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

A Peptide Derived from GAPDH Enhances Resistance to DNA Damage in Saccharomyces cerevisiae Cells

Social behaviors do not exist only in higher organisms but are also present in microbes that interact for the common good. Here, we report that budding yeast cells interact with their neighboring cells after exposure to DNA damage. Yeast cells irradiated with DNA-damaging UV light secrete signal peptides that can increase the survival of yeast cells exposed to DNA-damaging stress. The secreted peptide is derived from glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and it induced cell death of a fraction of yeast cells in the group. The data suggest that the GAPDH-derived peptide serves in budding yeast's social interaction in response to DNA-damaging stress. IMPORTANCE Many studies have shown that microorganisms, including bacteria and yeast, display increased tolerance to stress after exposure to the same stressor. However, the mechanism remains unknown. In this study, we report a striking finding that Saccharomyces cerevisiae cells respond to DNA damage by secreting a peptide that facilitates resistance to DNA-damaging stress. Although it has been shown that GAPDH possesses many key functions in cells aside from its well-established role in glycolysis, this study demonstrated that GAPDH is also involved in the social behaviors response to DNA-damaging stress. The study opens the gate to an interesting research field about microbial social activity for adaptation to a harsh environment.

Adaptive Role of Cell Death in Yeast Communities Stressed with Macrolide Antifungals

Microorganisms cooperate with each other to protect themselves from environmental stressors. An extreme case of such cooperation is regulated cell death for the benefit of other cells. Dying cells can provide surviving cells with nutrients or induce their stress response by transmitting an alarm signal; however, the role of dead cells in microbial communities is unclear. Here, we searched for types of stressors the protection from which can be achieved by death of a subpopulation of cells. Thus, we compared the survival of Saccharomyces cerevisiae cells upon exposure to various stressors in the presence of additionally supplemented living versus dead cells. We found that dead cells contribute to yeast community resistance against macrolide antifungals (e.g., amphotericin B [AmB] and filipin) to a greater extent than living cells. Dead yeast cells absorbed more macrolide filipin than control cells because they exposed intracellular sterol-rich membranes. We also showed that, upon the addition of lethal concentrations of AmB, supplementation with AmB-sensitive cells but not with AmB-resistant cells enabled the survival of wild-type cells. Together, our data suggest that cell-to-cell heterogeneity in sensitivity to AmB can be an adaptive mechanism helping yeast communities to resist macrolides, which are naturally occurring antifungal agents.

IMPORTANCE Eukaryotic microorganisms harbor elements of programmed cell death (PCD) mechanisms that are homologous to the PCD of multicellular metazoa. However, it is still debated whether microbial PCD has an adaptive role or whether the processes of cell death are an aimless operation in self-regulating molecular mechanisms. Here, we demonstrated that dying yeast cells provide an instant benefit for their community by absorbing macrolides, which are bacterium-derived antifungals. Our results illustrate the principle that the death of a microorganism can contribute to the survival of its kin and suggest that early plasma membrane permeabilization improves community-level protection. The latter makes a striking contrast to the manifestations of apoptosis in higher eukaryotes, the process by which plasma membranes maintain integrity.

Robin James Rowbury: 30th April 1937 – 11th July 2012

Robin Rowbury, Life Sciences Editor of Science Progress since 1995, died in July 2012 following a long battle with cancer. Despite his illness he continued working right up until May this year both writing and promoting the journal wherever he could. In this issue, we are proud to feature his final article in which he describes the multi-talented character who was Robert Hooke. However, first we are publishing some tributes to Robin whom we remember in the office as a gentle man with a very dry sense of humour, as well as a hard-working author and editor. We shall all certainly miss him.

Professor David Phillips CBE, FRSC who worked as Physical Sciences Editor alongside Robin describes him as a real gentleman and an excellent academic.

Dr David Smith, his lifelong colleague from University College London remembers Robin as a supportive friend and colleague, a great teacher, and a gifted research scientist. He influenced many lives, and numerous students owe their successful careers to his guidance.

These sentiment are reflected in three more tributes from former colleagues.

