The present study was planned to investigate the causative
agent of Oreochromis niloticus summer mortality that resulted in
significant economic losses in Port Said Governorate, Egypt
during 2017. Many bacterial isolates were identified, among
which Acinetobacter johnsonii has been selected to be the focus
of the current study. Acinetobacter johnsonii_Egy was isolated
and identified using the analytical profile index (API) 20 E,
sequencing 16S ribossomal RNA gene fragment and phylogenetic
analysis. A drug resistance evaluation revealed that A. johnsonii
was resistant to ampicillin, gentamicin, lincomycin, nalidixic
acid, tetracycline, and oxytetracycline among the tested
antibiotics. Experimental infection was performed and challenged
fish revealed similar clinical signs as seen in naturally infected
ones. The disease appears to be highly infectious and lethal,
causing 100% cumulative mortality during experimental
infections. Virulence genes (fimH, traT, and iutA) and antibiotic
resistance genes (qacED1, qnrS, sul1, dfrA, and aadA1) presence
was confirmed in the A. johnsonii_Egy isolate.
In conclusion, A. johnsonii is highly infectious to O. niloticus
and might have a great influence on transfer of the antibiotic
resistance in aquaculture. According to the available knowledge,
this study is first to report A. johnsonii as emerging opportunistic
pathogen in the Egyptian Nile tilapia (in Port Said).