Mycology

• LPS Structure and Bacterial Virulence:
• Lipopolysaccharide (LPS) in Gram-negative bacteria consists of Lipid A, core oligosaccharide, and the terminal O-antigen polysaccharide side chains.
• Smooth ($S$) Wild-Type Strains: Possess full-length O-antigen chains that sterically hinder the membrane attack complex (MAC, C5b-9) of complement from accessing the outer membrane, conferring serum resistance.
• Rough ($R$) Mutants: Genetic defects in LPS biosynthesis result in truncated core polysaccharides lacking the O-antigen. Lacking this steric shield, rough salmonellae are highly serum sensitive, easily lysed by complement, and rapidly cleared by phagocytosis.
• nalysis:
• Both (A) and (R) are correct, and (R) provides the mechanical explanation for (A). Hence, Option 1 is correct.

• Mechanism of Action of Rifampin (Rifampicin): Rifampin specifically binds to the $\beta$-subunit of bacterial DNA-dependent RNA polymerase (encoded by the rpoB gene), forming a stable drug-enzyme complex that sterically blocks the initiating phosphodiester bond and stalls the synthesis/elongation of nascent mRNA.
• Mechanisms of Fluoroquinolones:
• Ciprofloxacin, Gatifloxacin, Norfloxacin: Inhibit bacterial Type II topoisomerases—specifically DNA gyrase (gyrA/gyrB) and Topoisomerase IV (parC/parE)—blocking DNA supercoiling, replication fork progression, and DNA repair rather than inhibiting transcription directly.

Correct option: (1) Rifampin

• Comparative Biology of Bacterial Enterotoxins:
• Heat-Labile Toxin (LT) Family: The heat-labile toxin (LT-I, LT-II) of E. coli is an $AB_5$ holotoxin that is structurally and functionally homologous to Cholera toxin of Vibrio cholerae, as well as the enterotoxins of Campylobacter jejuni and Salmonella Typhimurium. All act by catalyzing the ADP-ribosylation of the $\alpha$-subunit of heterotrimeric stimulatory G protein ($G_{s\alpha}$), constitutively activating host adenylate cyclase and elevating intracellular cAMP.
• Contrasting Shiga Toxin (Stx): Shiga toxin of Shigella dysenteriae type 1 (and Shiga toxins Stx1/Stx2 of EHEC) is an $AB_5$ toxin with a completely different enzymology: the catalytic $A$ subunit is an RNA $N$-glycosidase that cleaves a specific adenine base ($A_{4324}$) from the 28S rRNA of the eukaryotic 60S ribosomal subunit, irreversibly halting host protein synthesis.

Correct option: (2) Shiga toxin of Shigella dysenteriae type 1