Asiatic acid and corosolic acid enhance the susceptibility of Pseudomonas aeruginosa biofilms to tobramycin

Asiatic acid and corosolic acid are two natural products identified as biofilm inhibitors in a biofilm inhibition assay. We evaluated the activities of these two compounds on Pseudomonas aeruginosa biofilms grown in rotating disk reactors (RDRs) in combination with tobramycin and ciprofloxacin. To determine the ruggedness of our systems, the antibiotic susceptibilities of these biofilms were assessed with tobramycin and ciprofloxacin. The biofilm bacteria produced in the RDR were shown to display remarkable tolerance to 10 mug/ml of ciprofloxacin, thus mimicking the tolerance observed in recalcitrant bacterial infections. These studies further demonstrate that a nonmucoid strain of P. aeruginosa can form a biofilm that tolerates ciprofloxacin at clinically relevant concentrations. Neither asiatic acid nor corosolic acid reduced the viable cell density of P. aeruginosa biofilms. However, both compounds increased the susceptibility of biofilm bacteria to subsequent treatment with tobramycin, suggesting asiatic acid and corosolic acid to be compounds that potentiate the activity of antibiotics. A similar statistical interaction was observed between ciprofloxacin and subsequent treatment with tobramycin.

Stress Response of Campylobacter spp. and its Role in Food Processing

In many temperate countries Campylobacter spp. are the most common bacterial causes of human infectious intestinal disease. Yet the aetiology of this infection has only partly been described. A majority of human campylobacteriosis cases are associated with food of animal origin. Despite being very sensitive to environmental stressors Campylobacter spp. are able to persist in the food chain and can pose a threat to the consumer. In this review, the survival potential and stress response of Campylobacter spp. in food will be summarized and the importance of food preservation technologies will be discussed.

Monitoring changes in nisin susceptibility of Listeria monocytogenes Scott A as an indicator of growth phase using FACS

Listeria monocytogenes has previously been shown to adapt to a wide variety of environmental niches, principally those associated with low pH, and this compromises its control in food environments. An understanding of the mechanism(s) by which L. monocytogenes survives unfavourable environmental conditions will aid in developing new food processing methods to control the organism in foodstuffs. The present study aimed to gain a further understanding of the physiological basis for the differential effects of one control strategy, namely the use of the lantibiotic nisin. Using propidium iodide (PI) to probe membrane integrity it was shown that L. monocytogenes Scott A was sensitive to nisin (8 ng mL(-)) but this was growth phase dependent with stationary phase cells (OD600=1.2) being much more resistant than exponential phase cells (OD600=0.38). We demonstrate that, using a combination of techniques including fluorescence activated cell sorting (FACS), the membrane adaptations underpinning nisin resistance are triggered much earlier (OD600<0.5) than the onset of stationary phase. The significance of these findings in terms of mechanism and application are discussed.

Influence of growth media on the sensitivity of Staphylococcus aureus and Pseudomonas aeruginosa to cationic biocides

In this study, the influence of culturing Staphylococcus aureus and Pseudomonas aeruginosa under different growth conditions on their inactivation by the cationic active compounds benzalkonium chloride, chlorhexidine digluconate and octenidine dihydrochloride was investigated. Cells were grown in non-agitated tryptone soya broth as well as on tryptone soya agar according to national and international standards for evaluating chemical disinfectants. In quantitative suspension tests, cells of both test organisms grown on agar were significantly more sensitive to all three biocides than cells grown in broth. The differences in antimicrobial activity were greater in the case of S. aureus than in the case of P. aeruginosa. With S. aureus cultures, differences in the reduction factor of up to 5 log steps were found, with P. aeruginosa up to 2.5 log steps. The results of our uptake tests performed with S. aureus and octenidine dihydrochloride indicated that the growth conditions and the associated different stress factors either had an influence on the composition of the cell surface of this test organism or induced the formation of an efflux system. Cells of S. aureus cultured in broth took up only one-fifth of the amount of biocide molecules compared to cells from agar cultures. These data correlated with the results of the suspension tests. A low uptake of biocides apparently led to a reduced killing rate. In contrast to S. aureus, no significant differences in the uptake of octenidine dihydrochloride by cells of P. aeruginosa could be observed. These cells took up the same amount of the antimicrobial substance, whether on agar or in broth. In view of these results, possible consequences should be considered prior to changing test regulations.

