MicroBiology MCQs with Answer and Explnation | Chapter 30

Answer: Ig A

IgA is the predominant antibody found in mucosal secretions, such as mucus, saliva, and tears. These secretions form a barrier against pathogens trying to enter the body through mucous membranes. IgA antibodies can neutralize pathogens directly or agglutinate them (clump them together) for easier removal by phagocytes (immune cells that engulf and destroy foreign particles).

The other options are incorrect:

• IgM: IgM is the largest antibody and is primarily involved in the early stages of an immune response. It is not well-suited for mucosal immunity due to its size and inability to efficiently penetrate mucus membranes.
• IgG: IgG is the most abundant antibody in the blood and is responsible for long-term immunity. However, it is not as effective as IgA in protecting mucosal surfaces.
• IgE: IgE is involved in allergic reactions and defense against parasites. It is not typically found in large amounts in mucosal secretions.

Answer: BSL 2

This level includes the handling of moderate-risk agents and is suitable for working with a broad range of laboratory microorganisms, including common pathogens such as Staphylococcus and certain enteric pathogens like E. coli. It involves more stringent safety measures than BSL 1.

The other options are incorrect:

• BSL 1 (Biosafety Level 1): BSL 1 is suitable for low-risk agents and does not typically include organisms of serious health concern. It is more basic and does not involve the pathogens mentioned.
• BSL 3 (Biosafety Level 3): BSL 3 is for handling indigenous or exotic agents that may cause serious or potentially lethal diseases. It involves additional precautions compared to BSL 2 and is not commonly used for organisms like HBV, Staphylococcus, and enteric pathogens.
• BSL 4 (Biosafety Level 4): BSL 4 is the highest level and is reserved for dangerous or exotic agents that pose a high risk of aerosol-transmitted infections and for which there is no available vaccine or therapy. It is not necessary for the organisms mentioned in the question.

Answer: Class II

Class II biological safety cabinets are the only type that sterilizes both the air entering and leaving the cabinet using HEPA filters. They provide protection for both the personnel working inside the cabinet and the environment outside.

The other options are incorrect:

• Class I: Class I cabinets exhaust contaminated air directly to the outside environment, meaning they primarily protect the personnel working inside but not the environment itself. They only use HEPA filters on the exhaust, not the intake.
• Class III: Class III cabinets are also known as glove boxes and provide the highest level of protection through complete isolation. They use a closed system with chemical disinfection and often multiple HEPA filters, but there is no open airflow in or out of the cabinet.
• Class IV: Class IV cabinets are similar to Class III but include additional features like gas-tight suits for operators. They also operate in complete isolation with no open airflow and utilize multiple HEPA filters for even stricter containment.

Answer: Toxic

Toxic chemicals can cause serious biological effects following inhalation, ingestion, or skin contact with even small amounts. They have the potential to harm living organisms.

The other options are incorrect:

• Corrosive: Corrosive chemicals can cause damage to living tissues upon contact, but the harm is primarily physical rather than a result of systemic biological effects.
• Carcinogenic: Carcinogenic chemicals have the potential to cause cancer after prolonged exposure. While they can be extremely hazardous, the effects are often long-term and may not manifest with small amounts or short-term exposure.
• Ignitable: Ignitable chemicals are substances that can catch fire easily. While they pose a different type of hazard, they do not necessarily cause serious biological effects through inhalation, ingestion, or skin contact in the same manner as toxic chemicals.

Answer: Adherence to surfaces

Bacterial fimbriae present on the outer cell surface are used for adherence to surfaces. They help bacteria attach to host tissues or other surfaces.

The other options are incorrect:

• Cellular activity: While fimbriae play a role in the activities of bacteria, they are specifically involved in adherence to surfaces rather than general cellular activity.
• Sexual reproduction: Bacterial fimbriae are not involved in sexual reproduction. Bacteria typically reproduce through processes such as binary fission or other forms of asexual reproduction.
• Adherence and exchange of genetic information: While adherence is correct, bacterial fimbriae are not directly involved in the exchange of genetic information. Conjugation pili, which are different structures, are responsible for the transfer of genetic material between bacteria.

Answer: Direct inoculation

Direct inoculation refers to the introduction of a pathogen directly into tissue or through a break in the skin or mucous membrane, bypassing the body’s natural defenses. This explains why needle sticks and broken glass can potentially lead to infections, as they create a direct pathway for pathogens to enter the body.

The other options are incorrect:

• Airborne: This route involves inhaling airborne pathogens, which wouldn’t be the case for needle sticks or broken glass.
• Ingestion: Consuming a pathogen is how ingestion-based infections occur, not through percutaneous (through the skin) injuries.
• Mucous membrane contact: While some pathogens can infect through mucous membranes like eyes or mouth, accidental needle sticks and broken glass typically involve skin penetration, hence falling under direct inoculation.

