Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2301: What is the clinical significance of the coagulase enzyme produced by Staphylococcus aureus?

A. It helps in identifying the bacteria
B. It coagulates blood, aiding in immune evasion (Correct Answer)
C. It breaks down red blood cells
D. It neutralizes stomach acids

Explanation: ***It coagulates blood, aiding in immune evasion*** - **Coagulase** functions by clotting plasma, primarily converting fibrinogen to **fibrin**, which forms a protective fibrin barrier around the bacteria. - This **fibrin clot** helps the bacteria evade phagocytosis by immune cells and localize the infection, contributing to abscess formation. *It helps in identifying the bacteria* - While the **coagulase test** is a crucial diagnostic tool for differentiating *Staphylococcus aureus* from other staphylococci (especially **coagulase-negative staphylococci**), its primary clinical significance relates to its role in pathogenesis, not just identification. - The test utilizes the enzyme's activity for identification, but the *function* of the enzyme in the body is linked to virulence. *It breaks down red blood cells* - The breakdown of red blood cells is typically associated with **hemolysins**, a different class of enzymes or toxins produced by various bacteria, including some strains of *Staphylococcus aureus*. - **Coagulase** does not directly cause **hemolysis**; its action is specifically on plasma proteins. *It neutralizes stomach acids* - Neutralizing stomach acids is a mechanism employed by certain bacteria (e.g., *Helicobacter pylori* with **urease**) to survive the acidic gastric environment. - **Coagulase** does not play a role in acid neutralization; its function is extracellular and involves plasma protein modification.

Question 2302: A 29-year-old male reports a red, painful lump on his back that has ruptured and drained pus. Which organism is the most likely cause?

A. Staphylococcus aureus (Correct Answer)
B. Pseudomonas aeruginosa
C. Streptococcus pyogenes
D. Corynebacterium minutissimum

Explanation: ***Staphylococcus aureus*** - *S. aureus* is the most common cause of **skin and soft tissue infections**, including **abscesses, boils, and carbuncles**, which present as red, painful lumps that may rupture and drain pus. - Its ability to produce **virulence factors** like coagulase and hemolysins contributes to its pathogenicity in these infections. *Pseudomonas aeruginosa* - *P. aeruginosa* typically causes infections in **immunocompromised individuals**, **burn victims**, or those with **indwelling medical devices**. - While it can cause skin infections, these often present differently, such as **ecthyma gangrenosum** or **folliculitis** in specific contexts like hot tub exposure, and are less commonly the cause of a spontaneous abscess in an otherwise healthy individual. *Streptococcus pyogenes* - *S. pyogenes* is known for causing **streptococcal pharyngitis** (strep throat), **impetigo**, and **cellulitis**, but it is less commonly associated with isolated, pus-draining abscesses. - While it can cause local skin infections, the classic presentation of a localized, fluctuant, pus-draining lesion is more characteristic of staphylococcal infection. *Corynebacterium minutissimum* - *C. minutissimum* is the causative agent of **erythrasma**, a superficial skin infection that appears as well-demarcated, reddish-brown patches, often in intertriginous areas. - It does not typically cause a painful, rupturing, pus-draining lump or abscess.

Question 2303: Which organism is the most common cause of neonatal urinary tract infections?

A. E. coli (Correct Answer)
B. S. aureus
C. Enterococcus
D. Anaerobes

Explanation: ***E. coli*** - **_Escherichia coli_** is the most frequent cause of **neonatal urinary tract infections (UTIs)**, accounting for a significant majority of cases (60-80%). - This is due to its prevalence in the **gastrointestinal tract** and ease of perineal contamination in infants, leading to ascending infections. *S. aureus* - **_Staphylococcus aureus_** is an important cause of skin and soft tissue infections, and in some cases, **sepsis**, but it is a relatively uncommon cause of primary neonatal UTIs. - While it can cause UTIs through **hematogenous spread**, especially in catheterized infants, it is not the most common causative agent. *Enterococcus* - **_Enterococcus_ species** are known causes of UTIs, particularly in hospitalized or immunocompromised patients, and can be isolated from neonatal UTIs. - However, they are **less common** than _E. coli_ in healthy neonates presenting with their first UTI. *Anaerobes* - **Anaerobic bacteria** are generally **not considered significant causes** of neonatal UTIs. - While they are part of the normal gut flora, their contribution to urinary tract infections is rare, as the urinary tract is typically aerobic.

Question 2304: A 22-year-old woman presents with dysuria, frequency, and urgency. A urine culture reveals a lactose-fermenting Gram-negative rod. What is the most likely causative agent?

