Additionally, it’s highly flammable and poses safety concerns.
• Ethanol: Similar to ether, ethanol can kill some microbes but not all, especially spores. It’s commonly used as a disinfectant, but not for sterilization.
• Chlorohexidine: This is a broad-spectrum antimicrobial agent mainly used for topical disinfection and preoperative skin preparation. It doesn’t effectively kill all microbes, particularly spores, and therefore isn’t considered a sterilizing agent.

Answer: Peyer’s patches

Peyer’s patches are specialized mucosal lymphoid tissues found in the small intestine and are a key component of the gut-associated lymphoid tissue (GALT). Their function is to sample antigens from the food and luminal contents of the intestine and initiate immune responses against potential pathogens.

The other options are incorrect:

• Thymus: The thymus is a primary lymphoid organ located in the chest and plays a crucial role in T cell development. While it contributes to the overall immune system, it is not directly connected to the digestive tract and not considered part of GALT.
• Tonsils: Tonsils are located in the throat and act as a first line of defense against inhaled and ingested pathogens. They are associated with MALT (mucosa-associated lymphoid tissue) but not specifically GALT, which focuses on the gut.
• Breast lymph nodes: Lymph nodes are distributed throughout the body and primarily involved in filtering lymph fluid and providing immune surveillance. While they might drain lymph from the breast tissue, they are not directly associated with the gut or GALT.

Answer: Prion

Among the listed options, prions are the most resistant to antiseptics, even surpassing bacterial spores. This is because prions are not living organisms; they are misfolded protein molecules that can induce other proteins to misfold as well, causing neurodegenerative diseases like Creutzfeldt-Jakob disease. Their lack of cellular machinery and specific structure makes them impervious to most antiseptics and disinfectants.

The other options are incorrect:

• Spore: Bacterial spores are highly resistant to harsh conditions, including antiseptics, due to their thick, multi-layered coat. However, some specialized disinfectants and high-pressure sterilization methods can effectively destroy them.
• Cyst: Some single-celled organisms like protozoa form cysts for protection against unfavorable conditions. These cysts can harbor some resistance to antiseptics but are generally less robust than bacterial spores.
• Fungus: Fungi vary in their susceptibility to antiseptics, but in general, they are not exceptionally resistant compared to prions. Some antifungal medications effectively target them.

Answer: Dolor

Dolor is a Latin term that translates to “pain.” In the context of the classical signs of inflammation (described by Celsus), dolor refers to the perception of pain associated with the inflammatory process. Pain or discomfort is a common symptom of inflammation, signaling tissue damage or irritation.

The other options are incorrect:

• Tumor: Tumor refers to swelling or an abnormal enlargement of a body part due to inflammation. It does not specifically indicate pain.
• Calor: Calor refers to heat. In the context of inflammation, it describes the localized increase in temperature due to increased blood flow to the affected area. While heat can be associated with inflammation, it is not a direct specification of pain.
• Rubor: Rubor refers to redness, which is another classic sign of inflammation. Redness results from increased blood flow and dilation of blood vessels. It does not specifically indicate pain but is a part of the overall inflammatory response.

Answer: Endotoxin

An exogenous pyrogen is a substance that originates outside the body and triggers fever. Among the listed options, endotoxin is the only one that fits this definition.

The other options are incorrect:

• Interleukin-1: This is an endogenous pyrogen, meaning it is produced within the body by immune cells in response to infection or tissue damage. It contributes to the fever response but is not itself an exogenous pyrogen.
• Complement: This is a complex system of proteins involved in the immune response but doesn’t directly cause fever.
• Interferon: This is another type of signaling molecule involved in the immune response but also doesn’t directly induce fever.

Answer: Histamine

Histamine is a potent inflammatory mediator that plays a crucial role in vasodilation, which is the widening of blood vessels. During an inflammatory response, histamine is released from mast cells and basophils, leading to increased blood flow to the affected area and contributing to the classic redness and warmth associated with inflammation.

