Surface versus volume synthesis governs growth-dependent efficacy of a β-lactam antibiotic

The efficacy of {beta}-lactam antibiotics depends strongly on bacterial growth rate. This can lead to poor correlation between in vivo action and in vitro assays, hindering effective prescribing - yet the mechanisms underlying growth-rate dependent {beta}-lactam action remain unclear. Here, we investigate growth-rate dependent action of mecillinam, a {beta}-lactam that targets the elongation-mediating PBP2 peptidoglycan transpeptidase enzyme, on Escherichia coli cells. We show that mecillinam alters the balance between the rates of cell surface area and volume synthesis in a growth-rate dependent manner. Under mecillinam treatment, cell volume increases exponentially at a rate [fi]xed by the growth medium, but the cells ability to produce new surface area is compromised by the antibiotic. On rich medium, this imbalance leads to lysis, but on poor medium, slow-growing cells reach a new balance between surface area and volume synthesis, allowing sustained growth even at concentrations of mecillinam far above the EUCAST MIC value. A mathematical model based on surface area vs volume synthesis can quantitatively explain growth-medium dependent differences in mecillinam killing, as well as rescue from killing when cell morphology is perturbed in a microfluidic device. {beta}-lactam antibiotic action is mechanistically complex, yet our work suggests that simple conceptual principles can help understand the interplay between molecular mechanism and cell physiology, potentially contributing to more effective use of these drugs.