Isolation, identification and pathogenicity study of emerging multi-drug resistant fish pathogen Acinetobacter pittii from diseased rohu (Labeo rohita) in India

Abstract

Growing numbers of disease outbreaks and a wider variety of pathogens have emerged as a result of aquaculture's expansion, intensification, and commercialization. In the current work, we isolated and identified a new multi-drug resistant pathogenic bacteria, Acinetobacter pittii strain COF_AHE14, that causes mass death in rohu (Labeo rohita) during a post-monsoon disease outbreak in the aquaculture farm of Tripura, India. The isolate was initially identified based on morphological and biochemical information, and the species was validated through molecular characterization of the 16S rRNA gene sequence and phylogenetic analysis. An experimental infectivity study in healthy rohu (Labeo rohita), haemato-biochemical and immune-pathophysiological analysis, histopathological study, and fulfilment of the Koch’s postulate all confirmed the pathogenicity of the isolated bacteria. The LD50 of A. pittii strain COF_AHE14 in an experimental infection was 4.1 × 106 CFU fish-1, and the clinical signs observed, such as ulcerative lesions and haemorrhages on the skin and tail and fin rot, were similar to those of the farm-diseased fish. Total RBC count, Hb, PCV, MCV, glucose, Na+, albumin, respiratory burst and anti-protease activity were significantly (p < 0.05) lowered in the infected group as compared to the control group. Whereas, the total WBC count, total protein, globulin, ALP, SGOT, SGPT, and K+ levels were significantly (p < 0.05) higher in the infected group as compared to the control group. Histopathological examination of the infected fish's kidney and liver revealed significant changes, indicating bacterial septicaemia. The antibiotic resistance pattern showed A. pittii strain COF_AHE14 was a multi-drug resistance, showing resistance to cephalexin, cefoxitin, nitrofurantoin, ampicillin, oxacillin, penicillin-G, bacitracin, and trimethoprim. This is the first account of A. pittii as a cause of mass death in rohu (L. rohita), and the current study revealed that, in addition to causing human clinical infections, A. pittii poses a potential threat to farmed fish.