Answer: 400 times bigger

The total magnification is calculated by multiplying the magnification of the eyepiece by the magnification of the objective lens. In this case, 10X (eyepiece) multiplied by 40X (objective lens) equals a total magnification of 400 times.

The other options are incorrect:

• 40 times bigger: This option incorrectly represents the magnification of the objective lens alone, not the total magnification.
• 4 times bigger: This option does not accurately represent the total magnification; it seems to reflect only the magnification of the objective lens.
• 14 times bigger: This value is not the correct calculation of the total magnification based on the given magnifications of the eyepiece and objective lens. The correct calculation is 10X (eyepiece) multiplied by 40X (objective lens) equals 400 times.

Answer: Sterilizer

A sterilizer is intended to destroy all microorganisms and their spores on inanimate surfaces. It achieves a higher level of microbial kill compared to disinfectants.

The other options are incorrect:

• Disinfectant: While disinfectants are used to eliminate or reduce the number of microorganisms on surfaces, they may not necessarily destroy all spores. Disinfectants are generally used on surfaces, instruments, or other non-living objects.
• Antiseptic: Antiseptics are used on living tissues to inhibit the growth of microorganisms and prevent infections. They are not typically used on inanimate surfaces with the goal of destroying spores.
• Antibiotic: Antibiotics are substances that inhibit the growth of bacteria, usually within the body. They are not used on surfaces and are not intended for the destruction of microorganisms on inanimate objects.

Answer: Diaphragm

The diaphragm of the microscope is used to change the amount of light entering the stage. It is an adjustable aperture located beneath the stage and is used to control the intensity and angle of the light reaching the specimen.

The other options are incorrect:

• Eyepiece: The eyepiece, also known as the ocular lens, is used for magnification and is not involved in adjusting the amount of light entering the stage.
• Nosepiece: The nosepiece holds the objective lenses and allows for their rotation to achieve different levels of magnification. It is not involved in adjusting the illumination.
• Coarse adjustment knob: The coarse adjustment knob is used for focusing the microscope by moving the stage up and down. It is not designed to control the amount of light entering the stage.

Answer: BSL-I

BSL-I is used for microorganisms that have no known pathogenic potential for healthy individuals, and Bacillus subtilis falls into this category. BSL-I is the lowest level of biosafety containment.

The other options are incorrect:

• BSL-II (Biosafety Level 2): BSL-II is used for agents associated with human disease, and it involves more precautions than BSL-I. Bacillus subtilis, in its typical form, is not considered a pathogen for healthy individuals.
• BSL-III (Biosafety Level 3): BSL-III is used for indigenous or exotic agents that may cause serious or potentially lethal diseases. Bacillus subtilis does not fall into this category as it is not pathogenic for healthy individuals.
• BSL-IV (Biosafety Level 4): BSL-IV is the highest level of containment and is reserved for dangerous or exotic agents that pose a high risk of aerosol-transmitted infections. Bacillus subtilis is not a pathogen that requires BSL-IV containment.

Answer: More turbid samples can be processed easily.

Membrane filtration methods are generally less suitable for processing turbid samples because the suspended particles can clog the filter and affect the accuracy of the results.

The other options are incorrect:

• Results are available in a shorter period of time: Membrane filtration methods can provide relatively quick results compared to some other methods, contributing to their efficiency.
• Larger volumes of sample can be processed: Membrane filtration allows for the processing of larger volumes of samples, making it advantageous for certain applications.
• The results are readily reproducible: Membrane filtration methods are known for producing reproducible results, contributing to the reliability of the technique.

Answer: Antiseptic

Antiseptics are chemicals used to inhibit the growth of microorganisms on living tissues, including the skin. They are applied to the skin to reduce the risk of infection.

The other options are incorrect:

• Disinfectant: Disinfectants are used on surfaces and inanimate objects to eliminate or reduce the number of microorganisms. They are not typically applied to living tissues.
• Sterilizer: Sterilizers are used to destroy all microorganisms and their spores, usually on inanimate surfaces or objects. They are not typically used directly on the skin.
• Moist heat: Moist heat is a method of sterilization that involves the use of steam or boiling water. While it is effective for sterilizing objects, it is not a chemical germicide used directly on the skin.

Answer: Corrosive

Corrosive substances cause visible destruction or irreversible damage to human skin upon contact. They have the potential to cause severe burns and damage to tissues.

The other options are incorrect:

• Toxic: Toxic substances can be harmful to health, but they may not necessarily cause visible destruction or irreversible damage to the skin upon contact.
• Carcinogenic: Carcinogenic substances have the potential to cause cancer after prolonged exposure but may not cause immediate visible destruction or damage to the skin upon contact.
• Explosive: Explosive substances can cause sudden release of energy, but they do not typically cause visible destruction or irreversible damage to human skin on contact. They pose a different type of hazard.

