ORIGINAL ARTICLE Cellular response mechanisms in Pseudomonas aeruginosa PseA during growth in organic solvents R. Gaur and S.K. Khare Enzyme and Microbial Biochemistry Lab, Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi, India Introduction Organic solvents are generally considered to be microbi- cidal agents, owing to their toxic effects on the cell mem- brane. They tend to accumulate in bacterial membranes and severely disturb the membrane structure (Kieboom et al. 1998). In recent years, few micro-organisms able to grow in high solvent concentrations have been isolated and defined as solvent-tolerant microbes. These have drawn considerable attention as attractive biosystems for solvent bioremediation and biotransformation in nonaqu- eous media (Sardessai and Bhosle 2004; Tang et al. 2008; Gupta and Khare 2009). These microbes are also per- ceived to be excellent sources of enzymes functional in solvents (Isken and de Bont 1998). Properties of solvent tolerance have been encountered mainly in the genera Pseudomonas, Bacillus, Flavobacterium, Rhodococcus and Staphylococcus (Ogino and Ishikawa 2001; Fang et al. 2006). The toxic effect of solvents is circumvented in these microbes through several adaptation strategies viz. (i) change in fatty acid conformation of the lipid bilayer from cis to trans and saturation of the fatty acid acyl chains, (ii) modification of the lipopolysaccharides of the Keywords cell membrane, lipase, Pseudomonas aeruginosa PseA, solvent hydrophobicity, solvent tolerance. Correspondence Sunil Kumar Khare, Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110 016, India. E-mail: skhare@rocketmail.com 2009 ⁄ 0503: received 16 March 2009, revised 21 May 2009 and accepted 27 May 2009 doi:10.1111/j.1472-765X.2009.02671.x Abstract Aims: Solvent-tolerant bacteria have emerged as a new class of micro-organ- isms able to grow at high concentrations of toxic solvents. Such bacteria and their solvent-stable enzymes are perceived to be useful for biotransformations in nonaqueous media. In the present study, the solvent-responsive features of a lipase–producing, solvent-tolerant strain Pseudomonas aeruginosa PseA have been investigated to understand the cellular mechanisms followed under sol- vent-rich conditions. Methods and Results: The solvents, cyclohexane and tetradecane with differing log P-values (3Æ2 and 7Æ6 respectively), have been used as model systems. Effect of solvents on (i) the cell morphology and structure (ii) surface hydrophobicity and (iii) permeability of cell membrane have been examined using transmission electron microscopy, atomic force microscopy and other biochemical tech- niques. The results show that (i) less hydrophobic (low log P-value) solvent cyclohexane alters the cell membrane integrity and (ii) cells adapt to organic solvents by changing morphology, size, permeability and surface hydrophobic- ity. However, no such changes were observed in the cells grown in tetradecane. Conclusions: It may be concluded that P. aeruginosa PseA responds differently to solvents of different hydrophobicities. Bacterial cell membrane is more per- meable to less hydrophobic solvents that eventually accumulate in the cyto- plasm, while highly hydrophobic solvents have lesser tendency to access the membrane. Significance and Impact of the Study: To the best of our knowledge, these are first time observations that show that way of bacterial solvent adaptability depends on nature of solvent. Difference in cellular responses towards solvents of varying log P-values (hydrophobicity) might prove useful to search for a suitable solvent for carrying out whole-cell biocatalysis. Letters in Applied Microbiology ISSN 0266-8254 372 Journal compilation ª 2009 The Society for Applied Microbiology, Letters in Applied Microbiology 49 (2009) 372–377 ª 2009 The Authors