MicroBiology MCQs with Answer and Explnation | Chapter 25

Answer: The heart and nervous system

The target cells of diphtheria toxin (not diphtherotoxin) are located in the heart and nervous system. Diphtheria toxin is produced by the bacterium Corynebacterium diphtheriae, and its action primarily affects these tissues. The toxin inhibits protein synthesis, leading to cell damage and tissue injury.

The other options are incorrect:

• The skin: The skin is not the primary target for diphtheria toxin. The toxin’s effects are more pronounced in the heart and nervous system.
• The skeletal muscles: While diphtheria toxin can have systemic effects, its primary targets are the heart and nervous system, not the skeletal muscles.
• The lungs: Diphtheria toxin primarily affects the heart and nervous system, and the lungs are not the main target for its toxic action.

Answer: Karyogamy

The fusion of nuclei in fungi is called karyogamy. It is a process where the nuclei from two different mating types fuse, leading to the formation of a diploid nucleus, which is a characteristic feature in the life cycle of fungi.

The other options are incorrect:

Progamy: Progamy refers to the condition in which the gametes (sex cells) are produced at different times, preventing immediate fusion. This is not the term used for the fusion of nuclei in fungi.
Microgamy: Microgamy refers to a form of sexual reproduction in plants where the sperm cell fuses with a small, specialized female cell. This term is not applicable to the fusion of nuclei in fungi.
Pregamy: Pregamy is not a recognized term in the context of fungal biology or the fusion of nuclei. It does not describe the specific process of nuclear fusion in fungi.

Answer: Adenine

Adenine is a purine. Purines are one of the two types of nitrogenous bases found in nucleotides, the building blocks of DNA and RNA. Adenine pairs with thymine in DNA and with uracil in RNA.

The other options are incorrect:

• Thymine: Thymine is a pyrimidine, not a purine. It pairs with adenine in DNA.
• Uracil: Uracil is a pyrimidine, not a purine. It pairs with adenine in RNA.
• Cytosine: Cytosine is a pyrimidine, not a purine. It pairs with guanine in both DNA and RNA.

Answer: Nonsense mutation

A nonsense mutation is a type of genetic mutation that leads to the premature termination of the synthesis of a protein. It occurs when a point mutation introduces a premature stop codon in the DNA sequence, causing the ribosome to stop translation earlier than expected. This results in a truncated and usually nonfunctional protein.

The other options are incorrect:

• Missense mutation: A missense mutation is a type of mutation where a single nucleotide change results in a codon that codes for a different amino acid. This may lead to an altered but still functional protein.
• Frameshift mutation: A frameshift mutation occurs when nucleotides are inserted or deleted in a DNA sequence, shifting the reading frame. This often leads to a completely different amino acid sequence and can result in a nonfunctional protein.
• Alternation: “Alternation” is not a term commonly used in the context of genetic mutations. The correct term for a mutation that introduces a premature stop codon is a nonsense mutation.

Answer: Toxoid

The anthrax vaccine is a toxoid vaccine. Toxoids are vaccines that are created from inactivated toxins produced by bacteria. In the case of anthrax, the vaccine is based on the inactivated toxin produced by Bacillus anthracis, the causative agent of anthrax. The inactivated toxin elicits an immune response that provides protection against the harmful effects of the toxin.

The other options are incorrect:

• Attenuated bacteria: Attenuated vaccines use live, weakened forms of the bacteria or virus. The anthrax vaccine is not based on attenuated bacteria but on inactivated toxin.
• Killed whole bacterial cells: Some vaccines use killed whole bacterial cells, but the anthrax vaccine specifically focuses on the inactivated toxin produced by Bacillus anthracis, not the whole bacterial cells.
• Recombinant: Recombinant vaccines are created using genetic engineering techniques to produce specific proteins or antigens. The anthrax vaccine is not a recombinant vaccine but is based on inactivated toxin.

Answer: Respiratory droplets

Tuberculosis, caused by Mycobacterium tuberculosis bacteria, primarily spreads through the air when someone with active lung or throat TB coughs, sneezes, sings, or talks. These tiny droplets containing the bacteria can be inhaled by others, potentially infecting them.

The other options are incorrect:

• Contaminated fomites: While the bacteria can survive on surfaces for some time, transmission through contaminated objects like utensils or linens is rare.
• Food: Mycobacterium tuberculosis doesn’t thrive in food, and foodborne transmission is practically non-existent.
• Vectors: Mosquitoes or other insects don’t transmit TB. While Mycobacterium tuberculosis can infect some animals, it’s not considered a significant source for human transmission.

Answer: Production of tetanospasmin

The major symptoms of tetanus, including muscle spasms and lockjaw, are caused by a toxin called tetanospasmin produced by the bacterium Clostridium tetani. This toxin interferes with nerve signals, causing muscle contractions and rigidity.

