MicroBiology MCQs with Answer and Explanations | Chapter 69

Answer: Proteus

Proteus is a genus of Gram-negative bacteria known for its swarming motility on culture medium. This swarming behavior allows them to efficiently explore their environment and locate resources.

The other options are incorrect:

• Salmonella: While Salmonella is a Gram-negative bacterium, it does not exhibit swarming motility.
• Clostridium: Clostridium is a genus of Gram-positive bacteria, but it typically forms colonies with a smooth, round morphology. It does not exhibit swarming motility.
• Staphylococci: Staphylococci are Gram-positive bacteria that typically form round, clustered colonies. They do not exhibit swarming motility.

Answer: Exaltation

Exaltation refers to the increase in virulence of a pathogen, meaning it becomes more capable of causing disease. This can happen through various mechanisms, such as acquiring new virulence factors or mutations that make it more resistant to the host’s immune system.

The other options are incorrect:

• Pathogenicity: Pathogenicity is the potential of a microorganism to cause disease. It’s a binary concept – a bacteria is either pathogenic or not. Exaltation refers to a change in virulence within a pathogenic organism.
• Attenuation: Attenuation is the opposite of exaltation. It describes the process of weakening a pathogen, making it less virulent and potentially useful for vaccines.
• Toxigenicity: Toxigenicity refers specifically to the ability of a bacteria to produce toxins, which are poisonous substances that contribute to disease. While toxin production can be a virulence factor, exaltation encompasses a broader range of mechanisms that can increase virulence.

Answer: 120oC

120°C is the standard temperature used in autoclaves for effective sterilization. This temperature, along with the accompanying steam pressure, is sufficient to kill most microorganisms, including bacteria, viruses, and fungi.

The other options are incorrect:

• 50°C: This temperature is too low to effectively kill most microorganisms.
• 100°C: This is the boiling point of water at standard pressure. While boiling can kill some bacteria, it’s not reliable for complete sterilization, especially for heat-resistant spores.
• 180°C: While temperatures this high can definitely sterilize, it’s not typically used in autoclaves due to potential damage to heat-sensitive materials being sterilized.

Answer: Both second and third

Spores, which are highly resistant structures produced by certain bacteria, are effectively killed by methods such as autoclaving (exposure to high temperature and pressure) and treatment with chemicals like glutaraldehyde. These methods penetrate the spore’s tough outer coat and destroy its viability.

The other options are incorrect:

• 70% alcohol: While alcohol (ethanol or isopropanol) is effective against many vegetative bacteria and some viruses, it is not reliably sporicidal. Spores are highly resistant and typically require more robust methods like autoclaving or chemical treatments for complete destruction.
• Glutaraldehyde: Glutaraldehyde is indeed effective against spores, making it a commonly used chemical for disinfection and sterilization in healthcare settings. However, it is not the only method for spore destruction.
• Autoclaving: Autoclaving is indeed effective against spores, as mentioned earlier. However, the statement “Both second and third” is correct because both glutaraldehyde and autoclaving are effective methods for killing spores.

Answer: Autoclaving

Autoclaving is the most common method for sterilizing glassware in laboratory settings. It involves subjecting the glassware to high-pressure steam at temperatures typically around 121°C, effectively killing microorganisms present on the surfaces of the glassware.

Incorrect Options:

• Hot air over: While hot air can be used for sterilization, it is not the most effective method for glassware. Hot air sterilization relies on exposure to dry heat at temperatures above 160°C for a prolonged period, which can be damaging to certain types of glassware and may not penetrate materials effectively.
• Incineration: Incineration involves burning materials at extremely high temperatures to destroy microorganisms and other contaminants. While it is a potent method of sterilization, it is not suitable for glassware, as glass can melt or shatter at the temperatures required for incineration.
• None of these: This option is incorrect because autoclaving is indeed a commonly used and effective method for sterilizing glassware in laboratory settings.

Answer: Tyndall

Tyndall isation, a method for sterilization using multiple cycles of heating and incubation, was named after its inventor, John Tyndall.

The other options are incorrect:

• Pasteur: While Louis Pasteur made significant contributions to microbiology, including germ theory, he is not credited with inventing Tyndallisation.
• Koch: Robert Koch is another prominent microbiologist known for his postulates and discoveries related to specific pathogens. He is not associated with Tyndallisation.
• Jenner: Edward Jenner is credited with developing the smallpox vaccine, a major breakthrough in immunology. He is not involved in sterilization techniques.

