Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2471: Which one of the following statements is true regarding the pathogenicity of Mycobacteria species?

A. M. Africanum is typically contracted from humans or monkeys.
B. M. marinum is primarily associated with skin infections. (Correct Answer)
C. M. Kansasii is primarily an environmental organism that rarely causes human disease.
D. M. tuberculosis and M. bovis have distinct pathogenic profiles in humans.

Explanation: ****M. marinum is primarily associated with skin infections.**** - *Mycobacterium marinum* is famously associated with "fish tank granuloma" or "swimming pool granuloma," causing **cutaneous lesions** following exposure to contaminated water. - Infection typically occurs through breaks in the skin from contact with infected fish, aquariums, or other aquatic environments. *M. Kansasii is primarily an environmental organism that rarely causes human disease.* - While *M. kansasii* is an environmental organism, it is a significant cause of **nontuberculous mycobacterial lung disease** in humans, resembling tuberculosis. - It does not rarely cause human disease; it's one of the more common causes of atypical mycobacterial infections. *M. Africanum is typically contracted from humans or monkeys.* - *M. africanum* is a species of *Mycobacterium tuberculosis* complex (MTBC) and is primarily a pathogen of **humans**, particularly in parts of West and East Africa. - It is **not typically contracted from monkeys**; the primary reservoir for *M. africanum* is humans. *M. tuberculosis and M. bovis have distinct pathogenic profiles in humans.* - Both *M. tuberculosis* and *M. bovis* are members of the MTBC and cause **tuberculosis in humans**, with *M. bovis* traditionally transmitted through contaminated dairy products. - While *M. tuberculosis* is the primary cause of human TB and *M. bovis* can cause extrapulmonary and pulmonary disease, their pathogenic profiles in humans are often **similar** in presentation with respect to the disease they cause, though modes of transmission differ.

Question 2472: The single most important laboratory test for determining the virulence of staphylococci is

A. Detection of coagulase (Correct Answer)
B. Mannitol fermentation
C. The catalase test
D. Hemolysis of sheep erythrocytes

Explanation: ***Detection of coagulase*** - The **coagulase enzyme** is a major virulence factor in *Staphylococcus aureus*, distinguishing it from less pathogenic staphylococci (coagulase-negative staphylococci). - Coagulase allows the organism to clot plasma, forming a fibrin layer that protects it from phagocytosis and immune detection. *Mannitol fermentation* - While *Staphylococcus aureus* can **ferment mannitol** (distinguishing it from some other staphylococci on mannitol salt agar), this characteristic is not directly linked to its virulence in the same critical way as coagulase. - Mannitol fermentation is a metabolic property, not a direct virulence factor that mediates host damage or evasion. *The catalase test* - The **catalase test** is used to differentiate staphylococci (catalase-positive) from streptococci (catalase-negative), but it does not differentiate between virulent and avirulent strains of staphylococci. - All staphylococci, both pathogenic and non-pathogenic, produce catalase to neutralize reactive oxygen species. *Hemolysis of sheep erythrocytes* - Many staphylococci (including *S. aureus*) produce **hemolysins**, which can lyse red blood cells. While a virulence factor, it is less specific and less critical than coagulase for determining overall virulence and pathogen identification. - Other bacteria also exhibit hemolysis, and some coagulase-negative staphylococci can also be hemolytic, making it a less definitive virulence marker for *Staphylococcus aureus* compared to coagulase.

Question 2473: Most common cause of traveler's diarrhea is:

A. Enterotoxigenic E. coli (Correct Answer)
B. Enteropathogenic E. coli
C. Enteroinvasive E. coli
D. Enterohemorrhagic E. coli

Explanation: ***Enterotoxigenic E. coli (ETEC)*** - **ETEC** is the most common cause of **traveler's diarrhea** worldwide, producing toxins that stimulate intestinal fluid secretion. - It causes a **watery diarrhea** without fever or significant inflammation, primarily through its **heat-labile** and **heat-stable toxins**. *Enterohemorrhagic E. coli (EHEC)* - **EHEC**, particularly O157:H7, is known for causing **hemorrhagic colitis** and **hemolytic uremic syndrome (HUS)**, not typically traveler's diarrhea. - Its pathogenicity is due to **Shiga toxins**, which damage intestinal epithelial cells and red blood cells. *Enteropathogenic E. coli (EPEC)* - **EPEC** is a significant cause of **infant diarrhea** in developing countries, leading to malabsorption. - It attaches to intestinal cells, causing effacement of microvilli, but does not produce toxins typically associated with traveler's diarrhea or invasiveness. *Enteroinvasive E. coli (EIEC)* - **EIEC** is responsible for a syndrome resembling **shigellosis**, involving invasion and destruction of intestinal epithelial cells. - It typically causes **dysentery** with fever, abdominal cramps, and bloody diarrhea, which is less common in traveler's diarrhea than watery symptoms.

Question 2474: Which of the following is the most diagnostically significant characteristic of Campylobacter jejuni?

