Bacteriology Practice Questions

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: 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 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.

Q: Which organism causes toxic shock syndrome?

Staphylococcus aureus. ***Staphylococcus aureus*** - **_S. aureus_** produces **toxic shock syndrome toxin-1 (TSST-1)** or other superantigens, which are potent activators of T cells, leading to a massive cytokine release and the systemic inflammatory response characteristic of **toxic shock syndrome**. - **TSST-1** is responsible for most cases of menstrual toxic shock syndrome, often associated with tampons, while non-menstrual cases can arise from surgical wounds, burns, or other local infections. *Pneumococcus* - **_Streptococcus pneumoniae_** (Pneumococcus) is a common cause of **pneumonia**, **otitis media**, and **meningitis**, but it does not produce toxins that cause toxic shock syndrome. - Its pathogenicity is primarily due to its **capsule**, which helps it evade phagocytosis, rather than superantigen production. *E. coli* - **_Escherichia coli_** is a gram-negative bacterium primarily known for causing **urinary tract infections**, **sepsis**, and **gastroenteritis** (e.g., traveler's diarrhea) through various toxins like **Shiga toxin** or **heat-labile/stable toxins**. - While it can cause severe systemic infections, it is not associated with the production of superantigens leading to toxic shock syndrome. *Enterococcus* - **_Enterococcus_** species (e.g., **_E. faecalis_**, **_E. faecium_**) are common causes of nosocomial infections, particularly **urinary tract infections**, **endocarditis**, and **wound infections**. - They are known for their **antibiotic resistance** but do not produce the specific superantigens associated with toxic shock syndrome.

Q: The following gram stain image shows which bacteria?

Staphylococcus aureus. ***Staphylococcus aureus*** - This image clearly shows **Gram-positive cocci** arranged in irregular, grape-like **clusters**, which is the classic microscopic appearance of *Staphylococcus* species, including *Staphylococcus aureus* - The purple color indicates they are **Gram-positive** bacteria - **Cluster arrangement** is the key distinguishing feature of staphylococci on Gram stain *Vibrio cholerae* - *Vibrio cholerae* is a **Gram-negative, comma-shaped bacillus** and would appear as pink/red, curved rods, not purple cocci - The morphology in the image is completely different from Vibrio species *Listeria monocytogenes* - *Listeria monocytogenes* is a **Gram-positive rod** (bacillus), not a coccus - It would appear as purple, rod-shaped bacteria, not spherical cocci - Its characteristic morphology is quite different from the cocci seen in the image *Streptococcus pneumoniae* - *Streptococcus pneumoniae* is a **Gram-positive coccus** but typically appears in **pairs (diplococci)** or short chains, often lancet-shaped - The bacteria in the image are predominantly in **clusters**, which differentiates them from *Streptococcus pneumoniae* - Streptococci arrange in chains or pairs, not grape-like clusters

Q: Vector of trench fever is which one of the following?

Louse. ***Louse*** - **Trench fever** is caused by the bacterium *Bartonella quintana* and is primarily transmitted to humans by the **body louse** (*Pediculus humanus corporis*). - The disease became notorious during World War I due to its prevalence among soldiers living in crowded, unhygienic conditions, which favored louse infestation. *Tick* - Ticks are vectors for various diseases such as **Lyme disease**, **Rocky Mountain spotted fever**, and **Ehrlichiosis**, but not trench fever. - These diseases are caused by different bacterial species and present with distinct clinical manifestations. *Mite* - Mites are known vectors for diseases like **scrub typhus** (transmitted by chiggers) and can cause conditions such as **scabies**, but they do not transmit trench fever. - Mites typically live on or in the skin of their hosts, and their biting patterns and associated pathogens differ from those of lice. *Rat flea* - **Rat fleas** (*Xenopsylla cheopis*) are the primary vectors for **bubonic plague** (caused by *Yersinia pestis*) and **murine typhus** (caused by *Rickettsia typhi*). - These diseases are distinct from trench fever in their causative agents, transmission cycles, and clinical presentations.

Q: Which of the following statements about Streptococcus is false?

Group A Streptococcus is catalase positive. ***Group A Streptococcus is catalase positive*** - This statement is false because all species of **Streptococcus**, including Group A Streptococcus (GAS), are **catalase-negative**. - The absence of the **catalase enzyme** is a key differentiating feature between streptococci and staphylococci, which are catalase-positive. *Group B causes neonatal meningitis* - **Group B Streptococcus (GBS)**, or *Streptococcus agalactiae*, is a well-known cause of **neonatal meningitis** and sepsis. - Infants can acquire GBS during passage through the birth canal from a colonized mother, leading to severe infections. *Group B lives in female genital tract* - GBS is a common commensal organism found in the **lower gastrointestinal** and **female genital tracts** of up to 30% of healthy women. - Its presence in the female genital tract poses a risk for transmission to neonates during childbirth. *Classification by Lancefield based on carbohydrate antigen* - The **Lancefield grouping system** classifies streptococci based on the presence of specific **carbohydrate antigens** in their cell walls. - This serological classification, developed by Rebecca Lancefield, divides streptococci into groups A, B, C, D, F, and G, which helps in identifying different species and their associated pathologies.

Question 1

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 2

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 3

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 4

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

Question 5

Which organism causes toxic shock syndrome?

Accepted Answer

Staphylococcus aureus

Answer

Pneumococcus

Answer

E. coli

Answer

Enterococcus

Question 6

The following gram stain image shows which bacteria?

Accepted Answer

Staphylococcus aureus

Answer

Listeria monocytogenes

Answer

Vibrio cholerae

Answer

Streptococcus pneumoniae

Question 7

Vector of trench fever is which one of the following?

Accepted Answer

Louse

Answer

Tick

Answer

Mite

Answer

Rat flea

Question 8

Which of the following statements about Streptococcus is false?

Accepted Answer

Group A Streptococcus is catalase positive

Answer

Group B causes neonatal meningitis

Answer

Group B lives in female genital tract

Answer

Classification by Lancefield based on carbohydrate antigen

Bacteriology — MCQs

Bacteriology — MCQs

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