39-year-old hunter with fever, chills, headache, painful ulcer, and swollen lymph nodes. An ulcer swab is cultured. Gram stain shows tiny Gram-negative coccobacilli; CHOC supplemented with cysteine yields small smooth colonies; BAP non-supplemented and MAC are not growth; Oxidase positive; Catalase positive with small bubbling. Which organism is suspected?

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!

Ulceroglandular tularemia from handling wild game is the hallmark scenario here. The organism that fits this picture is a tiny Gram-negative coccobacillus that is extremely fastidious about its growth requirements. It will form small, smooth colonies only on enriched media that provide a source of cysteine—such as cysteine-supplemented chocolate or heart-infusion type media. On standard non-supplemented blood agar or MacConkey, there’s little to no growth. The catalase test is positive with bubbling, which is a common finding for this organism, reinforcing the enzymatic profile you’d expect. The clinical setting—a hunter with a painful entry-site ulcer and regional lymphadenopathy—along with the distinctive culture requirement for cysteine-enriched media makes Francisella tularensis the most likely cause. Other organisms may share some features, but they don’t pair the same combination of ulceroglandular presentation and strict growth on cysteine-enriched media.