Bacteriology Practice Questions

Q: A 34-year-old pregnant female presented to the emergency room with fever, backache, headache, and myalgias. Blood cultures were collected and subjected to examination. Microbiological examination revealed positive motility at 25°C and catalase positivity, Esculin positivity, and positive growth in 6.5% NaCl. The colonies are as shown on the blood agar plate given below. What is the most common route of transmission of the above organism?

Ingestion of contaminated food. ***Ingestion of contaminated food*** - The symptoms (fever, backache, headache, myalgias) in a pregnant female, along with the microbiological findings (**motility at 25°C, catalase positive, esculin positive, growth in 6.5% NaCl**), are classic for **Listeria monocytogenes**. - **Listeria monocytogenes** is primarily transmitted through the consumption of foods contaminated with the bacterium, such as **deli meats**, **soft cheeses**, and **unpasteurized dairy products**. *Aerosol transmission is not a common route for this organism.* - **Listeria monocytogenes** is not typically spread via **aerosols**; its primary mode of transmission is **foodborne**. - Respiratory tract infections are not characteristic presentations of Listeria. *Fecal-oral transmission is not the primary route for this organism.* - While Listeria can be found in feces, direct **fecal-oral transmission** (e.g., poor hygiene) is not the most common or significant route for human infection. - The predominant route involves consuming **contaminated food products**. *Transmission through sexual contact is not associated with this organism.* - **Listeria monocytogenes** is not known to be a **sexually transmitted infection**. - Its infections are acquired environmentally, specifically through **food consumption**.

Q: After 5 days of birth, a baby developed poor feeding, convulsions, fever, high protein, low sugar, and low chloride levels in the cerebrospinal fluid. This is most likely due to –

Listeria monocytogenes infection. ***Listeria monocytogenes infection*** - **Listeria** is a significant cause of **neonatal meningitis**, typically manifesting within the first few days of life with symptoms like fever, poor feeding, and **convulsions**. - The CSF profile of **high protein**, **low glucose** (sugar), and **low chloride** is characteristic of **bacterial meningitis**, which **Listeria** commonly causes in neonates. *Tuberculosis meningitis* - **Tuberculous meningitis** in neonates is rare and typically presents after several weeks or months, not 5 days post-birth. - While it can cause similar CSF findings of high protein and low glucose, the very early onset in this case makes it less likely. *Leptospira interrogans infection* - **Leptospirosis** is usually acquired through contact with infected animal urine and is rare in neonates unless there's a specific maternal exposure, which is not indicated. - While it can cause meningitis, the clinical picture and rapid onset in a neonate make other bacterial causes more probable. *Mycoplasma pneumoniae infection* - **Mycoplasma pneumoniae** is primarily a cause of **respiratory tract infections**, especially atypical pneumonia. - While neurological complications like encephalitis exist, **meningitis** is very uncommon, and the typical CSF profile for bacterial meningitis (low glucose, high protein) is not usually associated with **Mycoplasma**.

Q: The most common cause of epididymitis is

Chlamydia infection. ***Chlamydia infection*** - **Chlamydia trachomatis** is the most frequent cause of epididymitis in sexually active men younger than 35 years. [1] - It is transmitted sexually and often co-occurs with **gonorrhea**, though Chlamydia alone is a significant pathogen. [1] *Gonococcal infection* - While **Neisseria gonorrhoeae** can cause epididymitis, it is less common than Chlamydia as a sole pathogen, though it frequently co-infects with Chlamydia. [1] - It typically results in more acute and severe symptoms, including a purulent urethral discharge. [1] *Proteus infection* - **Proteus species** are more common causes of epididymitis in older men, often associated with **urinary tract infections** or instrumentation. - This bacterial infection is typically seen in the context of other urinary system abnormalities rather than as a primary sexually transmitted infection. *Tubercular infection* - **Tuberculosis (TB) epididymitis** is uncommon in developed countries but can occur in endemic areas or immunocompromised individuals. - It typically presents with a more indolent course, often with a firm or nodular epididymis, and usually indicates systemic TB.