Answer: Ignitable

Any chemical that can burn and includes both combustible and flammable liquids is called ignitable. Ignitable substances can catch fire easily under certain conditions.

The other options are incorrect:

• Corrosive: Corrosive substances cause damage by chemical action, typically resulting in the corrosion or deterioration of materials. They are not specifically related to burning.
• Toxic: Toxic substances can be harmful to health, but the term does not specifically refer to the ability to burn or catch fire.
• Explosive: Explosive substances release energy suddenly, often with a violent force, but they are not necessarily characterized by their ability to burn or catch fire like ignitable substances.

Answer: Explosive

Explosive substances are reactive and unstable, readily undergoing violent chemical changes that release energy rapidly. This can result in an explosion.

The other options are incorrect:

• Toxic: Toxic substances may be harmful to health but are not necessarily characterized by violent chemical changes or explosions.
• Corrosive: Corrosive substances cause damage by chemical action, typically resulting in the corrosion or deterioration of materials. They are not specifically known for violent chemical changes leading to explosions.
• Ignitable: Ignitable substances can catch fire easily under certain conditions but are not necessarily associated with violent chemical changes that characterize explosions.

Answer: To prevent sample from contamination

The purpose of a biosafety cabinet in a microbiology laboratory is to provide a controlled, sterile environment that prevents contamination of samples, cultures, and laboratory personnel during microbiological procedures.

The other options are incorrect:

• To sterilize materials, such as media and glassware: Biosafety cabinets are not designed for the sterilization of materials. Autoclaves and other sterilization methods are used for this purpose.
• To provide a proper temperature for microbes to grow: While maintaining a controlled environment is important for microbiological work, the biosafety cabinet itself is not designed for controlling the temperature required for microbial growth. This is typically done using incubators.
• For long-term storage of microbes at low temperature: Biosafety cabinets are not intended for long-term storage. Microbes are usually stored in specialized equipment such as ultra-low temperature freezers for extended periods.

Answer: Graduated cylinder

A graduated cylinder is lab equipment used to accurately measure the volume of liquids. It has markings along its length that allow for precise volume measurements.

The other options are incorrect:

• Balance: A balance is used to measure the mass of an object, not the volume of liquids.
• Erlenmeyer flask: While Erlenmeyer flasks are commonly used in laboratories, they are not designed for precise volume measurements. They are more suitable for mixing, heating, and containing liquids.
• Ruler: A ruler is a tool for measuring length and is not appropriate for accurately measuring the volume of liquids.

Answer: Balance

A balance is used to measure the mass of an object, which is the amount of matter it contains. It provides a quantitative measure of the amount of substance present.

The other options are incorrect:

• Graduated cylinder: A graduated cylinder is used to measure the volume of liquids, not the amount of matter (mass) in an object.
• Thermometer: A thermometer is used to measure temperature and does not provide information about the amount of matter in an object.
• Autoclave: An autoclave is a device used for sterilization by applying heat and pressure. It is not designed to measure the amount of matter in an object but rather to eliminate microorganisms.

Answer: Incubator

An incubator is used to culture microbes at a specific temperature. It provides a controlled environment with a constant temperature for the growth of microorganisms.

The other options are incorrect:

• Autoclave: An autoclave is used for sterilization by applying heat and pressure, not for culturing microbes at a specific temperature.
• Hot air oven: A hot air oven is also used for sterilization through dry heat, and it is not designed for the specific temperature control needed for microbial growth.
• Desiccator: A desiccator is a sealed container used to maintain a low humidity environment. It is not designed for the culture of microbes at a specific temperature but rather for preserving moisture-sensitive substances.

Answer: Escherichia coli

E. coli is an example of an indicator coliform bacterial species commonly used to check the quality of drinking water. Its presence can indicate contamination by fecal matter and the potential presence of harmful pathogens.

The other options are incorrect:

• Staphylococcus aureus: Staphylococcus aureus is not typically used as an indicator of water quality. It is a bacterium associated with skin infections and food poisoning.
• Salmonella typhimurium: While Salmonella species can be indicators of fecal contamination, they are not as commonly used as E. coli for assessing water quality.
• Mycobacterium tuberculosis: M. tuberculosis is a bacterium responsible for causing tuberculosis in humans. It is not used as an indicator of water quality but is associated with respiratory infections.

Answer: Mixed culture

A mixed culture contains many different species in a sample. It consists of a combination of different microorganisms coexisting in the same environment.

The other options are incorrect:

• Broth culture: A broth culture contains a liquid medium with nutrients that supports the growth of microorganisms. It may contain a single species (pure culture) or multiple species (mixed culture).
• Pure culture: A pure culture contains only one species of microorganism. It is isolated to ensure the growth of a specific type of organism without interference from others.
• Streak plate culture: A streak plate culture is a method used to isolate individual colonies of microorganisms. It is a technique often employed to obtain pure cultures from a mixed sample.