The other options are incorrect:

• Multiplication of organisms at the site of infection: While multiplication at the wound site is part of the initial infection process, it doesn’t directly cause the major symptoms.
• Production of botulin toxin: Botulin toxin is associated with botulism, a different disease with distinct symptoms. Tetanus is caused by tetanospasmin.
• Superinfection due to antibiotic therapy: While antibiotic therapy can sometimes lead to superinfections, it’s not directly related to the major symptoms of tetanus. Antibiotics are actually used to treat tetanus alongside antitoxin therapy.

Answer: Antibiotic-induced colitis

Clostridium difficile (C. diff) is primarily associated with antibiotic-induced colitis, which is an inflammation of the inner lining of the colon (large intestine) caused by overgrowth of this bacterium. This overgrowth can occur after taking antibiotics, which disrupt the natural balance of gut bacteria, allowing C. diff to flourish and release toxins that damage the colon.

The other options are incorrect:

• Myonecrosis: Myonecrosis is a rare and serious condition involving tissue death in muscle. While C. diff can occasionally infect wounds, it’s not typically associated with myonecrosis.
• Food poisoning: Food poisoning is commonly caused by bacteria like Salmonella or E. coli, not C. diff. While C. diff can be present in some food, it’s not the typical culprit behind foodborne illness.
• Gas gangrene: Gas gangrene is a life-threatening infection caused by bacteria that thrive in oxygen-deprived environments. C. diff requires oxygen to grow and doesn’t cause gas gangrene.

Answer: Proteus vulgaris

Proteus vulgaris is known for its ability to swarm on a agar plate, covering the surface and making it difficult to distinguish individual colonies. This swarming behavior is characterized by the rapid and coordinated movement of bacteria across the agar surface.

The other options are incorrect:

• E. coli: Escherichia coli (E. coli) does not exhibit swarming behavior on agar plates. It typically forms distinct colonies.
• Shigella dysenteriae: Shigella dysenteriae does not swarm on agar plates. It forms isolated colonies on solid media.
• Salmonella typhi: Salmonella typhi does not exhibit swarming behavior. It forms discrete colonies on agar plates.

Answer: Surface immunoglobulin

Surface immunoglobulin, also known as B-cell receptor (BCR), is characteristic of B-cells but not T-cells. B-cells use surface immunoglobulins to recognize antigens, and when activated, they can differentiate into antibody-secreting plasma cells.

The other options are incorrect:

• Class I MHC: Both B-cells and T-cells express Class II MHC, not Class I. Class I MHC is mainly found on nucleated cells and presents antigens to cytotoxic T-cells.
• CD3: CD3 is a complex of proteins associated with the T-cell receptor (TCR). It is found on the surface of T-cells, not B-cells.
• Polyclonal activation by concanavalin A: Polyclonal activation by concanavalin A is a characteristic of T-cells, not B-cells. It is a lectin that stimulates T-cell proliferation.

Answer: Bacillus anthracis

Robert Koch developed his famous postulates based on his work isolating and characterizing the bacterium Bacillus anthracis, the causative agent of anthrax. He used his observations of this organism to establish a set of criteria for demonstrating the specific cause of infectious diseases.

The other options are incorrect:

• Bacillus cereus: While Bacillus cereus can cause food poisoning, it wasn’t the organism Koch used for his postulates.
• Clostridium tetani: This bacterium causes tetanus, and Koch studied it later in his career, but it wasn’t the basis for his initial postulates.
• Staphylococcus aureus: This common bacteria causes various infections, but Koch didn’t use it for his postulates.

Answer: Contaminated food

Most cases of listeriosis are transmitted through the consumption of contaminated food. Listeria monocytogenes, the bacterium responsible for listeriosis, can be found in certain foods, especially unpasteurized dairy products, raw vegetables, and processed meats. Infection occurs when individuals consume contaminated food containing viable Listeria bacteria.

The other options are incorrect:

• Insect vectors: Listeriosis is not typically transmitted by insect vectors. The main route of transmission is through contaminated food.
• Respiratory secretions: Listeriosis is not primarily transmitted through respiratory secretions. It is mainly associated with the ingestion of contaminated food.
• Transplacental: While listeriosis can lead to infections in pregnant women and transmission to the fetus through the placenta, the primary mode of transmission to the general population is through the consumption of contaminated food.

Answer: Mycobacterium leprae

Mycobacterium leprae is an obligate parasite. This means that it is a microorganism that can only survive and replicate within a host cell. Mycobacterium leprae is the bacterium responsible for causing leprosy (Hansen’s disease) and is known for its obligate intracellular lifestyle.

