Inner-membrane transporter genotype shapes susceptibility to non-lytic antimicrobials and bacterial fitness in Escherichia coli

Intracellularly acting antibacterials must cross the bacterial inner membrane to reach their targets. Using an isogenic Escherichia coli MG1655 panel carrying single and double deletions of sbmA, ygdD and mdtM , we assessed susceptibility to bleomycin, Oncocin, Api88 and Bac7(1–17), together with growth in vitro and competitive colonisation in the streptomycin-treated mouse intestine. SbmA was the dominant susceptibility determinant, whereas YgdD contributed most strongly to bleomycin and Api88 susceptibility. Deletion of ygdD or sbmA increased the bleomycin MIC four- and sixteenfold, respectively, while loss of both increased it 128-fold. Either single deletion also increased the Api88 MIC by more than 32-fold, whereas deletion of mdtM caused an eightfold increase. Single mutants grew similarly to wild type, whereas Δ ygdD Δ sbmA and Δ ygdD Δ mdtM had significantly longer doubling times; the largest effect was a 37% increase in Δ ygdD Δ sbmA . In mice, Δ sbmA and Δ mdtM showed reduced late competitive persistence. These findings support a compound-specific network of inner-membrane proteins whose fitness consequences depend on genetic background and environment.