Intracellular and extracellular components as bacterial thermometers, and early warning against thermal stress

Responses induced by cold or heat are triggered following detection of temperature changes by specific sensing molecules, complexes or structures. Low temperature responses are often induced following sensing of cold-induced falls in membrane fluidity, such changes turning-on or -off enzymic activities in membrane proteins, although ribosomes and DNA may also function in cold perception. Many thermal sensors are components of structures damaged by the heat, with sensing involving changes to ribosomes, DNA, intracellular proteins and, less commonly, membrane fluidity. Additionally, secreted proteins (extracellular sensing components, ESCs) detect temperature increases i.e. act as thermometers, with ESC activation in the medium, by the stimulus, converting such sensors to extracellular signalling molecules, the extracellular induction components (EICs), which induce thermal responses. Several ESC/EIC pairs trigger thermal responses, and have the unique property of giving early warning of the stress by diffusing to regions (and organisms) not yet exposed to elevated temperatures.

Enterobacterial responses to external protons, including responses that involve early warning against stress and the functioning of extracellular pheromones, alarmones and varisensors

Several striking findings, related to biological effects of external acidity, are reviewed here. The first of these relates to the role of PhoE in the penetration of H+ and protonated metabolites into the cell. PhoE is an anion pore and would not be expected to take up protons. The work reviewed here, however, shows that the loss or repression of PhoE leads to poor H+ passage through the outer membrane (OM), whilst derepression of PhoE leads to facilitated passage. It is now believed that H+ crosses through the PhoE pore in association possibly with oligopeptides, and that other protonated molecules, such as the acid tolerance EIC, use the same means to cross the OM. Additionally, several processes that form early warning systems against acidity are reviewed here. First, the properties of the acid tolerance EIC alarmones allow them to diffuse to regions not yet facing acid stress, and there give early warning and induce sensitive organisms to tolerance. Second, some agents, such as glucose, induce acid tolerance in organisms, long before these organisms are exposed to catabolically-produced acidity, preparing them, in advance, to resist this impending acid challenge. Third, the occurrence of multiple forms of ESCs (i.e. of varisensors) ensures that where organisms have been grown under conditions that sensitise them to acid stress, the ESCs formed are modified so as to be activated at much higher pH values, ensuring that lethality by acid is reduced or abolished. Fourthly, normally only EICs induce tolerance. Strikingly, however, pH 8.5 or 9.0-grown cells are induced to tolerance by ESC formed at pH 6.5. This is believed to provide another early warning system, protecting alkali-grown cells against sudden acidification of media. Two other finding reviewed here should be emphasised. First, the hydrophobic antibiotic novobiocin is ineffective against enterobacteria, due to its failure to penetrate the OM barrier. This only applies to cultures in pH 7.0 media, however, cells growing at pH 5.0 being exquisitely sensitive to novobiocin, due to a conformational change to the antibiotic at acidic pH, which allows ready penetration through the OM. Second, acidic pHs affect the synthesis and effects of another antibiotic, namely colicin V. Thus pH 5.0 prevents both synthesis of this agent and its effects on sensitive cells. Exposure to external acidity leads to numerous other effects, including those that influence growth, cell division, plasmid transfer and chemotaxis; these have also been reviewed here.

Identification of a novel stress resistance mechanism in Campylobacter jejuni

AIM

To study stress resistance mechanisms in Campylobacter spp.

METHODS AND RESULTS

Campylobacter strains were grown to the appropriate phase in Brucella broth. The cells were diluted into either cell-free spent medium (obtained by filtration of a grown culture) or a freshly prepared medium and the pH reduced to 4.5, a lethal pH value. At suitable time intervals survivors were enumerated on Campylobacter blood free selective agar base. The cell-free spent medium from mid-exponential and stationary phase had a protective effect on acid and thermal stress in Campylobacter jejuni CI 120, a natural isolate. The protective effect of the extracellular compound was not significantly inactivated by boiling, but was inactivated by proteinase.

CONCLUSIONS

The present study suggests that a protein (or proteins) accumulated by C. jejuni CI 120 during growth may play an active role in the induction of stress responses and that this protein is heat stable.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results indicate that C. jejuni CI 120, a natural isolate, has the ability to use extracellular signalling mechanisms to induce tolerance to stress factors. This is a major advancement in the understanding of the physiological basis for survival of C. jejuni in the environment.