Answer: Enrichment medium

Enriched medium is a culture medium supplemented with highly nutritious materials, such as blood serum or other enriched additives. It provides additional nutrients to support the growth of fastidious organisms that may have complex nutritional requirements.

The other options are incorrect:

• Differential medium: A differential medium is designed to distinguish between different types of microorganisms based on their biochemical characteristics. It is not necessarily supplemented with highly nutritious materials.
• General purpose medium: A general purpose medium provides the essential nutrients for the growth of a wide variety of microorganisms but may not be enriched with additional nutrients like serum.
• Enrichment medium: An enrichment medium is used to encourage the growth of a specific organism within a mixed culture. It may contain selective components but may not be specifically supplemented with highly nutritious materials.

Answer: Selective medium

Crystal violet agar is an example of a selective medium. It contains crystal violet, which inhibits the growth of certain microorganisms, allowing the selective growth of others.

The other options are incorrect:

• General purpose medium: A general purpose medium provides the essential nutrients for the growth of a wide variety of microorganisms. Crystal violet agar is selective rather than general purpose.
• Enriched medium: Enriched media are supplemented with additional nutrients, such as blood or serum, to support the growth of fastidious organisms. Crystal violet agar is not enriched but selective.
• Differential medium: A differential medium is designed to distinguish between different types of microorganisms based on their biochemical characteristics. Crystal violet agar is primarily selective, not differential.

Answer: Strictly prohibited for any use

Mouth pipetting is strictly prohibited in the microbiology lab. It poses a significant risk of exposure to harmful microorganisms and chemicals, as well as the potential for accidental ingestion.

The other options are incorrect:

• Dispensing liquid culture medium: Liquid culture medium is typically dispensed using sterile pipettes or other laboratory equipment. Mouth pipetting is not a safe practice and is strictly avoided in labs
• Dispensing water to wash glass slide: The use of mouth pipetting for dispensing water or any other substance is discouraged due to the risk of contamination and potential exposure to hazards.
• To transfer bacterial culture to fresh medium: Bacterial cultures are usually transferred using sterile techniques and proper laboratory equipment. Mouth pipetting is not considered safe and is prohibited in microbiology labs.

Answer: Optimum growth temperature

The temperature at which the rate of reproduction is most rapid is known as the optimum growth temperature. Different microorganisms have specific temperature ranges at which they grow best.

The other options are incorrect:

• Minimum growth temperature: The minimum growth temperature is the lowest temperature at which a microorganism can grow. Below this temperature, growth is limited or ceases.
• Maximum growth temperature: The maximum growth temperature is the highest temperature at which a microorganism can grow. Beyond this temperature, the organism’s growth is inhibited or stops.

Answer: 1.5 x 10⁸

To calculate the Colony Forming Units (CFU) per milliliter (ml), we can use the following formula:

CFU/ml = (Colonies per plate) / (Dilution factor * Volume added per plate)

In this case:

Colonies per plate = 75
Dilution factor = 10^-7 (since dilution is 10^-7)
Volume added per plate = 0.5 ml
Plugging these values into the formula:

CFU/ml = (75) / (10^-7 * 0.5) = (75) / (5 * 10^-8) = 15000 = 1.5 x 10^8

Therefore, the correct answer is 1.5 x 10^8 CFU/ml.

The other options are incorrect:

• 1.5 x 10⁶: This would be the answer if the volume added per plate was 1 ml instead of 0.5 ml.
• 1.5 x 10⁷: This is an overestimation because it doesn’t take into account the dilution factor.
• 7.5 x 10⁹: This is a significant overestimation and likely due to a miscalculation.

Answer: Both ‘Selective medium’ and ‘Differential medium’

MacConkey agar is an example of both a selective medium and a differential medium. It is selective because it inhibits the growth of Gram-positive bacteria and allows the growth of Gram-negative bacteria. It is differential because it contains lactose and pH indicators, allowing differentiation between lactose-fermenting and non-fermenting bacteria based on the color change of the medium.

The other options are incorrect:

• Enriched medium: MacConkey agar is not an enriched medium. Enriched media typically contain additional nutrients like blood or serum to support the growth of fastidious organisms.
• Selective medium: This option is partially correct. MacConkey agar is indeed a selective medium as it inhibits the growth of Gram-positive bacteria.
• Differential medium: This option is partially correct. MacConkey agar is indeed a differential medium as it allows for the differentiation of bacteria based on lactose fermentation, leading to a color change in the medium.

Answer: On the bottom-side of plate

Petri dishes or plates are commonly labeled on the bottom side to avoid interference with the growth or observation of the cultures on the agar surface.

