A very brief note on why bacterial evolution has physiology

The majority of bacteria live and evolve in surface biofilms. Both growth in biofilms and horizontal transfer of DNA are regulated by quorum‐sensing pheromone signals. The common regulation of bacterial surface growth and DNA transfers illustrates how physiology contributes to bacterial evolution. figure legend from Patterson et al. (2016) with permission from Creative Commons Attribution (CC BY 4.0). The bacterium Serratia marcescens regulates the expression of its CRISPR‐Cas adaptive anti‐viral defence systems by SmaR repressors, which can be overcome by a density‐dependent autoactivator system based on emission of an acyl homoserine lactone (AHL) quorum sensing pheromone (green arrows). Thus, transcription of CRISPR‐Cas is repressed at low cell density by SmaR, and the bacteria are quite sensitive to lethal bacteriophage infections. At higher cell concentrations, the AHL pheromone is strongly expressed, and it overcomes SmaR repression to facilitate the expression of the CRISPR‐Cas RNA‐directed defence molecules (blue blobs attached to the viral DNA). These defence molecules cleave the viral DNA and rescue the infected cells so that they survive the phage attack and can accept any other DNA that may be packaged in phage particles to complete a horizontal gene/DNA transfer (HGT). It has been known since the earliest days of bacterial genetics that a subset of virus particles carry host DNA and are capable of the genetic exchange process known as ‘transduction’ (Zinder 1958). In this example, the quorum sensing system controls not only bacterial growth but also the potential for evolutionary change by regulating CRISPR‐based adaptive bacterial immunity in a way that favours phage‐mediated transduction.