Implications of the VBNC State of B. cepacia and S. maltophilia on Bioreduction and Microbial Monitoring of ISS Potable Waters

Certain Eubacteria enter a viable but nonculturable (VBNC) state upon encountering unfavorable environmental conditions. VBNC cells do not divide on conventional media yet remain viable and in some cases retain virulence. Here, we describe the VBNC state of two opportunistic pathogens previously isolated from ISS potable waters, Burkholderia cepacia and Stenotrophomonas maltophilia. Artificially inoculated microcosms were exposed to the biocidal agents copper (CuSO₄) and iodine (I₂) in an attempt to induce nonculturablility. Viability was assessed via fluorescent microscopy (direct viable count assay coupled with BacLight™ staining) and metabolic activity was monitored by quantifying both intracellular ATP and transcribed rRNA (reverse transcriptase quantitative PCR). Culturablility was lost in both B. cepacia and S. maltophilia within two days of exposure to copper or high concentrations of iodine (6 or 8 ppm). Although both biocides appear effective in eliminating S. maltophilia populations, viable cells were still observed in B. cepacia microcosms through the end of the experiment. The ability of B. cepacia and other microbes to enter a VBNC state and circumvent detection by conventional cultivation-based techniques has serious repercussions. Accurate assessments of the microbial burden in these drinking water systems cannot be made when relying solely on conventional approaches. This study exemplifies the importance of developing and implementing cultivation-independent microbial monitoring techniques.