The other options are incorrect:

• Collagen: Collagen is a structural protein that provides support and flexibility to tissues. While it plays a role in wound healing and tissue repair, it doesn’t directly cause vasodilation.
• Complement C5a: While the complement system is involved in inflammation, C5a primarily attracts neutrophils and opsonizes pathogens. It doesn’t directly trigger vasodilation.
• Interferon: Interferons are signaling molecules involved in the antiviral and immune responses. They don’t directly induce vasodilation.

Answer: Tumor necrosis factor

TNF is an inflammatory mediator that stimulates chemotaxis, which is the movement of immune cells toward the site of infection or inflammation. TNF plays a key role in coordinating the immune response and recruiting white blood cells to the site of injury or infection.

The other options are incorrect:

• Serotonin: Serotonin is a neurotransmitter and vasoconstrictor. While it is involved in various physiological processes, it is not a primary mediator of chemotaxis.
• Granulocyte-Colony Stimulating Factor (G-CSF): G-CSF is a cytokine that stimulates the production and release of granulocytes, particularly neutrophils, from the bone marrow. While it is important for the immune response, it is not a direct mediator of chemotaxis.
• Interleukin-2 (IL-2): IL-2 is a cytokine that primarily regulates the activity of T lymphocytes. While it plays a role in immune activation, it is not a primary mediator of chemotaxis for immune cells.

Answer: Alpha, natural killer cells

Alpha interferon (IFN-α), produced by T lymphocytes (T cells), plays a role in activating natural killer (NK) cells. Natural killer cells are a type of immune cell that has the ability to recognize and destroy virus-infected cells. Interferons, including IFN-α, are signaling proteins that are part of the body’s antiviral defense mechanisms.
The other options are incorrect:

• Beta, Lymphocytes: Beta interferon (IFN-β) is primarily produced by fibroblasts and other cell types, not T lymphocytes. It is involved in antiviral responses but is not associated with the activation of lymphocytes.
• Gamma, Fibroblasts: Gamma interferon (IFN-γ) is produced by various immune cells, including T cells, and it has a role in activating macrophages and enhancing immune responses. However, it is not primarily associated with fibroblasts.
• Beta, Fibroblast: While fibroblasts can produce beta interferon (IFN-β), it is not primarily associated with the activation of fibroblasts. IFN-β has various roles in antiviral responses and immune regulation.

Answer: Membrane attack complex

The complement system is a complex series of protein interactions that aid in the immune response. While all the options listed play a role in this process, the final product and ultimate effector molecule of the complement system is the Membrane attack complex (MAC).

The other options are incorrect:

• Properdin: This is a protein involved in the alternative pathway of the complement cascade, but it is not the final product. It helps stabilize the formation of MAC.
• Cascade reaction: This refers to the sequence of protein activations that occur in the complement system leading to MAC formation. It’s the process, not the end product.
• Complement factor C9: This is one of the proteins that participates in the assembly of MAC, but it isn’t the complete complex itself. MAC requires several other proteins to form its functional structure.

Answer: The classic complement pathway

Tumor necrosis factor (TNF) is a potent inflammatory mediator involved in various immunological processes. However, unlike the other options, it’s not directly involved in the classic complement pathway.

The other options are incorrect:

• Chemotaxis of phagocytes: TNF can stimulate the production of chemotactic factors that attract phagocytes like neutrophils and macrophages to the site of inflammation.
• Fever: TNF is a key pyrogen, meaning it can induce fever by stimulating the release of prostaglandins in the hypothalamus.
• The inflammatory response: TNF plays a pivotal role in orchestrating the inflammatory response, activating other inflammatory mediators and promoting tissue repair.

Answer: Nutrient broth is basal media

Basal media, also known as basic or general-purpose media, provide the essential nutrients required for the growth of a wide range of microorganisms. Nutrient broth is a liquid basal medium composed of peptone, beef extract, and water. It is commonly used for the cultivation and maintenance of microorganisms in a liquid state.