Answer: Living cells

Viruses are obligate intracellular parasites, meaning they require a living host cell to replicate. They lack the machinery to grow and reproduce on their own. Lab media and broth, while providing nutrients, cannot support viral replication.

The other options are incorrect:

• Lab media: Lab media provides nutrients for bacterial growth, but it lacks the complex cellular machinery needed for viral replication.
• Broth: Similar to lab media, broth is a nutrient-rich liquid that can support bacterial growth but not viral replication.
• None of these: At least one of the options (living cells) is a valid method for cultivating viruses.

Answer: Only pathogenic forms can be removed

Pasteurization is a process of heating a food product to a specific temperature for a certain period to kill or inactivate pathogenic microorganisms present in the product. However, it does not necessarily remove all microorganisms, as some heat-resistant non-pathogenic forms may survive the process.

The other options are incorrect:

• All the microorganisms can be removed: Pasteurization is not guaranteed to remove all microorganisms present in a food product. While it reduces the microbial load and kills many bacteria, yeasts, and molds, it may not eliminate all microorganisms.
• Only non-pathogenic forms can be removed: Pasteurization primarily targets pathogenic microorganisms, but it can also reduce the numbers of non-pathogenic forms. However, it does not solely remove non-pathogenic forms.
• All of these are correct: This option is incorrect because pasteurization does not remove all microorganisms, and it primarily targets pathogenic forms rather than non-pathogenic forms.

Answer: Below 100oC

Pasteurization uses milder heat compared to sterilization. The specific temperature and time combination depends on the product being pasteurized. Typical pasteurization temperatures range from 63°C (145°F) to 72°C (161°F), all below the boiling point of water (100°C).

The other options are incorrect:

• Above 100°C: This temperature range is typically used for sterilization, which aims to eliminate all microorganisms.
• 100°C: While boiling can kill some bacteria, it’s not reliable for complete pasteurization, especially for heat-resistant bacteria.
• None of these: At least one of the options (below 100°C) is within the standard temperature range for pasteurization.

Answer: Enriched medium

An enriched medium is a type of growth medium that contains additional nutrients or growth factors beyond what is required for basic microbial growth. If any substance is used in excess in a medium other than nutrients, it is considered an enriched medium.

The other options are incorrect:

• Special medium: While a “special medium” could refer to a growth medium with specific characteristics tailored for particular microorganisms or purposes, it does not specifically denote the use of excess substances. It is a vague term compared to “enriched medium.”
• Enrichment medium: Enrichment medium typically refers to a medium used to encourage the growth of specific microorganisms by providing conditions favorable for their growth while inhibiting the growth of others. It does not necessarily imply the use of excess substances beyond what is required for basic microbial growth.
• None of these: This option is incorrect because if a substance is used in excess in a medium other than nutrients, it would indeed be considered an enriched medium.

Answer: Wilson and Blair

An indicator medium incorporates a specific chemical compound that changes color or produces a visible indicator based on the metabolic activity of microorganisms growing on it. Wilson and Blair medium, for example, contains thiosulfate and iron, which react with hydrogen sulfide produced by certain bacteria, causing black colonies to form.

The other options are incorrect:

• Nutrient Agar: Nutrient agar is a general-purpose medium that provides basic nutrients for microbial growth. It doesn’t contain any specific indicator for differentiating different types of microorganisms.
• Nutrient broth: Similar to nutrient agar, nutrient broth is a basic growth medium without any indicator properties.
• Czapeck-dox medium: This medium is formulated for fungi and some bacteria. It may contain specific carbon sources but doesn’t have a general indicator for differentiation.

Answer: Robertson cooked-meat medium

Robertson cooked-meat medium is a type of anaerobic medium specifically designed to cultivate anaerobic bacteria. It provides essential nutrients for bacterial growth and includes reducing agents that scavenge oxygen, creating an oxygen-depleted environment suitable for obligate and facultative anaerobes.

The other options are incorrect:

• Nutrient agar: Nutrient agar is a general-purpose medium that provides basic nutrients for microbial growth but doesn’t have any specific features to create an anaerobic environment.
• Nutrient broth: Similar to nutrient agar, nutrient broth is a basic growth medium that doesn’t have conditions suitable for anaerobic bacteria.
• Mac-Conkey’s agar: Mac-Conkey’s agar is a differential and selective medium used to differentiate lactose-fermenting and non-fermenting bacteria. It is not specifically designed for anaerobic growth.

Answer: Mac-Conkey’s Agar

MacConkey’s Agar is a selective and differential medium used to differentiate between lactose-fermenting and non-lactose fermenting bacteria, particularly gram-negative enteric bacteria. It contains lactose as a fermentable carbohydrate and pH indicators that cause lactose-fermenting colonies to appear pink/red, while non-lactose fermenters appear colorless.

