Involvement of oxidative stress and growth at high cell density in the viable but nonculturable state of Photorhabdus temperata ssp. temperata strain K122

Photorhabdus temperata ssp. temperata strain K122 represents a promising source of bioinsecticide. When cultured in an optimized medium, P. temperata exhibited restricted survival in terms of colony-forming ability on solid medium, which remained lower than the total cell counts. Membrane integrity assessment by flow cytometry showed that almost 100% of P. temperata cells were viable indicating that this bacterium enters in the viable but nonculturable state (VBNC). According to the double staining results, hydrogen peroxide was demonstrated to be responsible of P. temperata VBNC state. Addition of catalase or sodium pyruvate upon the inoculation of P. temperata on agar plates promoted the recovery of nonculturable cells up to 24 h incubation. Further, growth at high cell density enhanced the VBNC state of this bacterium. This should evidenced extracellular signals accumulation involved in quorum sensing mechanism. Elucidation of this state is interesting for both toxicity study and production of P. temperata useful as bioinsecticide.