All reactions described are those most typically encountered. Each laboratory must learn through experience how to interpret atypical results. Bismuth Sulfite Agar Plates may be read after 24h at 35 C. Incubate negative plates for a further 24h before discarding. Salmonella - colonies appear brown, grey, to black, sometimes with metallic sheen. Medium surrounding colony usually brown, darkening to black on further incubation.
Some strains produce green colonies with little or no darkening of surrounding medium.
Proteus spp - some strains produce red colonies. Pseudomonas - green or brownish colonies, flat, irregular. Levine's EMB Agar Incubate plates 24h at 35 C. E . Klebsiella and E . Salmonella - colourless colonies, no precipitated zone.