MicroBiology MCQs with Answer and Explanations | Chapter 75

Answer: Weil Felix

The Weil-Felix test is a blood test used to detect antibodies against Rickettsia, the bacteria that cause typhus fever. It’s a relatively inexpensive and simple test, although it has limitations in sensitivity and specificity compared to some newer methods.

The other options are incorrect:

• Mantoux test: This test is used to diagnose tuberculosis (TB) by checking for prior exposure to TB bacteria.
• Dick Test: This test is no longer used as it diagnosed susceptibility to scarlet fever, a disease now effectively treated with antibiotics.
• Shick test: Similar to the Dick Test, the Shick test is obsolete. It was used to assess susceptibility to diphtheria, another vaccine-preventable disease.

Answer: Urea breath test

The Urea Breath Test (UBT) is considered the most accurate non-invasive method for diagnosing H. pylori infection. It measures the presence of urease, an enzyme produced by H. pylori bacteria. The test is highly sensitive and specific, offering reliable results with minimal discomfort.

The other options are incorrect:

• Culture: While culturing a stomach tissue sample can directly identify H. pylori, it’s less accurate than the UBT. Culturing can be affected by factors like recent antibiotic use and requires a more invasive procedure (endoscopy) to obtain the sample.
• Biopsy urease test: This test, performed on a stomach tissue sample obtained by endoscopy, detects urease activity. However, it’s less sensitive than the UBT and requires an invasive procedure.
• Histopathology: This involves examining stomach tissue samples under a microscope to identify H. pylori bacteria or signs of inflammation it causes.

Answer: Mycobacterium TB

Acid-fast staining is a specific technique used to identify bacteria with cell walls rich in mycolic acids, a waxy substance. These waxy walls make the bacteria resistant to decolorization with acid during the staining process. M. tuberculosis, the bacteria that causes tuberculosis (TB), is a prime example of an acid-fast bacterium and the primary target of this diagnostic test.

The other options are incorrect:

• Ctetani: This is likely a misspelling. There’s no common bacteria named Ctetani. Acid-fast staining wouldn’t be used for diagnosing conditions caused by unknown or non-acid-fast bacteria.
• HIV/AIDS: HIV is a virus, not a bacterium. Acid-fast staining is specific to bacteria and wouldn’t be used for viral infections like HIV/AIDS.
• Meningococci: These are bacteria (Neisseria meningitidis) that cause meningitis.

Answer: Clostridium perfringens

Clostridium perfringens is a common type of bacteria that can cause food poisoning. It thrives in environments where cooked food is left at unsafe temperatures (between 40°F and 140°F) for extended periods, allowing the bacteria to multiply. When ingested, C. perfringens produces toxins that lead to symptoms like diarrhea and cramps.

The other options are incorrect:

• Clostridium botulism: This bacteria produces a potent neurotoxin that causes botulism, a serious illness affecting the nervous system. Symptoms are very different from food poisoning and can be life-threatening.
• Corynebacterium diphtheriae: This bacterium causes diphtheria, a respiratory illness characterized by throat swelling and difficulty breathing. It’s not typically associated with food poisoning.
• Clostridium tetani: This bacteria causes tetanus, a severe condition marked by muscle stiffness and spasms.

Answer: Autoclave

An autoclave is a pressurized chamber that uses steam at high temperature and pressure to sterilize equipment and materials, including culture media. This process effectively kills microorganisms and spores, ensuring a sterile environment for culturing cells or bacteria.

The other options are incorrect:

• Hot air oven: While hot air ovens can be used to sterilize some dry materials at high temperatures, they are not as effective as autoclaves for culture media. Hot air can struggle to penetrate liquids or reach all areas within containers, potentially leaving pockets of unsterilized media.
• Pasteurization: Pasteurization uses heat treatment at a lower temperature compared to autoclaving. It’s effective in reducing some harmful bacteria in food products but wouldn’t be sufficient for complete sterilization needed for culture media.

Answer: Sterilization

Sterilization is the complete elimination of all microorganisms, including their spores, from a surface, material, or environment. It ensures that no viable microorganisms remain, reducing the risk of contamination or infection.

