Enterobacter cloacae as a re-emerging pathogen affecting mullets (Mugil spp.): Pathogenicity testing, LD50, antibiogram, and encoded antimicrobial resistance genes

Enterobacter cloacae is regarded as a hazardous fish pathogen that causes severe diseases and major economic losses in the fish industry. In this study, 150 Mullet species, including 70 Mugil cephalus and 80 Mugil capito were collected randomly from Lake Temsah in Ismailia Governorate, Egypt during the period from May 2022 to July 2022. Subsequently, the collected samples were processed for clinical, post-mortem, and bacteriological analyses. The retrieved isolates were tested for antimicrobial susceptibility, pathogenicity, LD50, and PCR-based detection of antimicrobial resistance genes. The prevalence of E. cloacae among the examined fish was 28.7%. The highest prevalence was recorded in M. cephalus (38.6%) followed by M. capito (20%). Moreover, the liver was the most prominent infected organ (42.3%). Furthermore, 61.5% of the retrieved isolates were multidrug resistance (MDR) to seven antimicrobial classes and harbored blaTEM, blaCTX-M, tetA, aadA1, ereA, and sul1 resistance genes. Likewise, 13.5% of the tested isolates were MDR to six antimicrobial classes and possessed blaTEM, blaCTX-M, tetA, aadA1, and sul1 resistance genes. The experimentally infected fish with E. cloacae exhibited varying mortalities directly proportional to the inoculated doses (35%, 50%, 70%, and 80% for the inoculating doses of 105–8 CFU mL−1, respectively). As far as we know, this is the first investigation that highlighted the emergence of MDR E. cloacae in Mullet species that reveals a threat to the public health and the fish industry. Newly emerging MDR E. cloacae in Mullet species commonly harbored blaTEM, tetA, sul1, blaCTX-M, aadA1, and ereA resistance genes.