Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 2311: A newborn develops severe meningitis. Gram staining of the cerebrospinal fluid shows gram-positive cocci in chains. Which organism is most likely responsible?

A. Neisseria meningitidis
B. Staphylococcus aureus
C. Streptococcus agalactiae (Correct Answer)
D. Listeria monocytogenes

Explanation: ***Streptococcus agalactiae*** - **_Streptococcus agalactiae_** (Group B Streptococcus) is a common cause of **neonatal meningitis** and is characterized by **gram-positive cocci in chains**. - This organism is often acquired during **vaginal birth** from the mother's colonized birth canal. *Neisseria meningitidis* - **_Neisseria meningitidis_** causes **meningitis**, but it is characterized by **gram-negative diplococci**, not gram-positive cocci in chains. - While it can affect newborns, it is more commonly associated with **older infants, children, and young adults**. *Staphylococcus aureus* - **_Staphylococcus aureus_** is a **gram-positive coccus** that typically forms **clusters _(grape-like clusters)_**, not chains. - It is a common cause of skin infections, endocarditis, and osteomyelitis, but less commonly a primary cause of **neonatal meningitis** unless there's a predisposing factor like a shunt. *Listeria monocytogenes* - **_Listeria monocytogenes_** can cause **neonatal meningitis** and is acquired transplacentally or during birth. - However, it is a **gram-positive rod**, not a gram-positive coccus.

Question 2312: Which organism is most commonly associated with pyogenic osteomyelitis?

A. Staphylococcus aureus (Correct Answer)
B. Pseudomonas aeruginosa
C. Salmonella
D. Eikenella corrodens

Explanation: ***Staphylococcus aureus*** - This organism is the **most frequent cause** of osteomyelitis in individuals of all ages. - It often gains entry through **trauma**, **surgery**, or **hematogenous spread**. *Pseudomonas aeruginosa* - This is a common cause of osteomyelitis in specific populations, such as **intravenous drug users** or individuals with **puncture wounds** through footwear. - It's also frequently associated with osteomyelitis in patients with **diabetes** or after **bone graft surgery**. *Salmonella* - While *Salmonella* can cause osteomyelitis, it is primarily associated with individuals suffering from **sickle cell disease** due to impaired reticuloendothelial function. - It is a **less common** cause overall compared to *Staphylococcus aureus*. *Eikenella corrodens* - This gram-negative bacillus is typically involved in osteomyelitis following **human bite wounds** or **head and neck infections** due to its presence in oral flora. - It is a **rare cause** of osteomyelitis in the general population.

Question 2313: A 58-year-old male with chronic obstructive pulmonary disease presents with three weeks of productive cough and malaise. A chest X-ray shows a single lung cavity with an air-fluid level. Sputum culture grows filamentous bacteria with sulfur granules. What is the most likely pathogen?

A. Mycobacterium tuberculosis
B. Actinomyces israelii (Correct Answer)
C. Nocardia asteroides
D. Staphylococcus aureus

Explanation: ***Actinomyces israelii*** - The presence of **sulfur granules** in sputum, combined with a lung cavity and a history of COPD, is highly suggestive of **pulmonary actinomycosis**. - **Actinomyces israelii** is a normal inhabitant of the oral flora and can cause infection, particularly in individuals with compromised lung defenses or aspiration. *Mycobacterium tuberculosis* - While it can cause chronic productive cough, lung cavities, and is common in patients with impaired immunity, **Mycobacterium tuberculosis** does not produce **sulfur granules**. - It would typically be identified by **acid-fast staining** and specific culture methods, not by granules. *Nocardia asteroides* - **Nocardia** can cause pulmonary disease with cavities, especially in immunocompromised patients, but it typically presents with **partially acid-fast filaments** and does not form **sulfur granules**. - Infection often stems from inhalation of airborne organisms, and it can disseminate. *Staphylococcus aureus* - **Staphylococcus aureus** can cause lung abscesses and cavities (especially in patients with **COPD** or post-viral infections), but it typically does not produce **sulfur granules**. - Sputum cultures would reveal **Gram-positive cocci in clusters**, not filamentous bacteria.

Question 2314: A neonate presents with severe symptoms of meningitis. CSF analysis shows gram-positive rods. Considering the age of the patient, evaluate the most likely causative agent and the implications for treatment.

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

Explanation: ***Listeria monocytogenes*** - This is a common cause of **neonatal meningitis**, often acquired transplacentally or during birth, and presents as **gram-positive rods** in CSF. - Treatment typically involves **ampicillin**, as *Listeria* is intrinsically resistant to cephalosporins, which are commonly used for other bacterial meningitis. *Neisseria meningitidis* - While *N. meningitidis* can cause meningitis in neonates, it characteristically appears as **gram-negative diplococci** in CSF, not gram-positive rods. - Though highly virulent, its prevalence in neonates is lower compared to *Listeria* or Group B *Streptococcus*. *Streptococcus pneumoniae* - *S. pneumoniae* causes meningitis and appears as **gram-positive cocci** in pairs or chains, not rods. - Though it can affect neonates, it's more common in older infants and young children. *Haemophilus influenzae type B* - *H. influenzae type B* (Hib) typically presents as **gram-negative coccobacillary rods** in CSF. - The incidence of Hib meningitis has significantly decreased due to widespread vaccination, making it less likely, especially with gram-positive findings.