The other options are incorrect:

• Mycobacterium tuberculosis: While Mycobacterium tuberculosis is a pathogenic bacterium, it is not an obligate parasite. It can survive and replicate outside host cells under certain conditions.
• Corynebacterium diphtheriae: Corynebacterium diphtheriae is a bacterium responsible for diphtheria. It is not an obligate parasite as it can also exist outside of host cells.
• Clostridium difficile: Clostridium difficile is a bacterium associated with antibiotic-associated diarrhea and colitis. It is not an obligate parasite and can thrive in various environments.

Answer: Anthrax

Anthrax is considered a zoonosis, meaning it is an infectious disease that can be transmitted from animals to humans. Bacillus anthracis, the bacterium responsible for anthrax, can infect a variety of animals, and humans can contract the disease through contact with infected animals or their products.

The other options are incorrect:

• Gas gangrene: Gas gangrene, caused by Clostridium perfringens, is not typically considered a zoonosis. It is often associated with wound contamination rather than direct transmission from animals.
• Diphtheria: Diphtheria, caused by Corynebacterium diphtheriae, is not a zoonosis. It is primarily transmitted from person to person through respiratory droplets.
• Leprosy: Leprosy, caused by Mycobacterium leprae, is not typically considered a zoonosis. It is thought to be transmitted from person to person, and the exact route of transmission is not fully understood.

Answer: Binary Fission

Bacterial cells divide by binary fission. Binary fission is a form of asexual reproduction where a single bacterial cell divides into two identical daughter cells. This process is characterized by the replication of the bacterial DNA and the division of the cell into two separate entities.

The other options are incorrect:

• Budding: Budding is a form of asexual reproduction seen in certain yeast and other organisms, but it is not the method of bacterial cell division.
• Spores: While some bacteria can form spores as a survival strategy, spore formation is not the primary method of bacterial cell division. Spore formation is a specific reproductive strategy used under certain conditions.
• Sexual reproduction: Bacteria primarily reproduce through asexual processes such as binary fission. Sexual reproduction, involving the exchange of genetic material between two individuals, is not a common method in bacterial reproduction.

Answer: Paroxysmal coughing

A classic symptom of pertussis, also known as whooping cough, is paroxysmal coughing. Pertussis is characterized by severe and uncontrollable coughing fits (paroxysms), often followed by a distinctive “whooping” sound during inhalation. The coughing fits can be prolonged and may lead to exhaustion.

The other options are incorrect:

• Diarrhea: Diarrhea is not a classic symptom of pertussis. Pertussis primarily manifests with respiratory symptoms, including the characteristic paroxysmal cough.
• Convulsions: Convulsions are not a typical symptom of pertussis. The main features are related to respiratory distress caused by severe coughing.
• Headache: Headache is not a characteristic symptom of pertussis. The primary symptoms involve the respiratory system, particularly the severe and prolonged coughing episodes.

Answer: Both “Intestinal perforation” and “Liver abscesses”

Complications of typhoid fever can include both intestinal perforation and liver abscesses. Intestinal perforation is a serious complication where the wall of the intestine can rupture, leading to the leakage of intestinal contents into the abdominal cavity. Liver abscesses can also occur as a result of the spread of the Salmonella typhi bacteria to the liver.

The other options are incorrect:

• Neurological damage: Neurological damage is not a typical complication of typhoid fever. The primary complications are associated with the gastrointestinal system and other organs.

Answer: Contact with urine from an infected animal

Leptospira species are transmitted to humans through contact with the urine of infected animals. The bacteria are shed in the urine of infected reservoir hosts, such as rodents and livestock, and can enter the human body through mucous membranes or breaks in the skin.

The other options are incorrect:

• Animal bites: Leptospira species are primarily transmitted through contact with urine, not through animal bites.
• Arthropod vectors: Leptospirosis is not typically transmitted by arthropod vectors (e.g., mosquitoes or ticks). Direct contact with contaminated water or soil, or with the urine of infected animals, is the primary route.
• Inhalation: Inhalation is not a common route of transmission for Leptospira species. The primary mode is through contact with contaminated urine.

Answer: All of the above

Coliforms are used as indicator organisms of sewage pollution for several reasons:

• They are abundant in human intestines: Coliforms are naturally present in the gut microbiome of humans and warm-blooded animals. Their presence in water suggests fecal contamination from these sources.
• They ferment lactose: Most coliforms can ferment lactose, a sugar found in milk, which produces gas and acid. This characteristic makes them easy to detect and quantify in water samples.
• They are not pathogens: While some coliforms, like E. coli, can cause illness, the majority are not harmful. Their presence indicates the potential for more dangerous pathogens, like viruses and parasites, to be present as well.

Answer: Intoxication

A patient with nausea, vomiting, and diarrhea within a short timeframe after eating most likely has food intoxication. Food intoxication is caused by the ingestion of pre-formed toxins present in contaminated food. The rapid onset of symptoms, typically within a few hours, distinguishes intoxication from infections, which usually have a longer incubation period.