Evaluation of the pH-dependent, stationary-phase acid tolerance in Listeria monocytogenes and Salmonella Typhimurium DT104 induced by culturing in media with 1% glucose: a comparative study with Escherichia coli O157:H7

AIMS

To comparatively evaluate the adaptive stationary-phase acid tolerance response (ATR) in food-borne pathogens induced by culturing in glucose-containing media, as affected by strain variability and antibiotic resistance, growth temperature, challenge pH and type of acidulant.

METHODS AND RESULTS

Antibiotic resistant or sensitive strains of Listeria monocytogenes, Salmonella including S. Typhimurium DT104, and Escherichia coli O157:H7 were cultured (30 degrees C for 24 h; 10 degrees C for up to 14 days) in trypticase soya broth with yeast extract (TSBYE) with 1% or without glucose to induce or prevent acid adaptation, respectively. Cultures were subsequently exposed to pH 3.5 or 3.7 with lactic or acetic acid at 25 degrees C for 120 min. Acid-adapted cultures were more acid tolerant than nonadapted cultures, particularly those of L. monocytogenes and Salmonella. No consistent, positive or negative, influence of antibiotic resistance on the pH-inducible ATR or acid resistance (AR) was observed. Compared with 30 degrees C cultures, growth and acid adaptation of L. monocytogenes and S. Typhimurium DT104 at 10 degrees C markedly reduced their ATR and AR in stationary phase. E. coli O157:H7 had the greatest AR, relying less on acid adaptation. A 0.2 unit difference in challenge pH (3.5-3.7) caused great variations in survival of acid-adapted and nonadapted cells.

CONCLUSIONS

Culturing L. monocytogenes and Salmonella to stationary phase in media with 1% glucose induces a pH-dependent ATR and enhances their survival to organic acids; thus, this method is suitable for producing acid-adapted cultures for use in food challenge studies.

SIGNIFICANCE AND IMPACT OF THE STUDY

Bacterial pathogens may become acid-adapted in foods containing glucose or other fermentable carbohydrates. Low storage temperatures may substantially decrease the stationary-phase ATR of L. monocytogenes and S. Typhimurium DT104, but their effect on ATR of E. coli O157:H7 appears to be far less dramatic.

Induction of an adaptive tolerance response in the foodborne pathogen, Campylobacter jejuni

In this study we aimed to determine if Campylobacter had the ability to induce an adaptive tolerance response (ATR) to acid and/or aerobic conditions. Campylobacter jejuni CI 120 was grown to the appropriate phase in Brucella broth under microaerobic conditions. Cells were initially adapted to a mild stress (pH 5.5) for 5 h prior to challenge at pH 4.5, a lethal pH. Survival was examined by determining the numbers of viable cells on Campylobacter blood free selective agar base. Stationary phase cells adapted at pH 5.5 induced an ATR that enabled a 100-fold greater survival compared to an uninduced culture. Aerobic adaptation also protected the cells against acid challenge. The cross protection provided a 500-fold increase in survival when compared to unadapted cells. The incorporation of chloramphenicol during the induction period eliminated the ATR and resulted in death kinetics similar to an uninduced culture. These data suggest that Campylobacter spp. have the ability to induce an ATR to sublethal treatments, which increased their ability to withstand subsequent stresses.

Extracellular proteins as enterobacterial thermometers

Biological thermometers are cellular components or structures which sense increasing temperatures, interaction of the thermometer and the thermal stress bringing about the switching-on of inducible responses, with gradually enhanced levels of response induction following gradually increasing temperatures. In enterobacteria, for studies of such thermometers, generally induction of heat shock protein (HSP) synthesis has been examined, with experimental studies aiming to establish (often indirectly) how the temperature changes which initiate HSP synthesis are sensed; numerous other processes and responses show graded induction as temperature is increased, and how the temperature changes which induce these are sensed is also of interest. Several classes of intracellular component and structure have been proposed as enterobacterial thermometers, with the ribosome and the DnaK chaperone being the most favoured, although for many of the proposed intracellular thermometers, most of the evidence for their functioning in this way is indirect. In contrast to the above, the studies reviewed here firmly establish that for four distinct stress responses, which are switched-on gradually as temperature increases, temperature changes are sensed by extracellular components (extracellular sensing components, ESCs) i.e. there is firm and direct evidence for the occurrence of extracellular thermometers. All four thermometers described here are proteins, which appear to be distinct and different from each other, and on sensing thermal stress are activated by it to four distinct extracellular induction components (EICs), which interact with receptors on the surface of organisms to induce the appropriate responses. It is predicted that many other temperature-induced processes, including the synthesis of HSPs, will be switched-on following the activation of similar extracellular thermometers by thermal stimuli.