The other options are incorrect:

• Disinfection: Reduces germs, but doesn’t get all of them, especially spores.
• Incineration: Hot, but not guaranteed to kill everything.
• Lysis: Breaks cells, but not all microorganisms form cells like that.

Answer: Disinfection

Disinfection is the process of reducing the number of pathogens from any article, surface, or medium. It aims to kill or inactivate a wide range of microorganisms, including bacteria, viruses, and fungi, but may not necessarily eliminate all microbial spores.

The other options are incorrect:

• Sterilization: Wipes out all microbes, including tough spores.
• Incineration: Burning destroys some germs, but might not get all of them.
• Lysis: Breaks cells, but not effective against all microorganisms.

Answer: Mycobacterium tuberculosis

The bacterium Mycobacterium tuberculosis is the causative agent of Tuberculosis (T.B.). It primarily affects the lungs but can also affect other parts of the body.

The other options are incorrect:

• Mycobacterium leprae: This bacteria causes leprosy, a chronic infectious disease affecting the skin and nerves.
• Treponema pallidum: This bacterium is responsible for syphilis, a sexually transmitted infection.
• Borrelia species: These bacteria belong to the genus Borrelia and cause various tick-borne illnesses like Lyme disease.

Answer: Treponemapallidum

Treponema pallidum is the bacterium responsible for causing syphilis, a sexually transmitted infection that progresses through multiple stages if left untreated.

The other options are incorrect:

• Mycobacterium tuberculosis: Mycobacterium tuberculosis is the bacterium responsible for causing tuberculosis (TB), a bacterial infection primarily affecting the lungs.
• Mycobacterium leprae: Mycobacterium leprae is the bacterium responsible for causing leprosy, also known as Hansen’s disease, which primarily affects the skin and nerves.
• Borrelia species: Borrelia species are responsible for causing diseases such as Lyme disease and relapsing fever, but they are not associated with syphilis.

Answer: Both Boiling & Autoclaving

Both boiling and autoclaving are moist heat methods of sterilization. Boiling involves heating water to its boiling point (100°C or 212°F) and immersing the items to be sterilized in the boiling water for a specified period. Autoclaving, on the other hand, utilizes high-pressure steam at temperatures above 121°C (250°F) to sterilize items within a sealed chamber.

The other options are incorrect:

• Hot Air Oven: Hot air ovens utilize dry heat, not moist heat, for sterilization. They operate at temperatures typically ranging from 160°C to 180°C (320°F to 356°F) and are suitable for sterilizing items that can withstand high temperatures but are not suitable for sterilizing materials sensitive to heat and moisture.

Answer: 121°C for 15 min

The temperature range commonly used for autoclaving is 121°C (250°F) for 15 minutes. This combination of high temperature and pressure is effective in achieving sterilization by killing microorganisms, including their spores.

The other options are incorrect:

• 100°C for 10 min: Boiling water reaches 100°C, but this temperature isn’t sufficient for sterilization, especially for spores.
• 100°C for 30 min: While longer boiling times can increase effectiveness, 100°C still doesn’t guarantee complete sterilization.
• 121°C for 30 min: While 121°C is the standard temperature for autoclaving, 15 minutes is the minimum recommended time for sterilization. Longer durations might be used depending on the load content.

Answer: Lactic acid

Lactic acid is the first organic acid typically produced by microbial fermentation, particularly by lactic acid bacteria, during the fermentation of carbohydrates.

The other options are incorrect:

• Acetic acid: While acetic acid is another common product of fermentation, it often comes after lactic acid production.
• Butyric acid: Butyric acid can also be a fermentation product, but not necessarily the first one formed.

Answer: Flagella at both ends

Amphitrichous flagella refer to flagella that are located at both ends of the bacterial cell.

The other options are incorrect:

• Flagella all-round the cell: This describes peritrichous flagella, where flagella are distributed around the entire cell.
• Flagella at one end: This describes monotrichous flagella, where there’s only one flagellum at one end.
• Flagella in tufts: This describes lophotrichous flagella, where multiple flagella are clustered at one or both poles of the cell.