The other options are incorrect:

• On the top of the plate: Labeling on the top of the plate may obstruct the view and interfere with the observation of cultures growing on the agar surface.
• On the side of the plate: Labeling on the side might be less convenient and may not be easily visible when the plate is stacked or stored.
• On the inside of the plate: Labeling on the inside of the plate is not practical, as it would be covered by agar and not visible during normal use in a laboratory setting.

Answer: Separation of a single colony

In microbiology, the goal of isolation is to obtain a pure culture, meaning a culture containing only one type of microorganism. This involves separating a single colony from a mixed culture (containing multiple types) and transferring it to a fresh sterile medium. By ensuring only one cell is transferred, subsequent growth will result in a culture containing only its descendants, a pure culture.

The other options are incorrect:

• Purification of culture: While separation is critical for purification, the isolation process itself doesn’t directly involve purifying the culture.
• Introduction of inoculum: Inoculum refers to the material introduced into a culture medium to initiate growth. While inoculum is involved in setting up isolation, it’s not the primary aim.
• To grow microorganism on a surface: Growing microorganisms is a natural consequence of successful isolation, but it’s not the defining factor of the process.

Answer: Enrichment culture technique

This method involves providing conditions that selectively favor the growth of the target organism (Salmonella in this case) while inhibiting the growth of competing microorganisms. Fecal samples are often enriched in selective broth media that promote the growth of Salmonella. After enrichment, the sample can be streaked onto selective agar plates for isolation and further identification.

The other options are incorrect:

• Crowded-plate technique: There is no widely recognized technique known as the “crowded-plate technique” for the isolation of Salmonella. It may be a term that is not commonly used in microbiology.
• Pour plate technique: The pour plate technique involves mixing a microbial sample with a liquid agar medium before solidification. It is used for enumerating microorganisms in a sample but is not specifically designed for the isolation of Salmonella.
• Gradient-plate technique: Similar to the crowded-plate technique, there is no widely recognized method referred to as the “gradient-plate technique” for the isolation of Salmonella. It may not be a commonly used or accepted term in microbiology.

Answer: storage in a freezer at ultra-low temperatures

Long-term storage of microbial samples is best achieved when carried out properly in a freezer at ultra-low temperatures. Ultra-low freezers typically operate at temperatures below -70°C or even lower, providing a stable and optimal environment for preserving microbial cultures for extended periods.

The other options are incorrect:

• Storage in a freezer: While a regular freezer can be used for short-term storage, it may not maintain the extremely low temperatures required for long-term preservation compared to ultra-low freezers.
• Storage in a refrigerator on an agar slant: Refrigerators are not suitable for long-term storage at ultra-low temperatures. Storing microbial samples on agar slants in a regular refrigerator may allow for some short-term preservation, but it is not ideal for long-term storage.
• Storage on a petri plate at room temperature: Storage of microbial samples on a petri plate at room temperature is not recommended for long-term preservation. Room temperature conditions are not conducive to maintaining the viability of microbial cultures over an extended period.

Answer: Plate-count method and membrane-filter count

Both the plate-count method and the membrane-filter count are used for viable counts of a culture.
Plate-count method: In this method, diluted samples are spread or poured onto a solid agar medium, and viable cells form visible colonies. The number of colonies is then used to estimate the number of viable cells in the original sample.

Membrane-filter count: In this method, the sample is filtered through a membrane filter, and the filter is placed on a suitable agar medium. Viable cells on the filter grow into visible colonies, and the number of colonies is used to estimate the number of viable cells in the original sample.

The other options are incorrect:

• Plate-count method: This option is correct for viable count and is included in the correct answer.
• Membrane-filter count: This option is correct for viable count and is included in the correct answer.
• Direct microscopic count: Direct microscopic count involves counting all cells, both viable and non-viable, and does not specifically distinguish between them. It is not suitable for viable count estimation.

Answer: Ammonification

Ammonification is the process by which soil microbes decompose nitrogen-containing organic matter from dead plants and animals, ultimately releasing ammonium (NH3) as the end product. This process recycles nitrogen back into the soil, making it available for plant uptake and other microbial activities.

The other options are incorrect:

• Denitrification: This process converts nitrates (NO3-) into gaseous nitrogen (N2) and contributes to nitrogen loss from the ecosystem. While it involves nitrogenous compounds, it doesn’t result in NH3 formation.
• Nitrogen fixation: This process converts atmospheric nitrogen (N2) into ammonia (NH3) by specialized bacteria and archaea. However, it doesn’t involve the breakdown of organic matter, rather the fixation of gaseous nitrogen.
• Nitrification: This process involves the two-step conversion of ammonium (NH3) into nitrates (NO3-) by different soil bacteria. While it starts with NH3, it doesn’t result in its final accumulation.