The other options are incorrect:

• Wilson & lair: There is no commonly known medium called “Wilson & lair.” It may be a misspelling or a less-known medium. Without further context, it is difficult to provide information about it.
• Blood Agar: Blood Agar is a general-purpose medium used for the cultivation of a wide range of microorganisms, particularly those that require additional nutrients provided by blood. It is not specifically designed to differentiate lactose fermentors.
• Tetra thionate broth: Tetra thionate broth is a selective enrichment medium used for the isolation of Salmonella species from clinical samples. It is not typically used to differentiate lactose fermentors.

Answer: Streak-plate

The streak plate method is the most common and effective way to isolate a pure culture of bacteria from a mixed sample. It works by progressively diluting the sample across the surface of an agar plate with a sterile inoculation loop. This dilution allows individual bacterial cells to become separated. As these single cells divide and grow, they form discrete colonies, each derived from a single parent cell. By selecting a well-isolated colony, you can be fairly certain it represents a pure culture of a single bacterial strain.

The other options are incorrect:

• Agar slant: An agar slant can be used to grow and maintain existing cultures, but it’s not ideal for isolating pure cultures from mixed samples. With a slant, there’s less surface area for dilution, making it difficult to achieve separation of individual cells.
• Both first & second: While both methods have their uses, the streak plate technique offers a more reliable way to achieve a pure culture from a mixed sample compared to an agar slant.
• None of these: At least one of the options (streak plate) is a valid method for obtaining a pure culture.

Answer: Inoculation loop

An inoculation loop is a small, handheld tool with a wire loop at the end, typically made of nichrome or platinum. It is used to aseptically transfer small amounts of culture media or bacterial samples from one container to another.

The other options are incorrect:

• Slant: An agar slant is a type of culture container, not a tool for transferring cultures.
• Needle: Needles are primarily used for piercing or injecting, not for aseptic transfer of cultures. They can damage bacterial cells and are not the preferred method.
• Autoclave: An autoclave is a device used for sterilization at high pressure and steam, not for transferring cultures.

Answer: Inoculum

An inoculum refers to a small amount of a pure culture that is introduced into a fresh medium to initiate growth. It’s essentially a starter culture used to propagate bacteria in a new environment.

The other options are incorrect:

• Suspension: While a suspension can refer to a mixture containing microorganisms, it doesn’t necessarily imply a pure culture. A suspension from a mixed culture could contain various types of microorganisms.
• Dilution: Dilution refers to the process of thinning a culture by adding sterile liquid medium. While an inoculum might be prepared from a diluted culture, dilution itself isn’t the final product (inoculum) used for initiating growth.
• None of these: At least one of the options (inoculum) accurately describes the bacterial culture prepared from a pure culture.

Answer: Isolation

Isolation refers to the process of separating a single colony of microorganisms from a mixed population. This is typically done using techniques such as streak-plating or dilution plating, where the colony is physically separated from others to obtain a pure culture.

The other options are incorrect:

• Pure-culturing: Pure-culturing refers to the cultivation of microorganisms in a medium that supports the growth of only a single type of organism, resulting in a pure culture. While isolation is a step in the process of obtaining a pure culture, pure-culturing specifically refers to the cultivation aspect.
• Separation: While separation is a general term that could describe the process of isolating a single colony, it is not commonly used in microbiology to denote this specific step. Isolation is the more precise term for this process.
• Both first and second: This option is incorrect because while both pure-culturing and isolation are related to obtaining a pure culture, isolation specifically refers to the process of separating a single colony from a mixed population, which is the more direct answer to the question.

Answer: Incubation

Incubation is the period after inoculation when a culture is allowed to grow and multiply. During incubation, the culture is placed in a controlled environment with optimal temperature, humidity, and sometimes gas composition (for specific bacteria) to facilitate optimal growth.

The other options are incorrect:

• Inoculation: Inoculation refers to the act of introducing a small amount of culture (inoculum) into a fresh medium to initiate growth. It’s the step before the incubation period.
• Incineration: Incineration involves burning something at high temperatures to destroy it. It’s not related to culturing microorganisms.
• Isolation: Isolation is the process of separating a single colony from a mixed culture. It can be a step before incubation, but it doesn’t refer to the growth period itself.

Answer: Hot air oven

Hot air ovens can achieve temperatures sufficient for sterilization, including 160°C for one hour. This temperature range is effective in killing most microorganisms, including bacteria, viruses, and spores.

