Bacteriology, NEET PG & INI-CET Microbiology MCQs

Explanation

Both toxins ADP-ribosylate elongation factor-2, halting protein synthesis and killing the cell. Diphtheria toxin also produces a pseudomembrane in the throat and can cause myocarditis.

Explanation

Tetanospasmin blocks release of the inhibitory neurotransmitters GABA and glycine from spinal interneurons, causing unopposed motor activity with rigidity and spasms such as lockjaw and opisthotonos. Botulinum toxin by contrast blocks acetylcholine release, causing flaccid paralysis.

Explanation

Cholera toxin ADP-ribosylates the Gs protein, permanently activating adenylate cyclase and raising cAMP. This drives massive chloride and water secretion into the gut lumen, producing rice-water stools.

Explanation

Staphylococcus aureus is coagulase-positive, distinguishing it from coagulase-negative staphylococci such as S. epidermidis. All staphylococci are catalase-positive, which separates them from streptococci.

Explanation

Group A streptococcus, S. pyogenes, is beta-haemolytic, producing complete clearing on blood agar, and is bacitracin-sensitive. S. pneumoniae and viridans streptococci are alpha-haemolytic, giving partial greenish haemolysis.

Explanation

In neonates, Group B Streptococcus, Escherichia coli and Listeria monocytogenes are the leading causes of meningitis. In older children and adults, S. pneumoniae and N. meningitidis predominate.

Explanation

The high mycolic-acid content of the mycobacterial cell wall makes it acid-fast, resisting decolourisation by acid-alcohol. This waxy wall also confers resistance to many disinfectants and to drying.

Explanation

H. pylori produces urease, which splits urea into ammonia and neutralises gastric acid locally. The urea breath test detects this activity by measuring labelled carbon dioxide produced from ingested urea.

Explanation

Pseudomonas aeruginosa produces the blue-green pigment pyocyanin and a characteristic fruity, grape-like odour. It is an important cause of burn-wound, hospital-acquired and ventilator-associated infections.

Explanation

Clostridioides difficile overgrows after antibiotic use, producing toxins A and B that cause pseudomembranous colitis. Diagnosis is by stool toxin testing or PCR, and treatment is oral vancomycin or fidaxomicin.

Explanation

The Weil-Felix test detects cross-reacting antibodies against Proteus OX antigens in rickettsial infections. It is a non-specific test, now largely replaced by more specific serology.

Explanation

Blood culture is most sensitive in the first week of enteric fever. Stool and urine cultures become positive later, in the second and third weeks. The Widal test has limited specificity and is supportive only.

Explanation

Treponema pallidum, a spirochaete, causes syphilis and is visualised by dark-ground microscopy as it is too thin for routine light microscopy. Serology uses non-treponemal tests such as VDRL and RPR alongside treponemal tests such as TPHA.

Explanation

Endotoxin is the lipid A portion of lipopolysaccharide in the gram-negative outer membrane. Released on cell lysis, it triggers cytokine release, fever and septic shock. Unlike exotoxins it is not secreted and cannot be converted into a toxoid.