A culture from a leg wound for a 70-year-old man indicated lactose-negative colonies on MacConkey agar. What is the most probable organism based on the biochemical reactions?

When evaluating the given scenario where a leg wound culture yielded lactose-negative colonies on MacConkey agar, it’s essential to consider the characteristics of the organisms listed.

Lactose-negative colonies indicate that the bacteria do not ferment lactose, which is a critical factor in differentiating among the possible organisms. Proteus vulgaris and Proteus mirabilis, both members of the Enterobacteriaceae family, are indeed lactose-negative. However, they are not typically associated with a signal for identifiable biochemical reactions beyond lactose fermentation.

Serratia marcescens is also lactose-negative and is known for its distinctive red pigment and ability to produce a variety of biochemical reactions. It can grow in various stable temperatures and is notable for its involvement in wound infections, especially in older adults. The biochemical characteristics often highlight Serratia marcescens due to its unique ability to produce certain enzymes and proteins that assist in establishing infections, which could align with the presentation of a leg wound.

On the other hand, Enterobacter cloacae is generally lactose-positive and therefore would not fit the description of a lactose-negative organism. The distinctive features of Serratia marcescens, along with its potential to be associated with wound infections, define it as a plausible candidate in this context