UV radiation-induced enterobacterial responses, other processes that influence UV tolerance and likely environmental significance

The ability of enterobacteria to become UV-tolerant is important because such tolerance may enable organisms to resist irradiation in the environment, in water treatment, in shell-fish, in stages of food processing, and at locations in the domestic, commercial and hospital environment The mechanism for regulation of tolerance induction and SOS response induction has been studied for many years, and is well understood, except for the early stages of induction. Such early stages, namely sensing of the stimulus (UV irradiation) and the way in which such sensing leads to signal production, have until now been poorly understood. The claim has been made that DNA is the sensor and that either damage to DNA or production of SS regions in DNA (following interaction of UV with DNA) triggers the signal that sets in train RecA activation and other stages of tolerance induction. This claimed induction mechanism is a "classical" one in the sense that it involves intracellular sensing (by DNA) of the stressing stimulus (UV), and production of an intracellular signalling molecule. It is not, however, firmly established as the mechanism for initiation of UV tolerance induction and SOS response induction. The results reviewed here give firm evidence for a different and unique mechanism for sensing of UV and production of the signal. These results establish without doubt that, for UV tolerance induction, the UV sensor is an extracellular protein, which is a UV tolerance-specific extracellular sensing component (ESC). This component is formed by unstressed cells and on interacting with the stimulus (UV) in the medium, is converted to the tolerance induction signalling molecule, which is a UV tolerance-specific extracellular induction component (EIC). It is this extracellular signal which interacts with the sensitive organisms and triggers tolerance induction. This pair of extracellular components (ECs) may offer the only means of switching-on such tolerance induction; certainly they offer the only known way for early warning to be given of impending UV challenge. Thus, the EIC can diffuse from a region of UV stress to a stress-free region and there warn organisms of impending stress and prepare them to resist it. As indicated here, UV irradiation not only induces UV tolerance, but also switches-on acid tolerance, alkali tolerance and thermotolerance responses. The fact that all three responses involve ESC/EIC pairs strongly supports the view that functioning of such EC pairs form the major, if not the only, means for UV tolerance induction. The UV tolerance-specific ESC can detect other stresses and becomes activated, leading to cross-tolerance responses. Of particular interest, this ESC acts as a biological thermometer, detecting increases in temperature, such increases leading to gradually increasing formation of the EIC and, accordingly, gradual increases in UV tolerance. This UV tolerance-specific ESC can also detect other stresses e.g. acting as a pH sensor. In all cases, on activation, the EIC formed (from this specific ESC) only induces UV tolerance. It is proposed that the interaction of EICs with stress-sensitive organisms should be examined, and it is suggested that such EICs may, directly or indirectly, interact with and activate the same stress response regulators as are used to detect internal stressors and which, on activation, also trigger the switching-on of stress responses. For example, EICs either a in a protonated or oxidised state (formed by activation of ESCs by H+ or H2O2) or b produced by irradiation, may lead to protonation or oxidation or other forms of activation of the appropriate regulator (e.g. Fur or OxyR or RecA etc), leading to response induction.