Answer: All the above

Robert Koch truly was a giant in the field of microbiology. Here’s why all the statements are true:

• He is known as the father of bacteriology: Along with Louis Pasteur, Robert Koch’s work on isolating and identifying specific pathogens laid the foundation for modern bacteriology.
• The causative organism of cholera, Vibrio cholera was identified by him: In 1883, Koch successfully isolated and identified Vibrio cholerae, the bacterium responsible for cholera.
• He discovered the hypersensitivity phenomenon: While not his primary area of focus, Koch did contribute to the understanding of hypersensitivity reactions through his work on tuberculin, a derivative of Mycobacterium tuberculosis used for TB diagnosis.

Answer: All the above

Louis Pasteur made significant contributions to microbiology in several ways:

• Techniques of Pasteurization: He developed the process of pasteurization, using heat treatment to eliminate harmful bacteria from beverages like milk and wine.
• Process of Fermentation: Pasteur’s work on fermentation showed it was caused by living microorganisms, not spontaneous generation. This understanding revolutionized microbiology.
• Rabies, cholera and anthrax vaccines: Pasteur pioneered the development of vaccines for rabies, chicken cholera, and anthrax, paving the way for vaccines against numerous infectious diseases.

Answer: Hansen’s bacilli

Mycobacterium leprae, the bacteria that causes leprosy, is also known as Hansen’s bacilli. This name honors Gerhard Armauer Hansen, the scientist who first identified the bacterium in 1873.

The other options are incorrect:

• Golden bacilli: This term isn’t typically used for Mycobacterium leprae. Some bacteria might be described as golden due to their appearance under certain staining techniques, but this isn’t specific to M. leprae.
• Koch’s bacillus: This refers to Mycobacterium tuberculosis, the bacteria responsible for tuberculosis. While both are Mycobacterium species, they cause different diseases.
• Magical bacilli: This is a fanciful term not used in scientific microbiology.

Answer: Koch’s bacillus

Mycobacterium tuberculosis is widely known as Koch’s bacillus. This name honors Robert Koch, the scientist who discovered this bacterium in 1882.

The other options are incorrect:

• Golden bacilli: This term isn’t specific to Mycobacterium tuberculosis. Some bacteria might appear golden under certain staining techniques, but it’s not a definitive characteristic.
• Magical bacilli: This is a fanciful term not used in scientific microbiology.
• Hansen’s bacilli: This refers to Mycobacterium leprae, the bacteria that causes leprosy, a different disease.

Answer: Staphylococcus

Acid-fast organisms are those that resist decolorization by acid-alcohol after staining with carbolfuchsin dye. Staphylococcus bacteria do not exhibit this characteristic.

The other options are incorrect:

• Mycobacterium tuberculosis: This is a well-known example of an acid-fast bacterium.
• Nocardia: These are also acid-fast bacteria sometimes causing opportunistic infections.
• Isospora belli: Isospora belli is a protozoan parasite and is not an acid-fast organism. It does not have the characteristic cell wall structure that confers acid-fastness.

Answer: Mesosome

Mesosomes are structures in bacteria that are involved in cellular respiration, as well as other functions such as DNA replication and cell division.

The other options are incorrect:

• Ribosome: Ribosomes are cellular structures for protein synthesis, not directly involved in respiration.
• Pili: Pili are hair-like structures used for attachment or conjugation (genetic exchange), not respiration.
• Flagella: Flagella are appendages for bacterial movement, not respiration.

Answer: All the above

All the statements are true regarding bacteria.

• Respiration, cell division and sporulation are functions of mesosome: The infolding of the plasma membrane in bacteria (mesosome) increases surface area and is involved in these processes.
• Capsule in bacteria enables adherence to surfaces and protects against phagocytosis: The capsule is a slimy layer surrounding some bacteria that helps them stick to surfaces and resist being engulfed by immune cells.
• Heterotrophs are bacteria that are unable to synthesize their own food materials: Heterotrophs are organisms that require organic compounds from other sources for nutrition, and this applies to many types of bacteria.

Answer: All the above

All the statements are true regarding cocci.