The other options are incorrect:

• Shigellosis: Shigellosis is a bacterial infection caused by Shigella species. It usually has a longer incubation period (1-7 days) before symptoms appear.
• E. coli gastroenteritis: E. coli gastroenteritis can result from the ingestion of pathogenic strains of Escherichia coli. The onset of symptoms is generally not as rapid as in food intoxication.
• Salmonellosis: Salmonellosis is a bacterial infection caused by Salmonella species. Like shigellosis, it often has a longer incubation period (6 hours to 6 days) before symptoms develop.

Answer: Enlarged lymph node

The bubo of bubonic plague is an enlarged and painful lymph node. Bubonic plague is caused by the bacterium Yersinia pestis, which is transmitted through the bite of an infected flea. The bacteria then multiply in the lymph nodes, causing inflammation and the characteristic development of a painful, swollen, and tender lymph node, known as a bubo.

The other options are incorrect:

• Ulcer where the flea bite occurred: While flea bites can lead to skin lesions, the bubo itself is not an ulcer at the site of the flea bite.
• Granuloma in the skin: A granuloma is a type of inflammatory response, but in the context of bubonic plague, the characteristic feature is the enlarged lymph node, not a granuloma in the skin.
• Infected sebaceous gland: The bubo in bubonic plague is not associated with an infected sebaceous gland. It is specifically an enlarged and inflamed lymph node.

Answer: All of these

Escherichia coli can display all three types of antigens: capsular, flagellar, and somatic.

• Capsular antigens: These are polysaccharide molecules that form a capsule around the bacterial cell. They help E. coli evade the immune system and contribute to its virulence. There are many different types of capsular antigens, and their presence can determine the specific E. coli serotype.
• Flagellar antigens: These are proteins that form the flagella, hair-like appendages that allow E. coli to move. Flagellar antigens can also be classified into different serotypes, and they influence the bacterium’s motility and ability to spread.
• Somatic antigens: These are also known as lipopolysaccharides (LPS) and are integrated into the outer membrane of the bacterial cell wall. They play a role in maintaining the cell structure and protecting it from certain environmental factors. Somatic antigens can also be categorized into different serogroups based on their structure.

Answer: A fluctuating pattern of fever

The cardinal manifestation of human brucellosis is a fluctuating pattern of fever. Brucellosis is caused by bacteria of the genus Brucella, and it typically presents with recurrent, undulating fevers, along with symptoms such as sweating, weakness, and muscle aches.

The other options are incorrect:

• Vomiting and diarrhea: Vomiting and diarrhea are not typical cardinal manifestations of brucellosis. Gastrointestinal symptoms are less common in this infection.
• A pseudo-membrane in the throat: The presence of a pseudo-membrane in the throat is associated with infections like diphtheria, not brucellosis.
• Peeling of the skin on the palms and soles: Peeling of the skin on the palms and soles is not a characteristic manifestation of brucellosis. It may be seen in conditions such as toxic shock syndrome or certain viral infections.

Answer: Non-glucose fermenting

Non-glucose fermenting is not a characteristic of the coliform group. Coliform bacteria are characterized by their ability to ferment lactose with the production of acid and gas. Non-glucose fermenting bacteria would not fit the typical profile of coliforms.

The other options are incorrect:

• Lactose fermenting: Lactose fermenting is a characteristic of the coliform group. The ability to ferment lactose is a key feature used in their identification.
• Oxidase negative: Coliform bacteria are typically oxidase-negative. This is a characteristic feature that helps distinguish them from certain other groups of bacteria.
• Gram-negative rods: Coliform bacteria are Gram-negative rods. This is another characteristic feature used in their classification.

Answer: Salmonella typhi

Salmonella typhi was not discovered by Robert Koch. Salmonella typhi is associated with typhoid fever, and its discovery is credited to Karl Joseph Eberth, who first identified the bacterium in the tissues of individuals with typhoid fever.

The other options are incorrect:

• Bacillus anthracis: Robert Koch is credited with the discovery of Bacillus anthracis, the bacterium responsible for causing anthrax.
• Mycobacterium tuberculosis: Robert Koch is also credited with the discovery of Mycobacterium tuberculosis, the bacterium responsible for causing tuberculosis.
• Vibrio cholerae: Robert Koch did significant work on Vibrio cholerae, including identifying its causative role in cholera. However, he did not exclusively discover it; others had contributed to the understanding of cholera before Koch.

Answer: Secondary syphilis

The stage of syphilis characterized by a disseminating rash, alopecia (hair loss), lymphadenopathy (swollen lymph nodes), and flu-like symptoms is secondary syphilis. This stage occurs several weeks to a few months after the appearance of the primary chancre in primary syphilis.

