Comparative Bacteriological, Molecular, and Pathological Studies on Streptococcus agalactiae, Psychrobacter, and Enterococcus Bacteria Co-Infecting Cultured Oreochromis niloticus and Mugil cephalus

Infectious diseases, especially bacterial diseases, are regarded as one of the primary problems in aquaculture systems, resulting in significant economic losses and impeding aquaculture's sustainability. Different fish species exhibit varying degrees of susceptibility to bacterial pathogens due to differences in immune competence, anatomical barriers, and genetic resistance. Subsequently, the purpose of this study is to assess the severity of the pathological alterations caused by bacterial infection in Oreochromis niloticus (O. niloticus) and Mugil cephalus (M. cephalus). Equal numbers (305) from each species of O. niloticus and M. cephalus were sampled from private farms in Egypt and examined for any gross lesions. From the examined fishes, 19 and 11 that showed the most severe gross changes were collected from O. niloticus and M. cephalus, respectively. Postmortem examination and internal organs were examined for both bacteriological and histopathological analysis. Streptococcus agalactiae (S. agalactiae) was the most predominant pathogen isolated from both diseased O. niloticus (42.1%) and M. cephalus (45.4%), followed by Psychrobacter spp. Other pathogens such as Enterococcus faecalis (E. faecalis) and Enterococcus faecium (E. faecium) were isolated at lower frequencies. This highlights S. agalactiae as a key infectious agent in both cultured fish species. The examined O. niloticus and M. cephalus exhibited numerous pathological alterations. O. niloticus and M. cephalus differ significantly in the severity of their lesions. These variations might be attributed to host-specific and pathogen-related factors, such as anatomical and physiological differences, innate immune competence, and bacterial virulence expression variation in different hosts, which need further investigations.