The other options are incorrect:

• Autoclave: While autoclaves are the most common method for sterilization in a laboratory setting, they typically use high-pressure steam at around 121°C (250°F). While some autoclaves can reach higher temperatures, 160°C is not the standard setting.
• Laminar flow: A laminar flow cabinet provides a sterile work environment but doesn’t generate the high temperatures required for sterilization. It protects cultures from airborne contaminants but doesn’t sterilize equipment or materials.
• Incubator: Incubators are designed to maintain optimal growth temperatures for microorganisms, typically ranging from 30°C to 37°C. They are not intended for sterilization.

Answer: Both second and third

An autoclave utilizes a combination of moist heat and steam under pressure for sterilization. Here’s why both aspects are important:

• Moist heat: Moist heat, in the form of high-pressure steam, is more effective at killing microorganisms compared to dry heat. Moisture helps denature proteins and disrupt cell membranes of microbes, leading to their death.
• Steam under pressure: The pressure component plays a crucial role. Increased pressure elevates the boiling point of water, allowing the steam to reach a higher temperature within the autoclave chamber. This higher temperature significantly enhances the microbicidal effect of the moist heat.

The other options are incorrect:

• Dry heat: Dry heat sterilization uses hot air at high temperatures (typically above 160°C) to kill microorganisms. While effective, it’s not the principle used in autoclaves.

Answer: Bacillus stearothermophilus

Bacillus stearothermophilus is a heat-resistant bacterium known for its ability to form spores that can withstand high temperatures, including moist heat.

The other options are incorrect:

• Bacillus subtilis: While Bacillus subtilis can form spores, these spores are not as heat-resistant as those of Bacillus stearothermophilus.
• Coxiella burnetti: This bacterium can survive harsh conditions but primarily thrives within intracellular environments and doesn’t form spores known for extreme heat resistance.
• Pseudomonas: This is a genus of bacteria that does not typically form heat-resistant spores.

Answer: All of the above

he disinfectivity of a disinfectant depends on a combination of factors. Here’s how each one plays a role:

• Concentration of the substance: Higher concentrations of disinfectant generally lead to greater effectiveness in killing microorganisms.
• Time of action: The longer a disinfectant is in contact with microbes, the more likely it is to inactivate them.
• pH of the medium and temperature suitable for the chemical: Disinfectants have optimal pH and temperature ranges for their activity. Deviating from these can significantly reduce their effectiveness.

Answer: Both first and third

Both formaldehyde and glutaraldehyde are highly effective disinfectants classified as aldehydes. They work by denaturing proteins and disrupting cellular processes in microorganisms.

The other options are incorrect:

• Acetaldehyde: While an aldehyde, acetaldehyde has a weaker antimicrobial effect compared to formaldehyde and glutaraldehyde. It’s not typically used as a disinfectant.

Answer: Gram positive

Acridine dyes, such as acriflavine and proflavine, are more effective against Gram-positive bacteria compared to Gram-negative bacteria. This is because the dyes can penetrate the cell wall of Gram-positive bacteria more easily due to their thicker peptidoglycan layer, leading to disruption of cellular functions and inhibition of growth.

The other options are incorrect:

• Gram negative: Acridine dyes are generally less effective against Gram-negative bacteria compared to Gram-positive bacteria due to the presence of an outer membrane in Gram-negative bacteria, which acts as a barrier against the dyes.
• Mycoplasmas: Mycoplasmas are a group of bacteria that lack a cell wall. Acridine dyes may have some effectiveness against mycoplasmas, but they are not specifically known to be more effective against them compared to other bacterial groups.
• Rickettsiae: Rickettsiae are a group of Gram-negative bacteria that are intracellular parasites. Acridine dyes may have limited effectiveness against them, but they are not specifically known to be more effective against rickettsiae compared to other bacterial groups.

Answer: Ethylene oxide

Ethylene oxide is a gas commonly used as a sterilizing agent. It effectively kills microorganisms, including bacteria, viruses, and fungi, by alkylating their essential cellular components.

The other options are incorrect:

• Oxygen: While oxygen can support some sterilization processes (like flaming loops), it’s not generally used as a standalone sterilizing agent.
• Nitrogen: Nitrogen is an inert gas and doesn’t possess properties necessary for sterilization.
• Carbon tetrachloride: While once used, carbon tetrachloride is a toxic substance not recommended for sterilization due to safety concerns and limited effectiveness against microbes.