A. Grows at 42°C (Correct Answer)
B. Obligate aerobe
C. Oxidase negative
D. Non-motile

Explanation: ***Grows at 42°C*** - The ability to grow optimally at **42°C (thermophilic growth)** is a key discriminating characteristic for *Campylobacter jejuni*, differentiating it from other common enteric pathogens. - This property is exploited in selective media (e.g., Skirrow agar) incubated at this higher temperature to aid in its isolation from fecal samples. *Obligate aerobe* - *Campylobacter jejuni* is actually **microaerophilic**, meaning it requires reduced oxygen levels (typically 5% O2) for optimal growth, not completely aerobic conditions. - Obligate aerobes require atmospheric oxygen concentrations for survival, which is contrary to the growth requirements of *C. jejuni*. *Oxidase negative* - *Campylobacter jejuni* is **oxidase positive**, which means it produces the enzyme cytochrome c oxidase. - An oxidase negative result would rule out *Campylobacter jejuni* and is characteristic of other organisms like *Escherichia coli*. *Non-motile* - *Campylobacter jejuni* is **highly motile** due to its characteristic single polar flagellum, which contributes to its invasiveness in the intestinal mucosa. - Non-motility would be inconsistent with the typical microscopic appearance and pathogenic mechanisms of *C. jejuni*.

Question 2475: Which of the following disorders would be more likely associated with Staphylococcus saprophyticus rather than Staphylococcus aureus?

A. Burns
B. Tension pneumothorax
C. Osteomyelitis
D. Acute cystitis (Correct Answer)

Explanation: ***Acute cystitis*** - **Staphylococcus saprophyticus** is a common cause of **urinary tract infections (UTIs)**, particularly acute cystitis, in young sexually active women. - This bacterium has a high affinity for **uroepithelial cells**, facilitating its colonization and subsequent infection of the bladder. *Tension pneumothorax* - A **tension pneumothorax** is a medical emergency characterized by air accumulation in the pleural space, leading to lung collapse and mediastinal shift. - It is typically caused by trauma or iatrogenic factors, not directly by bacterial infection from either *Staphylococcus saprophyticus* or *Staphylococcus aureus*. *Burns* - Burn wounds are highly susceptible to bacterial colonization and infection, with **Staphylococcus aureus** being a primary pathogen in this context. - *Staphylococcus saprophyticus* is rarely associated with burn wound infections. *Osteomyelitis* - **Osteomyelitis**, an infection of the bone, is most frequently caused by **Staphylococcus aureus** via hematogenous spread or direct inoculation. - *Staphylococcus saprophyticus* is not a common pathogen in osteomyelitis.

Question 2476: Which of the following statements about Enterohemorrhagic E. coli (EHEC) is false?

A. Sereny test is positive (Correct Answer)
B. Commonly causes HUS
C. Bloody diarrhea and HUS linked to O157:H7
D. Produces Shiga-like toxin

Explanation: ***Sereny test is positive*** - The **Sereny test** assesses the invasiveness of bacteria by observing their ability to invade and replicate in epithelial cells, typically in guinea pig conjunctiva. - *Enterohemorrhagic E. coli* (**EHEC**) is **non-invasive**; its pathogenicity stems from its production of **Shiga-like toxins**, not from cellular invasion, making a positive Sereny test statement false. *Commonly causes HUS* - **EHEC**, particularly serotype **O157:H7**, is a well-known cause of **Hemolytic Uremic Syndrome (HUS)**, especially in children and the elderly. - The **Shiga-like toxins** produced by EHEC are absorbed into the bloodstream, damaging endothelial cells and leading to the characteristic triad of **hemolytic anemia**, **thrombocytopenia**, and **acute kidney injury** seen in HUS. *Bloody diarrhea and HUS linked to O157:H7* - **EHEC O157:H7** is the most common serotype associated with outbreaks of **bloody diarrhea** and the subsequent development of **HUS**. - The toxins cause widespread damage to the intestinal lining, leading to **hemorrhagic colitis** (bloody diarrhea), and can then disseminate to distant organs like the kidneys, causing HUS. *Produces Shiga-like toxin* - The primary virulence factor of **EHEC** is the production of **Shiga-like toxins** (also known as **verotoxins**), specifically Stx1 and Stx2. - These toxins inhibit protein synthesis in eukaryotic cells, leading to cell death and tissue damage, which contribute to the severe gastrointestinal symptoms and systemic complications like HUS.

Question 2477: Which of the following agents of meningitis can grow on chocolate agar but not on blood agar?