The other options are incorrect:

• Primary syphilis: Primary syphilis is characterized by the appearance of a painless chancre at the site of infection. It is the initial stage of syphilis.
• Tertiary syphilis: Tertiary syphilis is a later stage of the disease and is characterized by the development of gummas (soft, non-cancerous growths), cardiovascular complications, and neurosyphilis.
• Congenital syphilis: Congenital syphilis occurs when a pregnant woman with syphilis passes the infection to her baby. It can result in a variety of symptoms and complications in the newborn.

Answer: Mycoplasma

Mycoplasma would be unsusceptible to penicillin. Mycoplasma species lack a cell wall, and penicillin primarily targets the synthesis of bacterial cell walls. Since Mycoplasma lacks this target, it is inherently resistant to penicillin.

The other options are incorrect:

• Leptospira: Leptospira is susceptible to penicillin. It has a cell wall, and penicillin can affect the synthesis of its cell wall.
• Chlamydia: Chlamydia is intracellular and has a unique developmental cycle. While penicillin is not the first-line treatment for Chlamydia, it can still have some activity against certain species.
• Rickettsia: Rickettsia is susceptible to certain antibiotics, including tetracyclines and chloramphenicol. While penicillin is not typically used, it may have some limited activity against certain species.

Answer: Borrelia burgdorferi, ticks

Lyme disease is caused by the bacterium Borrelia burgdorferi and is primarily spread through the bite of infected ticks. The black-legged tick (Ixodes scapularis) and the western black-legged tick (Ixodes pacificus) are common vectors for transmitting the bacteria to humans.

The other options are incorrect:

• Borrelia recurrentis, lice: Borrelia recurrentis is associated with relapsing fever, and its transmission is often associated with lice bites, not ticks.
• Borrelia hermsii, ticks: Borrelia hermsii is also associated with relapsing fever, and it is transmitted by soft ticks, not the same ticks associated with Lyme disease.
• Borrelia burgdorferi, chiggers: Borrelia burgdorferi is the correct causative agent of Lyme disease, but it is primarily spread by ticks, not chiggers.

Answer: Antibody producing B cells

Antibody-producing B cells do not express MHC class II molecules on their surface. Their primary function is to produce antibodies that directly neutralize pathogens or target them for destruction by other immune cells. MHC class II molecules are crucial for antigen presentation to T cells, which B cells are not involved in.

The other options are incorrect:

• Cytotoxic T cells: These cells directly kill infected cells and tumors and express MHC class II molecules to present antigens to CD4+ T helper cells for optimal activation.
• Dendritic cells: These professional antigen-presenting cells express high levels of MHC class II molecules on their surface to effectively present captured antigens to T cells.
• Macrophages: These phagocytes also express MHC class II molecules for antigen presentation to T cells, alongside their role in engulfing and digesting pathogens.

Answer: Tryptophan

The indole test indicates the cleavage of tryptophan. In this test, bacteria that possess the enzyme tryptophanase can break down tryptophan, an amino acid, into indole, pyruvic acid, and ammonia. The presence of indole is detected by adding Kovac’s reagent to the bacterial culture, resulting in the formation of a red color.

The other options are incorrect:

• Lactose: The indole test is not related to the cleavage of lactose. Lactose fermentation is typically assessed through other tests, such as the phenol red broth test.
• Glucose: The indole test is not indicative of the cleavage of glucose. Glucose metabolism is often tested using various media like glucose broth.
• Tyrosine: The indole test is specific to the cleavage of tryptophan and does not provide information about the cleavage of tyrosine. Tyrosine metabolism is evaluated using different biochemical tests.

Answer: Dental caries

Dental caries, commonly known as cavities, is the most common chronic disease affecting humans worldwide. It affects people of all ages and backgrounds, with estimates suggesting that nearly 90% of adults have experienced at least one cavity in their lifetime.

The other options are incorrect:

• Walking pneumonia: While a common respiratory infection, it affects a smaller portion of the population compared to dental caries and is not considered a chronic disease.
• Strep throat: Also a relatively common bacterial infection, but again, it affects a smaller portion of the population compared to dental caries and is not chronic.
• Tuberculosis: A serious infectious disease, but not as widespread as dental caries, particularly in developed countries with effective public health measures.

Answer: Genitourinary tract infection

Ureaplasma, a genus of tiny bacteria lacking a cell wall, is most commonly associated with genitourinary tract infections (UTIs), particularly non-gonococcal urethritis (NGU) in men and non-gonococcal cervicitis (NGC) in women. These infections often exhibit symptoms like burning urination, pelvic pain, and abnormal discharge.

The other options are incorrect:

• Atypical pneumonia: Ureaplasma has been implicated in some cases of atypical pneumonia, especially in immunocompromised individuals. However, its role is not fully established, and other pathogens like Mycoplasma pneumoniae are more commonly responsible.
• Tracheobronchitis: Ureaplasma detection in tracheobronchitis cases is rare and its clinical significance is unclear. Further research is needed to determine its potential role.
• Influenza-like illness: Ureaplasma is not typically associated with influenza-like symptoms like fever, chills, and muscle aches. These symptoms are more commonly caused by viral infections like influenza or the common cold.

