Vita-PAMPs: signatures of microbial viability

Can the innate immune system detect and respond to microbial viability? Using bacteria as a model, we found that indeed the very essence of microbial infectivity, viability itself, can be detected, and notably, in the absence of the activity of virulence factors. The microbial molecule that serves as the signature of viability is bacterial messenger RNA (mRNA), common to all bacteria, and without which bacteria cannot survive. Prokaryotic mRNAs also differ from eukaryotic mRNAs in several ways, and as such, these features all fulfill the criteria, and more, for a pathogen-associated molecular pattern (PAMP) as originally proposed by Charles Janeway. Because these mRNAs are lost from dead bacteria, they belong to a special class of PAMPs, which we call vita-PAMPs. Here we discuss the possible receptors and pathways involved in the detection of bacterial mRNAs, and thus microbial viability. We also consider examples of vita-PAMPs other than bacterial mRNA.