Culture media for the differentiation of isolates of Yersinia ruckeri, based on detection of a virulence factor.

Strains of the bacterial fish pathogen Yersinia ruckeri were identified with the API 20E system and distinguished on the basis of whole cell agglutination with antisera, sorbitol fermentation and polymyxin B sensitivity. Strains which were shown to possess the virulence-associated heat-sensitive factor (HSF) were shown to grow preferentially on culture media containing sodium dodecyl sulphate (SDS) and to produce a creamy deposit around the colonies. By contrast, strains lacking this factor (HSF-) grew poorly and without forming a deposit. Enhancement of the differentiation between the two types was shown by the incorporation of Coomassie brilliant blue dye into agar containing 1% SDS, and the uptake of Coomassie blue and Congo red was shown to be temperature-dependent. Most strains tested were shown to belong to serotype I, and were sensitive to polymyxin and did not ferment sorbitol. With the medium developed most serotype I strains but not those of other serotypes were shown to possess HSF. It is suggested that the medium is used in epidemiological studies of Y. ruckeri.