Version 1: Antimicrobial activity of Serratia marcescens endosymbionts of Rhabditis nematodes against selected antibiotic resistant bacteria

Serratia marcescens is a gram-negative non-sporulating gamma proteobacteria of the family Enterobacteriaceae. It is a symbiont of the Oscheius nematodes. The bacterium has shown capacity to kill insect pests, thus the need to evaluate its efficacy against antibiotic tolerant bacterial pathogens. Serratia marcescens bacteria were isolated from hemolymph of previously nematode-infected Galleria mellonella larvae. Cultures were made by streaking on selective media then incubated at 28°C for 48hrs. Antimicrobial bioassays were then performed on Staphylococcus aureus, Methicillin-resistant S. aureus (MRSA), Escherichia coli, Bacillus cereus and Pseudomonas aeruginosa. Genomic DNA was isolated using the Cetyl Trimethyl Ammonium Bromide method followed by amplification of the 16S rRNA gene using B27F and 1492R primers. The two strains scored 99.26% and 98.22% identity to Serratia marcescens strains B195 and RS, respectively. No significant differences were observed between the two isolates with respect to antimicrobial efficacy towards the test pathogens. In general, the least susceptible pathogen was P. aeruginosa with a mean diameter of 13.00 mm followed by MRSA (14.83 mm), S. aureus (15.92 mm), E. coli (21.09 mm) and B. cereus (26.92 mm). Serratia marcescens should be exploited in the management of antibiotic resistant human pathogens.