Role of Toxin-Antitoxin-Regulated Persister Population and Indole in Bacterial Heat Tolerance

YafQ is an endoribonuclease toxin that degrades target gene transcripts such as that of tnaA, a gene encoding tryptophanase to synthesize indole from tryptophan. DinJ is the cognate antitoxin of YafQ, and the YafQ-DinJ system was reported to regulate persister formation by controlling indole production in Escherichia coli. In this study, we investigated the role of YafQ-DinJ, indole production, and persister population in bacterial heat tolerance. yafQ (Delta yafQ), dinJ (Delta dinJ), and tnaA (Delta tnaA) single-gene knockout mutants showed approximately 10-fold higher heat tolerance than wild-type (WT) E. coli BW25113. Persister fractions of all mutants were slightly larger than that of the WT. Interestingly, these persister cells showed an approximately 100-fold higher heat tolerance than normal cells, but there was no difference among the persister cells of all mutants and the WT in terms of heat tolerance. Indole and its derivatives promoted a drastic reduction of bacterial heat tolerance by just 10 min of pretreatment, which is not sufficient to affect persister formation before heat treatment. Surprisingly, indole and its derivatives also reduced the heat tolerance of persister cells. Among the tested derivatives, 5-iodoindole exhibited the strongest effect on both normal and persister cells. IMPORTANCE Our study demonstrated that a small persister population exhibits significantly higher heat tolerance than normal cells and that this small fraction contributes to the heat tolerance of the total bacterial population. This study also demonstrated that indole, known to inhibit persister formation, and its derivatives are very promising candidates to reduce the heat tolerance of not only normal bacterial cells but also persister cells.