A Single Promoter Inversion Switches Photorhabdus Between Pathogenic and Mutualistic States

Abstract

Twin Tales of Two Toxins The luminescent bacterium, Photorhabdus luminescens , is carried in the gut of an insect-parasitic nematode as a stealth weapon. By using an allele swapping technique, Somvanshi et al. (p. 88 ) investigated the promoter-switching mechanism that flips the bacterium from the almost dormant M forms, which stick to the adult nematode's posterior gut, into the motile, luminous P forms, which are armed with the toxic virulence factors needed to overcome the insect prey of the nematode. Similar switches may operate in bacteria that flip between harmless commensals and lethal pathogens. The bio-control agent Bacillus thuringiensis also kills insects by means of a crystal toxin, which allows the bacteria to penetrate the host gut and access nutrients. Release of nutrients also allows bacterial cheats that do not make toxin, to grow and outcompete the toxin-producing colonizers. In field experiments, Raymond et al. (p. 85 ) found that, consequently, the bacterial population becomes less virulent. Because these type of virulence factors are secreted from the cell and are widespread in pathogens, such social interactions may affect the fitness and constrain the virulence of many toxin-producing bacteria.