Question 2315: A patient presents with fever, chills, and a maculopapular rash. Blood culture reveals gram-negative rods that are lactose-fermenting. What is the most likely organism?

A. Salmonella typhi
B. Shigella flexneri
C. Escherichia coli (Correct Answer)
D. Yersinia pestis

Explanation: ***Escherichia coli*** - *Escherichia coli* is a common **lactose-fermenting gram-negative rod** and can cause systemic infections leading to fever, chills, and rash, especially in cases of bacteremia. - While many infections are localized, **urosepsis** or other systemic *E. coli* infections can present with these non-specific symptoms. *Salmonella typhi* - *Salmonella typhi* is a **non-lactose-fermenting** gram-negative rod, primarily causing typhoid fever, characterized by a **rose spot rash** and prolonged fever. - The organism's metabolic characteristics rule it out as the culprit in this case. *Shigella flexneri* - *Shigella flexneri* is another **non-lactose-fermenting** gram-negative rod, primarily known for causing **dysentery** with bloody diarrhea. - Systemic infection with rash and fever is less common, and its fermentation pattern does not match the clinical finding. *Yersinia pestis* - *Yersinia pestis* is a **non-lactose-fermenting** gram-negative rod responsible for plague, presenting as **buboes**, pneumonia, or septicemia, often without a widespread maculopapular rash. - The distinctive clinical presentation and fermentation characteristics do not align with the patient's symptoms.

Question 2316: A patient with chronic alcoholism presents with a swollen and painful knee. A Gram stain of the synovial fluid reveals gram-negative rods. What is the most likely causative agent?

A. Escherichia coli (Correct Answer)
B. Pseudomonas aeruginosa
C. Staphylococcus aureus
D. Streptococcus pneumoniae

Explanation: ***Escherichia coli*** - The presence of **gram-negative rods** in synovial fluid rules out gram-positive organisms and points to enteric or environmental gram-negative bacteria. - Among the available options, **E. coli** is the most common gram-negative organism causing septic arthritis, typically through **hematogenous spread** from urinary tract or gastrointestinal sources. - **Chronic alcoholism** increases susceptibility to various bacterial infections due to impaired immunity, making gram-negative septic arthritis more likely. - *Note: Klebsiella pneumoniae would be more classically associated with alcoholism and gram-negative septic arthritis, but is not among the options.* *Pseudomonas aeruginosa* - While *Pseudomonas* is a gram-negative rod and can cause septic arthritis, it is more specifically associated with **IV drug abuse**, **penetrating trauma**, or **nosocomial infections**. - Less commonly causes septic arthritis in alcoholics compared to enteric gram-negative organisms. *Staphylococcus aureus* - **Most common** cause of septic arthritis overall (accounts for 40-60% of cases), but it is a **gram-positive coccus in clusters**, not a gram-negative rod. - The Gram stain findings definitively rule out this organism despite its high prevalence. *Streptococcus pneumoniae* - This is a **gram-positive diplococcus** (lancet-shaped pairs), not a gram-negative rod. - The Gram stain result is inconsistent with this pathogen, ruling it out despite its association with immunocompromised states including alcoholism.

Question 2317: A 5-year-old child presents with fever and an erythematous rash. A throat culture grows beta-hemolytic colonies that are catalase-negative. What is the likely causative organism?

A. Staphylococcus aureus
B. Streptococcus pyogenes (Correct Answer)
C. Corynebacterium diphtheriae
D. Neisseria meningitidis

Explanation: ***Streptococcus pyogenes*** - The combination of **fever**, **erythematous rash**, and **beta-hemolytic, catalase-negative colonies** from a throat culture strongly points to *Streptococcus pyogenes* (Group A Strep), which causes **scarlet fever**. - Its **beta-hemolytic** property on blood agar indicates complete lysis of red blood cells, and it is consistently **catalase-negative**, differentiating it from staphylococci. *Staphylococcus aureus* - While *Staphylococcus aureus* can cause skin infections and rashes, it is typically **catalase-positive** and often **coagulase-positive**, which contradicts the laboratory findings. - *S. aureus* is known for causing localized infections like **abscesses** and **impetigo**, rather than the diffuse erythematous rash seen in scarlet fever. *Corynebacterium diphtheriae* - This bacterium is known for causing **diphtheria**, characterized by a **pseudomembrane** in the throat and systemic toxicity, but it is **not beta-hemolytic** and has a characteristic club-shaped morphology, not detected by the given information. - *C. diphtheriae* grows well on selective media like **Loeffler's** or **tellurite agar**, and its presumptive identification relies on Gram stain and biochemical tests, not just hemolysis and catalase. *Neisseria meningitidis* - *Neisseria meningitidis* causes **meningitis** and **meningococcemia**, often with a petechial or purpuric rash, but it is a **Gram-negative coccus**, not beta-hemolytic, and has distinctive growth requirements (e.g., CO2). - It is **catalase-positive** and **oxidase-positive**, features absent from the description of the causative organism.

