What is the likely organism from blood cultures revealing lactose-negative, motile gram-negative rods in a urinary tract infection case?

• A. Proteus vulgaris
• B. Proteus mirabilis
• C. Serratia marcescens
• D. Klebsiella pneumoniae

Answer: (A)

In the context of lactose-negative, motile gram-negative rods associated with a urinary tract infection, Proteus vulgaris emerges as a likely organism. Both Proteus vulgaris and Proteus mirabilis are recognized for their lactose-negative characteristic, but the distinctive biochemical and pathogenic profiles can help differentiate them.

Proteus species, including vulgaris, are known for their swarming motility on agar plates and for producing urease, which can lead to elevated urine pH, a common feature in urinary tract infections. Gram staining reveals the characteristic appearance of gram-negative rods. The virulence factors and behavior of Proteus vulgaris make it significant in urological infections, where it can cause complicated cases, typically in settings of catheter use or urinary tract abnormalities.

The other options, Serratia marcescens and Klebsiella pneumoniae, do not fit the description as well. Serratia is typically lactose-negative but is also often associated with a characteristic red pigment and primarily forms non-motile colonies, whereas Klebsiella is lactose-positive and tends to produce mucoid colonies due to its capsule. Therefore, the characteristics of motility and lactose fermentation effectively narrow it down to identifying Proteus vulgaris in this scenario.