WBR0301
| Author | [[PageAuthor::Rim Halaby, M.D. [1]]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Microbiology |
| Sub Category | SubCategory::Infectious Disease |
| Prompt | [[Prompt::A lab technician is conducting a test to speciate a gram positive organism he obtained from an operating room wound swab. After isolating the organism, he inoculates it perpendicularly to streaks of a beta-hemolytic staphylococcus on sheep blood agar plates and incubates them at 37 C for 18 hours. While examining the plates, he notices the pattern seen below. Which of the following is the most likely isolated organism?
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| Answer A | AnswerA::Streptococcus agalactiae |
| Answer A Explanation | AnswerAExp::Streptococcus agalactiae or Group B streptococcus are associated with a positive CAMP test and produce a distinct "arrowhead" of hemolysis when plated with beta-hemolytic Staphylococcus aureus. |
| Answer B | AnswerB::Streptococcus viridans |
| Answer B Explanation | AnswerBExp::Streptococcus viridans is not associated with a positive CAMP test. |
| Answer C | AnswerC::Streptococcus pyogenes |
| Answer C Explanation | AnswerCExp::Streptococcus pyogenes is not associated with a positive CAMP test. |
| Answer D | AnswerD::Staphylococcus epidermidis |
| Answer D Explanation | AnswerDExp::Staphylococcus epidermidis is not associated with a positive CAMP test. |
| Answer E | AnswerE::Listeria monocytogenes |
| Answer E Explanation | AnswerEExp::Listeria monocytogenes is not associated with a positive CAMP test. |
| Right Answer | RightAnswer::A |
| Explanation | [[Explanation::Streptococcus agalactiae or Group B streptococcus (GBS) is a spherical-shaped gram-positive bacterium that is bacitracin resistant and has the ability to hydrolyze sodium hippurate. Clinically, it is the most common cause of neonatal sepsis. The CAMP test is an important test used in the identification of GBS species. Isolates of suspected GBS bacteria are usually inoculated perpendicularly to streaks of a beta-hemolytic staphylococcus on sheep blood agar plates and incubated at 37 C. The bacteria are allowed to grow overnight and examined for the pattern of beta hemolysis. GBS typically produce a distinct "arrowhead" of hemolysis at the point where the streak meets the staphyloccocal inoculum. This is indicative of a positive CAMP reaction, identifying GBS. This enhancement of the beta-hemolysis is due to the CAMP factor only GBS possess.
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| Approved | Approved::No |
| Keyword | WBRKeyword::Streptococcus agalactiae, WBRKeyword::Group B streptococcus, WBRKeyword::CAMP test, WBRKeyword::Beta-Hemolysis |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |
