An alternative physiological role for the EmhABC efflux pump in Pseudomonas fluorescens cLP6a

Efflux assays conducted with 14C-phenanthrene showed that EmhABC activity is affected by incubation temperature. Increased phenanthrene efflux was measured in cLP6a cells grown at 10°C and decreased efflux was observed at 35°C compared with cells grown at the optimum temperature of 28°C. Membrane fatty acids in cLP6a cells were substantially altered by changes in growth temperature and in the presence of tetracycline. Changed membrane fatty acids and increased membrane permeability were associated with ~30-fold increased expression of emhABC in cLP6a cells grown at 35°C, and with increased extracellular free fatty acids. Growth of P. fluorescens cLP6a at supra-optimal temperature was enhanced by the presence of EmhABC compared to strain cLP6a-1.Combined, these observations suggest that the EmhABC efflux pump may be involved in the management of membrane stress effects such as those due to unfavourable incubation temperatures. Efflux of fatty acids replaced as a result of membrane damage or phospholipid turnover may be the primary physiological role of the EmhABC efflux pump in P. fluorescens cLP6a.Efflux pumps of the resistance-nodulation-division (RND) superfamily contribute to antibiotic resistance, virulence and solvent tolerance in Gram-negative bacteria [1-3]. The clinical significance of RND efflux pumps and their relevance to bioremediation necessitate understanding the factors influencing their expression and activity. Previous studies seeking the inducers of genes encoding RND efflux pumps focussed on known substrates of the pumps [4,5]. However, such studies showed that substrates are often not inducers, and the pumps are present in bacterial cells that have not been exposed to antibiotics or solvents [5-7]. Furthermore, genes encoding RND efflux pumps can be induced by stress responses such as ribosome disruption or membrane-damaging agents [4,7-9]. These observations suggest a physiological function for RND efflux systems beyond the transport of antibiot