Cross-talk involving extracellular sensors and extracellular alarmones gives early warning to unstressed Escherichia coli of impending lethal chemical stress and leads to induction of tolerance responses

摘要

The aim is to review how extracellular chemical stresses are sensed and how this switches on stress responses. Intracellular sensors function in many inducible systems, but might lead to delayed induction where the stressing agent is in the growth medium. It is clear, however, that extracellular stress sensors would lead to early warning of extracellular stress. This review shows that, for many responses, especially responses to change in pH, sensors which detect extracellular stresses are themselves extracellular (extracellular sensing components, ESCs), and that these ESCs are converted (activated) by stress, in the absence of bacteria, to extracellular induction components (EICs). EICs act as alarmones, warning unstressed organisms of impending challenge and preparing them to resist such challenge. Because they are small molecules, EICs can diffuse to parts of the bacterial environment not yet reached by the stressing components or conditions, so that bacteria are pre-warned of the stress and prepared to resist it. Thus, it is this intercellular communication (cross-talk) which gives early warning of stress. The ESCs and EICs for a specific response are similar in properties, but ESCs cannot induce stress tolerance unless activated. All ESCs studied exist in more than one form, the form synthesized depending on conditions during synthesis. Each form shows a distinct pH profile for activation, and such altered pH responsiveness allows a second type of early warning against stress. Growth conditions for organisms appear to modify their response to extracellular agents. Components which function as ESCs for organisms grown under one condition act as EICs for those grown under another, with specific and modifiable cell surface receptors interacting with the EICs. The properties of ESCs and EICs allows them to resist conditions which are lethal to the producing organisms and, accordingly, killed bacterial cultures can confer stress tolerance on living cells.