Question 2318: Nagler's reaction is exhibited by which organism?

A. Clostridium tetani
B. Clostridium botulinum
C. Clostridium perfringens (Correct Answer)
D. Clostridium septicum

Explanation: ***Clostridium perfringens*** - The **Nagler's reaction** is a specific test used to detect the presence of **alpha toxin (lecithinase/phospholipase C)** produced by *Clostridium perfringens*. - The test is performed on **egg yolk agar** where one half of the plate contains **specific antitoxin** that neutralizes the alpha toxin. - A positive result shows **opacity** (due to lecithin breakdown) on the side without antitoxin, while the side with antitoxin remains **clear**, confirming the specificity of the reaction. - This is a **pathognomonic test** for *C. perfringens*. *Clostridium tetani* - This organism produces **tetanospasmin**, a potent neurotoxin responsible for tetanus, but does not produce the lecithinase detected by Nagler's reaction. - Identification typically involves clinical presentation of **tetanus** and specific toxin detection assays, not Nagler's reaction. *Clostridium botulinum* - *Clostridium botulinum* produces **botulinum toxin**, which causes botulism, and does not exhibit Nagler's reaction. - Its identification focuses on detecting the specific neurotoxin in patient samples or food. *Clostridium septicum* - While also a clostridial species, *Clostridium septicum* is associated with **myonecrosis** and **gas gangrene**, particularly in immunocompromised patients, but it does not produce the alpha toxin in a manner that would result in a positive Nagler's reaction. - Its detection usually involves culture and molecular methods, not Nagler's reaction.

Question 2319: A 30-year-old immunocompromised patient presents with chronic diarrhea, weight loss, and malabsorption. A biopsy of the small intestine reveals macrophages filled with acid-fast bacilli. Which pathogen is most likely responsible?

A. Mycobacterium avium complex (Correct Answer)
B. Cryptosporidium
C. Giardia lamblia
D. Entamoeba histolytica

Explanation: ***Mycobacterium avium complex*** - The combination of **chronic diarrhea**, **weight loss**, **malabsorption**, and **macrophages filled with acid-fast bacilli** in an immunocompromised patient is classic for *Mycobacterium avium complex* (MAC) infection. - MAC typically causes **disseminated disease** in immunocompromised individuals, often affecting the gastrointestinal tract and leading to villous atrophy and malabsorption. *Cryptosporidium* - While *Cryptosporidium* causes **chronic diarrhea** and can be severe in immunocompromised patients, it is a **protozoan parasite**, not a bacterium, and would not present as acid-fast bacilli. - Diagnosis typically involves detecting **oocysts** in stool samples, not intracellular microscopic acid-fast bacteria in tissue. *Giardia lamblia* - *Giardia lamblia* causes **malabsorption** and **diarrhea** but is also a **protozoan parasite** with distinct trophozoite and cyst forms, not acid-fast bacilli. - It primarily affects the small intestine but does not form intracellular acid-fast inclusions within macrophages. *Entamoeba histolytica* - *Entamoeba histolytica* causes **amebic dysentery** and liver abscesses, characterized by **bloody diarrhea**, but it is a **protozoan parasite** and would not be seen as acid-fast bacilli. - Microscopic examination of biopsies would reveal **trophozoites** ingesting red blood cells, not acid-fast bacteria in macrophages.

Question 2320: A 55-year-old man with a history of intravenous drug use presents with tricuspid valve endocarditis. Which organism is most likely responsible?

A. Staphylococcus aureus (Correct Answer)
B. Streptococcus viridans
C. Haemophilus influenzae
D. Enterococci

Explanation: ***Staphylococcus aureus*** - **_Staphylococcus aureus_** is the most common cause of **endocarditis** in individuals with **intravenous drug use (IVDU)**, particularly affecting the **tricuspid valve**. - This is due to its high prevalence on the **skin** and ability to form **biofilms** on damaged valves. *Streptococcus viridans* - **_Streptococcus viridans_** is a common cause of subacute bacterial endocarditis, often in patients with **pre-existing valvular disease** (e.g., mitral valve). - It is less commonly associated with **IVDU** and **tricuspid valve involvement** compared to _S. aureus_. *Haemophilus influenzae* - **_Haemophilus influenzae_** is a rare cause of **endocarditis**, belonging to the **HACEK group** of organisms, which are typically associated with **culture-negative endocarditis**. - It is not commonly seen in the setting of **intravenous drug use** or **tricuspid valve infection**. *Enterococci* - Endocarditis caused by **_Enterococci_** is often associated with **nosocomial infections**, **urinary tract infections**, or in patients with **pre-existing gastrointestinal or genitourinary conditions**. - While it can occur in IVDU, it is less frequent than _S. aureus_ as the causative agent for **tricuspid valve endocarditis**.

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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