The other options are incorrect:

• Solid media are enrichment media: This statement is incorrect. Solid media can be either basal or enriched. Enriched media contain additional nutrients to support the growth of specific organisms with particular nutritional requirements.
• Agar adds nutrient to media: This statement is incorrect. Agar is not a nutrient; it is a solidifying agent used to solidify liquid media. It does not contribute nutritional content to the medium.
• Chocolate agar is selective media: This statement is incorrect. Chocolate agar is considered an enriched medium rather than a selective medium. It contains additional nutrients, such as heated blood, to support the growth of fastidious organisms, but it is not designed to selectively inhibit the growth of specific types of microorganisms.

Answer: Prions, bacterial spores, bacteria

Bacterial spores, particularly those produced by certain species of Bacillus and Clostridium, are highly resistant to various sterilization methods. Bacteria, including vegetative cells, are generally less resistant than spores but still require effective sterilization measures. Prions, which are misfolded proteins associated with certain diseases, are relatively more resistant to conventional sterilization methods.

The other options are incorrect:

• Bacterial spores, bacteria, Prions: This is the correct order, as explained above.
• Bacteria, Prions, Bacterial spores: This option is incorrect. Bacterial spores are more resistant than bacteria, so the order is not accurate.
• Prions, bacteria, bacterial spores: This option is incorrect. Bacterial spores are more resistant than bacteria, so the order is not accurate.

Answer: Pathogen Associated Molecular Patterns

PAMPs are molecular structures or patterns that are associated with microbes, such as bacteria, viruses, and fungi. These patterns are recognized by the innate immune system, including phagocytes (such as macrophages). When phagocytes recognize PAMPs, they initiate an immune response to eliminate the invading microorganisms.

The other options are incorrect:

• Pyrogen: Pyrogens are substances that induce fever. While fever is a response to infection, it is not a signaling molecule recognized by phagocytes. Instead, pyrogens act on the hypothalamus to elevate body temperature.
• Complement: The complement system is a part of the immune system that enhances the ability of antibodies and phagocytic cells to clear microbes and damaged cells. However, complement itself is not a signaling molecule recognized by phagocytes.
• Lectin: Lectins are proteins that can bind to carbohydrates. While lectins are involved in various biological processes, they are not typically signaling molecules recognized by phagocytes in the context of microbial recognition. PAMPs are more specific in this context.

Answer: Bacillus stereothermophilus

Bacillus stereothermophilus, specifically the strain ATCC 7953, is commonly used as a biological indicator in autoclaves. This bacterium is chosen because it forms heat-resistant spores that are more resistant to the sterilization process. If spores of Bacillus stereothermophilus are killed after autoclaving, it indicates that the autoclave has effectively sterilized the contents.

The other options are incorrect:

• Clostridium tetani: While Clostridium tetani is a spore-forming bacterium, it is not typically used as an indicator in autoclaves. It is associated with tetanus and is not as commonly employed for testing autoclave efficacy.
• Bacillus anthracis: Bacillus anthracis, the causative agent of anthrax, is also a spore-forming bacterium. However, it is not commonly used as an indicator in autoclaves due to safety concerns associated with handling this pathogenic species.
• Clostridium botulinum: Clostridium botulinum is a spore-forming bacterium that produces the botulinum toxin. It is not commonly used as an indicator in autoclaves, likely due to safety considerations and the availability of other suitable indicator organisms like Bacillus stereothermophilus.

Answer: Both “Selective medium” and “Differential medium”

MacConkey agar is commonly used in microbiology to isolate and differentiate lactose-fermenting Gram-negative bacteria, especially members of the family Enterobacteriaceae. It contains crystal violet and bile salts, making it selective for Gram-negative bacteria. It also contains lactose and pH indicators (neutral red), allowing for the differentiation of lactose-fermenting bacteria from non-lactose fermenters based on the color change of the colonies.