Answer: All of these

While the use of some heavy metal salts as disinfectants has declined due to toxicity concerns, historically, and in some limited cases today, several have been used:

• Thiomersal: This organic mercury compound was once used as a preservative in vaccines but is now phased out due to safety concerns.
• Phenyl mercury nitrate: This was used as a topical antiseptic but is no longer recommended due to its toxic nature.
• Mercurochrome: This red-colored antiseptic solution containing mercury is still available in some regions but its effectiveness is limited and its use discouraged due to potential mercury toxicity.

Answer: Sub-cultures

Subcultures are new cultures derived from a previously existing culture (often a stock culture) by transferring a small amount of the inoculum (microbial suspension) into fresh media. This process allows for maintaining the culture, propagating it for further use, or isolating specific colonies.

The other options are incorrect:

• Stock cultures: These are cultures that are maintained for long-term storage and are not typically used for routine work. They are often stored with special techniques like freezing or lyophilization (freeze-drying) to minimize growth and preserve viability. Penetrating the stock culture with an inoculation loop could contaminate it and reduce its longevity.
• Stab cultures: While a technique for inoculating cultures, it involves inserting the inoculation loop vertically into the agar medium, not penetrating it with the suspension.
• None of these: Subculturing is a common microbiological technique for maintaining and propagating microbial cultures.

Answer: Dilution

The streak plate method relies on the principle of dilution to achieve its primary goal: obtaining isolated colonies of microorganisms. By progressively streaking a loopful of inoculum across the agar surface, the number of cells deposited in each subsequent streak zone is diluted. This dilution allows for individual, well-separated colonies to grow from single or a small number of viable cells, facilitating their identification and further study.

The other options are incorrect:

• Separation: While separation of colonies is the desired outcome, dilution is the underlying principle that enables it.
• Streaking: Streaking is the physical technique employed to achieve dilution on the plate’s surface.
• Isolation: Isolation of pure cultures is the ultimate aim of the streak plate method, but dilution is the key factor that makes it possible.

Answer: All of the above

All the listed options are commonly used culture media for fungi:

• Potato Dextrose Agar (PDA): This is a general-purpose medium containing potato extract and dextrose (sugar) that supports the growth of a wide range of fungi.
• Sabouraud’s agar: Another general-purpose medium specifically formulated for cultivating fungi. It contains peptone and dextrose, providing essential nutrients for fungal growth.
• Czapekdox agar: This medium is designed for fungi with specific nutritional needs. It contains a mineral base, sucrose (sugar), and sodium nitrate as a nitrogen source, catering to a broader range of fungal requirements compared to PDA and Sabouraud’s agar.

Answer: 65o C for 30 min.

While some actinomycete species can form spores with varying heat tolerance, 65°C for 30 minutes is a more typical range for inactivating most actinomycete spores.

The other options are incorrect:

• 52°C for 30 min: Some heat-resistant actinomycete spores might survive at this temperature, but 65°C is generally considered a more reliable range for killing most.
• 70°C for 30 min: While possible for some highly heat-resistant species, 70°C for 30 minutes might be unnecessarily harsh for many applications and could potentially damage heat-sensitive components in a sterilization process.
• 43°C for 30 min: This temperature is too low to effectively kill most actinomycete spores. They can typically survive and germinate at room temperature (around 20-25°C).

Answer: Thermoduric

Thermoduric bacteria are a type of bacteria that can withstand the heat treatment used in pasteurization (typically around 72°C for 15 seconds) but cannot grow at even higher temperatures. Pasteurization is intended to kill most harmful bacteria in milk and other products, but thermoduric bacteria can survive this process.

The other options are incorrect:

• Thermophiles: These bacteria thrive at high temperatures (usually above 50°C) and wouldn’t be affected by pasteurization temperatures.
• Extreme thermophiles: These are a sub-category of thermophiles that grow optimally at even higher temperatures (often exceeding 80°C) and wouldn’t be affected by pasteurization.
• Facultative thermophiles: These bacteria can grow at a wide range of temperatures, including both moderate and high temperatures. While some might survive pasteurization, they can also grow at higher temperatures unlike thermoduric bacteria.

Answer: 7 to 12

Alkaliphiles are microorganisms that thrive in alkaline environments. They grow optimally at pH values between 7 and 12, indicating that they prefer environments with high pH levels.

The other options are incorrect:

• 1 to 6: This pH range is acidic and would not support the growth of alkaliphiles, which require alkaline conditions.
• 6 to 9: While this pH range includes neutral and slightly alkaline conditions, it does not extend high enough to encompass the optimal pH range for alkaliphiles.
• 1 to 11: While alkaliphiles can tolerate pH levels up to 11, they prefer and grow optimally at pH values higher than 7. This option includes acidic and neutral pH values that would not support the growth of alkaliphiles.

