Different bacteria are known to enter the VBNC state in response to different factors, all of which a cell would normally encounter in natural environments. These include such stresses as elevated or reduced temperature, elevated or reduced osmotic (e.g., salt) concentrations, nutrient starvation, levels of oxygen, and even certain intensities of light.1 In all cases, the inducers of the VBNC state appear to be environmental factors which are potentially injurious to a given bacterial species. For example, while entry into the VBNC state by V. vulnificus (optimum temperature of 37°C) is induced by low (< 10°C) temperature incubation,9 the reverse is true for Pseudomonas fluorescens. One strain of this bacterium, which prefers low temperatures, enters the VBNC state at 37°C.10
The time required for cells to enter the VBNC state varies markedly with the bacterium and the inducing conditions. Reports of months being required are not uncommon, while others have reported days for the same bacteria. In general, times of a few days to a month seem typical. One factor which has been shown to have a dramatic effect on the time required for lab-grown cells to become nonculturable is the "age" of the cells. We have shown that, whereas V vulnificus cells from the logarithmic phase of growth generally require less than 10 days to become completely nonculturable at 5°C, those taken from the stationary phase require over a month. Indeed, a direct correlation between time required to become nonculturable and the age of the population has been demonstrated.11
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