A Comparative Study of the Causative Agent of a Mycobacterial Disease of Salmonoid Fishes1

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Abstract
Modifications of the standard Ziehl-Neelsen and Fite-Faraco methods have been attempted. Under the present experimental conditions, the Ziehl-Neelsen method has proved to be unsatisfactory for staining acid-fast organisms in salmonoid fish tissue sections. The Fite-Faraco method of deparaffinizing sections with a mixture of light oil and xylene was modified by the use of extra heavy mineral oil, decolorizing with 3% sulfuric acid, and counterstaining with 0.1% methylene blue. The modified Fite-Faraco technique is the method of choice for staining these organisms. A comparison of the staining characteristics of human tubercle bacilli and the acid-fast organism observed in fish tissue indicates that differences between the 2 are sufficient to preclude any attempt to equate them. A difference in the staining characteristics of the acid-fast bacillus in adult and juvenile Chinook salmon (Oncorhynchus tshawytscha) has been noted and an explanation was postulated on the basis of differences in oil content of the 2 groups. Attention has been brought to the observation that standard methods of staining may not be the best for detection of atypical organisms in tissue sections.