Antibiotic persistence does not cause phenotypic heterogeneity in tolerance of Escherichia coli to formaldehyde stress but can preserve it through time

The phenomenon of phenotypic heterogeneity, where an isogenic population expresses varying phenotypes, has been uncovered for an expanding variety of organisms and traits. This heterogeneity in phenotype can be physiologically relevant, such as the ability for antibiotic persistence to allow populations to survive lethal conditions due to rare cells being in a slow or non-growing state. Recently, it was discovered that Methylorubrum extorquens, a facultative methylotroph, possesses a continuous spectrum of phenotypic states conferring tolerance to formaldehyde. Formaldehyde is a toxin produced by M. extorquens during growth on methanol. The phenotypic tolerance allowed rapid growth on levels of formaldehyde which were lethal to the majority of the population. Transcriptomics indicated this may be due to upregulation of proteins that attenuate oxidative stress and protein damage rather than increasing formaldehyde oxidation to prevent accumulation. These data suggested that heterogeneity to formaldehyde stress may be present even in non-methylotrophic organisms that do not routinely produce large quantities of formaldehyde, and thus widely distributed across bacteria. To investigate this, we tested Escherichia coli for heterogeneity to formaldehyde stress during growth on glucose. Like M. extorquens, E. coli populations have a wide, continuous range of formaldehyde tolerance thresholds and this tolerance was reversible. Several other features, however, were different from what was found for M. extorquens. Most E. coli growth occurred after formaldehyde levels had dropped, suggesting that persistence could be the cause. The dynamics of antibiotic persistence and formaldehyde tolerance were both tracked but found to not be correlated. On the other hand, the persister cell state can maintain formaldehyde tolerance. These data suggest that persistence can preserve phenotypic heterogeneities in other traits, further expanding its potential role in helping cells survive environmental stressors.