A. Neisseria meningitidis
B. Haemophilus influenzae (Correct Answer)
C. Moraxella catarrhalis
D. Escherichia coli

Explanation: ***Haemophilus influenzae*** - *Haemophilus influenzae* requires **factor X (hemin)** and **factor V (NAD)** for growth, both of which are present in chocolate agar but not typically in blood agar. - On blood agar, *H. influenzae* can exhibit **satellite phenomenon** around *Staphylococcus aureus* colonies, which lyse red blood cells and release necessary factors. *Neisseria meningitidis* - *Neisseria meningitidis* can grow on **blood agar (enriched with CO2)** as well as chocolate agar. - It is a **fastidious organism** but does not have the same strict Factor X and V requirements as *Haemophilus influenzae* that would prevent its growth on blood agar. *Moraxella catarrhalis* - *Moraxella catarrhalis* is **not fastidious** and can readily grow on **blood agar** and chocolate agar. - It is known for its characteristic **hockey-puck adherence** to the agar when pushed. *Escherichia coli* - *Escherichia coli* is a **non-fastidious bacterium** and grows well on both **blood agar** and **MacConkey agar**. - Its ability to ferment lactose causes it to appear as **pink colonies** on MacConkey agar.

Question 2478: Cholera is caused by?

A. Enterococcus
B. E.coli
C. Vibrio cholerae O1 (Correct Answer)
D. Vibrio parahemolyticus

Explanation: ***Vibrio cholerae O1*** - **Cholera** is primarily caused by **toxigenic strains** of *Vibrio cholerae*, with serogroup **O1** being the most common and historically significant cause of epidemics and pandemics. - This bacterium produces **cholera toxin**, which leads to profuse watery diarrhea by acting on the intestinal epithelial cells. *Enterococcus* - **Enterococcus** species are common inhabitants of the human gastrointestinal tract and are often associated with **nosocomial infections**, such as urinary tract infections, endocarditis, and bacteremia. - They do not typically cause the severe, watery diarrhea characteristic of cholera. *Vibrio parahemolyticus* - *Vibrio parahemolyticus* is a common cause of **foodborne illness**, particularly associated with consuming **raw or undercooked seafood**. - It causes a form of gastroenteritis characterized by abdominal pain, watery diarrhea, nausea, vomiting, and fever, but it is distinct from cholera. *E.coli* - While various strains of **Escherichia coli (E. coli)** can cause diarrheal diseases (e.g., enterotoxigenic *E. coli* causing traveler's diarrhea, enterohemorrhagic *E. coli* causing bloody diarrhea), **E. coli** is not the causative agent of cholera. - The pathogenesis and specific toxins produced differ significantly from those of *Vibrio cholerae*.

Question 2479: Which of the following organisms is known to have sterols in its cell membrane?

A. Mycoplasma (Correct Answer)
B. Bacillus
C. Streptococcus
D. Spirillum

Explanation: ***Mycoplasma*** - **Mycoplasma** species are unique among bacteria for lacking a **cell wall** and incorporating **sterols** from their host into their cell membrane. - This incorporation of sterols contributes to the stability and integrity of their **cytoplasmic membrane**, which is vital due to the absence of a rigid cell wall. *Bacillus* - **Bacillus** species, like most bacteria, possess a rigid **peptidoglycan cell wall** and do not contain **sterols** in their cell membranes. - Their cell membrane is primarily composed of **phospholipids** and proteins, lacking the eukaryotic characteristic of sterols. *Streptococcus* - **Streptococcus** species are Gram-positive bacteria characterized by a thick **peptidoglycan cell wall** and do not have **sterols** in their cell membranes. - The cell wall provides structural support and protection, making sterols unnecessary for membrane stability. *Spirillum* - **Spirillum** species are Gram-negative bacteria that have a typical bacterial **cell wall** (including an outer membrane and peptidoglycan layer) and lack **sterols** in their membranes. - The presence of a strong cell wall negates the need for sterols to maintain membrane rigidity.

Question 2480: Which of the following is the MOST CHARACTERISTIC feature of Campylobacter?

A. Comma-shaped (Correct Answer)
B. Non-motile
C. Encapsulated
D. Spore-forming

Explanation: ***Comma-shaped*** - **Campylobacter** species are classically described as **comma-shaped** or S-shaped (seagull-shaped) **gram-negative rods** when viewed under a microscope. - This distinctive curved morphology is the **most characteristic feature** used in their identification. - The characteristic "darting motility" combined with this morphology makes identification straightforward. *Encapsulated* - **Campylobacter** species are generally **not encapsulated**, differentiating them from other pathogenic bacteria like *Streptococcus pneumoniae*. - Encapsulation is a **virulence factor** that helps bacteria evade phagocytosis but is not typical for **Campylobacter**. *Non-motile* - **Campylobacter** species are **highly motile**, possessing a **single polar flagellum** at one or both ends. - Their characteristic **darting motility** contributes to pathogenicity and ability to penetrate the intestinal mucus layer. - This motility is a crucial feature allowing movement through viscous environments. *Spore-forming* - **Campylobacter** species are **non-spore-forming** bacteria. - Spore formation is characteristic of certain gram-positive bacteria like *Bacillus* and *Clostridium* species. - This helps differentiate Campylobacter from spore-forming enteric pathogens.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

Practice Questions

Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

Practice Questions

Spirochetes

Practice Questions

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