Ionic environment and nutrients affect the cysB-dependent conditional susceptibility of E. coli to mecillinam

Antibiotic efficacy is influenced by environmental context. Urinary E. coli isolates lacking a functional cysB gene have been shown to be resistant to the {beta}-lactam antibiotic mecillinam under high-osmolarity conditions, suggesting that increased fluid intake might synergize with drug treatment. In this study, we further explore E. coli susceptibility to mecillinam across growth conditions. We show that while osmolarity has a mild protective effect on cysB knockout and reference strain, it is the presence of certain salts that leads to the resistant phenotype in the mutant. Such conditional resistance is specific to mecillinam and rapidly wanes when salts are removed. In agreement with our previous work showing that slow growing cells escape mecillinam-induced bursting by maintaining a small size, for both strains we observe low susceptibility to mecillinam in scarcely nutritious pooled human urine. As we supplement urine with nutrients, we observe a sharp transition to susceptibility when growth rate surpasses 0.6 h-1. Growth in the presence of salt or uncharged osmolites decreases cell size of both reference strain and cysB knockout offering a potential explanation to the observed mild protective effects of osmolarity. Our findings give insights into the mechanism of cysB dependent susceptibility to mecillinam and, due to the dual impact of nutrients and osmolites on treatment, allow us to reformulate ideas on potential diet recommendations. While increased fluid intake will decrease urine osmolarity, it will also reduce nutrient content, limiting mecillinam efficacy. We suggest instead that reducing sodium intake during mecillinam treatment might avoid this trade-off.