A Single Amino Acid Substitution in Elongation Factor G Can Confer Low-Level Gentamicin Resistance in .

The continued emergence of Neisseria gonorrhoeae isolates which are resistant to first-line antibiotics has reinvigorated interest in alternative therapies such as expanded use of gentamicin (Gen). We hypothesized that expanded use of Gen promotes emergence of gonococci with clinical resistance to this aminoglycoside. To understand how decreased susceptibility of gonococci to Gen might develop, we selected spontaneous low-level Gen-resistant (Gen) mutants (Gen MIC = 32 μg/mL) of the Gen-susceptible strain FA19. Consequently, we identified a novel missense mutation in , which encodes elongation factor G (EF-G), causing an alanine (A) to valine (V) substitution at amino acid position 563 in domain IV of EF-G; the mutant allele was termed . Transformation analysis showed that could increase the Gen MIC by 4-fold. While possession of did not impair either gonococcal growth or protein synthesis, it did result in a fitness defect during experimental infection of the lower genital tract in female mice. Through bioinformatic analysis of whole-genome sequences of 10,634 international gonococcal clinical isolates, other alleles were frequently detected, but genetic studies revealed that they could not decrease Gen susceptibility in a similar manner to . In contrast to these diverse international alleles, the encoded A563V substitution was detected in only a single gonococcal clinical isolate. We hypothesize that the rare occurrence of in N. gonorrhoeae clinical isolates is likely due to a fitness cost during infection, but compensatory mutations which alleviate this fitness cost could emerge and promote Gen in global strains.