Answer: Bactericidal

Penicillin is a bactericidal drug, meaning it kills bacteria. It does this by interfering with the growth of their cell walls, specifically targeting a protein called PBP (penicillin-binding protein) responsible for linking new cell wall components. Without this crucial protein functioning, bacteria cannot build strong cell walls, leading to their death as internal pressure bursts the weakened membrane.

The other options are incorrect:

• Bacteriocins: Bacteriocins are proteinaceous toxins produced by bacteria to inhibit the growth of closely related bacterial strains. They are not drugs like penicillin.
• Bacteriostatic: Bacteriostatic drugs inhibit the growth and reproduction of bacteria but do not necessarily cause their death. Penicillin, however, is bactericidal.
• Bacteria inhibiting: “Bacteria inhibiting” is a general term and doesn’t specifically describe the mode of action of penicillin. Penicillin is more accurately described as bactericidal.

Answer: Toxin disrupts protein synthesis

The primary virulence factor for Escherichia coli (E. coli) is often associated with toxins that disrupt protein synthesis. E. coli strains can produce toxins, such as Shiga toxins or heat-stable and heat-labile enterotoxins, which interfere with the host cell’s protein synthesis machinery, leading to various pathogenic effects.

The other options are incorrect:

• Inflammatory response: While E. coli infections can elicit an inflammatory response, the primary virulence factor is often associated with specific toxins rather than the general inflammatory response.
• cAMP inducing toxin: Some E. coli strains can produce toxins that induce an increase in cyclic AMP (cAMP) levels in host cells, but this is not the primary virulence factor. The disruption of protein synthesis is more central to their pathogenicity.
• Superantigen: Superantigens are proteins that can provoke an exaggerated immune response, but they are not typically considered the primary virulence factor for E. coli.

Answer: Gram Positive Cell Wall

Lysozymes are enzymes found in various bodily fluids, including sweat, tears, and saliva. They are known for their ability to break down peptidoglycan, a major component of the bacterial cell wall. However, they are particularly effective against Gram-positive bacteria due to the specific structure of their peptidoglycan layer.

The other options are incorrect:

• Gram-negative cell wall: Gram-negative bacteria have an outer membrane surrounding their cell wall, which provides additional protection against lysozymes. This outer membrane allows them to resist the enzyme’s action more effectively than Gram-positive bacteria.
• Viruses: Viruses do not have cell walls, so lysozymes cannot target them. They are effective against bacteria due to their specific action on peptidoglycan, which is absent in viruses.
• Parasites: Parasites are eukaryotic organisms and do not have cell walls similar to bacteria. While lysozymes might play a role in immune defense against some parasites, their main target remains bacteria.

Answer: Cell membranes

Mycoplasmas attack the cell membranes of host cells. These bacteria lack a cell wall, which makes them different from other bacteria, and they are known for attaching to and invading host cell membranes. Mycoplasmas can disrupt cell membrane integrity and affect the function of the host cell.

The other options are incorrect:

• Nucleus: Mycoplasmas do not primarily attack the nucleus of host cells. Their main target is the cell membrane.
• Ribosomes: Mycoplasmas are not known for attacking ribosomes. Their pathogenicity is more associated with interactions at the cell membrane level.
• Mitochondria: Mycoplasmas typically do not target mitochondria. Their main mode of interaction is with the host cell membrane.

Answer:

Both Rickettsia and Chlamydia share the characteristic of being obligate intracellular bacteria, meaning they cannot survive and reproduce outside of host cells. They rely on the host cell machinery for essential metabolic processes and replication. This obligate intracellular lifestyle makes them unique among bacteria and contributes to their virulence and the types of diseases they cause.

The other options are incorrect:

• Free of a cell wall: While both Rickettsia and Chlamydia lack a typical bacterial cell wall, they do have other outer layers that provide some protection and contribute to their pathogenicity. Therefore, the absence of a cell wall is not exclusive to these two genera.
• The cause of eye infections: While both can cause eye infections (e.g., trachoma for Chlamydia, epidemic typhus for Rickettsia), this is not their sole or defining characteristic. They also have a wide range of other potential target organs and associated diseases.
• Carried by arthropod vectors: While some Rickettsia and Chlamydia species are transmitted by arthropod vectors (e.g., ticks for Rickettsia, mosquitoes for Chlamydia trachomatis), others have different transmission routes. This is not a universal characteristic of these genera.

Answer: Elementary body

The infectious stage of Chlamydia is the elementary body. Chlamydia has a unique biphasic developmental cycle that involves infectious elementary bodies and non-infectious reticulate bodies. The elementary bodies are the extracellular, infectious form that can survive outside the host and initiate infection.

