A culture from a leg wound grew motile, lactose-negative colonies. What is the most probable organism?

• A. Proteus vulgaris
• B. Serratia marcescens
• C. Proteus mirabilis
• D. Enterobacter cloacae

Answer: (B)

In this scenario, the organism in question is identified based on the characteristics of being motile and lactose-negative. Among the listed options, Serratia marcescens is known to be a lactose-negative, motile organism.

Serratia marcescens is part of the Enterobacteriaceae family and is commonly associated with various infections, including those involving wounds. The ability to move (motility) and the negative reaction to lactose fermentation are key characteristics that guide the identification process.

In contrast, the other organisms listed may not fit the criteria as clearly. For instance, Proteus vulgaris and Proteus mirabilis are both motile and lactose-negative, yet they are more commonly associated with urinary tract infections rather than wound infections. Additionally, Enterobacter cloacae is typically lactose-positive and is often found in different clinical settings. Thus, considering the combination of motility and lactose-negative fermentation, Serratia marcescens stands out as the most probable organism responsible for the growth observed in the leg wound culture.