Answer: Both first and second

Both osmophiles and halophiles can grow at high salinity, although through different mechanisms:

• Osmophiles: These microorganisms can tolerate and even require high concentrations of solutes (not just salts) in their environment. They have adapted mechanisms to maintain their internal osmotic balance in such environments.
• Halophiles: These microorganisms specifically require high salt concentrations (often sodium chloride) for growth. They have adapted internal mechanisms to utilize these salts and maintain their cellular functions in saline environments.

Answer: Salmonella typhi

Non-lactose fermenting colonies seen on MacConkey’s agar are typically indicative of pathogens such as Salmonella typhi, which do not ferment lactose. MacConkey’s agar is a selective and differential medium used to isolate and differentiate lactose-fermenting bacteria (which produce pink colonies) from non-lactose fermenters (which produce colorless or transparent colonies).

The other options are incorrect:

• Escherichia coli: Escherichia coli is a lactose-fermenting bacterium and typically produces pink colonies on MacConkey’s agar due to its ability to ferment lactose.
• Klebsiella pneumoniae: Klebsiella pneumoniae is another lactose-fermenting bacterium commonly found in the Enterobacteriaceae family. It produces pink colonies on MacConkey’s agar.
• Shigella shigae: Shigella species are non-lactose fermenters; however, the correct spelling for the species is Shigella dysenteriae. Shigella dysenteriae typically produces colorless colonies on MacConkey’s agar, indicating non-lactose fermentation.

Answer: Salmonella typhosa

Wilson and Blair agar is a selective and differential medium specifically designed to isolate and differentiate Salmonella species, particularlyThe other options are incorrect: from other bacteria commonly found in clinical samples. The medium contains ingredients that inhibit the growth of many other bacteria while allowing Salmonella to grow. Additionally, the medium allows for the identification of Salmonella based on their ability to produce hydrogen sulfide (H2S), which creates a characteristic blackening around the colonies.

The other options are incorrect:

• Staphylococci: Wilson and Blair medium is not selective for Staphylococci. These bacteria can grow on the medium, but there won’t be any specific differentiating characteristics to identify them from Salmonella.
• Vibrio cholerae: This medium is not ideal for isolating Vibrio cholerae. While some strains might grow, the selective components favor Salmonella.
• Shigella shigae: While Shigella and Salmonella share some similarities, Wilson and Blair agar is not specifically designed for isolating Shigella. Other media are better suited for this purpose.

Answer: All of the above

A comprehensive laboratory diagnosis of enteric fever utilizes a combination of tests to increase accuracy and capture the bacteria at different stages of infection.

• Blood Culture: This is the gold standard for diagnosing enteric fever. It directly detects Salmonella typhi in the bloodstream during the early stages of the illness. However, blood cultures can sometimes be negative, especially if antibiotics have already been administered.
• Urine and Stool Culture: While less sensitive than blood cultures, these tests can sometimes detect Salmonella in the urine or stool during later stages of the infection, particularly in chronic carriers.
• Widal Test: This is a serological test that detects antibodies against Salmonella typhi. However, the Widal test has limitations. It can be positive in people who have been vaccinated against typhoid fever or who have had a previous infection. Additionally, it takes time for the body to develop antibodies, so the Widal test may be negative in the early stages of the illness.

Answer: Shiga

The genus Shigella is named after Kiyoshi Shiga, a Japanese bacteriologist. In 1897, during a dysentery outbreak, Shiga successfully isolated and identified the bacteria responsible for the illness, which is now known as Shigella.

The other options are incorrect:

• Schmitz: While there may have been other researchers involved in studying dysentery around the same time, Shiga is credited with the initial isolation of Shigella.
• Sonnei: C. Sonne is another researcher whose name is associated with a specific type of Shigella bacteria (Shigella sonnei). However, he wasn’t the first to isolate the genus itself.
• Robert Koch: Robert Koch was a prominent microbiologist who developed Koch’s postulates, which are used to confirm the germ theory of disease. While Koch’s work played a vital role in microbiology, Shiga is credited with the specific discovery of Shigella.

Answer: S.typhimurium

Most strains of Salmonella typhimurium are capable of producing gas during fermentation. They can ferment various sugars, including glucose, with the byproduct being carbon dioxide (CO2) which appears as gas bubbles in culture media.

