Ampicillin Resistance is Increased in Escherichia coli K12 relA and spoT mutants but Sub-inhibitory Pretreatment Does Not Induce Adaptive Resistance

The genes relA and spoT of Escherichia coli are required for the production of guanosine tetraphosphate, a critical molecule for the adoption of a stringent state that minimizes harm during metabolic and environmental stress. Subinhibitory treatments of antibiotics have been shown to result in subsequent antibiotic resistance, an effect also seen after stringency is induced through amino acid starvation. A ΔrelA mutant has been shown to lose this acquired resistance, suggesting a role for the stringent response in antibiotic resistance following sublethal exposure to the same antibiotic. This study examined an isogenic ΔrelAΔspoT strain to test the role of relA and spoT involvement in this possible mechanism of adaptive antibiotic resistance. Using minimal inhibitory concentration assays, wild-type, ΔrelA and ΔrelAΔspoT strains were grown in the presence of inhibitory ampicillin following a sub-inhibitory pretreatment of ampicillin, with growth rates measured through spectrophotometric analysis. These results demonstrate that pretreatment with sub-inhibitory levels of ampicillin does not induce resistance to subsequent inhibitory ampicillin concentrations in wild-type cells, while presence of relA appears to be decrease ampicillin resistance, possibly due to its role in peptidoglycan synthesis.