The other options are incorrect:

• Enrichment medium: MacConkey agar is not an enrichment medium. Enrichment media typically contain specific nutrients that promote the growth of a particular group of microorganisms while inhibiting others.
• Selective medium: This is the correct answer. MacConkey agar is a selective medium due to the inclusion of components like crystal violet and bile salts, which inhibit the growth of Gram-positive bacteria and favor the growth of Gram-negative bacteria.
• Differential medium: This is the correct answer. MacConkey agar is a differential medium as it allows for the differentiation of bacteria based on their ability to ferment lactose, resulting in color changes in the colonies.

Answer: Transduction

Transduction refers to the process by which genetic material is transferred from one bacterial cell to another by a bacteriophage (a virus that infects bacteria). In this context, the virus acts as a vector, carrying bacterial DNA from one host cell to another during the viral replication cycle. Transduction is a natural mechanism that contributes to genetic recombination in bacteria.

The other options are incorrect:

• Transfection: Transfection is the introduction of foreign DNA into eukaryotic cells, typically using non-viral methods such as liposomes or electroporation. It is not specific to virus-mediated transfer in bacteria.
• Transformation: Transformation is the process by which a cell takes up extracellular genetic material, such as DNA, and incorporates it into its own genome. In bacteria, this can occur naturally or through artificial means, but it does not involve a viral vector.
• Transcription: Transcription is the process by which RNA is synthesized from a DNA template. It is not related to the transfer of DNA between cells, and it does not involve viral mediation.

Answer: Incorporating part of host DNA

Bacteria can acquire characteristics through various mechanisms, such as taking up soluble DNA from the environment (transformation), bacteriophage-mediated transfer of genetic material (transduction), and direct cell-to-cell transfer of genetic material through conjugation. However, bacteria do not typically acquire characteristics by incorporating part of their own host DNA. The mechanisms mentioned involve the acquisition of genetic material from external sources.

The other options are incorrect:

• Taking up soluble DNA from the environment: This is a valid mechanism known as transformation, where bacteria can take up extracellular DNA from their surroundings and incorporate it into their genome.
• Through bacteriophages: Bacteriophages are viruses that infect bacteria and can transfer genetic material from one bacterium to another during the viral replication cycle. This process is known as transduction.
• Through conjugation: Conjugation involves the direct transfer of genetic material (plasmids) from one bacterial cell to another through a physical bridge called a pilus. It is a mechanism of horizontal gene transfer.

Answer: Biofilm formation

Biofilms are communities of microorganisms, including bacteria, that adhere to surfaces and produce a slimy matrix composed of extracellular polymeric substances (EPS). This matrix helps protect the bacterial cells within the biofilm from external threats, including antibiotics. The slimy matrix can act as a physical barrier, making it more challenging for antibiotics to penetrate and reach the bacterial cells embedded in the biofilm. Consequently, biofilm formation is associated with increased antibiotic resistance.

The other options are incorrect:

• Co-aggregation: Co-aggregation is a phenomenon where different bacterial species aggregate and adhere to each other. It is not specifically associated with antibiotic resistance due to slime production.
• Mutation evolving in altered target site for antibiotic: This refers to a mechanism where bacteria develop mutations in the target sites of antibiotics, making the drugs less effective. It is a form of antibiotic resistance but is not directly related to slime production.
• Mutation evolving a target bypass mechanism: This option suggests the development of mutations that allow bacteria to bypass the target of antibiotics. While it represents a mechanism of antibiotic resistance, it is not specifically associated with slime production.

Answer: Involved in multidrug resistance transfer

Plasmids are small, circular DNA molecules that can exist independently of the chromosomal DNA in bacteria. Some plasmids carry genes that provide bacteria with various advantages, including resistance to multiple antibiotics. Plasmids can be transferred between bacteria, allowing for the spread of antibiotic resistance genes.

