Enumeration of viable vole bacilli by direct Agar phase-contrast microscopsy

Abstract

A problem encountered in estimating viability of the vole bacillus, J1!(ycobacterium tuberculosis val'. muris, by surface colony counts has been the inevitable delay with which final counts are obtained (Leach and Wells, 1956). At least four weeks' incubation is required even with strains adapted to optimal growth on oleic acid-albumin-agar medium, and furthermore this long incubation period is conducive to contamination, particularly by moulds which normally develop too slowly at 37°C. to affect cultures offaster-growing organisms. In order to overcome these difficulties an alternative method for estimating viability of vole bacilli was sought. Alternatives to the colony count method have been described for others of the mycobacteria, but these have relied mainly on such chemical procedures as the reduction of methylene blue (Cardoso and Almeida, 195I) or of tetrazolium dyes (Vandiviviere and others, 1952; Hanks, 1951). Such methods fail to give a count in terms of numbers or proportion of viable organisms and some method was required in which the proportion of bacteria capable of multiplying could be measured.