• Cocci are spherical shaped bacteria: Coccus (plural: cocci) refers to bacteria with a round or spherical shape.
• Cocci present in chain is known as streptococci: Streptococci are cocci that grow in chains linked end-to-end.
• Cocci present in the shape of grapes is known as staphylococci: Staphylococci are cocci that grow in clusters resembling grapes.

Answer: Vibrio

Comma-shaped bacteria are known as Vibrio. They are curved or comma-shaped bacteria.

The other options are incorrect:

• Staphylococci: Staphylococci are spherical bacteria that often form clusters resembling grapes, not comma-shaped.
• Streptococci: Streptococci are spherical bacteria that typically form chains, not comma-shaped.
• Bacillus: Bacillus bacteria are rod-shaped, not comma-shaped.

Answer: Prions

Prions are the most resistant to sterilization methods due to their unique structure and resistance to heat, chemicals, and radiation.

The other options are incorrect:

• Tubercle bacilli: While tubercle bacilli, such as Mycobacterium tuberculosis, are resistant to certain sterilization methods, they are not as resistant as prions.
• Viruses: While some viruses can be resistant to certain sterilization methods, they are generally not as resistant as prions.
• Spores: Bacterial spores, such as those produced by Clostridium and Bacillus species, are highly resistant to many sterilization methods, but they are generally not as resistant as prions.

Answer: Venkatraman-Ramakrishanan (VR) medium

🔹 VR medium is the recommended transport medium for stool specimens suspected to contain Vibrio cholerae.
It preserves the organism’s viability during transport until culture.

• Buffered glycerol saline → unsuitable (glycerol is toxic to V. cholerae)
• Nutrient broth / Blood agar → not transport media

Answer: Chocolate agar

Chocolate agar is an enriched medium commonly used for the cultivation of fastidious microorganisms, particularly those requiring additional nutrients and growth factors.

The other options are incorrect:

• Selenite F broth: Selenite F broth is a selective enrichment medium used for the isolation of Salmonella species from clinical specimens, not an enriched medium.
• Peptone water: Peptone water is a simple, non-selective liquid medium composed of peptone and water, not an enriched medium.
• MacConkey agar: MacConkey agar is a selective and differential medium used for the isolation and differentiation of Gram-negative bacteria, particularly members of the Enterobacteriaceae family, based on lactose fermentation, but it is not an enriched medium.

Answer: Disinfection

Disinfection does not directly affect microbial growth; instead, it aims to reduce the number of microorganisms on surfaces or in the environment.

The other options are incorrect:

• Moisture: Microbes need water for their metabolic processes and survival. Lack of moisture significantly hinders their growth.
• Darkness: While some microbes require light for certain functions (phototrophs), many can grow in darkness. Darkness itself doesn’t prevent microbial growth.
• Temperature: Temperature plays a crucial role in microbial growth. Each microbe has a specific temperature range for optimal growth.

Answer: 1-2%

The agar concentration required to prepare nutrient agar is typically in the range of 1-2%.

The other options are incorrect:

• 4-6%: This concentration is too high for preparing nutrient agar and would result in a gel that is too stiff and difficult for microbial growth.
• 0.25-0.5%: This concentration is too low for preparing nutrient agar and may not provide sufficient solidification of the medium.
• 0.5-1%: While this concentration is closer to the correct range, it is still lower than the typical agar concentration used for preparing nutrient agar.

Answer: Retrovirus

HIV (Human Immunodeficiency Virus) is a retrovirus, meaning it contains RNA as its genetic material and uses reverse transcriptase enzyme to convert its RNA into DNA for replication.

The other options are incorrect:

• DNA virus: DNA viruses use DNA as their genetic material and don’t require reverse transcriptase. Fungus: HIV is not a fungus. Fungus is a eukaryotic organism that can cause infections but is entirely different from a virus.
• Bacteria: Bacteria are single-celled prokaryotic organisms, much simpler than viruses. HIV is a complex virus with a specific structure and mode of replication.

Answer: Chromosome

MRSA (Methicillin-resistant Staphylococcus aureus) resistance is mediated by genes located on the bacterial chromosome.

