Effect of temperature and plasmid carriage on nonculturability in organisms targeted for release

Abstract

The ability to track genetically modified bacteria released into the environment is essential for assessing their persistence and dispersal. Some bacteria can enter a ‘viable but nonculturable’ (VBNC) state in which the cells remain viable while losing the ability to grow on routine culture media. Thus, VBNC cells are not detectable by standard plating methods. In order to determine what conditions, if any, induce this state in Pseudomonas fluorescens, Pseudomonas syringae, and Escherichia coli, cells were ‘marked’ with lux genes, either chromosomally or on one of two different plasmids. Variations in temperature, but not nutrient or NaCl concentrations, affected culturability of these strains and induced the VBNC state. The temperature which induced the VBNC state in the two pseudomonads depended on whether or not the cell carried one of the two lux-marked plasmids. This effect was shown not to be due to the presence of the lux genes, as their removal from the plasmid had no effect on entry into the VBNC state. Instead, the effect appeared to depend on the location of the plasmid DNA, as a strain of P. fluorescens with the same plasmid integrated into the chromosome behaved identically to the parent strain. The fact that plasmids may have such a dramatic effect on culturability has significant implications for the monitoring of genetically modified bacteria intended for environmental release.