Isolation of<i>Mycobacterium abscessus</i>from Japanese Medaka

Abstract

Japanese medaka Oryzias latipes were cultured at the U.S. Army Biomedical Research and Development Laboratory for use as aquatic toxicity test animals. During routine histological examination, granulomatous lesions were detected, and a low prevalence of Mycobacterium sp. was suspected in the culture system. Fish maintained under experimental and breeding conditions were cultured for acid-fast bacteria. Modified Middlebrook 7H 10 medium with albumin, dextrose, and catalase (ADC) enrichment was used to recover mycobacteria from whole-fish homogenates via three isolation procedures. Highest isolation rates were obtained by submerging whole fish in individual bags of modified broth at a 1:10 (weight : Volume) dilution for 1 h, then homogenizing the whole fish and plating diluted samples on solid medium using a plate count method. Medaka, ranging in age from 2 to 27 months, were positive for Mycobacterium abscessus with prevalences of 25%–100%. Mean bacterial counts for all groups examined ranged from 6.7 × 102 to 4.5 × 108 colony-forming units per gram of fish. Throughout the study, fish remained apparently healthy, exhibited no clinical signs of piscine mycobacteriosis, and suffered no significant mortality.