Extracellular Sensing and Signalling Pheromones Switch-on Thermotolerance and Other Stress Responses in Escherichia Coli

Abstract

The findings reviewed here overturn a major tenet of bacterial physiology, namely that stimuli which switch-on inducible responses are always detected by intracellular sensors, with all other components and stages in induction also being intracellular. Such an induction mechanism even applies to quorum-sensed responses, and some others which involve functioning of extracellular components, and had previously been believed to occur in all cases. In contrast, for the stress responses reviewed here, triggering is by a quite distinct process, pairs of extracellular components being involved, with the stress sensing component (the extracellular sensing component, ESC) and the signalling component, which derives from it and induces the stress (the extracellular induction component, EIC), being extracellular and the stimulus detection occurring in the growth medium. The ESCs and EICs can also be referred to as extracellular sensing and signalling pheromones, since they are not only needed for induction in the stressed culture, but can act as pheromones in the same region activating other organisms which fail to produce the extracellular component (EC) pair. They can also diffuse to other regions and there act as pheromones influencing unstressed organisms or those which fail to produce such ECs. The cross-talk occurring due to such interactions, can then switch-on stress responses in such unstressed organisms and in those which cannot form the ESC/EIC pair. Accordingly, the ESC/EIC pairs can bring about a form of intercellular communication between organisms. If the unstressed organisms, which are induced to stress tolerance by such extracellular components, are facing impending stress challenge, then the pheromonal activities of the ECs provide an early warning system against stress. The specific ESC/EIC pairs switch-on numerous responses; often these pairs are proteins, but non-protein ECs also occur and for a few systems, full induction needs two ESC/EIC pairs. Most of the above ECs needed for response induction are highly resistant to irreversible inactivation by lethal agents and conditions and, accordingly, many killed cultures still contain ESCs or EICs. If these killed cultures come into contact with unstressed living organisms, the ECs again act pheromonally, altering the tolerance to stress of the living organisms. It has been claimed that bacteria sense increased temperature using ribosomes or the DnaK gene product. The work reviewed here shows that, for thermal triggering of thermotolerance and acid tolerance in E. coli, it is ESCs which act as thermometers.

Quorum sensing and biofilm formation in Streptococcal infections

“…Changes in external pH can significantly influence many physiological parameters, such as energy coupling, ion transport, proton movement, and export of metabolic products, thereby triggering numerous secondary signals. During growth at pH 5.0, Escherichia coli can signal stress tolerance to other unadapted cells by secreting a proteinlike molecule, termed extracellular induction component (EIC) (30). Although the signal molecule remains unidentified, induction of stress (including acid) adaptation in E. coli presumably involves cell-cell communication.…”

Habituation of Foodborne Pathogens Exposed to Extreme pH Conditions: Genetic Basis and Implications in Foods and Food Processing Environments

“…EICs are formed from a precursor molecule, termed an extracellular sensing component (ESC), that is thought to be necessary for induction of several stress responses (Rowbury, 2002;Rowbury & Goodson, 2001) (Table 1). Rowbury (2001b) detected an ESC in culture media after growth of a non-pathogenic strain of E. coli at pH 7.0-9.0.…”

“…Problems of adverse pH and bacterial strategies to combat it have been reviewed (Dilworth & Glenn, 1999;Rowbury & Goodson, 2001). The phenomenon of increased resistance of bacteria to stress conditions following exposure to sublethal pH stress can result from changes in one or more metabolic activities or functions.…”

Use of bioluminescence to evaluate the effects of rapid cooling on recovery of Salmonella enterica serovar Typhimurium DT104 after heat treatment

“…It has been long established that bacteria exposed to sublethal heat become more heat resistant (Rowbury & Goodson, 2001). On the other hand extending the heating time, by cooling less rapidly, may render unacceptable the appearance of fresh food, post pasteurisation.…”