The role of ppGpp in modulating the evolution of fluoroquinolone resistance in Shigella sonnei

Shigella causes shigellosis that requires antibiotic treatment in severe cases. Sub-lethal antibiotic concentrations can promote resistance, but their effect on antibiotic-sensitive bacteria prior to resistance development is unknown. This study investigated the effects of sublethal norfloxacin challenges on a norfloxacin-sensitive strain Shigella sonnei. We show that low levels of norfloxacin do not change the antimicrobial resistance genes and norfloxacin targets expression. Instead, multiple ribosomal protein genes were downregulated, which could be attributed to a decrease in the ribosomal protein promoter activity, modulated by ppGpp. Under sublethal norfloxacin stress and high ppGpp levels, the cells underwent filamentation suggesting that ppGpp regulates the response of S. sonnei to norfloxacin. We speculate that high ppGpp levels play roles in modulating the development of antibiotic resistance in the sensitive strain. Indeed, the adaptive evolution of S. sonnei in the presence of norfloxacin revealed that the evolved strain developed a mutation (Asp87Tyr) in gyrA gene conferring reduced norfloxacin susceptibility and reduced filamentation. These results collectively suggest that ppGpp may not be involved in regulating the mutant's response to norfloxacin exposure.