Q: Thumb print appearance in a culture film smear is seen in which of the following organisms?

Bacillus anthracis. ***Bacillus anthracis*** - The "**thumb print**" or "**boxcar**" appearance refers to the characteristic microscopic morphology seen in **Gram-stained smears** of *B. anthracis*. - The bacilli appear as large, rectangular, Gram-positive rods with **square ends** (resembling boxcars), often arranged in chains. - This distinctive appearance aids in the laboratory diagnosis of **anthrax**. - On culture plates, colonies show a "**Medusa head**" appearance with irregular edges. *Bordetella pertussis* - Colonies on **Bordet-Gengou agar** exhibit a "**mercury drop**" or "**pearl-like**" appearance—smooth, glistening, dome-shaped colonies. - This is different from the "thumb print" morphology, which refers to microscopic rather than colonial appearance. - *B. pertussis* is the causative agent of **whooping cough** (pertussis). *Brucella species* - Produce small, smooth, translucent, slightly raised colonies on enriched media. - Lack any distinctive "thumb print" morphology in smears. - Causative agents of **brucellosis** (undulant fever), a zoonotic infection. *Clostridium perfringens* - Large, Gram-positive rods that may appear boxcar-shaped but are typically associated with **double zone of hemolysis** on blood agar. - Colonies are large, flat, and spreading, not exhibiting the classical "thumb print" identification feature. - Common cause of **gas gangrene** and food poisoning.

Q: Infection in burns is most commonly due to:

Pseudomonas. ***Pseudomonas*** - **_Pseudomonas aeruginosa_** is the **most common cause of infection in burn wounds**, particularly in hospitalized patients with significant burns. - It thrives in the moist environment of burn wounds and produces virulence factors including elastase, exotoxin A, and biofilm that lead to tissue destruction and invasive infection. - Pseudomonas is the leading cause of **burn sepsis and burn-related mortality**, and shows resistance to many common antibiotics. - Characteristic **green pigment (pyocyanin)** and **grape-like odor** may be noted in infected wounds. *Staphylococci* - **_Staphylococcus aureus_** is a common early colonizer of burn wounds due to its ubiquitous presence on normal skin. - More prevalent in **minor, superficial burns** and in the immediate post-injury period. - While clinically significant, it is **not the most common overall** in major burn infections. *Streptococci* - **Group A Streptococcus** (e.g., **_Streptococcus pyogenes_**) can cause burn wound infections, particularly cellulitis and invasive soft tissue infections. - Generally less common than Pseudomonas or Staphylococcus in burn wound epidemiology. *Candida* - **_Candida_ species** (primarily **_Candida albicans_**) are important fungal pathogens in burn wounds. - Typically occur in patients with **extensive burns, prolonged hospitalization, or broad-spectrum antibiotic use**. - Usually represent late infections and indicate immunosuppression.

Q: Which of the following statements about Mycoplasma pneumonia is false?

It can be easily cultured from sputum on routine media.. ***It can be easily cultured from sputum on routine media.*** - *Mycoplasma pneumoniae* is a **fastidious organism** that lacks a cell wall, making it **extremely difficult to culture** using standard laboratory methods. - It requires **specialized media** (PPLO agar, Eaton's agar) and **prolonged incubation** (2-3 weeks), so **routine sputum cultures cannot detect it**. - In clinical practice, diagnosis relies on **serology (IgM antibodies)**, **PCR**, or **cold agglutinin testing** rather than culture. *The treatment of choice is erythromycin.* - **Macrolides** such as erythromycin, azithromycin, and clarithromycin are indeed the **first-line antibiotics** for *Mycoplasma pneumoniae* infections. - Due to the **absence of a cell wall**, *Mycoplasma* is naturally resistant to beta-lactam antibiotics (penicillins, cephalosporins). - **Tetracyclines** and **fluoroquinolones** are alternative options. *ESR may be elevated.* - An **elevated Erythrocyte Sedimentation Rate (ESR)** is a common non-specific indicator of inflammation. - *Mycoplasma pneumoniae* infection causes inflammation in the respiratory tract and can lead to a systemic inflammatory response, thus **ESR is typically elevated**. *It can be diagnosed by serum cold agglutinins.* - **Cold agglutinins** are IgM autoantibodies that agglutinate red blood cells at cold temperatures (4°C) and are produced in response to *Mycoplasma pneumoniae* infections. - Develop in **50-75% of cases**, usually appearing in the **second week** of illness. - A titer of **≥1:64** or a **4-fold rise** supports the diagnosis, though it is **not entirely specific** and may be seen in other conditions (EBV, lymphoma).

