Survivability and long-term preservation of bacteria in water and in phosphate-buffered saline*

AIMS: To evaluate the suitability of using sterile water and phosphate-buffered saline (PBS) for preservation of bacteria pathogenic to plants or humans. METHODS AND RESULTS: The stationary-phase bacterial cells collected from rich agar media were transferred to 10 ml of sterile water or PBS (pH 7.2) containing KH2PO4, 15.44 microm; NaCl, 1.55 mm; Na2HPO4, 27.09 microm in a screw-cap tube. The tubes were sealed with parafilm membranes and stored in the dark and at room temperature. Almost all the bacteria tested (148 strains), including Pseudomonas fluorescens, P. viridiflava, Erwinia spp., Xanthomonas campestris, Cytophaga johnsonae, Salmonella spp., Yersinia enterocolitica, Escherichia coli O157:H7, Listeria monocytogenes and Staphylococcus aureus, survived in water for at least several months and up to 16 years. A vast majority of the Gram-negative bacteria tested survived equally well in water and in PBS for at least 30 weeks. However, the populations of two Gram-positive bacteria G(+), L. monocytogenes and Staph. aureus, declined more rapidly in water than in PBS. CONCLUSIONS: Plant- and human-pathogenic bacteria can be preserved in pure water or PBS for several years. G(+) bacteria appear to survive better in PBS than in water. SIGNIFICANCE AND IMPACT OF THE STUDY: The method described here is a simple and economical means for preservation of bacterial cultures, which is especially useful for laboratories not equipped with the lyophilizer or ultra-low freezer. Long-term survival of food-borne pathogens in water underlines the importance of water as a potential vehicle for transmitting the diseases.