The other options are incorrect:

• Plasmid: MRSA resistance is primarily mediated by chromosomal genes, not plasmids.
• Transposons: While transposons can contribute to antibiotic resistance in bacteria, MRSA resistance is primarily mediated by chromosomal genes.

Answer: Peritrichous

Peritrichous refers to bacteria with flagella distributed all around the cell, allowing for efficient movement in any direction.

The other options are incorrect:

• Amphitrichous: This describes bacteria with flagella at opposite ends of the cell.
• Monotrichous: This refers to bacteria with a single flagellum at one end.
• Lophotrichous: This describes bacteria with a cluster of flagella at one or both poles of the cell.

Answer: Capsule

India ink is used in a negative staining technique to visualize the capsule surrounding some bacteria. The ink particles are too large to penetrate the capsule, creating a clear halo around the bacteria against a dark background.

The other options are incorrect:

• Cell wall: India ink wouldn’t be very effective in specifically highlighting the cell wall.
• Nucleus: Bacteria lack a true nucleus and wouldn’t be stained by India ink.
• Flagella: Flagella are too thin to be visualized using India ink staining.

Answer: Mesophilic

Mesophilic bacteria thrive in moderate temperatures, typically between 20°C and 45°C (68°F and 113°F). This range encompasses the human body temperature (around 37°C) and many environmental conditions suitable for bacterial growth.

The other options are incorrect:

• Capnophilic: These bacteria require increased carbon dioxide (CO2) levels for growth, not a specific temperature range.
• Psychrophilic: These are cold-loving bacteria that prefer temperatures near freezing and wouldn’t grow optimally at 25-40°C.
• Thermophilic: These bacteria thrive at high temperatures, often above 50°C, and wouldn’t be suited for the 25-40°C range.

Answer: Spherical

Cocci are bacteria with a round or spherical shape, resembling balls or spheres.

The other options are incorrect:

• Rod: This describes bacilli, another common bacterial shape resembling rods or cylinders.
• Comma: This describes vibrio, a curved, comma-shaped bacterium.
• Spiral: This describes spirilla, a slender, helical or spiral-shaped bacterium.

Answer: Coxiella burnetii

Coxiella burnetii, the bacterium that causes Q fever, is known to be highly resistant to pasteurization methods like the Holder method.

The other options are incorrect:

• Mycobacterium: While some Mycobacterium species like M. tuberculosis can be somewhat resistant to pasteurization, they are typically inactivated by the Holder method.
• Brucella: Brucella species, causing brucellosis, are generally susceptible to pasteurization and would be inactivated by the Holder method.
• Salmonella: Salmonella bacteria, a common cause of foodborne illness, are readily inactivated by pasteurization methods like Holder pasteurization.

Answer: Filtration

Filtration is the preferred method for sterilizing heat-labile media because it physically removes microbes without compromising the delicate components of the media.

The other options are incorrect:

• Pasteurization: Pasteurization uses heat treatment to eliminate some pathogens but wouldn’t be suitable for heat-labile media as it might damage essential components.
• Autoclaving: Autoclaving uses high pressure and steam for sterilization, which is too harsh for heat-labile media.
• Tyndallization: This method involves repeated cycles of heating to a moderate temperature, allowing for growth, and then killing the remaining spores.

Answer: Protein

Exotoxins are typically proteinaceous substances produced and secreted by certain bacteria.

The other options are incorrect:

• Heat labile: While exotoxins can be heat labile, this characteristic is not definitive of exotoxins as some may be heat stable.
• More antigenic: Exotoxins are indeed antigenic, but they are not necessarily more antigenic than other bacterial components.
• Non-enzymatic: Exotoxins are often enzymatic, as they exert their toxic effects by enzymatic activity within the host.

Answer: Neutralized by antitoxin

Exotoxins are neutralized by antitoxins, which are antibodies that specifically bind to and neutralize the toxic effects of exotoxins.

The other options are incorrect:

• Produced only by gram positive bacteria: Exotoxins can be produced by both gram-positive and gram-negative bacteria.
• Less potent than endotoxin: Exotoxins can be highly potent, sometimes more so than endotoxins, depending on the specific toxin and its target.
• Pyrogenic: Exotoxins are not necessarily pyrogenic; they do not always induce fever as endotoxins do. Some exotoxins may have pyrogenic effects, but it is not a defining characteristic.

