Isolation and Characterization of Novel Lytic Phages to Combat Multidrug-Resistant E. coli and Salmonella spp.

Objectives: Escherichia coli and some Salmonella enterica serovars are zoonotic pathogens affecting livestock and humans. These pathogens cause significant loss of productivity in livestock, severe morbidity and mortality in humans, and have high antibiotic resistance profiles. Therefore, the exploitation of lytic phages for therapeutic purposes is important for eliminating these resistant bacterial strains. Methods: Thirty-four bacterial stock isolates comprised of 23 E. coli and 11 Salmonella spp. strains were evaluated for antimicrobial susceptibility to seven antibiotics using the Kirby-Bauer disk diffusion test. The antibiotics included Ciprofloxacin, Trimethoprim-Sulphamethoxazole, Gentamycin, Imipenem, Ceftriaxone, Cefotaxime, and Ofloxacin. Twelve (12/23) E. coli and (2/11) Salmonella spp. exhibited antimicrobial resistance. Selected six (6/12) drug-resistant E. coli strains were subjected to three different phages (PA5, EHEC005, C11S1A) for efficacy and host range assay. Similarly, two (2/2) resistant Salmonella strains were exposed to one Salmonella phage A23 for efficacy and host range assay. The E. coli (C11S1A) phage, which infected most bacterial hosts, was evaluated for optimal efficiency at various pH and temperatures. Results: E. coli isolates had the highest resistance 12/23 (52%) compared to Salmonella spp. 2/11(18%) (p<0.05). Most resistance was against Trimethoprim-Sulphamethoxazole (44%) and (9%) for E. coli and Salmonella spp., respectively. Furthermore, E. coli (C11S1A) phages killed all the Escherichia coli strains, while Salmonella phage A23 only lysed the host bacteria. The E. coli (C11S1A) phages were highly efficacious at 37 0 C and pH 7.4. Conclusion: The successful isolation of novel lytic E. coli (C11S1A) phages, which killed all the E. coli strains tested, demonstrates the potential for therapeutic purposes for humans and livestock. J Microbiol Infect Dis 2021; 11(4):183-190.