[A false positive reaction to lactose in polycarbohydrate media]

In combined media for primary identification Salmonella typhi are detectable by the lactose test only within a certain range of proteolytic activities, which fact is explained by specific features of these media. Reduced proteolytic activity and thiosulfate reductase activity in S. typhi cultures resulted in false-positive lactose test and false-positive hydrogen sulfide production test, this leading to identification of these cultures as Escherichia in accordance with the universally acknowledged classification scheme. Taking this feature into consideration, the author has additionally isolated 20 typical S. typhi strains of the 22 cultures isolated in the laboratory.