Micrococcus luteus, an emerging opportunistic pathogen in farmed Nile tilapia, Oreochromis niloticus in Andhra Pradesh, India

Impact of opportunistic bacterial pathogens and their antimicrobial resistance continues to increase in aquaculture, posing significant public health concerns. The present study aimed to investigate the incidence of mortality in cultured Nile tilapia, O. niloticus in Andhra Pradesh, India. Diseased samples exhibiting exophthalmia, swollen abdomen, hemorrhages and pale gills were collected to isolate pathogenic bacteria. The bacteria were characterized using cultural, biochemical characteristics and 16 S rRNA gene sequence. The isolate had 99.66% homology with Micrococcus luteus (GenBank accession no. PP659810). The cumulative mortality LD 50 was calculated as 1.39 × 10 5 CFU per fish. Histopathological alterations revealed hyperplasia and fusion of gill lamellae, enlarged hepatocytes, degenerative renal tubules, fibrous lesions, ellipsoidal compression of white pulp, degeneration of splenic tissue, and melano-macrophage centres. Antibiogram studies revealed that M. luteus showed varying degrees of resistance to different antibiotics, with a multiple antibiotic resistance index of 0.62 ± 0.3. Mortality rate in cultured farm and experimental infection is 30% and 70%, respectively. These findings highlight that M. luteus can be considered as one of the potential bacterial pathogens in Nile tilapia. Our findings highlight that Good Aquaculture Practices (GAP), biosecurity, disease surveillance, and the One Health Approach are essential for tackling AMR- and disease-related issues. This report forms the first record of emergence of M. luteus infection in cultured Nile tilapia, O. niloticus in India.