Answer: Sterilization

The process of killing all microorganisms along with their spores is called sterilization. Sterilization aims to eliminate or destroy all forms of microbial life, including bacterial spores, viruses, and fungi.

The other options are incorrect:

• Sanitization: Sanitization involves reducing the microbial population to safe levels as determined by public health standards. It does not necessarily achieve the complete elimination of all microorganisms, including spores.
• Disinfection: Disinfection refers to the reduction of the microbial population to a level that is considered safe for public health. It may not necessarily result in the complete destruction of all microbial spores.
• Antisepsis: Antisepsis is the application of antimicrobial substances to living tissue (e.g., skin) to inhibit or destroy microorganisms. While it is aimed at preventing infection, it may not achieve the level of microbial destruction associated with sterilization.

Answer: Staphylococcus aureus

S. aureus is a Gram-positive bacteria, while the other listed options are all Gram-negative. Most commonly used antimicrobial susceptibility tests are designed for Gram-negative bacteria and may not accurately reflect the activity against Gram-positive organisms like S. aureus.

The other options are incorrect:

• Salmonella typhimurium: This is a Gram-negative bacteria commonly associated with foodborne illness and is susceptible to many antibiotics.
• Pseudomonas aeruginosa: This is another Gram-negative bacteria known for its multidrug resistance, but several antimicrobials still show activity against it.
• Escherichia coli: This is a common Gram-negative bacteria found in the intestines and is susceptible to a wide range of antibiotics.

Answer: Enrichment culture

Isolation of antibiotic-producing microorganisms often involves enriching the environment to favor their growth over other, non-producing bacteria. Enrichment culture provides specific nutrients and conditions that selectively enable antibiotic-producers to thrive and dominate the sample. This allows for easier detection and isolation of these valuable microorganisms.

The other options are incorrect:

• Pour plate: This technique spreads the entire sample over an agar plate, resulting in dense growth of all types of bacteria present, making it difficult to identify antibiotic producers.
• Crowded-plate: Similar to pour plate, this technique also results in dense growth and competition, masking the potential presence of antibiotic-producing isolates.
• Streak plate: While streak plate helps isolate individual colonies, it doesn’t directly enrich for antibiotic-producing bacteria and may not guarantee their successful isolation.

Answer: Dextrose

Dextrose is another name for glucose. The Triple Sugar Iron (TSI) agar test differentiates bacteria based on their ability to ferment and produce gas from three specific sugars: glucose, lactose, and sucrose. Since glucose is already included in the test medium, dextrose would be redundant and not provide additional information for differentiation.

The other options are incorrect:

• Sucrose: This sugar is included in the TSI agar and its fermentation produces an acidic end product, turning the medium yellow.
• Glucose: As mentioned, glucose (dextrose) is already present in the TSI agar for fermentation testing.
• Lactose: This sugar is also included in the TSI agar and its fermentation produces both acid and gas, causing the medium to slant, change color, and potentially have gas bubbles.

Answer: To differentiate b/w fermentation of glucose and lactose

The Triple Sugar Iron (TSI) test is designed to differentiate bacterial species based on their ability to ferment specific sugars, produce hydrogen sulfide (H2S), and generate gas. While it can differentiate between fermentation of lactose and sucrose based on the color change and slant formation in the medium, the design doesn’t directly distinguish between glucose and lactose fermentation.

The other options are incorrect:

• To differentiate between fermentation of lactose and sucrose: This is one of the primary functions of the TSI test. Lactose fermentation leads to acid production that slants the medium and turns it yellow, while sucrose fermentation also produces acid but doesn’t slant the medium.
• To observe H2S production: The TSI medium contains iron salts that react with H2S produced by certain bacteria, forming a black precipitate. Therefore, the test can indeed observe H2S production.
• To observe gas production from glucose fermentation: Gas production caused by glucose fermentation can be seen as bubbles in the medium, which is another key function of the TSI test.

Answer: The surface area/volume ratio

The small size of cells can be attributed to the surface area/volume ratio. As cells become smaller, the surface area relative to the volume increases, facilitating more efficient exchange of materials with the environment.

The other options are incorrect:

• The rate of diffusion: While the rate of diffusion is important for cellular processes, it is not the primary factor determining the small size of cells. The surface area/volume ratio plays a more significant role.
• The number of mRNAs that can be produced by the nucleus: The number of mRNAs produced by the nucleus is not directly related to the size of cells. Cell size is more influenced by structural and functional considerations, such as the need for efficient exchange of materials.
• All of the above: While the rate of diffusion and the number of mRNAs can have biological significance, they are not the primary factors determining the small size of cells. The surface area/volume ratio is a more critical factor in this context.

Answer: 10-1

The dilution is calculated by dividing the volume of the initial sample (food + water) by the volume of the aliquot of the initial sample taken for further analysis. In this case, the dilution is
20ml180ml​=10-1.

