Bacteriology Practice Questions

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.

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: A cook prepares sandwiches for 10 people going for a picnic. Eight out of them develop severe gastroenteritis within 4-6 hours of consuming the sandwiches. It is likely that on investigation, the cook is found to be the carrier of -

Staphylococcus aureus. ***Staphylococcus aureus*** - The rapid onset of symptoms (4-6 hours) and the development of severe gastroenteritis in multiple individuals after consuming common food items (sandwiches) strongly suggest a **preformed toxin ingestion**. - **_Staphylococcus aureus_** is a common cause of food poisoning due to its ability to produce enterotoxins that are heat-stable and cause rapid onset of nausea, vomiting, and diarrhea. *Salmonella typhi* - **_Salmonella typhi_** causes typhoid fever, which typically has an incubation period of **1-3 weeks**, much longer than the 4-6 hours seen in this case. - The symptoms of typhoid fever are also more systemic, including high fever, headache, and abdominal pain, rather than acute gastroenteritis with rapid onset. *Vibrio cholerae* - **_Vibrio cholerae_** causes cholera, characterized by **profuse watery diarrhea** with a typical incubation period of **1-5 days**. - The rapid onset of symptoms in this scenario (4-6 hours) does not align with the incubation period of cholera. *Entamoeba histolytica* - **_Entamoeba histolytica_** causes amoebiasis, which has an incubation period ranging from **several days to weeks or even months**. - It typically presents with **bloody diarrhea** and abdominal pain, and its slow onset is inconsistent with the acute event described.

Q: Polyribosyl ribitol phosphate (PRP) antigen is present in the capsule of which H. influenzae serotype?

Serotype b. ***Serotype b*** - **Polyribosyl ribitol phosphate (PRP)** is the unique capsular polysaccharide of *Haemophilus influenzae* **serotype b (Hib)**. - The presence of this antigen is crucial for diagnostic tests and vaccine development against Hib infections. *Serotype A* - *H. influenzae* serotype A possesses a **different capsular polysaccharide** structure than PRP. - Infections by serotype A are far **less common** and typically less severe than those caused by Hib. *Serotype C* - The capsule of *H. influenzae* serotype C is composed of a **distinct polysaccharide**, not PRP. - Serotype C infections are relatively **rare** and do not exhibit the same level of invasive disease as Hib. *Serotype D* - *H. influenzae* serotype D has a **capsular polysaccharide distinct** from PRP. - This serotype is infrequently associated with human disease and is **not targeted by the Hib vaccine**.

Q: A forceps-delivered premature baby develops fever, abnormal behavior, and a bulging fontanelle 5 days after birth. What is the likely causative organism?

Listeria monocytogenes. ***Listeria monocytogenes*** - Among the options provided, *Listeria monocytogenes* is the most appropriate answer for **neonatal meningitis** in a **premature infant** at 5 days of life. - *Listeria* accounts for approximately **5-10% of neonatal meningitis** cases and is associated with **vertical transmission** during birth, particularly in premature infants. - The symptoms of fever, abnormal behavior, and bulging fontanelle are classic signs of **bacterial meningitis** in neonates. - **Note**: While *Listeria* is the correct answer here, **Group B Streptococcus (GBS)** and **E. coli** are actually the most common causes of early neonatal meningitis (first week of life), with GBS being the leading pathogen. However, these are not among the given options. *Staphylococcus aureus* - *S. aureus* is not a common cause of early neonatal meningitis acquired during birth. - It typically causes **skin and soft tissue infections**, **osteomyelitis**, or **catheter-related sepsis** in neonates. - Meningitis due to *S. aureus* usually occurs following **neurosurgical procedures**, **shunt infections**, or in the setting of bacteremia, not as primary birth-related infection. *Neisseria meningitidis* - *N. meningitidis* is **rare in the neonatal period** (first 28 days of life), particularly in the first week. - It becomes a more significant cause of meningitis in **older infants (>3 months), children, and adolescents**. - Typically associated with **respiratory transmission** and close contacts, not birth-related transmission. *Streptococcus pneumoniae* - *S. pneumoniae* is **uncommon in neonatal meningitis**, especially in the first week of life. - It becomes a leading cause of bacterial meningitis in **infants >2-3 months** and children. - Neonatal meningitis is predominantly caused by organisms acquired during birth: **GBS (most common), E. coli**, and *Listeria monocytogenes*.

Q: A patient presents with a history of unresponsive fever and cough. Chest X-ray reveals pneumonia. Sputum examination shows gram-positive, partially acid-fast bacteria with branching filaments that grows on sheep blood agar. What is the most likely etiologic agent?