The other options are incorrect:

• S. typhi: Salmonella typhi, the cause of typhoid fever, is generally non-gas producing. It ferments glucose but does not produce gas as a byproduct.
• S. enteritidis: Similar to S. typhi, Salmonella enteritidis is typically non-gas producing. It ferments various sugars but doesn’t generate gas during the process.
• S. choleraesuis: Salmonella choleraesuis is also generally non-gas producing. While it ferments some sugars, gas is not a common byproduct.

Answer: Salmonella spp.

The Kauffmann-White scheme, also known as the serotyping scheme, is used to classify and differentiate various serovars of Salmonella species based on their surface antigens, particularly the O (somatic) and H (flagellar) antigens. This scheme helps in identifying different strains of Salmonella for epidemiological and diagnostic purposes.

The other options are incorrect:

• Shigella spp.: While Shigella and Salmonella are both Gram-negative enteric bacteria, they are distinct genera. There’s a separate classification scheme for Shigella.
• E. coli: Escherichia coli is another Gram-negative bacterium commonly found in the intestines. While some E. coli strains can cause illness, the Kauffmann-White scheme is specific for Salmonella serotyping.
• None of these: The description directly mentions the scheme being used to classify bacteria, and Salmonella spp. is the only option listed that the scheme is designed for.

Answer: Pink coloured colonies

Escherichia coli (E. coli) is a lactose-fermenting bacterium. MacConkey Agar contains lactose as a sugar source and a pH indicator (phenol red). When E. coli ferments lactose, it produces lactic acid, which lowers the pH of the surrounding medium. The decreased pH causes the phenol red indicator to turn pink, resulting in pink coloured colonies of E. coli on MacConkey Agar.

The other options are incorrect:

• Colorless colonies: Colorless colonies would indicate that E. coli is not fermenting lactose, which is not typical for this bacterium.
• Greenish pigmentation: Greenish pigmentation isn’t a characteristic color associated with E. coli on MacConkey Agar. Some other bacteria might produce greenish colonies on this medium depending on specific pigments they produce.
• Medusa head appearance: Medusa head appearance refers to a specific colony morphology with a central raised core and peripheral fringed edges. This is not typically seen with E. coli on MacConkey Agar.

Answer: Potassium tellurite medium

Corynebacterium diphtheriae, the bacterium that causes diphtheria, requires special culture media containing potassium tellurite for selective isolation. Potassium tellurite inhibits the growth of most bacteria but allows for the growth of C. diphtheriae, which reduces tellurite to its elemental form, resulting in characteristic black colonies.

The other options are incorrect:

• LJ medium: While LJ medium is used for some bacteria, it’s not suitable for C. diphtheriae.
• Mac Conkey’s medium: This medium is not selective for C. diphtheriae.
• PDA medium: This medium is not suitable for growing C. diphtheriae.

Answer: LJ medium

LJ medium, also known as Löwenstein-Jensen medium, is the most widely used and recommended culture medium for Mycobacterium tuberculosis. It provides the specific nutrients and environment needed for this slow-growing bacteria to thrive.

The other options are incorrect:

• Mac Conkey’s medium: This medium is designed for differentiating lactose-fermenting from lactose-non-fermenting bacteria and is not suitable for culturing Mycobacterium tuberculosis.
• Wilson Blair medium: This medium is used for detecting motility and gas production in intestinal bacteria and is not appropriate for growing Mycobacterium tuberculosis.

Answer: Foot pad of mice

The footpad of mice is the most commonly used and reliable method for culturing Mycobacterium leprae, the causative agent of leprosy. This method provides a suitable environment for the slow-growing bacteria to replicate.

The other options are incorrect:

• Armadillo’s brain: While armadillos can naturally contract leprosy, their brain is not a standard or recommended medium for culturing Mycobacterium leprae in a laboratory setting.
• Liver of guinea pig: Although guinea pigs can be susceptible to leprosy infection, the footpad of mice is generally preferred for culturing the bacteria due to its higher sensitivity and efficiency.
• Any of the above: While the other options might be used in research settings, the footpad of mice remains the gold standard for diagnostic purposes.

Answer: Robertson’s cooked meat medium

Robertson’s Cooked Meat Medium (RCM) is a rich, complex culture medium that provides the ideal environment for growth of many Clostridia species. It contains essential nutrients, including meat fragments, which stimulate the growth of these anaerobic bacteria.

The other options are incorrect:

• Löwenstein-Jensen (LJ) medium: While LJ medium is excellent for culturing Mycobacterium tuberculosis, it’s not suitable for Clostridia spp. due to its different nutritional requirements and aerobic conditions.
• Mac Conkey’s Medium: This medium is designed to differentiate lactose-fermenting from lactose-non-fermenting bacteria and is not formulated for anaerobic bacteria like Clostridia spp.

