The lack of OmpF, but not OmpC, contributes to increased antibiotic resistance in Serratia marcescens
Serratia marcescens is a microorganism that constitutes one of the primary causes of nosocomial outbreaks in hospitals. One characteristic of S. marcescens clinical isolates is the high resistance to antimicrobials used in the clinic. Recent reports have attributed antibiotic resistance to altered porin expression. In this study, S. marcescens Db11 isogenic porin mutants were generated using the generalized transducing phage IF3 to move marked target-genes between isogenic strain backgrounds,
... ior to removal of the antibiotic resistance cassette by Flp-FRT strategy. Mutants for three classical porins were obtained and the effect of ompF and ompC deletion on antimicrobial resistance was evaluated by MIC. The use of this method avoided the incorporation of additional resistance markers and is an alternative strategy to create clean unmarked Serratia mutant strains. The lack of OmpF, but not OmpC, significantly increased MIC values to the βlactam drugs such as ampicillin and cefoxitin as well as to nitrofurantoin. Genetic deletion of both ompF and ompC did not compromise the integrity of the bacterial cell envelope in optimal growth conditions, suggesting that other outer-membrane porins may function in a compensatory role to facilitate nutrient uptake and cell envelope integrity. S. marcescens is a pathogen of C. elegans and can be used to study host response to bacterial infections. The host model Caenorhabditis elegans was used in this study to investigate if porin deficits affected bacterial virulence. When porin mutants were evaluated in the C. elegans host model, the virulence of the single porin mutant strains increased in comparison to the wild-type. This study demonstrated that mutations of ompF and ompC did not attenuate S. marcescens virulence, but rather demonstrated a hypervirulent phenotype when they were iii assessed in C. elegans. The absence of OmpF and OmpC porins in S. marcescens appeared to increase the bacterial invasion of C. elegans nematode tissue. Further studies are required to fully investigate the hypervirulent phenotype of these mutant strains. This study reveals that decrease of outer membrane permeability due to porin mutation alters antimicrobial resistance and does not generate virulence attenuation in S. marcescens Db11. iv ACKNOWLEDGEMENTS First and foremost, I would like to thank Dr. Elizabeth Worobec for providing me with the opportunity, guidance and support in starting PhD studies on Serratia marcescens in her research laboratory. I would like to acknowledge the support provided by Faculty of Science Graduate Scholarships, Faculty of Graduate Studies Travel Awards and NSERC. I would also like to thank Drs. Ann Karen Brassinga and Ivan Oresnik for agreeing to become my co-supervisors, during my second year of PhD studies, in place of Dr. Worobec when she became Director of University 1 at the University of Manitoba in 2011, and when she relocated to Kwantlen Polytechnic University (British Columbia) as Dean of Science and Holticulture in 2012. Drs. Brassinga and Oresnik assisted me in redefining my thesis objectives to include the Serratia marcescens Db11 strain as the basis for genetic studies on porins and infection modeling in C. elegans, and keeping me on track to complete my thesis research in a timely fashion. I am also grateful for the stipend support provided by the Faculty of Science for the last three years of my PhD studies.