Q: A 65-year-old diabetic presents with necrosis of the external auditory meatus and foul-smelling discharge. What is the probable organism associated with this condition?

Pseudomonas aeruginosa. ***Pseudomonas aeruginosa*** - This clinical presentation, involving a **diabetic patient** with external auditory canal **necrosis** and **foul-smelling discharge**, is classic for **malignant otitis externa**. - **Pseudomonas aeruginosa** is the most common causative organism for malignant otitis externa, thriving in moist environments and common in immunocompromised individuals like diabetics. *Haemophilus influenzae* - While it can cause otitis media, **Haemophilus influenzae** is not typically associated with **malignant otitis externa** or the severe necrosis and foul-smelling discharge seen in this patient. - It primarily causes **respiratory tract infections**, including acute otitis media, and is less common in severe, necrotizing external ear infections. *Streptococcus pyogenes* - **Streptococcus pyogenes** is a common cause of pharyngitis and skin infections (e.g., cellulitis), but it is not typically linked to **malignant otitis externa**. - This organism does not usually cause the extensive tissue necrosis and foul-smelling discharge characteristic of this condition. *E. coli* - **E. coli** is a common pathogen in urinary tract infections and intra-abdominal infections, but it is rarely a cause of **otitis externa**, especially the severe, necrotizing form. - It would not typically produce the specific clinical picture of external auditory meatus necrosis and foul-smelling discharge in a diabetic.

Q: Which of the following is the LEAST likely to cause appendicitis-like syndrome?

Yersinia pestis. ***Yersinia pestis*** - While *Yersinia pestis* is a bacterium from the *Yersinia* genus, it is primarily known as the causative agent of **plague** (bubonic, pneumonic, septicemic). - It does not typically cause an appendicitis-like syndrome but rather systemic infections with **lymphadenopathy** (buboes), **fever**, and severe illness. - This is the **LEAST likely** organism among the options to present with appendicitis-like symptoms. *Yersinia enterocolitica* - This bacterium is a well-known cause of **yersiniosis**, which commonly presents with **acute ileitis** and **mesenteric lymphadenitis**, mimicking appendicitis. - Symptoms include **fever**, **abdominal pain** (often in the right lower quadrant), **diarrhea**, and sometimes vomiting, making differentiation from appendicitis clinically challenging. *Yersinia pseudotuberculosis* - Similar to *Yersinia enterocolitica*, this organism causes **mesenteric lymphadenitis** and **terminal ileitis**, leading to an appendicitis-like presentation. - It often results in **abdominal pain** localized to the **right lower quadrant**, **fever**, and sometimes a rash (erythema nodosum), mimicking acute appendicitis. *Campylobacter jejuni* - A common cause of bacterial gastroenteritis that can also cause **acute ileitis** and **mesenteric adenitis**. - Can present with **right lower quadrant pain** mimicking appendicitis, especially in children and young adults. - Associated with bloody diarrhea, fever, and crampy abdominal pain.

Q: What mechanism does Chlamydia use to evade killing by the immune system?