Answer: Streptococcus pyogenes

Streptococcus pyogenes, also known as Group A Streptococcus, is usually sensitive to penicillin. Penicillin is often the first-line treatment for infections caused by this bacterium.

The other options are incorrect:

• Staphylococcus aureus: Staphylococcus aureus can be resistant to penicillin due to the production of beta-lactamase enzymes.
• Enterococcus faecalis: Enterococcus faecalis can exhibit varying degrees of resistance to penicillin.
• Staphylococcus epidermidis: Staphylococcus epidermidis can be resistant to penicillin due to the production of beta-lactamase enzymes.

Answer: Group-B

Group B Streptococcus (Streptococcus agalactiae) is the leading cause of neonatal meningitis in newborns.

The other options are incorrect:

• Group-A: Group-A Streptococcus (Streptococcus pyogenes) is not a common cause of neonatal meningitis; it is more associated with infections such as pharyngitis and skin infections.
• Group-C: Group-C Streptococcus is not a common cause of neonatal meningitis.
• Group-D: Group-D Streptococcus includes various species, but none are commonly associated with neonatal meningitis.

Answer: Clostridia

Clostridia, specifically certain species like Clostridium perfringens and Clostridium botulinum, are gram-positive bacteria responsible for food poisoning.

The other options are incorrect:

• Mycoplasmas: Mycoplasmas are not typically associated with food poisoning; they are a different group of bacteria known for causing infections such as pneumonia and urinary tract infections.
• Pseudomonas: Pseudomonas is a gram-negative bacterium that is not commonly associated with food poisoning.
• All of the above: This option is incorrect as mycoplasmas and pseudomonas are not typically responsible for food poisoning.

Answer: Staphylococci

Methicillin-resistant Staphylococcus aureus (MRSA) is the most common type of methicillin-resistant bacteria. Staphylococci are gram-positive bacteria.

The other options are incorrect:

• Streptococci: While some Streptococcus species can develop antibiotic resistance, they are not typically associated with methicillin resistance.
• Pseudomonas: These are gram-negative bacteria and do not develop resistance to methicillin in the same way as Staphylococci.
• Haemophilus: These are also gram-negative bacteria and not commonly known for methicillin resistance.

Answer: Staphylococcus saprophyticus

Staphylococcus saprophyticus is a common cause of urinary tract infections (UTIs) in young, sexually active women.

The other options are incorrect:

• Staphylococcus aureus: While S. aureus can cause UTIs, it’s less frequent than S. saprophyticus in this specific context.
• Staphylococcus epidermidis: This species is not a typical culprit for UTIs in sexually active women.
• Streptococcus viridians: These are gram-positive bacteria but are not a common cause of UTIs, especially in sexually active women

Answer: Clindamycin

Clindamycin is one of the primary antibiotics used in treating toxic shock syndrome (TSS) because it helps suppress the production of toxins by bacteria, specifically Staphylococcus aureus and Streptococcus pyogenes, which are common causes of TSS.

The other options are incorrect:

• Vancomycin: While vancomycin is a powerful antibiotic effective against various bacteria, it doesn’t specifically target toxin production like clindamycin.
• Cephalosporins: Cephalosporins are a broad spectrum antibiotic class effective against many bacteria, but they may not be the most effective choice for TSS as they don’t necessarily target toxin production.
• Streptomycin: Streptomycin is not typically used for TSS treatment as it’s less effective against the bacteria commonly associated with TSS.

Answer: Pseudomonas

Pseudomonas is an organism known for its considerable resistance to antiseptics, disinfectants, and antibiotics, making it challenging to treat infections caused by this bacterium.

The other options are incorrect:

• E.coli: While Escherichia coli can develop resistance to antibiotics, it is not typically known for considerable resistance to antiseptics and disinfectants.
• Staphylococci: Staphylococci, particularly Staphylococcus aureus, can develop antibiotic resistance, but their resistance to antiseptics and disinfectants is not as pronounced as that of Pseudomonas.
• Streptococci: Streptococci can develop antibiotic resistance, but they are generally more susceptible to antiseptics and disinfectants compared to Pseudomonas.

