Harr Microbiology Practice Test 2026 – 400 Free Practice Questions to Pass the Exam

The scenario describes an organism isolated from a cat scratch that forms smooth gray colonies, shows no hemolysis, and tests positive for both catalase and oxidase. The key here is the association with a cat scratch and the specific biochemical characteristics specified.

Pasteurella spp. is commonly associated with animal bites and scratches, particularly from cats and dogs. It typically presents as small, smooth, gray colonies on culture media, and its lack of hemolysis aligns with this organism's profile. Being both catalase and oxidase positive further supports the identification of Pasteurella, as these biochemical characteristics are consistent with this group of bacteria.

Other options present distinct profiles that do not match as accurately with the details provided. For example, Capnocytophaga spp. can also be associated with dog bites but has a different colony appearance and is often facultatively anaerobic. Proteus spp. typically generates a swarming growth pattern on media and is often urease positive, thus not fitting this scenario. Pseudomonas spp. can produce a range of colors and hemolytic activity on certain media, but it is also known for having a distinct colony morphology and would not typically match with the specified characteristics.

This specificity in both the clinical context