Nocardia. **Nocardia** - *Nocardia* species are **gram-positive**, **partially acid-fast, branching filamentous bacteria** that can cause pneumonia, especially in immunocompromised individuals. - They grow well on standard laboratory media like **sheep blood agar**, and their **branching morphology** is a key diagnostic feature. *Actinomyces* - *Actinomyces* species are also **gram-positive, branching filamentous bacteria** but are **not acid-fast** and are typically **anaerobic**, not growing well on sheep blood agar in the presence of oxygen. - They are commonly associated with cervicofacial abscesses and draining sinuses, often described as having "sulfur granules." *Aspergillus* - *Aspergillus* is a **fungus**, characterized by **septate hyphae with acute angle branching**, and would not be described as gram-positive bacteria. - It is a common cause of pneumonia, particularly in immunocompromised patients, but the sputum microscopic description clearly points to a bacterial etiology. *Pneumococci* - *Streptococcus pneumoniae* (Pneumococci) are **gram-positive cocci** that typically appear in pairs (**diplococci**) or short chains, **not branching filamentous structures**. - They are a very common cause of bacterial pneumonia but do not exhibit partial acid-fastness or the filamentous morphology described.

Q: Which of the following statements about Staphylococcus epidermidis is true?

Forms biofilms on medical devices. ***Forms biofilms on medical devices*** - *Staphylococcus epidermidis* is a common cause of **nosocomial infections** associated with medical devices such as catheters, prosthetic joints, and heart valves. - Its ability to form **biofilms** allows it to adhere to these surfaces, evade host defenses, and resist antibiotic treatment. *Sensitive to methicillin* - While some strains of *S. epidermidis* may be sensitive, many strains are **methicillin-resistant** (MRSE), which is a significant clinical concern. - MRSE owes its resistance to the **_mecA_ gene**, which encodes for an altered penicillin-binding protein (PBP2a). *The only coagulase negative staphylococcus of clinical significance* - Although *S. epidermidis* is the most common and clinically significant **coagulase-negative staphylococcus (CoNS)**, other CoNS species, such as *Staphylococcus saprophyticus* (a cause of UTIs) and *Staphylococcus lugdunensis* (can cause endocarditis), are also clinically significant. - The classification "coagulase-negative" simply distinguishes them from *Staphylococcus aureus*, which produces coagulase. *Produces exotoxins* - While *S. aureus* is known for producing a wide array of potent **exotoxins** that contribute to its pathogenicity (e.g., toxic shock syndrome toxin, exfoliatin), *S. epidermidis* generally does not produce significant exotoxins. - Its pathogenicity primarily stems from its ability to form **biofilms** and its resistance to antibiotics.

Question 1

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 2

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 3

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

Question 4

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 5

A cook prepares sandwiches for 10 people going for a picnic. Eight out of them develop severe gastroenteritis within 4-6 hours of consuming the sandwiches. It is likely that on investigation, the cook is found to be the carrier of -

Accepted Answer

Staphylococcus aureus

Answer

Salmonella typhi

Answer

Vibrio cholerae

Answer

Entamoeba histolytica

Question 6

Polyribosyl ribitol phosphate (PRP) antigen is present in the capsule of which H. influenzae serotype?

Accepted Answer

Serotype b

Answer

Serotype A

Answer

Serotype C

Answer

Serotype D

Question 7

A forceps-delivered premature baby develops fever, abnormal behavior, and a bulging fontanelle 5 days after birth. What is the likely causative organism?

Accepted Answer

Listeria monocytogenes

Answer

Staphylococcus aureus

Answer

Neisseria meningitidis

Answer

Streptococcus pneumoniae

Question 8

All of the following statements about Staphylococcus aureus are true, except:

Accepted Answer

Most common source of infection is cross infection from infected people.

Answer

About 30% of general population is healthy nasal carriers.

Answer

Methicillin resistance is chromosomally mediated.

Answer

Epidermolysin and TSS toxin are superantigens.

Question 9

A patient presents with a history of unresponsive fever and cough. Chest X-ray reveals pneumonia. Sputum examination shows gram-positive, partially acid-fast bacteria with branching filaments that grows on sheep blood agar. What is the most likely etiologic agent?

Accepted Answer

Nocardia

Answer

Aspergillus

Answer

Pneumococci

Answer

Actinomyces

Question 10

Which of the following statements about Staphylococcus epidermidis is true?

Accepted Answer

Forms biofilms on medical devices

Answer

The only coagulase negative staphylococcus of clinical significance

Answer

Produces exotoxins

Answer

Sensitive to methicillin

Bacteriology — MCQs

Bacteriology — MCQs

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