Increases the levels of intracellular cylic GMP

Acts through the opoid receptors

lnhibits the enzyme phosphodiesterase

How Much Do You Really Know About Microbiology?

1.

With reference to Bacteroides fragilis the following statements are true except-

A.

B. fragilis is the most frequent anaerobe isolated from clinical samples.
B.

B. fragilis is not uniformly sensitive to metronidazole
C.

The lipopolysaccharide formed by B. fragilis is structurally and functionally different from the conventional endotoxin
D.

Shock and disseminated intravascular coagulation are common in Bacteriodes bacteremia

Correct Answer
D. Shock and disseminated intravascular coagulation are common in Bacteriodes bacteremia

Explanation
Bacteroides:

Gram negative anaerobic bacilli
Most common anaerobic pathogen isolated from clinical specimens
Virulence factors include capsular polysaccharides and lipopolysaccharide endotoxin
Lipopolysaccharide endotoxin is much less toxic compared to other gram negative bacteria
Hence infections do not directly produce signs of sepsis, which is common in other gram negative bacteria
Some are resistant to metronidazole

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2.

A child had pustular lesion on leg. On gram staining gram positive cocci are seen. To establish the diagnosis of Group A streptococcal erythroderma, the test used is

A.

Bile solubility test
B.

Catalase test
C.

Optochin sensitivity
D.

Bacitracin sensitivity

Correct Answer
D. Bacitracin sensitivity

Explanation
The correct answer is Bacitracin sensitivity. This test is used to establish the diagnosis of Group A streptococcal erythroderma. Group A streptococci are sensitive to bacitracin, so if the bacteria are inhibited by bacitracin, it suggests that the infection is caused by Group A streptococci. The other tests listed, such as bile solubility, catalase, and optochin sensitivity, are not specific for Group A streptococci and are used for different purposes.

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3.

Cholera toxin:

A.

Increases the levels of intracellular cylic GMP
B.

Acts through the opoid receptors
C.

Causes continued activation of adenylate cyclase
D.

Lnhibits the enzyme phosphodiesterase

Correct Answer
C. Causes continued activation of adenylate cyclase

Explanation
Cholera toxin causes continued activation of adenylate cyclase. Adenylate cyclase is an enzyme responsible for producing cyclic adenosine monophosphate (cAMP) from adenosine triphosphate (ATP). Increased levels of cAMP lead to the activation of protein kinase A, which in turn leads to the phosphorylation of various proteins and subsequent cellular responses. Therefore, cholera toxin's ability to cause continued activation of adenylate cyclase results in increased cAMP levels and subsequent cellular effects.

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4.

A patient admitted to an ICU is on central venous line for the last one week. He is on ceftazidime and amikacin. After 7 days of antibiotics he develops a spike of fever an his blood culture is positive for gram positive cocci in chains, which are catalase negative. Following this, vancomycin was started but the culture remained positive for the same organism even after 2 weeks of therapy. The most likely organism causing infection is:

A.

Staphylococcus aureus
B.

Viridans streptococci
C.

Enterococcus faecalis
D.

Coagulase negative Staphylococcus

Correct Answer
C. Enterococcus faecalis

Explanation
The most likely organism causing the infection is Enterococcus faecalis. This is indicated by the fact that the patient developed a fever and the blood culture was positive for gram positive cocci in chains, which are catalase negative. Enterococcus faecalis is a gram positive cocci that often forms chains and is catalase negative. Additionally, the fact that the culture remained positive for the same organism even after 2 weeks of therapy with vancomycin suggests that the organism is resistant to this antibiotic, which is a characteristic of Enterococcus faecalis.

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5.

An infant had high grade fever and respiratory distress at the time of presentation to the emergency room. The sample collected for blood culture was subsequently positive showing growth of a haemolytic colonies. On Gram staining these were gram positive cocci. In the screening test for identification, the suspected pathogen is likely to be susceptible to the following agent:

A.

Bacitracin
B.

Novobiocin
C.

Optochin
D.

Oxacillin

Correct Answer
C. Optochin

Explanation
Optochin is likely to be the susceptible agent for the suspected pathogen. This is because the question mentions that the Gram staining showed gram positive cocci, and optochin is a screening test used to identify Streptococcus pneumoniae, which is a common gram positive cocci pathogen. Optochin sensitivity is a characteristic feature of Streptococcus pneumoniae, making it the correct answer in this case.

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6.

ln the gut, anaerobic bacteria outnumber the aerobes by a ratio of

A.

10:1
B.

100:1
C.

1000:1
D.

10,000:1

Correct Answer
C. 1000:1

Explanation
In the gut, there is a higher population of anaerobic bacteria compared to aerobes. This is supported by the fact that the ratio of anaerobic bacteria to aerobes is 1000:1. This means that for every 1 aerobe, there are 1000 anaerobic bacteria present in the gut.

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7.

A veterinary doctor had pyrexia of unknown origin. His blood culture in special laboratory media was positive for gram-negative short bacilli which was oxidase positive. Which one of the following is the likely organism grown in culture?

A.

Pasteurella spp
B.

Francisella spp
C.

Bartonella spp
D.

Brucella spp

Correct Answer
D. Brucella spp

Explanation
Brucella is a gram negative bacillus and is oxidase positive. Infection is associated with exposure to infected animals or their products. Individuals at high risk of acquiring infection are farmers, veterinarians and workers in slaughter houses. Patients frequently present with pyrexia of unknown origin.

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8.

A young boy had a flea bite while working in a wheat grain godown. After 5 days he developed fever and had axillary lymphadenopathy. A smear was sent to the laboratory to perform a specific staining. Which one of the following staining method would help in the identification of the suspected pathogen:

A.

Albert staining
B.

Ziehl Neelsen staining
C.

McFadyen’s staining
D.

Wayson staining

Correct Answer
D. Wayson staining

Explanation
Flea bite might have been by a rat flea as wheat grain godowns are frequented by rats. Fever and axillary lymphadenopathy are characteristic features of bubonic plague – which is spread by bite of rat flea. Wayson stain is used to demonstrate Yersinia pestis which is the causative organism for plague.

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9.

Which one of the following statement is true regarding pathogenicity of Mycobacteria species?

A.

M. tuberculosis is more pathogenic than M. bovis to humans
B.

M. Kansasii can cause a disease indistinguishable from tuberculosis
C.

M Africanum infection is acquired from an environmental source
D.

M. Marinum is responsible for tubercular lymphadenopathy

Correct Answer
B. M. Kansasii can cause a disease indistinguishable from tuberculosis

Explanation
M. tuberculosis and M. bovis are equally pathogenic to humans. M Africanum is part of M. tuberculosis complex together with M.tuberculosis, M.bovis and M.microti. Environmental sources are present in case of non tuberculous mycobacteria and atypical mycobacteria. They are not seen in case of bacteria belonging to M.tuberculosis complex.

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10.

A VDRL reactive mother gave birth to an infant. All of the following would help in determining the risk of transmission to the infant except:

A.

TPHA test on the serum sample of the mother
B.

TPHA test on the serum sample of the infant
C.

VDRL on the paired serum sample of the infant and mother
D.

Time interval between the treatment of the mother and her delivery

Correct Answer
B. TPHA test on the serum sample of the infant

Explanation
Serological testing for syphilis is not reliable in case of neonates. Negative serology does not rule out the possibility of infection.

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