The other options are incorrect:

• Reticulate body: The reticulate body is the intracellular, non-infectious form of Chlamydia. It is involved in replication within the host cell but is not the infectious stage.
• Vegetative cell: The term “vegetative cell” is not commonly used to describe Chlamydia. The infectious stage is specifically referred to as the elementary body.
• Both “Reticulate body” and “Vegetative cell”: The infectious stage is the elementary body, not the reticulate body or a hypothetical “vegetative cell.”

Answer: Neisseria meningitidis

Endotoxin is responsible for symptoms caused by Neisseria meningitidis. Neisseria meningitidis is a Gram-negative bacterium, and endotoxin (lipopolysaccharide or LPS) is a component of the outer membrane of Gram-negative bacteria. The release of endotoxin can trigger a strong immune response and lead to symptoms such as fever, shock, and inflammation, as seen in meningococcal infections.

The other options are incorrect:

• Streptococcus pyogenes: Streptococcus pyogenes is a Gram-positive bacterium, and endotoxin is not a major component of Gram-positive bacterial cell walls.
• Clostridium tetani: Clostridium tetani is a Gram-positive bacterium, and endotoxin is not responsible for the symptoms associated with tetanus.
• Bacillus anthracis: Bacillus anthracis is a Gram-positive bacterium, and endotoxin is not the primary factor responsible for symptoms in anthrax infections.

Answer: Eosinophils

Eosinophils function primarily as phagocytes in parasitic infections. These white blood cells are especially effective against helminthic parasites. Eosinophils release toxic granules containing enzymes and proteins that are toxic to parasites, helping to defend the host against parasitic infections.

The other options are incorrect:

• Lymphocytes: Lymphocytes play a key role in the adaptive immune response, including the production of antibodies and the coordination of immune responses. They are not primarily phagocytes.
• Basophils: Basophils are involved in allergic reactions and the release of histamine. While they play a role in the immune response, they are not primary phagocytes.
• Neutrophils: Neutrophils are important phagocytes and play a crucial role in the defense against bacterial and fungal infections. However, eosinophils are more specifically associated with parasitic infections.

Answer: Painless chancre

A painless chancre is characteristic of primary syphilis. A chancre is a sore or ulcer that typically appears at the site of initial infection with the bacterium Treponema pallidum. The painless nature of the chancre is a notable feature of primary syphilis.

The other options are incorrect:

• Painful chancre: Primary syphilis is associated with a painless chancre, not a painful one.
• Several painful ulcers in genital region: Primary syphilis is typically characterized by a single painless chancre rather than multiple painful ulcers.
• Several painless ulcers in genital region: While multiple ulcers can occur in conditions like herpes, primary syphilis is characterized by a single, painless chancre.

Answer: CD8 + T lymphocytes

MHC I molecules present peptides derived from intracellular proteins. CD8+ T lymphocytes have T cell receptors (TCRs) specifically designed to recognize these MHC I-peptide complexes and trigger immune responses against infected or abnormal cells.

The other options are incorrect:

• B-lymphocytes: These recognize antigens through B cell receptors (BCRs) and primarily participate in humoral immunity. They don’t interact with MHC I.
• CD4+ lymphocytes: These recognize MHC II molecules bound to peptides originating from extracellular or internalized antigens. They play a role in activating other immune cells, but not through direct MHC I recognition.
• Monocytes: These are phagocytic cells and antigen presenters. They express MHC I but don’t have TCRs, so they can’t recognize these molecules themselves.

Answer: T-helper cells

APCs (Antigen Presenting Cells) specialize in capturing, processing, and presenting antigens to T lymphocytes. T-helper cells, however, are themselves T lymphocytes and play a different role in the immune response by assisting the activation of other T cells and immune cells. Their primary function is not antigen presentation.

The other options are incorrect:

• B-cells: While not technically considered APCs, B-cells do play a role in antigen presentation to some extent. They can internalize antigens, process them, and present them on their surface to activate T cells.
• Dendritic cells: Highly specialized APCs, dendritic cells are masters at antigen capture, processing, and presentation. They efficiently activate both CD4+ and CD8+ T lymphocytes.
• Macrophages: Another type of APC, macrophages are phagocytic cells that internalize antigens and present them to T cells. They play a crucial role in innate immunity and antigen presentation.

Answer: di-sulphide bond

Light chains and heavy chains in antibodies (immunoglobulins) are joined by disulfide bonds. Disulfide bonds are strong covalent bonds formed between the sulfur atoms of cysteine residues in the polypeptide chains. These bonds contribute to the overall structure and stability of the antibody molecule.

The other options are incorrect:

• Covalent bond: While disulfide bonds are a type of covalent bond, not all covalent bonds involve sulfur atoms or specifically connect light and heavy chains in antibodies.
• Hydrogen bond: Hydrogen bonds are weaker interactions compared to covalent bonds and are not typically involved in joining the light and heavy chains of antibodies.
• Ionic bond: Ionic bonds involve the transfer of electrons between atoms with different electronegativities, leading to the formation of charged ions. Ionic bonds are not the primary type of bond involved in connecting light and heavy chains in antibodies.