A. Escherichia coli (Correct Answer)
B. Staphylococcus saprophyticus
C. Proteus mirabilis
D. Enterococcus faecalis

Explanation: **_Escherichia coli_** - *E. coli* is the most common cause of **uncomplicated urinary tract infections (UTIs)**, especially in young women. - It is a **lactose-fermenting Gram-negative rod** commonly found in the gastrointestinal tract, fitting the urine culture findings. *Staphylococcus saprophyticus* - While *S. saprophyticus* is a significant cause of UTIs in young women, it is a **Gram-positive coccus**, not a Gram-negative rod. - It is also **coagulase-negative** and typically novobiocin-resistant, differentiating it from other staphylococci. *Proteus mirabilis* - *Proteus mirabilis* is a **Gram-negative rod** and a known cause of UTIs, particularly those associated with **struvite stones** due to its urease activity. - However, it is typically a **non-lactose fermenter**, which contradicts the urine culture result. *Enterococcus faecalis* - *Enterococcus faecalis* is a common cause of UTIs, especially in catheterized patients or those with healthcare-associated infections. - However, it is a **Gram-positive coccus**, differentiating it from the Gram-negative rod identified in the culture.

Question 2305: A 2-year-old child presents with fever and a maculopapular rash. Blood culture grows gram-positive cocci in chains that are catalase-negative. Which organism is likely responsible?

A. Staphylococcus epidermidis
B. Streptococcus pyogenes (Correct Answer)
C. Neisseria meningitidis
D. Enterococcus faecium

Explanation: ***Streptococcus pyogenes*** - The presentation of fever and a **maculopapular rash** in a child, along with **gram-positive cocci in chains** that are **catalase-negative**, is highly indicative of *Streptococcus pyogenes* infection. - *S. pyogenes* is a common cause of **scarlet fever**, which presents with a characteristic maculopapular rash (scarlatiniform rash) and is caused by erythrogenic toxins produced by the bacteria. *Staphylococcus epidermidis* - *Staphylococcus epidermidis* is a **coagulase-negative staphylococcus** that is part of normal skin flora and is more commonly associated with medical device-related infections. - While it is a gram-positive coccus, it typically grows in **clusters** and is **catalase-positive**, differentiating it from the stated findings. *Neisseria meningitidis* - *Neisseria meningitidis* is a **gram-negative diplococcus** and is **catalase-positive**, making it inconsistent with the gram stain and catalase results. - It causes **meningitis** and **meningococcemia**, often with a petechial or purpuric rash, not typically a maculopapular rash in this context. *Enterococcus faecium* - *Enterococcus faecium* is a **gram-positive coccus that grows in chains** and is **catalase-negative**, which is consistent with the initial bacteriological findings. - However, while it can cause various infections, it is not typically associated with a maculopapular rash in a 2-year-old child as a primary presentation like *S. pyogenes* in scarlet fever.

Question 2306: Which pathogen is the most common cause of acute otitis media in children?

A. Streptococcus pneumoniae (Correct Answer)
B. Pseudomonas aeruginosa
C. Haemophilus influenzae
D. Moraxella catarrhalis

Explanation: ***Streptococcus pneumoniae*** - This pathogen is the most common bacterial cause of **acute otitis media (AOM)**, accounting for approximately 25-50% of cases in children. - Its prevalence underscores the importance of vaccination (pneumococcal vaccine) in preventing childhood ear infections. *Haemophilus influenzae* - While a significant cause of AOM, particularly in the post-pneumococcal vaccine era, *H. influenzae* is the **second most common** bacterial pathogen, making up about 20-40% of cases. - Non-typeable *H. influenzae* strains are often implicated. *Moraxella catarrhalis* - *M. catarrhalis* is another common bacterial cause of AOM but is less frequent than *S. pneumoniae* or *H. influenzae*, typically responsible for about 10-20% of cases. - It often causes milder infections compared to the other two primary pathogens. *Pseudomonas aeruginosa* - *P. aeruginosa* is an **uncommon cause** of acute otitis media in healthy children. - It is more typically associated with **otitis externa** (swimmer's ear) or chronic otitis media, especially in children with tympanostomy tubes.

Question 2307: A patient with a cat bite presents with a swollen, tender hand and fever. A Gram stain of the wound shows gram-negative rods. What is the most likely pathogen?

A. Staphylococcus aureus
B. Pasteurella multocida (Correct Answer)
C. Bartonella henselae
D. Capnocytophaga canimorsus

Explanation: ***Pasteurella multocida*** - ***Pasteurella multocida*** is a common commensal in the oral flora of cats and dogs, making it the most frequent cause of infection following **animal bites**. - The presentation of rapid onset of **swelling**, **pain**, and **inflammation** around a cat bite, accompanied by **gram-negative rods** on stain, is highly characteristic of ***Pasteurella*** infection. *Staphylococcus aureus* - While ***Staphylococcus aureus*** can cause skin and soft tissue infections, it typically presents with **gram-positive cocci** in clusters, not gram-negative rods. - Although common in human skin infections, it is less typical for a direct animal bite infection unless secondary contamination occurs. *Bartonella henselae* - ***Bartonella henselae*** is the causative agent of **cat scratch disease**, presenting primarily as **lymphadenopathy** often some weeks post-scratch. - It does not typically cause immediate, localized cellulitis with gram-negative rods visible on stain, as seen in this clinical scenario. *Capnocytophaga canimorsus* - ***Capnocytophaga canimorsus*** is a gram-negative rod found in the oral flora of dogs and, less commonly, cats, which can cause severe infections, particularly in immunocompromised individuals. - While it can cause rapid-onset infection from a bite, ***Pasteurella multocida*** is significantly more common in routine animal bite infections.

