Antibiotics Change the Growth Rate Heterogeneity and Morphology of Bacteria

A better understanding of the system-level effects of antibiotics is necessary to fight the rise of antibiotic resistance. Utilising the Multipad Agarose Plate (MAP), we monitor the growth rate and cell morphology of three clinically relevant species ( E . coli, S . aureus and P . aeruginosa ) after exposure to 11 different concentrations of 13 antibiotics (for a total of 24 microbe-antibiotic combinations). As the drug dose approaches the MIC and regardless of the mode of action, our results show a consistent increase in growth rate heterogeneity. Remarkably, drugs that affect protein synthesis consistently show the opposite trend, reducing heterogeneity. We hypothesize that growth rate heterogeneity under antibiotic treatment might therefore depend on the functional distance of the target from ribosomal activity, which is key in determining growth rate. Low heterogeneity is desirable from a clinical perspective, as the opposite is often associated to persistence and antibiotic survival. For all of the antibiotics and species tested, we also find a striking and non-trivial correlation between morphological alterations and growth inhibition. This observation allows us to introduce a new morphological parameter, MOR50, that enables the estimation of minimum inhibitory concentration (MIC) for antibiotic susceptibility testing (AST) with a single snapshot after 2.5 hours of incubation. In addition to introducing a novel, resource-efficient, rapid AST method, our findings shed new light on the effects of antibiotic perturbations on bacteria at the system level that might inform treatment design.