166 Bacteria in nature are exposed to variations in temperature, and are affected by the availability of nutrients and water and the presence of toxic molecules. Their reactions to these changes require a series of rapid adaptive responses. Although transcriptional regulation is of primary importance in these responses, translational regulation and even activation of ‘silenced’ enzymes are critical for survival in changing environments. Bacteria have developed a series of mechanisms at the membrane structure level to cope with high concentrations of solvents. In addition, solvent-tolerant strains express highly effective efflux pumps to remove solvents from the cytoplasm. Desiccation tolerance is based on the synthesis and accumulation of osmoprotectants together with changes in fatty acid composition to preserve membrane structure. Both cold shock and heat shock responses are mainly regulated at a post- transcriptional level, translation efficiency in the case of cold shock and mRNA half-life and σ 32 stability in the case of heat shock. Addresses Consejo Superior de Investigaciones Científicas, Estación Experimental del Zaidin, Department of Plant Biochemistry, Profesor Albareda 1, 18008 Granada, Spain *e-mail: jlramos@eez.csic.es Current Opinion in Microbiology 2001, 4:166–171 1369-5274/01/$ —see front matter © 2001 Elsevier Science Ltd. All rights reserved. Abbreviations CSP cold shock protein Cti cis→trans isomerase ECF extracytoplasmic function HSP heat shock protein σ factor sigma factor Introduction Bacteria in the environment are exposed to a series of vari- able factors, such as large alterations in temperature and availability of nutrients and water, and the presence of toxic molecules that originate from their abiotic and biotic envi- ronment (even deleterious molecules originating from their own metabolism) that can make their living conditions far from optimal. Survival in this changing environment requires a wide range of fast, adaptive responses. In the majority of cases, the bacterial response leads to transcriptional acti- vation of genes whose products cope with a given physico-chemical stress. Gene regulators respond to specific signals (such as environmental and cellular signals) by stim- ulating or inhibiting transcription, translation or some other event in gene expression, so that the rate of synthesis of gene products is appropriately modified. Microorganisms able to offer a successful physiological/biochemical adaptation are better suited to colonizing the changing niche. In this short review, we focus on how Gram-negative bacteria respond to a series of abiotic stresses such as exposure to toxic organic compounds, desiccation and changes in temperature. Response to organic solvents Organic solvents with logP ow (the logarithm of the parti- tioning coefficient of a solvent in a defined octanol–water mixture) values between 1.5 and 3 are extremely toxic to microorganisms, because they bind to the cells and disturb the structure and functioning of the membranes. This is often followed by cell lysis and death. Nevertheless, sev- eral solvent-tolerant bacteria, most of them belonging to the genus Pseudomonas, have been isolated, and possible tolerance mechanisms have been reported [1]. Low cell surface hydrophobicity and alterations in membrane com- ponents could play an important role in preventing the accumulation of organic solvent molecules in the mem- brane [2]. Some other mechanisms participate in re-adjusting membrane fluidity, altered after solvent expo- sure. Among them, cis→trans isomerization of fatty acids, the change in the saturated : unsaturated fatty acid ratio and changes in the phospholipid head-group composition are the most important. The data obtained with a cis→trans isomerase (Cti) null mutant of Pseudomonas putida DOT-T1E indicates that the Cti is an important factor in preventing initial cell damage. This periplasmic enzyme is synthesized constitutively [3 • ]. The effective removal of the solvent from the cytoplasm or membrane is one of the major tolerance mechanisms. The export of organic solvents across the two membranes of the cell envelope of Gram-negative bacteria in a single energy- coupled step has been shown to be one of the most successful mechanisms for solvent tolerance in different bacteria. Several efflux pumps belonging to the RND family have been described as being involved in organic solvent extrusion ([4–6]; A Rojas, E Duque, G Mosqueda et al., unpublished data). In P. putida S12, expression of the solvent resistance protein ABC (SrpABC) pump is induced by aromatic and aliphatic solvents and alcohols, whereas general stress conditions such as pH, temperature, NaCl or the presence of organic acids do not induce srp transcription [4]. Three efflux pumps, encoded by toluene tolerance genes (ttg), have been described in P. putida DOT-T1E: TtgABC, TtgDEF and TtgGHI. The TtgABC and TtgGHI efflux pumps are able to extrude toluene, styrene, ethylbenzene, propylben- zene and xylenes, whereas the TtgDEF pump only extrudes toluene and styrene (A Rojas, E Duque, G Mosqueda et al., unpublished data). The ttgDEF operon Responses of Gram-negative bacteria to certain environmental stressors Juan L Ramos*, María-Trinidad Gallegos, Silvia Marqués, Maria-Isabel Ramos-González, Manuel Espinosa-Urgel and Ana Segura