How Much Do You Really Know About Microbiology?

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.

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.

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.

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.

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.

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

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.

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.

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.

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