Inhibiting phagolysosome fusion. ***Inhibiting phagolysosome fusion*** - *Chlamydia* **prevents the fusion of the phagosome** (containing the bacterium) with the lysosome, thus avoiding the degradative enzymes and low pH environment that would kill it. - This mechanism allows *Chlamydia* to **replicate intracellularly** within a protected vacuole called an **inclusion body**. - This is the **most direct mechanism** by which *Chlamydia* evades intracellular killing. *Molecular mimicry in immune evasion* - **Molecular mimicry** involves pathogens displaying antigens similar to host tissues, leading to **autoimmune damage** rather than evasion of killing. - While this may contribute to chronic complications of *Chlamydia* infection, it is not the primary mechanism *Chlamydia* employs to directly **evade intracellular destruction** by immune cells. *Causing cell membrane perforation* - **Cell membrane perforation** would typically lead to cell lysis and release of intracellular contents, which is counterproductive for an **obligate intracellular pathogen** like *Chlamydia*. - Instead, *Chlamydia* **maintains host cell integrity** to establish its replicative niche within the inclusion body. *Producing immune-modulating factors* - While *Chlamydia* does produce **immune-modulating factors** to suppress host immune responses, this is a **secondary mechanism** that complements its survival strategy. - The question specifically asks about evading **killing**, which is most directly achieved by **preventing phagolysosome fusion** and the resulting exposure to lysosomal enzymes. - Immune modulation affects the broader immune response but does not directly prevent the bacterium from being killed once inside a phagosome.

Question 1

A 34-year-old pregnant female presented to the emergency room with fever, backache, headache, and myalgias. Blood cultures were collected and subjected to examination. Microbiological examination revealed positive motility at 25°C and catalase positivity, Esculin positivity, and positive growth in 6.5% NaCl. The colonies are as shown on the blood agar plate given below. What is the most common route of transmission of the above organism?

Accepted Answer

Ingestion of contaminated food

Answer

Aerosol transmission

Answer

Sexual contact

Answer

Fecal-oral transmission

Question 2

After 5 days of birth, a baby developed poor feeding, convulsions, fever, high protein, low sugar, and low chloride levels in the cerebrospinal fluid. This is most likely due to –

Accepted Answer

Listeria monocytogenes infection

Answer

Tuberculosis meningitis

Answer

Leptospira interrogans infection

Answer

Mycoplasma pneumoniae infection

Question 3

The most common cause of epididymitis is

Accepted Answer

Chlamydia infection

Answer

Proteus infection

Answer

Tubercular infection

Answer

Gonococcal infection

Question 4

Thumb print appearance in a culture film smear is seen in which of the following organisms?

Accepted Answer

Bacillus anthracis

Answer

Brucella species

Answer

Bordetella pertussis

Answer

Clostridium perfringens

Question 5

Infection in burns is most commonly due to:

Accepted Answer

Pseudomonas

Answer

Candida

Answer

Staphylococci

Answer

Streptococci

Question 6

Which of the following statements about Mycoplasma pneumonia is false?

Accepted Answer

It can be easily cultured from sputum on routine media.

Answer

The treatment of choice is erythromycin.

Answer

ESR may be elevated.

Answer

It can be diagnosed by serum cold agglutinins.

Question 7

In differentiating pathogenic from non-pathogenic streptococcal strains in a throat culture, which of the following is the MOST important factor?

Accepted Answer

Beta-hemolysis pattern on blood agar plates

Answer

M protein presence

Answer

Surface protein differences

Answer

Lancefield grouping based on C-carbohydrate antigen

Question 8

A 65-year-old diabetic presents with necrosis of the external auditory meatus and foul-smelling discharge. What is the probable organism associated with this condition?

Accepted Answer

Pseudomonas aeruginosa

Answer

Streptococcus pyogenes

Answer

E. coli

Answer

Haemophilus influenzae

Question 9

Which of the following is the LEAST likely to cause appendicitis-like syndrome?

Accepted Answer

Yersinia pestis

Answer

Yersinia enterocolitica

Answer

Yersinia pseudotuberculosis

Answer

Campylobacter jejuni

Question 10

What mechanism does Chlamydia use to evade killing by the immune system?

Accepted Answer

Inhibiting phagolysosome fusion

Answer

Causing cell membrane perforation

Answer

Molecular mimicry in immune evasion

Answer

Producing immune-modulating factors

Bacteriology — MCQs

Bacteriology — MCQs

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