Extracellular sensing components and extracellular induction component alarmones give early warning against stress in Escherichia coli

Abstract

The work reported here follows from the proposal that, for efficient induction of numerous extracellular stress responses, cultures contain extracellular stress-sensing molecules, termed extracellular sensing components (ESCs). These are directly converted to extracellular induction components (EICs) by stresses, thus providing an early warning system against stress, with very rapid responses occurring on exposure to increasing levels of stress. Although some stress responses appear to involve activation of intracellular sensors, the proposed ESCs and EICs function for many stress tolerance and sensitization responses and for several cross-tolerance and cross-sensitization responses. Because EICs can induce responses in unstressed cells, and because they are small molecules that can diffuse away from the site of formation, they can be considered to be 'alarmones', both warning unstressed organisms of future stress and preparing both stressed and unstressed ones to resist it. Therefore, EICs produced by one group of organisms could affect another group i.e. there could be 'cross-talk' (cell-to-cell communication) with other organisms in an area, to which the EICs diffuse, that has not yet faced the stress. In particular, stimuli that switch on acid tolerance, alkali tolerance, pH sensitization responses and alkylhydroperoxide tolerance are detected by ESCs; these molecules can give rise to EICs in the presence of the stress without organisms needing to be present. Not only does the ESC-EIC interconversion allow rapid switching on of responses, but for some responses it also allows rapid switching off. For some ESCs, the sensor can be modified by the culture conditions, modification leading to altered responsiveness to stress; such sensor changes appear to have evolved to allow the most efficient responses to stress to occur, under defined sets of conditions. In addition, the receptors on the organisms that interact with EICs are modified by culture conditions, so that extracellular components that function as ESCs for some cultures can act as EICs for others. In view of their role in early warning of stress, EICs and ESCs are likely to have important functions in the natural environment, especially in natural waters, in foods and food preparation and production, in hospital, domestic and commercial locations, and in the animal and human body. Findings of major importance relate to the extreme stress tolerance of some EICs. For example, because the acid-tolerance EIC formed at pH 5.0 is a heat-resistant molecule, heat-killed suspensions of acid-tolerant cultures can confer acid tolerance on living E. coli; cultures killed by extreme acidity and alkalinity and by exposure to high levels of UV irradiation or novobiocin are also able to confer acid tolerance on living E. coli. Extracellular components that inhibit induction of stress responses also occur in enterobacteria, since it has been found that AMP and HCO3-, which inhibit acid-tolerance induction, do so by forming extracellular agents that block the functioning of EICs. Similar agents to the above EICs and ESCs may occur in other non-stress-related processes. Systems using these extracellular components are quite distinct in their properties from quorum-sensing systems in Gram-negative bacteria and from those systems that use small peptides in intercellular communication and which induce virulence-related enzyme synthesis in Staphylococcus aureus and competence in streptococci and bacilli. Additionally, probably because the ESCs have evolved to become modified by cultural conditions, the components in the stress-related systems, although relatively small proteins, are much larger than the extracellular components used in the quorum-sensing processes and related systems. It is possible that the extracellular 'protectants' of Nikolaev, which protect E. coli from stress, act similarly to the EICs described here, e.g. by inducing stress tolerance. The antimutagenic factor of Vorobjeva may act similarly, although there is no evidence, so far, to suggest that it acts by inducing tolerance to mutagens.