What is the most likely organism in a cancer patient’s blood cultures showing lactose-negative, motile, gram-negative rods before antibiotic therapy?

In the context of a cancer patient’s blood cultures showing lactose-negative, motile, gram-negative rods, the identification of Proteus vulgaris as the likely organism is appropriate due to its distinctive characteristics.

Proteus vulgaris is known for being motile, which is often evidenced by a swarming growth pattern on culture media. This feature, along with its gram-negative morphology and lactose-negative status on MacConkey agar, aligns with the described culture results. It is frequently implicated in urinary tract infections and can also be associated with bacteremia, especially in immunocompromised patients, such as those undergoing cancer treatment.

While Proteus mirabilis shares many characteristics with Proteus vulgaris, including being motile and lactose-negative, its clinical significance and typical presentations differ. Additionally, Serratia marcescens is lactose-negative but is primarily associated with opportunistic infections and produces a red pigment, which does not apply here. Klebsiella pneumoniae, on the other hand, is known to be lactose-positive and typically does not fit the criteria of being lactose-negative.

Therefore, the characteristics of motility, gram-negative rods, and lactose-negative growth strongly suggest that Proteus vulgaris is the most likely organism in this clinical context.