Genes that Aid Ecological Fitness of Shewanella oneidensis MR-1 in Sediments Reveals the Value of Antibiotic Resistance

Dept. of Botany and Microbiology, University of Oklahoma, Norman, Oklahoma 73019; Dept. of Microbiology and Immunology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73190

^* To whom correspondence should be addressed. Email: krumholz{at}ou.edu.

	Abstract

Environmental bacteria persist in various habitats, yet little is known about the genes that contribute to growth and survival in their respective ecological niches. Signature tagged mutagenesis (STM) of Shewanella oneidensis MR-1 coupled with a screen involving incubations of mutant strains in anoxic aquifer sediments allowed us to identify 47 genes that enhance sediment fitness. Gene function inferred from annotations provides us with insight into physiological and ecological processes that environmental bacteria use while growing in sediment ecosystems. Identification of the mexF gene and other potential membrane efflux components by STM demonstrated that homologues of multidrug resistance genes present in pathogens are required for sediment fitness of non-pathogenic bacteria. Further studies with a mexF deletion mutant demonstrated that the mutidrug resistance pump encoded by mexF is required for resistance to antibiotics, including chloramphenicol and tetracycline. Chloramphenicol-adapted cultures exhibited mutations in the gene encoding a TetR family regulatory protein, indicating a role for this protein in regulating expression of the mexEF operon. The relative importance of mexF for sediment fitness suggests that antibiotic efflux may be a required process for bacteria living in sediment systems.