The other options are incorrect:

• Imparts capsule formation: Plasmids are not typically involved in capsule formation. Capsules are structures composed of polysaccharides that surround some bacteria and contribute to virulence, but their formation is usually governed by chromosomal genes.
• Imparts pili formation: While some plasmids may carry genes related to the formation of pili (appendages that aid in bacterial adhesion), it is not a universal function of plasmids. The presence of pili-related genes is specific to certain plasmids and bacterial strains.

Answer: Is a virus which invades bacteria

viruses that invade bacteria. Bacteriophages, or phages for short, are viruses that specifically infect and replicate within bacterial cells. They have a structure consisting of a protein coat (capsid) surrounding genetic material (either DNA or RNA). Bacteriophages attach to specific receptors on the surface of bacterial cells, inject their genetic material into the bacteria, and use the host’s cellular machinery to replicate and produce more phages.

The other options are incorrect:

• Can transmit toxin to bacteria: Bacteriophages do not transmit toxins to bacteria. They primarily infect bacteria to replicate and do not involve the transfer of toxins.
• Bacterial which transmits DNA to another bacteria: This description is more characteristic of the process of bacterial conjugation, where one bacterium transfers DNA to another through direct cell-to-cell contact. Bacteriophages do not transfer DNA between bacteria.
• Causes transformation of bacteria: The process of transformation in bacteria involves the uptake of free DNA from the environment and its incorporation into the bacterial genome. Bacteriophages do not cause transformation but rather play a role in transduction, where they transfer bacterial DNA from one cell to another during their life cycle.

Answer: Transduction

Transduction is a mechanism by which bacteriophages (viruses that infect bacteria) transfer genetic material from one bacterium to another. During the replication cycle of the bacteriophage, bacterial DNA may be packaged into the phage capsid instead of viral DNA. When the phage infects another bacterium, it injects this bacterial DNA, which may include drug resistance genes, into the new host bacterium. As a result, transduction can contribute to the spread of drug resistance among bacterial populations.

The other options are incorrect:

• Transformation: Transformation involves the direct uptake of extracellular DNA by a bacterial cell. It is not typically associated with drug resistance transfer by bacteriophage.
• Conjugation: Conjugation is a process where genetic material is transferred from one bacterial cell to another through direct cell-to-cell contact. It is mediated by a structure called a pilus. Conjugation is not a mechanism used by bacteriophages.
• Convocation: “Convocation” is not a recognized term in the context of genetic transfer or bacterial processes. It may be a typographical error.

Answer: Is coagulase positive

Coagulase is an enzyme produced by some Staphylococcus species, and the coagulase test is used to differentiate between S. aureus (coagulase-positive) and S. epidermidis (coagulase-negative). The presence of coagulase in S. aureus can cause the formation of clots in blood plasma, while S. epidermidis lacks this ability.

The other options are incorrect:

• Forms white colonies: The color of colonies is not a reliable characteristic for differentiating between S. aureus and S. epidermidis. Colony color can be influenced by various factors, and it is not a specific distinguishing feature.
• A common cause of UTI: Both S. aureus and S. epidermidis are not typically common causes of urinary tract infections (UTIs). UTIs are more commonly associated with other bacteria, such as Escherichia coli.
• Causes endocarditis of prosthetic valve: S. aureus is indeed associated with causing endocarditis, including infective endocarditis of prosthetic valves. However, this statement does not differentiate between S. aureus and S. epidermidis, as both can be involved in endocarditis.

Answer: Staphylococcus aureus

S. aureus produces heat-stable toxins (enterotoxins) that can cause rapid-onset food poisoning. When food, particularly protein-rich foods like sandwiches, is contaminated with S. aureus and allowed to sit at room temperature, the bacteria can multiply and produce toxins. Ingesting the toxins with the contaminated food can lead to symptoms of gastroenteritis, including nausea, vomiting, and abdominal cramps, within a few hours.