Answer: Streptococcus bovis

Streptococcus bovis (formerly Streptococcus bovis biotype I) is a bacterium frequently linked to endocarditis in patients with colon cancer. It’s part of the gut flora and can enter the bloodstream through compromised areas in the colon caused by cancer, leading to infection of heart valves.

The other options are incorrect:

• Staphylococcus epidermidis: While S. epidermidis can cause endocarditis, it’s less commonly associated with colon cancer. It’s typically linked to healthcare-related infections involving medical devices.
• Streptococcus sanguis: S. sanguis is a common cause of endocarditis in general, but the association with colon cancer is weaker compared to Streptococcus bovis.
• Streptococcus mutans: S. mutans is primarily associated with dental caries (cavities) and not typically linked to endocarditis.

Answer: Klebsiella

Klebsiella pneumoniae is a common lactose fermenter. During lactose fermentation, Klebsiella breaks down lactose sugar into lactic acid and other products.

The other options are incorrect:

• Pseudomonas: Pseudomonas aeruginosa is not a lactose fermenter. It utilizes different carbon sources for energy.
• Salmonella: Salmonella enterica serovar Typhi (commonly known as Salmonella Typhi) is not a lactose fermenter. It ferments other sugars but not lactose.
• Shigella: Shigella sonnei, a common cause of dysentery, is not a lactose fermenter. It also utilizes different sugars for energy.

Answer: Cholera

Rice-water stool, a hallmark symptom of cholera, is characterized by its whitish, opaque appearance resembling water used to cook rice. It occurs due to massive fluid secretion in the intestines caused by the Vibrio cholerae bacteria.

The other options are incorrect:

• Bacillary dysentery: While dysentery can cause diarrhea with mucus or blood, it typically doesn’t present with the characteristic rice-water appearance associated with cholera.
• Salmonella infection: Salmonella infections can cause diarrhea, but the stool consistency and appearance are usually different from rice-water stool seen in cholera.
• Pseudomembranous colitis: This condition, often caused by antibiotic use, can cause various diarrheal presentations, but rice-water stool is not a typical feature.

Answer: Streptococcus mutans

Streptococcus mutans is the key player in dental caries development. It thrives on sugars in the mouth, producing acids that demineralize tooth enamel, leading to cavities.

The other options are incorrect:

• Streptococcus pyogenes: This bacterium is more commonly associated with strep throat and skin infections and isn’t a major culprit in tooth decay.
• Streptococcus faecalis: While it can be present in the mouth, S. faecalis is primarily a gut inhabitant and doesn’t significantly contribute to cavities.
• Streptococcus agalactiae: This bacterium is linked to infections in newborns and pregnant women and has no role in dental caries.

Answer: Enterotoxigenic Escherichia coli

Enterotoxigenic E. coli (ETEC) is the most common culprit behind traveler’s diarrhea. It produces toxins that disrupt the intestinal function, leading to watery diarrhea.

The other options are incorrect:

• Enteroinvasive E. coli (EIEC): EIEC causes a more severe form of diarrhea with bloody stools, less frequent compared to traveler’s diarrhea caused by ETEC.
• Enterohemorrhagic E. coli (EHEC): EHEC is known for causing bloody diarrhea and is not the typical cause of traveler’s diarrhea.
• Enteropathogenic E. coli (EPEC): EPEC is another cause of diarrhea, particularly in children, but it’s less prevalent than ETEC in traveler’s diarrhea.pen_spark

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Answer: Parvo virus

Parvoviruses are some of the smallest viruses known, with a diameter around 18-22 nanometers. They are even smaller than some large protein molecules.

The other options are incorrect:

• Poxvirus: Poxviruses are relatively large viruses, ranging from 170-300 nanometers in diameter.
• Herpesvirus: Herpesviruses are also on the larger side, typically between 120-200 nanometers in diameter.
• Adenovirus: Adenoviruses fall in the medium size range for viruses, with a diameter of around 70-90 nanometers.