Practice question

Aminoglycosides such as gentamicin, streptomycin and amikacin are polycationic molecules whose uptake depends on proton-motive force and electron transport, explaining reduced activity against anaerobes and biofilms. Once concentrated inside bacterial cytoplasm, they lodge with exceptionally high affinity in the decoding center formed by helix 44 of 16S rRNA within the 30S small ribosomal subunit at the A-site where aminoacyl-tRNA selection and proofreading occur during translation. The drug forces an abnormal closed conformation of rRNA, impairing proofreading and inducing misreading of mRNA codons, premature termination, frameshifting and accumulation of aberrant, misfolded membrane proteins. These defective proteins insert into inner membrane, increasing permeability, dissipating membrane potential and promoting further drug uptake, a positive feedback loop that contributes to concentration-dependent bactericidal action and post-antibiotic effect. Aminoglycosides also block formation of the 70S initiation complex comprising mRNA, formyl-methionyl-tRNA and subunits and inhibit translocation of peptidyl-tRNA from A to P site. Synergy with cell-wall agents that enhance entry underlies combination therapy for Gram-negative sepsis.