The other options are incorrect:

• Salmonella typhi: While it causes typhoid fever, symptoms typically take longer than 4-6 hours to develop (around 1-2 weeks).
• Entamoeba histolytica: It can cause amoebic dysentery, but symptoms can vary and may not always include severe gastroenteritis. Bloody diarrhea is also more common with amoebic dysentery than staphylococcal food poisoning.
• Vibrio cholerae: This bacterium causes cholera, characterized by severe watery diarrhea, not typically associated with sandwiches or rapid onset of symptoms.

Answer: MECA gene

S. aureus exhibits methicillin resistance through the presence of the MECA gene. This gene encodes a penicillin-binding protein (PBP2a) with low affinity for β-lactam antibiotics like methicillin, enabling cell wall synthesis even in their presence.

The other options are incorrect:

• β-lactamase: These enzymes can break down β-lactam antibiotics, but are not involved in methicillin resistance in S. aureus.
• AMPC gene: This gene encodes another penicillin-binding protein, but it does not confer methicillin resistance.
• Porin development: Porins are channels in the bacterial cell wall, and mutations affecting them can decrease antibiotic susceptibility, but this is not the primary mechanism of methicillin resistance in S. aureus.

Answer: S. aureus

This form of osteomyelitis occurs when bacteria, often from an infection elsewhere in the body, spread through the bloodstream and reach the bones, leading to bone infection. S. aureus is a major causative agent of acute hematogenous osteomyelitis, particularly in children. The bacteria can adhere to the bone, causing inflammation, pain, and, if left untreated, potential bone destruction.

The other options are incorrect:

• Streptococcus pneumoniae: While Streptococcus pneumoniae can cause various infections, it is not a common cause of acute hematogenous osteomyelitis. This bacterium is more frequently associated with respiratory infections such as pneumonia.
• E. coli: Escherichia coli (E. coli) is a Gram-negative bacterium and is not a common cause of acute hematogenous osteomyelitis. E. coli is more commonly associated with urinary tract infections and gastrointestinal infections.
• Pneumococcus: “Pneumococcus” typically refers to Streptococcus pneumoniae. While this bacterium can cause respiratory infections and pneumonia, it is not a common cause of acute hematogenous osteomyelitis.

Answer: Hyaluronidase

Hyaluronidase is an enzyme produced by S. aureus that facilitates the breakdown of hyaluronic acid, a major component of the connective tissue in the skin. By breaking down hyaluronic acid, hyaluronidase helps the bacterium spread and move through the tissues more easily. This enzyme is involved in the tissue invasion and can contribute to the persistence of S. aureus in the skin.

The other options are incorrect:

• Catalase: Catalase is an enzyme produced by many bacteria, including S. aureus. It helps in the breakdown of hydrogen peroxide, providing protection against the host’s oxidative defenses. However, catalase is not directly involved in the prolonged presence of S. aureus in the skin.
• Coagulase: Coagulase is an enzyme produced by S. aureus that promotes the formation of blood clots. While coagulase is important for certain aspects of S. aureus pathogenesis, it is not directly responsible for the prolonged presence of the bacterium in the skin.
• DNAase: DNAase is an enzyme that breaks down DNA. It plays a role in the breakdown of extracellular DNA, which can be part of S. aureus biofilms. However, it is not a primary factor in the prolonged presence of S. aureus in the skin.

Answer: All of these

All three listed pathogens can cause infective endocarditis (IE) eight months after prosthetic valve replacement, albeit with varying frequencies and mechanisms:

• Staphylococcus aureus: More common in early-onset (within 2 months) prosthetic valve IE due to its virulence and ability to form biofilms on artificial surfaces. Can still occur eight months post-surgery, typically through bloodstream spread from other colonized sites.
• Staphylococcus epidermidis: More prominent in late-onset (> 2 months) prosthetic valve IE due to its persistent colonization of skin and medical devices. Can form persistent biofilms on prosthetic valves, leading to delayed infections.
• Streptococcus viridans: Can cause prosthetic valve IE at any time after surgery, including eight months later. Often originates from oral flora and enters the bloodstream during routine activities like brushing teeth.