Micrococcus luteus (M. luteus) is an emerging opportunistic fish pathogen. This study was carried out to investigate incidence of M. luteus infection among Oreochromis niloticus (O. niloticus) farms at Al Dakhla city, New Valley Governorate, Egypt through clinical and bacteriological examinations and to determine antimicrobial susceptibility of M. luteus isolates and the minimum inhibitory concentration (MIC) of silver nanoparticles (AgNPs) and zinc oxide nanoparticles (ZnONPs) against M. luteus isolates. O. niloticus samples (n=150) were collected from several farms at Al Dakhla city from March to October 2019 and they were clinically and bacteriologically examined. Suspected isolates were identified as M. luteus by the morphological and biochemical characteristics then by 16S rRNA gene sequencing and phylogenetic analysis where 14 isolates were identified as M. luteus from the examined O. niloticus with prevalence of 9.3%. M. luteus isolates were biochemically identical except in arginine dehydrolase, Vogus-Proskauer and urease tests results. O. niloticus infected with M. luteus showed excessive skin pigmentation, loss of scales, hemorrhages and ulcers on the body, congestion and rot of fins, corneal opacity, exophthalmia and congested enlarged liver, spleen and kidney. Antimicrobial susceptibility of M. luteus isolates revealed that they were sensitive to penicillin, ampicillin/sulbactam, amoxicillin/clavulanic acid, norfloxacin, chloramphenicol and tetracycline, while were resisantant to cefotaxime, amikacin, tobramycin, erythromycin and ciprofloxacin. Furthermore, it was found that MIC of AgNPs against M. luteus isolates was 100µg/ml while M. lutues isolates were resistant to all ZnONPs concentrations used in this study. This study reported fish infection with M. luteus for the first time in Egypt and its results will be a starting point for the further studies to investigate prevalence and distribution of M. luteus infection among fishes in Egypt and the plan of its prevention.

Micrococcus luteus (M. luteus) is an emerging opportunistic fish pathogen that results in considerable economic losses among the infected fish. In the present study, a total number of 300 cultured Nile tilapia, Oreochromis niloticus were collected from concrete ponds farms in Alkharga and Aldakhla oases, New Valley governorate during the period from March to July 2019 and subject to clinical and bacteriological examination. The infected fish exhibited excessive skin pigmentation or depigmentation, scales loss, skin hemorrhages, body ulcers, fins congestion and rot, corneal opacity, and exophthalmia. Internally, congested enlarged liver, spleen, and kidney were observed., The prevalence of M. luteus infection was 10.3% and a total number of 31 isolates were recovered from infected fish. These isolates were biochemically similar except for Arginine hydrolysis, Vogus-Proskauer and Urease tests. The isolation and biochemical characterizations suggested that the isolates belong to the genus Micrococcus. The recovered M. luteus isolates were pathogenic to O. niloticus, this is the first report of M. luteus infection in O. niloticus in Egypt.