A culture from a decubitus ulcer shows a gram-negative facultative bacillus with acid slant and acid butt on TSI; Indole positive; Citrate negative; Urease negative; ONPG positive; Voges-Proskauer negative. Which organism is most likely?

• A. Enterobacter cloacae
• B. Escherichia coli
• C. Citrobacter koseri
• D. Providencia stuartii

Answer: (B)

Interpreting a panel of biochemical tests helps distinguish lactose-fermenting Enterobacteriaceae and identify the organism most likely responsible for the wound infection.

The TSI result showing acid in both slant and butt means the organism ferments lactose (A/A), not just glucose. Indole positivity points toward organisms like Escherichia coli (and some Citrobacter), with E. coli being the classic indole producer. The citrate test being negative argues against organisms that commonly use citrate as a carbon source (which would often be citrate positive). Urease negativity excludes urease-producing species such as Proteus. A positive ONPG test confirms lactose permease activity and lactose utilization capability, which fits lactose fermenters such as E. coli. Voges-Proskauer negativity further supports E. coli, since many lactose-fermenters like Enterobacter and Klebsiella are VP positive.

Putting these results together, the profile aligns best with Escherichia coli: lactose fermentation with indole production, inability to utilize citrate, lack of urease activity, lactose permease presence (ONPG positive), and VP negative.