Efficient resuscitation of early-stage viable but non-culturable cells of Vibrio cholerae using treatment with proteolytic enzymes

Vibrio cholerae, the etiological agent of cholera, is ubiquitous in environmental brackish waters. Exposure to low water temperatures induces the bacterium to enter a viable but non-culturable (VBNC) state. In this study, a stepwise decrease in water temperature to 4°C was found to delay the transition to the non-culturable state compared to an abrupt temperature drop, suggesting that V. cholerae cells partially adapt to low temperatures. V. cholerae VBNC cells maintained at 4°C gradually lost their ability to revert to a culturable state. However, VBNC cells in the early stage of dormancy were efficiently resuscitated following treatment with proteolytic enzymes, including proteinase K. The abundance of culturable V. cholerae cells in brackish estuarine waters was quantified using the most probable number (MPN)–quantitative polymerase chain reaction (qPCR) method. Although culturable cells were undetectable in samples treated with bovine serum albumin, they were estimated at 93 and 1,500 MPN/mL in two water samples collected on different days and pre-incubated with proteinase K. Similarly, the abundance of Vibrio species increased markedly following treatment with this enzyme. Additionally, cells of Vibrio species were enumerated by the plating method using CHROMagar Vibrio plates. Consistent with the results of the MPN–qPCR method, treatment with proteinase K resulted in over a 100-fold increase in colony formation. Collectively, these findings suggest that treatment with proteinase K is effective for resuscitating and quantifying V. cholerae VBNC cells in environmental water samples.