PubMed1976
The reactions of lysine, ornithine and arginine decomposition are often difficult to read in Falkow's medium because either the decolorization of the indicator or the lack of sharp colour differences between positive and negative reactions. In such cases Nessler's reagent may be a useful aid. A volume of about 0.2 ml is added to the cultures after 4 days incubation through the mineral oil layer by means of a pipette. A positive reaction is indicated by an immediate white precipitation in case of lysine and ornithine decarboxylation, and a white or brownish precipitate which indicates arginine decomposition. A delayed opacity should be regarded as a negative reaction. Only unequivocal reactions should be considered. The specificity of the reactions was tested with pure substances of compounds which are formed by the decomposition of lysine, ornithine and arginine. Further studies of bacterial cultures in Falkow's medium and in a synthetic, amino acid containing medium without peptone gave identical results and showed that peptone derivates do not cause a false positive reaction with Nessler's reagent (Table 1). Comparative studies on 605 strains of Enterobacteriaceae and Vibrio in Falkow's medium with and without added Nessler's reagent gave corresponding results except some strains of Escherichia coli and Citrobacter freundii with different arginine reactions (Table 2). Strains of these species mostly decolorized the indicator thereby hindering the recognition of either a true positive or a true negative reaction. In these cases, however, the results obtained after addition of Nessler's reagent corresponded closely to the percentage of positive reactions cited in the literature.