How killed enterobacterial cultures can activate living organisms to resist lethal agents or conditions

A major aim in many areas of microbiology is to ensure sterility, and even where this is impossible, to reduce the number of viable organisms occurring in particular environments to an absolute minimum. This applies in the aquatic environment, where e.g. water treatment must ensure as complete absence of viable microbes as possible. It is also crucial in food processing and production; many food constituents contain appreciable numbers of viable organisms, even potential pathogens, and the number must be greatly reduced and in many situations, the presence of viable organisms totally abolished. Cleaning of food production components and surfaces must also kill associated microbes. In domestic, hospital and commercial situations, similar disinfection is critical. Ultimately, the aim is to ensure, if possible, sterility, with the assurance that microbial problems cannot occur if organisms are absent. Additionally, however, it has been implicitly assumed that killed organisms and even killed cultures cannot (except in minor and trivial ways) influence the behaviour of living organisms that later enter the environment. The work reviewed here challenges that view and in fact disproves it. The findings described show that killed enterobacterial cultures, which prior to killing had phenotypically gained the ability to resist potentially lethal stresses, can pass on such ability to living organisms that later enter their environment i.e. that such killed cultures can convey a baleful legacy to living ones. This phenomenon is so widespread that it is clear that it has significance for enterobacterial survival in natural waters, in foods and in food production, in the domestic, commercial and hospital situation, and in the animal and human body. In fact, in this last area, the likely effect of killed cultures appears to be of appreciable public health importance. Here, the ability of appropriate killed cultures to transfer tolerance to acidity, alkalinity and thermal stress is described, as well as their ability to pass on sensitisation to acid and alkali. Other work reviewed suggests that killed cultures can almost certainly transfer the ability to tolerate hydrogen peroxide, ultraviolet irradiation and metal ions. The serious implications of this phenomenon are further emphasised by the fact that numerous killing methods produce cultures effective in tolerance response transfer. All the evidence suggests that it is extracellular components (extracellular sensing components, ESCs, and extracellular induction components, EICs), in the killed cultures which are involved in stress response transfer, and that the actual stress response induction process depends on interaction of living organisms with EICs from the killed cultures. It is of note that ESCs and EICs survive in killed cultures because of their extreme resistance to irreversible inactivation by lethal levels of stressing agents and conditions. This is in contrast to the fact that EC activation, namely the conversion of ESC to EIC occurs on exposure to very low levels of stressors. Not only is this the case, but in fact high levels of stressors (e.g. those that kill organisms) generally fail to convert ESC to EIC.

Bacterial responses to alkaline stress

Studies of bacterial adaptation to alkaline pH have been less extensive to date compared with those of acidic pH. Recent development of novel methods for global analysis of gene expression under various conditions revealed that many genes were induced at high pH. These data led us to question why so many genes are required for adaptation to alkaline pH. The internal pH of bacteria growing at extremely high pH remains unclear because the methods for measuring interior acidic deltapH developed to date are not so accurate, but it is generally accepted that cytoplasmic pH increases with medium alkalization, although the increase is lower than that of the change in medium pH. Therefore, activities of enzymes working in neutral cytoplasm may decrease with cytoplasmic alkalization under extreme alkaline conditions. Based on these findings, we propose in this article that genes whose products have an optimum activity at high pH are induced under alkaline stress to compensate for the decrease in activities of systems functioning at neutral pH.

Exposure to non-acid fresh meat decontamination washing fluids sensitizes Escherichia coli O157:H7 to organic acids

AIMS

To investigate whether Escherichia coli O157:H7 maintains acid tolerance in water meat decontamination washing fluids.

METHODS AND RESULTS

A rifampicin-resistant derivative of E. coli O157:H7 strain ATCC 43895 was inoculated (10(5) cfu ml(-1)) in spray-washings from meat sprayed with cold (10 degrees C) or hot (85 degrees C) water, stored at 10 degrees C for up to 14 days, and its acid tolerance was assessed at 2 and 8 days by exposure to broth or new washings adjusted to pH 3.5 or 3.7 with lactic or acetic acid. The pathogen survived in the water washings, but it was outgrown by the natural, Pseudomonas-like flora, and it was sensitized to acid.

CONCLUSIONS

The acid tolerance of E. coli O157:H7 decreases following exposure to non-acid, but otherwise stressful, conditions prevailing in water meat washings at 10 degrees C.

SIGNIFICANCE AND IMPACT OF THE STUDY

These findings suggest that the more intense use of water-based technologies should be included in meat decontamination strategies because they may contribute to enhanced meat safety by inducing acid sensitization in E. coli O157:H7.

Extracellular sensing and signalling pheromones switch-on thermotolerance and other stress responses in Escherichia coli

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.