Question 2308: A 65-year-old man presents with fever and confusion. CSF analysis shows high protein and low glucose levels. Gram stain reveals small, pleomorphic, gram-negative coccobacilli. What is the most likely causative organism?

A. Haemophilus influenzae (Correct Answer)
B. Listeria monocytogenes
C. Streptococcus pneumoniae
D. Neisseria meningitidis

Explanation: ***Haemophilus influenzae*** - The combination of **fever**, **confusion**, **high CSF protein**, **low CSF glucose**, and **small, pleomorphic, gram-negative coccobacilli** on Gram stain strongly points to *Haemophilus influenzae* meningitis. - While vaccination has reduced its incidence, *H. influenzae* can still cause **meningitis** in unvaccinated individuals, particularly in older adults or those with underlying conditions. *Listeria monocytogenes* - This organism typically causes meningitis in **neonates**, **elderly individuals**, and **immunocompromised patients**. - On Gram stain, *Listeria* appears as **gram-positive rods**, which is inconsistent with the description of gram-negative coccobacilli. *Streptococcus pneumoniae* - *S. pneumoniae* is a common cause of bacterial meningitis, presenting with similar CSF findings (high protein, low glucose). - However, it appears as **gram-positive cocci in pairs or chains** on Gram stain, not gram-negative coccobacilli. *Neisseria meningitidis* - *N. meningitidis* also causes meningitis with high protein and low glucose in CSF, and it is a **gram-negative coccus** (often in pairs). - However, the description **"pleomorphic coccobacilli"** is more characteristic of *Haemophilus influenzae* than the typically more uniform cocci of *Neisseria*.

Question 2309: A patient presents to the clinic with a fever and cough. A chest X-ray shows lobar pneumonia. Which organism is most likely responsible?

A. Streptococcus pneumoniae (Correct Answer)
B. Haemophilus influenzae
C. Mycoplasma pneumoniae
D. Legionella pneumophila

Explanation: ***Streptococcus pneumoniae*** - This is the **most common cause** of community-acquired bacterial pneumonia and is frequently associated with **lobar pneumonia** on chest X-ray. - Patients typically present with acute onset of **fever, cough**, and often **pleuritic chest pain**. *Haemophilus influenzae* - While it can cause pneumonia, especially in patients with **chronic lung disease** (e.g., COPD), it is a less common cause of classic **lobar pneumonia** than *S. pneumoniae*. - Often presents with features similar to other bacterial pneumonias but may also cause **epiglottitis** or **meningitis**. *Mycoplasma pneumoniae* - This typically causes **atypical pneumonia**, often referred to as "walking pneumonia," which tends to have a more **insidious onset** and milder symptoms. - Chest X-rays usually show **interstitial infiltrates**, not distinct lobar consolidation. *Legionella pneumophila* - This can cause severe pneumonia, often associated with a history of **exposure to contaminated water sources** (e.g., air conditioning systems). - Clinical features may include **hyponatremia** and **gastrointestinal symptoms** (diarrhea), which are not mentioned here, and while it can cause lobar consolidation, it is less common than *S. pneumoniae*.

Question 2310: In a 60-year-old diabetic patient with necrotizing fasciitis, what is the most likely causative agent?

A. Staphylococcus aureus
B. Streptococcus pyogenes (Correct Answer)
C. Clostridium perfringens
D. Pseudomonas aeruginosa

Explanation: ***Streptococcus pyogenes*** - **Group A Streptococcus (GAS)**, particularly *S. pyogenes*, is the most common cause of **Type II necrotizing fasciitis**, a severe, rapidly progressing soft tissue infection. - Diabetic patients are at higher risk for severe infections due to **impaired immune response** and compromised microcirculation, making them susceptible to aggressive bacterial agents. *Staphylococcus aureus* - While *S. aureus*, especially **MRSA**, can cause skin and soft tissue infections, it is less common as the primary causative agent for typical **necrotizing fasciitis** compared to *S. pyogenes*. - *S. aureus* is more frequently associated with **abscesses, cellulitis, and wound infections**, rather than the rapidly spreading tissue destruction seen in classical necrotizing fasciitis. *Clostridium perfringens* - *C. perfringens* is the primary cause of **gas gangrene (clostridial myonecrosis)**, a distinct form of severe soft tissue infection characterized by gas production in tissues. - While also a rapidly progressive and life-threatening infection, it presents with specific signs like **crepitus** and is not typically the first suspected agent for general necrotizing fasciitis without these features. *Pseudomonas aeruginosa* - *P. aeruginosa* is often implicated in **healthcare-associated infections**, infections in immunocompromised patients, and **burn wounds**. - While it can cause serious soft tissue infections, it is not the most common causative agent of **community-acquired necrotizing fasciitis** in a diabetic patient without specific risk factors like extensive burn injuries or prolonged hospitalization.

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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