Isolation and Characterization of Lytic Bacteriophages against Aeromonas dhakensis Isolated from Water in Thailand

Phage therapy has emerged as a potential solution to the issue of multidrug-resistant bacteria. In this study, a novel bacteriophage vB_AdhS_M4 , which infects Aeromonas dhakensis, was isolated. A. dhakensis strains were isolated from water samples in Thailand and identified by biochemical and 16S rDNA sequence analysis. Four isolates, namely AM, NGP8, AK3, and SBKN4 were identified as A. dhakensis and used as hosts for phage isolation. Only 1 phage, vB_AdhS_M4, was obtained using A. dhakensis AM as host. Phage vB_AdhS_M4 morphology, host range, one-step growth curve, pH and temperature stability and genome were investigated. Transmission electron microscopy revealed that vB_AdhS_M4 has an icosahedral head of 64.8 ± 0.51 nm and a long tail of 185.4 ± 0.48 nm, suggesting that it belongs to the Siphoviridae family. Phage vB_AdhS_M4 had a latent period of 50 mins and a mean burst size of approximately 48 phage particles per infected cell. Our study on host-range determination demonstrated that this phage was specific for infecting A. dhakensis. Phage vB_AdhS_M4 was stable within a pH range of 4-7 and at temperatures ranging from 4 to 45 °C. The complete genome of Aeromonas phage vB_AdhS_M4 is 61,429 bp in length, with a G+C content of 61.7% and 77 open reading frames. In vitro A. dhakensis growth inhibition were observed with vB_AdhS_M4 at various MOIs. The greatest reduction in cell count occurred during the first 6-12 h of incubation with the phage, compared to the uninfected bacterial control, at all MOIs tested. Although the bacteria regrew after 24 h of incubation with the phage, the bacterial count remained lower than that of the control throughout the entire 48-h period. Based on these findings, it appears that vB_AdhS_M4 could be a valuable tool for biocontrol of A. dhakensis in aquaculture.