Answer: Nagler’s test

Nagler’s reaction is a specific test used to identify Clostridium perfringens (formerly known as Clostridium welchii) based on its ability to ferment lactose and produce gas bubbles within a lactose-containing medium. This rapid test helps differentiate C. perfringens from other Clostridium species.

The other options are incorrect:

• Elek’s gel precipitation test: This test is used to identify and differentiate Shigella and enteroinvasive Escherichia coli (EIEC) based on their ability to produce specific toxins that diffuse through a semi-solid agar medium.
• Weil-Felix test: This test, though historically used, is no longer considered reliable for diagnosing rickettsial infections. It relies on antibodies produced against unrelated Proteus bacteria that cross-react with some Rickettsia species.
• Bacitracin test: This test is used to differentiate between gram-positive and gram-negative bacteria. Clostridium perfringens, being gram-positive, is typically resistant to bacitracin.

Answer: Lecithinase

Nagler’s reaction specifically detects the presence of lecithinase, an enzyme also known as phospholipase C. This enzyme is produced by Clostridium perfringens and breaks down lecithin, a component found in egg yolk.

The other options are incorrect:

• Coagulase: This enzyme is produced by certain bacteria, like Staphylococcus aureus, and helps them clot blood plasma. Nagler’s reaction is not designed to detect coagulase.
• Hyaluronidase: This enzyme breaks down hyaluronic acid, a component of connective tissues. While some bacteria produce hyaluronidase, Nagler’s reaction specifically targets lecithinase activity.
• None of these: Since Nagler’s reaction relies on the breakdown of lecithin by lecithinase, this is the primary target it detects.

Answer: 8-12 hours

Clostridium perfringens (Cl. welchii) typically has an incubation period of 8-12 hours within the human gastrointestinal tract. This timeframe allows for the ingested spores to germinate, grow, and produce toxins that cause food poisoning symptoms.

The other options are incorrect:

• 7-10 hours: While Cl. welchii can grow within this timeframe, 8-12 hours is generally considered the standard incubation period for consistent growth and toxin production leading to illness.
• 5-7 hours: This incubation period might be too short for widespread growth and toxin production necessary for causing foodborne illness symptoms in most cases.
• 2-4 hours: This is significantly shorter than the typical incubation period required for Cl. welchii to germinate, grow, and produce enough toxin to cause noticeable symptoms.

Answer: 7-10 days

The average incubation period of tetanus, the infectious disease caused by the bacterium Clostridium tetani, is typically 7-10 days. However, the incubation period can vary depending on factors such as the site and severity of the wound, as well as the individual’s immune response.

The other options are incorrect:

• 2-3 days: This incubation period is too short for typical tetanus symptoms to develop. The toxin produced by the bacteria needs more time to reach and affect the nervous system.
• 14-21 days: While possible in rare cases, tetanus usually has a shorter incubation period. A longer incubation period might suggest a different cause for the symptoms.
• 3-4 weeks: This incubation period is too long for most tetanus cases. Very rarely might such a long incubation occur, but 7-10 days is the accepted range for most tetanus infections.

Answer: Staphylococcus

Salt agar, specifically Mannitol Salt Agar (MSA), is a selective and differential medium commonly used to isolate and identify Staphylococcus species. The high salt concentration (around 7.5%) in MSA inhibits the growth of most other bacteria, while some Staphylococci can tolerate this harsh environment and grow.

The other options are incorrect:

• Streptococcus: While some Streptococcus species might exhibit some growth on salt agar, it’s not a selective medium for them. They can grow on other, less selective media.
• Vibrio: Vibrio species typically require specific media with different salt concentrations and additional components for optimal growth. They are generally inhibited by the high salt content in MSA.
• Shigella: Similar to Vibrio, Shigella species have different growth requirements and are not specifically targeted by the high salt concentration in MSA. They can grow on other media better suited for their needs.

Answer: Blastospore

A blastospore is a single-celled asexual fungal spore that develops through a budding process. A small protrusion emerges from the parent cell wall and enlarges until it separates to form a new, independent spore.

The other options are incorrect:

• Chlamydospore: These are thick-walled resting spores produced by some fungi under unfavorable conditions. They are not formed by budding.
• Arthospore: These are multicellular asexual spores formed by fragmentation of a fungal hyphae (filament). They don’t involve budding.
• Conidia: This is a general term for asexual fungal spores produced by various mechanisms, including budding (blastospores).