Recovery of culturability of an HOCl-stressed population of Escherichia coli after incubation in phosphate buffer: resuscitation or regrowth?

An Escherichia coli population harvested in exponential phase at about 10(8) cells/ml was treated in phosphate buffer with HOCl at concentrations ranging from 0.4 to 1 mg/liter (7.7 to 19 microM). The HOCl stress resulted in the appearance of three cell subpopulations: a majority of dead (nonrespiring) cells, a few culturable cells (10(2) to 10(4)), and about 10(7) viable but nonculturable cells. In the absence of any added exogenous nutrient, a culturable population could be recovered after 1 day of incubation in phosphate buffer, and such a population would reach a cell density close to 10% of the initial density of the stressed population, whatever the initial number of survivors. When a small number of untreated cells were mixed with the stressed population, growth of the untreated cells was observed, demonstrating that damaged cells provided nutrients. Similarly, a filtrate and a disrupted-cell filtrate of the stressed population supported growth of untreated cells with the same efficiency. The number of CFU (untreated or stressed) at plateau phase depended on the initial density of the stressed cells. Taken together, these results suggest that recovery in phosphate buffer of an HOCl-stressed population is in large part due to growth of a few culturable cells at the expense of damaged cells. However, comparison of the growth rates of the stressed culturable population and of untreated bacteria growing in filtrate showed significantly faster growth of the stressed cells, a fact not fully compatible with the hypothesis that recovery is only the simple growth of survivors. We suggest, therefore, that in addition to growth of the few culturable stressed cells, there is repair and growth of some mildly injured viable but nonculturable cells.