Deletion of gene encoding methyltransferase (gidB) confers high-level antimicrobial resistance in Salmonella.

The glucose-inhibited division gene (gid)B, which resides in the gid operon, was thought to have a role in the modulation of genes similar to that of gidA. Recent studies have indicated that GidB is a methyltransferase enzyme that is involved in the methylation of the 16S ribosomal RNA (rRNA) in Escherichia coli. In this study, we investigated the role of GidB in susceptibility to antibiotics and the overall biology of Salmonella. A gidB isogenic mutant of Salmonella was constructed and subsequently characterized under different conditions. Our data indicated that growth and invasion characteristics of the gidB mutant were similar to those of the wild type (WT). The gidB mutant was outgrown by the WT in a competitive growth assay, indicating a compromised overall bacterial fitness. Under the stress of nalidixic acid, the gidB mutant's motility was significantly reduced. Similarly, the mutant showed a filamentous morphology and smaller colony size compared with the rod-shaped and large colonies of the WT in the presence of nalidixic acid. Most importantly, deletion of gidB conferred high-level resistance to the aminoglycoside antibiotics streptomycin and neomycin. A primer extension assay determined the methylation site for the WT to be at G527 of the 16S rRNA. A lack of methylation in the mutant indicated that GidB is required for this methylation. Taken together, these data indicate that the GidB enzyme has a significant role in the alteration of antibiotic susceptibility and the modulation of growth and morphology under stress conditions in Salmonella.