Answer: C3bBb

Complement component C3 can be cleaved by the C3 convertase enzyme, which is formed by the sequential action of C3b and Factor Bb. This enzymatic complex, known as C3bBb, cleaves C3 into C3a and C3b. C3b plays a crucial role in various immune responses, including opsonization, phagocytosis, and the formation of the membrane attack complex.

The other options are incorrect:

• C3b: C3b is a component of the C3 convertase complex, and it does not cleave C3. Instead, it is involved in opsonization and the formation of the C5 convertase.
• Factor B: Factor B is involved in the formation of the C3 convertase (C3bBb) by binding to C3b and being cleaved by Factor D to produce the active C3 convertase.
• Factor D: Factor D is involved in the cleavage of Factor B to form the active C3 convertase (C3bBb). It does not directly cleave C3.

Answer: During the first year of life

The thymus, a crucial organ for T cell development, reaches its maximal size and activity within the first year of life, typically between 3-12 months. Following this peak, it gradually involutes (shrinks) throughout adolescence and adulthood.

The other options are incorrect:

• Teenage years (puberty): While the thymus remains functionally active during puberty, its size actually starts to decrease around this time.
• Between 40 and 50 years of age: The involution process continues significantly throughout adulthood, with the thymus becoming much smaller and less active by this age range.
• After 70 years of age: By this point, the thymus is very small and functionally inactive, contributing minimally to T cell production.

Answer: Interferons

Interferons mediate an initial response to viral infections by the innate immune system. Interferons are signaling proteins released by infected cells to trigger the protective defenses of neighboring cells. They play a crucial role in inhibiting the replication of viruses, enhancing the activity of natural killer cells, and promoting the maturation of dendritic cells.

The other options are incorrect:

• Complement components: Complement components are part of the innate immune system and contribute to the defense against various pathogens, but they are not specific to viral infections and have a broader role in immune responses.
• T and B lymphocytes: T and B lymphocytes are components of the adaptive immune system. While they are crucial for a specific and targeted immune response, they are not part of the initial innate immune response to viral infections.
• Cytokines: Cytokines are signaling molecules that play various roles in immune responses, including communication between immune cells. However, interferons are a specific subset of cytokines that are particularly important in the antiviral response of the innate immune system.

Answer: Cytokines

Cytokines act as messenger molecules that link the innate and adaptive immune systems. They are produced by various immune cells in response to pathogens or other stimuli and transmit signals across cell types, instructing them to perform specific functions.

The other options are incorrect:

• Complement components: While complement plays a role in opsonization and inflammation, it doesn’t directly act as a communication link between the two immune systems.
• T and B lymphocytes: These are lymphocytes of the adaptive immune system and wouldn’t act as messengers for the innate response.
• Interferons: While interferons are crucial cytokines specifically targeting viral infections, they belong to a broader category of these messenger molecules.

Answer: Microglial cells

Microglial cells are resident macrophages in the central nervous system (CNS). They are specialized immune cells that play a crucial role in immune surveillance and defense within the brain and spinal cord. Microglial cells are involved in maintaining the health of the CNS, responding to injury or infection, and participating in the regulation of immune responses within the brain.

The other options are incorrect:

• Kupffer cells: Kupffer cells are macrophages found in the liver, not in the central nervous system.
• Alveolar cells: Alveolar cells are cells lining the alveoli of the lungs and are not macrophages resident in the CNS.
• Langerhans cells: Langerhans cells are dendritic cells found in the skin and mucous membranes. They are not resident macrophages in the CNS.

Answer: Cathelicidin

Neutrophils employ a process known as the respiratory burst to generate reactive oxygen species (ROS) and reactive nitrogen species (RNS) to kill bacteria. In anaerobic conditions (low oxygen levels), neutrophils rely more on the generation of ROS through the enzyme respiratory burst oxidase. This enzyme is responsible for producing superoxide and other reactive oxygen species, contributing to the oxidative burst that helps eliminate bacteria.

The other options are incorrect:

• IL-12: Interleukin-12 (IL-12) is a cytokine that plays a role in the immune response, particularly in promoting the differentiation of T cells. However, it is not directly involved in the generation of reactive oxygen species by neutrophils.
• Nitric oxide: Nitric oxide (NO) is produced by various immune cells, including macrophages, and has antimicrobial properties. However, neutrophils primarily use respiratory burst oxidase for generating ROS.
• Cathelicidin: Cathelicidins are antimicrobial peptides that can be produced by neutrophils and other cells to combat bacterial infections. However, they do not generate reactive oxygen species as part of the anaerobic response.