Hypo-osmotic stress is an anticipatory trigger of heat-resistance in presumptive extraintestinal pathogenic Escherichia coli isolated from treated sewage.

Extraintestinal pathogenic Escherichia coli (ExPEC) are responsible for a variety of human infections (urinary tract infections, septicemia, meningitis) and can be routinely isolated from treated sewage. However, the phenotypic properties mediating treatment resistance in ExPEC remain elusive. Herein, we examined heat-resistance in presumptive wastewater ExPEC (W-ExPEC) surviving sewage chlorination or full-scale wastewater treatment. Interestingly, heat-resistance in W-ExPEC was triggered by exposure to hypo-osmotic conditions (i.e., sterile distilled water), resulting in a 10- to 1,000-fold increase in heat-resistance compared to cells exposed to iso-osmotic conditions (i.e., phosphate buffered saline). Remarkably, hypo-osmotic induction of heat resistance occurred extremely fast, in as little as 30 s, and was reversible, demonstrating the phenotypic plasticity of this stress response. Hypo-osmotic stressed W-ExPEC strains survived 58 °C temperatures for up to 20 min - this compared to the clinical reference ExPEC strain, CTF073, which became non-culturable after only 5 min of exposure at this temperature (>8 log10 decline in culturability). The upper thermotolerance level of W-ExPEC (defined as the temperature where culturability was lost after 5 min of exposure) was 62 °C, compared to 58 °C for CFT073. The finding that osmotic stress acts as an anticipatory inducer of heat resistance in W-ExPEC is novel, providing some insights into the possible mechanisms triggering a treatment resistant phenotype in W-ExPEC. The evolution of treatment resistance is worrying prospect for public health, given that waste treatment is a hallmark of infectious disease control in modern society.