Bacteriology Practice Questions

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 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: 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: Endotoxins have all but one of the following properties?

Produced by gram positive bacteria. ***Produced by gram positive bacteria*** - **Endotoxins** are exclusively associated with the **outer membrane of gram-negative bacteria**, making this statement incorrect. - **Gram-positive bacteria** produce **exotoxins**, not endotoxins. *Produced by gram negative bacteria* - **Endotoxins are integral components of the outer membrane** of gram-negative bacteria, specifically their **lipopolysaccharide (LPS)** layer. - Upon bacterial cell lysis or growth, these endotoxins are released and can cause a potent immune response. *Heat stable* - **Endotoxins** are known for their **heat stability**, meaning they can withstand high temperatures (autoclaving for several hours) without losing their toxic activity. - This property makes them particularly challenging to eliminate from contaminated materials. *Cannot be toxoided* - **Endotoxins cannot be effectively toxoided** (inactivated to lose toxicity but retain immunogenicity) to produce vaccines. - This is a key difference from **exotoxins**, which can often be toxoided to create vaccines (e.g., diphtheria and tetanus toxoids).

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.

Q: A 10-month-old child presents with a two-week history of fever, vomiting, and alteration of sensorium. A cranial CT scan reveals basal exudates and hydrocephalus. What is the most likely etiological agent?

Mycobacterium tuberculosis. ***Mycobacterium tuberculosis*** - The combination of **basal exudates** (inflammation at the base of the brain) and **hydrocephalus** on CT scan in a child with prolonged fever and altered sensorium is highly characteristic of **tuberculous meningitis**. - **Tuberculous meningitis** typically has a more insidious onset, and the thick exudates around the basal cisterns can obstruct CSF flow, leading to hydrocephalus. *Cryptococcus neoformans* - While it can cause chronic meningitis, **Cryptococcus neoformans** is more common in **immunocompromised individuals** (e.g., HIV/AIDS patients), which is not indicated here. - CT scans typically show **gelatinous pseudocysts** or dilation of Virchow-Robin spaces rather than prominent basal exudates in early stages. *Listeria monocytogenes* - Primarily affects **neonates**, **immunocompromised adults**, and the elderly. - Although it causes meningitis and encephalitis, **basal exudates** and significant **hydrocephalus** are less common than with tuberculosis. *Streptococcus pneumoniae* - This is a common cause of **acute bacterial meningitis** in children, often presenting with a rapid onset and severe symptoms. - While it can cause hydrocephalus, the presence of prominent **basal exudates** with a subacute course (two weeks) points away from typical pneumococcal meningitis.

Q: Which of the following can be utilized by Salmonella as a sole carbon source?

Citrate. ***Citrate*** - Most *Salmonella* species can utilize **citrate as a sole carbon source**, which is demonstrated by the **Simmons citrate test**. - This is a key biochemical characteristic used in the identification of *Salmonella* species, particularly in differentiating it from other Enterobacteriaceae. - The ability to utilize citrate is encoded by genes for citrate permease and citrate lyase, allowing the organism to transport and metabolize citrate even in the absence of other carbon sources. *Niacin* - **Niacin** (Vitamin B3) is a precursor to NAD and NADP, essential coenzymes for metabolic pathways. - However, most *Salmonella* species can synthesize niacin and do not require it as an exogenous growth factor or sole carbon source. - Niacin is not a characteristic metabolic substrate used for *Salmonella* identification. *Vitamin B-12* - **Vitamin B12** (cobalamin) is a coenzyme for certain metabolic reactions. - *Salmonella* can synthesize vitamin B12 and does not require it as an exogenous sole carbon source. - While *Salmonella* can acquire and use B12, it is not the characteristic substrate for identification purposes. *Tryptophan* - **Tryptophan** is an amino acid that *Salmonella* can metabolize, producing indole as a byproduct in some species. - However, tryptophan is not typically used as a sole carbon source for *Salmonella* growth. - While tryptophan metabolism is important in host-pathogen interactions, it is not the defining metabolic characteristic of *Salmonella* used in laboratory identification.

Q: Which of the following statements about the lepromin test is true?

It is an important aid to classify type of leprosy disease. ***It is an important aid to classify type of leprosy disease*** - The **lepromin test** assesses the host's cellular immune response to *Mycobacterium leprae* antigens, which is crucial for differentiating between **tuberculoid (positive)** and **lepromatous (negative)** forms of leprosy. - A positive lepromin test indicates a strong cell-mediated immune response, characteristic of the more contained and less infectious **tuberculoid leprosy**, while a negative test suggests a weak or absent response, typical of the widespread and highly infectious **lepromatous leprosy**. - The test is **not diagnostic** but helps in **classification and prognosis** of leprosy. *It is negative in most children below 2 years of age* - The lepromin test is typically **negative in young children** (usually below 2 years) because their cell-mediated immune system is still developing and may not have fully matured. - This negativity reflects **immunological immaturity** rather than disease status, and the response gradually develops with age and potential exposure to mycobacterial antigens. *It is a diagnostic test* - The lepromin test is **not a diagnostic test** for active leprosy infection; it indicates the **immunological response** of the host, not the presence of disease. - Diagnosis of leprosy relies on clinical features (skin lesions, nerve involvement) and detection of **acid-fast bacilli** in skin smears or biopsy specimens. *BCG vaccination may convert lepra reaction from negative to positive* - While BCG vaccination can provide cross-immunity against mycobacteria, the statement oversimplifies the relationship between BCG and lepromin reactivity. - The lepromin test uses specific *Mycobacterium leprae* antigens, and while some cross-reactivity may occur, BCG vaccination **does not reliably convert** lepromin negativity to positivity in a clinically predictable manner.

Question 1

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 2

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 3

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 4

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 5

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 6

Endotoxins have all but one of the following properties?

Accepted Answer

Produced by gram positive bacteria

Answer

Heat stable

Answer

Produced by gram negative bacteria

Answer

Cannot be toxoided

Question 7

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

Question 8

A 10-month-old child presents with a two-week history of fever, vomiting, and alteration of sensorium. A cranial CT scan reveals basal exudates and hydrocephalus. What is the most likely etiological agent?

Accepted Answer

Mycobacterium tuberculosis

Answer

Cryptococcus neoformans

Answer

Streptococcus pneumoniae

Answer

Listeria monocytogenes

Question 9

Which of the following can be utilized by Salmonella as a sole carbon source?

Accepted Answer

Citrate

Answer

Niacin

Answer

Tryptophan

Answer

Vitamin B-12

Question 10

Which of the following statements about the lepromin test is true?

Accepted Answer

It is an important aid to classify type of leprosy disease

Answer

It is a diagnostic test

Answer

BCG vaccination may convert lepra reaction from negative to positive

Answer

It is negative in most children below 2 years of age

Bacteriology — MCQs

Bacteriology — MCQs

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