The other options are incorrect:

• 10-2: This is not the correct dilution. The dilution factor is determined by dividing the volume of the initial sample by the volume of the aliquot taken, which results in a dilution of 10-1 in this case.
• 10-3: This is not the correct dilution. The dilution factor is 10-1, not 10-3.
• 10-4: This is not the correct dilution. The dilution factor is 10-1, not 10-4.

Answer: Koch and Pasteur

Both Robert Koch (1843-1910) and Louis Pasteur (1822-1895) were prominent scientists working in the field of microbiology during the late 19th century. They made significant contributions to our understanding of infectious diseases and developed techniques for isolating and identifying pathogens. Their lives overlapped considerably, with Koch’s career starting around the time Pasteur was already well established.

The other options are incorrect:

• Darwin and Woese: Charles Darwin (1809-1882) lived in the 19th century, while Carl Woese (1928-2012) was active in the latter half of the 20th century. Although their work focused on different areas of biology (evolution and microbiology, respectively), their lives did not overlap.
• Van Leeuwenhoek and Ricketts: Antoni van Leeuwenhoek (1632-1723) was a pioneering microscopist from the 17th century, while Howard Taylor Ricketts (1871-1910) was an American pathologist studying infectious diseases in the early 20th century. Their lives were separated by almost 150 years.
• Berg and Hooke: Paul Berg (b. 1926) is a prominent biochemist from the late 20th and early 21st centuries, while Robert Hooke (1635-1703) was a 17th-century polymath known for his work in microscopy and other fields. Their lives were separated by around 200 years.

Answer: Habitats which are extreme environments

While both archaea and bacteria share many features, one defining characteristic setting them apart is their ability to thrive in extreme environments. Archaea are called extremophiles due to their unique adaptations that allow them to survive in harsh conditions like boiling hot springs, deep ocean trenches, and highly acidic or alkaline environments. Bacteria, while diverse, generally don’t possess the same tolerance to such extremes.

The other options are incorrect:

• Absence of a nuclear membrane: Both archaea and bacteria are prokaryotes. This means they lack a true nucleus and membrane-bound organelles. While their nuclear material is contained in a region called the nucleoid, the absence of a membrane isn’t unique to archaea.
• Presence of a cell wall: Both archaea and bacteria have cell walls, although the composition of these walls differs. Bacterial cell walls typically contain peptidoglycan, while archaeal cell walls can have various components like proteins or polysaccharides. This difference is significant, but not the primary distinguishing feature.
• Cytoplasmic ribosomes that are 70S: Both archaea and bacteria have 70S ribosomes, responsible for protein synthesis. While other domains of life (eukaryotes) have 80S ribosomes, this particular characteristic doesn’t differentiate archaea from bacteria.

Answer: Christian Gram

Christian Gram (1853-1916), a Danish physician and bacteriologist, developed the Gram staining technique in 1884. This method categorizes bacteria based on their cell wall composition, using a specific sequence of dyes to differentiate them into Gram-positive and Gram-negative types. His groundbreaking work revolutionized bacterial identification and understanding, impacting diagnosis and treatment of infectious diseases.

The other options are incorrect:

• Alfred Gram: While Christian Gram’s father’s name was Frederik Tram, there is no record of an “Alfred Gram” being associated with the staining technique.
• Robert Cooke: Robert Cooke (1823-1890) was an American physicist and inventor, primarily known for advancements in telegraphy and telephone technology. His work had no connection to Gram staining.
• Louis Pasteur: Louis Pasteur (1822-1895) was a French chemist and microbiologist known for his contributions to vaccination and germ theory. While a pioneer in microbiology, he did not develop the Gram staining technique.

Answer: Evolution

Evolution is considered the most unifying concept in biology. It provides a framework that explains the diversity of life on Earth. The theory of evolution, proposed by Charles Darwin and Alfred Russel Wallace, has become a central and unifying principle that ties together various biological disciplines.

The other options are incorrect:

• Taxonomy: Taxonomy is the science of classification and naming of living organisms. While important for organizing biological diversity, it is not as unifying as the concept of evolution, which explains the underlying processes of diversity.
• Anatomy: Anatomy is the study of the structure of organisms. While it is essential for understanding the form and function of living organisms, it is not as broad and unifying as the concept of evolution.
• Genetics: Genetics is the study of heredity and the variation of inherited characteristics. While genetics is a fundamental aspect of biology, evolution provides a broader framework that includes genetic processes but also extends to the origin and diversity of species.

Answer: All of these

• Subspecies: Bacterial subspecies refer to groups within a species that differ from each other but not to the extent of being classified as separate species. This subdivision is based on specific characteristics, such as biochemical or genetic differences.
• Bio-varieties: Bio-varieties (biotypes) are bacterial groups that are distinguishable by biochemical or physiological characteristics. These variations may not be significant enough to warrant classification as distinct subspecies.
• Sero-varieties: Sero-varieties (serovars) are bacterial variants identified based on differences in their antigenic properties, particularly their surface antigens. This classification is often used for bacteria like Salmonella and other pathogens.

