Bacteriophage infection reveals pre-emptive cooperative dormancy in stationary phase Escherichia coli

Bacterial cultures entering stationary phase (SP) undergo complex physiological alterations, helping the cells to survive when medium resources are exhausted. The onset of various stress responses underlying SP physiology is generally believed to be due to reactions of individual cells to the environmental cues such as starvation, toxic metabolites, etc. The SP physiological state makes bacteria unsuitable for the replication of most bacteriophages; however, some phages are able to infect and multiply in them. We investigated Escherichia coli phage DH23 growth in SP cultures of E. coli MG1655 infected at different hours post inoculation (ages). Under our conditions the cells enter SP at about 6 h, but the phage replication was possible till 11h and then dropped abruptly by 13h of culture aging, indicating an abrupt physiological change to deeper dormancy during SP. The onset of this phage-inhibiting middle-SP dormancy (MSPD) turned out to be mediated by intercellular communication mediated by the middle-SP signal particles (MSP) which are larger than 100 kDa and contain both RNA and DNA. These MSP are inactivated by RNAse or by DNAse, enabling phage growth in 14h-old cultures and lifts the tolerance of such cultures to kanamycin. Moreover, the RNAse or DNAse treatment makes 24h culture spent media suitable for additional cell growth, and cultures initiated in fresh LB supplemented with RNAse reach an OD600 about 1.5 times higher compared to the untreated control. This indicates that MSP signaling influences cell physiology well before the MSPD onset and helps the population cease growth pre-emptively to save about 1/3 of medium resources to support the viability during SP.