Which organism best matches pink mucoid colonies on MacConkey agar with TSI A/A with gas and spot indole positivity?

Which organism best matches pink mucoid colonies on MacConkey agar with TSI A/A with gas and spot indole positivity?

• A. Klebsiella pneumoniae
• B. Escherichia coli
• C. Klebsiella oxytoca
• D. Serratia marcescens

Answer: (C)

MacConkey lactose fermentation with a capsule-rich, mucoid appearance points toward a Klebsiella species. The TSI result of acid slant/acid butt with gas confirms carbohydrate fermentation with gas production, which is typical for many lactose-fermenters like Klebsiella. The indole test then separates K. oxytoca from K. pneumoniae, since K. oxytoca is indole positive and K. pneumoniae is indole negative. Therefore, the combination of pink mucoid colonies on MacConkey, A/A with gas on TSI, and spot indole positivity best matches Klebsiella oxytoca. The other organisms either lack indole positivity (K. pneumoniae), are not strongly mucoid lactose fermenters (Serratia marcescens or E. coli in typical isolates), or don’t fit the pigment and colony morphology profile.

MacConkey lactose fermentation with a capsule-rich, mucoid appearance points toward a Klebsiella species. The TSI result of acid slant/acid butt with gas confirms carbohydrate fermentation with gas production, which is typical for many lactose-fermenters like Klebsiella. The indole test then separates K. oxytoca from K. pneumoniae, since K. oxytoca is indole positive and K. pneumoniae is indole negative. Therefore, the combination of pink mucoid colonies on MacConkey, A/A with gas on TSI, and spot indole positivity best matches Klebsiella oxytoca. The other organisms either lack indole positivity (K. pneumoniae), are not strongly mucoid lactose fermenters (Serratia marcescens or E. coli in typical isolates), or don’t fit the pigment and colony morphology profile.