Answer: Transcription and translation are coupled

In both archaea and eukarya, transcription (the synthesis of RNA from DNA) and translation (the synthesis of proteins from RNA) are not spatially separated in the cell. They are coupled processes, meaning that translation can begin while the mRNA is still being synthesized.

The other options are incorrect:

• Translation is inhibited by diphtheria toxin: This statement is incorrect. Diphtheria toxin inhibits protein synthesis by inactivating elongation factor-2 (EF-2), but it is not a characteristic of protein synthesis in archaea and eukarya.
• Proteins are synthesized from D-, rather than L-, isomers of amino acids: This statement is incorrect. In both archaea and eukarya, proteins are synthesized from L-amino acids, not D-amino acids.
• The initiator tRNA is charged with N-formylmethionine: This statement is specific to bacteria, not archaea or eukarya. In bacteria, the initiator tRNA is charged with N-formylmethionine, but in archaea and eukarya, it is charged with methionine

Answer: Bacterial spores

Bacterial spores, particularly endospores produced by certain bacteria like Bacillus and Clostridium species, exhibit the maximum resistance to physical and chemical agents. The spore form provides protection against harsh environmental conditions.

The other options are incorrect:

• Viruses: While some viruses may exhibit resistance to certain conditions, they are generally more susceptible to physical and chemical agents compared to bacterial spores.
• Mold spores: Mold spores can be resistant to some conditions, but they are not as resistant as bacterial spores. Bacterial spores have a unique structure and protective mechanisms that make them more resilient.
• E. coli: Escherichia coli (E. coli) is a bacterium, and its resistance to physical and chemical agents is generally lower compared to bacterial spores. E. coli is not known for forming spores like some other bacteria.

Answer: Corynebacterium

The virulence of Corynebacterium is linked to its cell wall, an exotoxin (PLD – phospholipase D), and a protective antigen (CP40). Corynebacterium diphtheriae, in particular, is known for causing diphtheria, and the mentioned virulence factors play crucial roles in its pathogenicity.

The other options are incorrect:

• Mycobacterium: While Mycobacterium species, such as Mycobacterium tuberculosis, have unique cell wall structures, their virulence is not specifically linked to an exotoxin (PLD) and a protective antigen (CP40). Mycobacteria are known for causing diseases like tuberculosis.
• Campylobacter: Campylobacter species are primarily associated with gastrointestinal infections. Their virulence is not specifically linked to the mentioned combination of cell wall, exotoxin, and protective antigen.
• Brucella: Brucella species cause brucellosis, and their virulence is associated with various factors, including their ability to survive within macrophages. The mentioned combination of cell wall, exotoxin, and protective antigen is not a characteristic of Brucella.

Answer: Enterohemorrhagic E. coli (EHEC)

E. coli O157:H7 is an example of Enterohemorrhagic E. coli (EHEC). This strain of E. coli is known for producing toxins that can lead to severe gastrointestinal symptoms, including bloody diarrhea.

The other options are incorrect:

• Enterotoxigenic E. coli (ETEC): ETEC is associated with traveler’s diarrhea and is characterized by the production of enterotoxins. It does not include E. coli O157:H7.
• Enteropathogenic E. coli (EPEC): EPEC is known for causing attaching and effacing lesions on intestinal cells. It is not synonymous with E. coli O157:H7.
• Enteroinvasive E. coli (EIEC): EIEC is capable of invading and multiplying within intestinal cells, causing inflammation. E. coli O157:H7 is not classified as enteroinvasive.

Answer: Eukaryotes evolved from bacteria

Genetic and biochemical similarities between contemporary cyanobacteria and eukaryotic chloroplasts support the theory that eukaryotes evolved from bacteria. This concept is known as endosymbiotic theory, which suggests that mitochondria and chloroplasts in eukaryotic cells originated from free-living bacteria engulfed by ancestral eukaryotic cells.

The other options are incorrect:

• Eukaryotes evolved from archaea: The endosymbiotic theory proposes a bacterial origin for mitochondria and chloroplasts, not archaea.
• Oxygenic photosynthesis first evolved in eukaryotes: Oxygenic photosynthesis first evolved in cyanobacteria, and the endosymbiotic theory suggests that eukaryotic chloroplasts originated from cyanobacteria through endosymbiosis.
• Cyanobacteria arose from chloroplasts which escaped from plant cells: The endosymbiotic theory suggests the opposite—chloroplasts originated from cyanobacteria engulfed by ancestral eukaryotic cells. Cyanobacteria did not arise from chloroplasts.