In a viable but nonculturable (VBNC) state, bacteria are no longer culturable on standard laboratory media, but still, remain a pathogenic potential and present possible health risks. In this study, we investigated ampicillin's ability, which is commonly used in dairy cattle disease treatment, to induce Cronobacter sakazakii into the VBNC state. After treatment with ampicillin, the counts of culturable cells decreased from 108 CFU/mL to an undetected level 7-30 days post-treatment. Meanwhile, viable cells were still approximately 104-105 cells/mL, and could be resuscitated under appropriate conditions. Fluorescence microscopy showed that VBNC cell maintained apparent cellular integrity, but that the morphology of VBNC cells differed visibly from that of normal cells. Moreover, the respiratory chain activity of VBNC cells were confirmed by flow cytometry (FCM) analysis, suggesting that cells in a VBNC state were physiologically active. Finally, transcriptomics analysis and real-time PCR (qPCR) validation were used to explore the underlying mechanisms of VBNC cell formation. Over-expression of relA, lon, ppx, and ppk in the toxin-antitoxin (TA) trigger system contributed to VBNC cell formation. In the TA trigger system, RelA and exopolyphosphatases/guanosine pentaphosphate phosphohydrolases (PPX/GPPA) synthesize ppGpp, which activates polyphosphate kinase (PPK), the cellular enzyme that accumulates plyphosphate (PolyP). PolyP combines with and stimulates Lon to degrade the antitoxins, thereby activating the toxins that induce a VBNC state. The results of our research will facilitate a better understanding of the survival strategies that bacteria develop to deal with ampicillin pressure and the health risks associated with VBNC Cronobacter sakazakii induced by antibiotics.
Survival strategy of Cronobacter sakazakii against ampicillin pressure: Induction of the viable but nonculturable state.