Which organism is associated with a painful genital ulcer and a Gram stain described as small Gram-negative coccobacilli in a school-of-fish arrangement?

Prepare for the CAMLPR Microbiology Test. Utilize flashcards and engaging multiple-choice questions, each equipped with descriptions and explanations. Ready yourself for an excellent performance!

Painful genital ulcers with a distinctive smear finding point to chancroid, caused by Haemophilus ducreyi. On Gram stain of lesion exudate, this organism appears as small Gram-negative coccobacilli arranged in a characteristic “school-of-fish” pattern, a clue that helps differentiate it from other bacteria. Haemophilus influenzae can also be a Gram-negative coccobacillus, but it’s primarily a respiratory pathogen and not associated with painful genital ulcers. Escherichia coli is a gut-associated Gram-negative rod and does not cause chancroid. Treponema pallidum causes painless ulcers in syphilis and is not visualized well on routine Gram stain. Therefore, the described organism is Haemophilus ducreyi.