Antibacterial activity associated with the entomopathogenic nematode-dwelling bacteria Xenorhabdus and Photorhabdus

Aims: Xenorhabdus and Photorhabdus are Gram-negative bacteria symbiotically associated with entomopathogenic nematodes, considered potential sources of naturally occurring antibiotics. The aim of this study was to determine the antibacterial activity of Xenorhabdus and Photorhabdus against 15 strains of pathogenic bacteria. Methodology and results: Ethyl acetate extractions were performed on whole cell suspensions of selected Xenorhabdus and Photorhabdus isolates using a rotary vacuum evaporator. A disc diffusion method was used to evaluate the antibacterial activity of serially diluted extracts to determine the minimal inhibitory concentrations (MIC) and minimal bactericidal concentrations (MBC) of test bacterial species. Xenorhabdus stockiae bPP39.5_TH inhibited the growth of up to 5 strains of Gram-negative bacteria (Acinetobacter baumannii strain AB324 (XDR), Pseudomonas aeruginosa strain PB30 (MDR), Escherichia coli ATCC 35218, Klebsiella pneumoniae ATCC 700603 and K. pneumoniae strain PB21) and 4 strains of Gram-positive bacteria (Staphylococcus aureus ATCC 20475, S. aureus strain PB36 (MRSA), S. aureus strain PB57 (MRSA), Enterococcus faecalis ATCC 51299). The extract from X. stockiae bPP39.5_TH exhibited MIC and MBC ranging between 7.81 and 15.63 mg/mL, respectively, against several strains of pathogenic bacteria. In addition, 3 other X. stockiae isolates exhibited similar antibacterial growth-inhibitory effects. In contrast, X. japonica bHND30.5_TH and all Photorhabdus isolates had little or no growth-inhibitory effects on any of the pathogenic bacteria tested. Conclusion, significance and impact of study: This study demonstrates that X. stockiae possesses growth-inhibitory properties against several pathogenic bacteria strains. These bacteria may serve as natural sources of potentially novel bioactive compounds, which could be exploited in the development of new antibiotics.