WBR0301
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| Author | [[PageAuthor::Serge Korjian M.D. (Reviewed by Alison Leibowitz)(Reviewed by Serge Korjian)]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Microbiology |
| Sub Category | SubCategory::Infectious Disease |
| Prompt | [[Prompt::A lab technician conducts a test to speciate a gram positive organism, which he obtained from a cerebrospinal fluid sample. He first isolates the organism, 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 displayed below. Which of the following organisms is most likely isolated?
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| Answer A | AnswerA::''Streptococcus agalactiae'' |
| Answer A Explanation | [[AnswerAExp::Streptococcus agalactiae or Group B Streptococcus (GBS) isolates typically have a positive CAMP test. The GBS inoclua produce distinct "arrowhead" shaped zones of enhanced hemolysis where the streak meets the staphyloccocal inoculum.]] |
| 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. It produces a beta-hemolytic toxin that may enlarge the area of hemolysis produced by the staphylococcal inoculum, but the characteristic "arrowhead" is not seen.]] |
| Answer D | AnswerD::''Staphylococcus epidermidis'' |
| Answer D Explanation | [[AnswerDExp::Staphylococcus epidermidis is not associated with a positive CAMP test.]] |
| Answer E | AnswerE::''Clostridium perfringens'' |
| Answer E Explanation | [[AnswerEExp::Clostridium perfringens is classically associated with a positive reverse CAMP test. The test uses the same principle to identify Clostridium perfringens species by streaking GBS species perpendicularly to suspected C. perfringens isolates on blood agar. A positive test is also characterized by "arrowhead" shaped hemolytic zones at the meeting points of the inocula. Beta-hemolytic Staphylococcus are not used in the identification of Clostridium perfringens.]] |
| Right Answer | RightAnswer::A |
| Explanation | [[Explanation::Streptococcus agalactiae or Group B Streptococcus (GBS) is a spherical-shaped, gram-positive bacterium, which is resistant to bacitracin, suspected isolates are inoculated perpendicularly to streaks of a beta-hemolytic staphylococcus on sheep blood agar plates and then incubated at 37 °C. Following overnight growth, the plate is examined for the hemolysis pattern. GBS typically manifests with a distinct "arrowhead" shaped zones of enhanced hemolysis where the streak meets the staphyloccocal inoculum. The hemolysis pattern is indicative of a positive CAMP test. The characteristic pattern of hemolysis occurs in the presence of CAMP factor, a weakly hemolytic protein produced by GBS, which acts synergistically with staphylococcal beta-hemolysin. The CAMP test can also be used to identify Listeria species particularly Listeria monocytogenes. Educational Objective: The CAMP test is used to identify Streptococcus agalactiae or Group B Streptococcus (GBS) isolates. The GBS inoclua produce distinct "arrowhead" shaped zones of enhanced hemolysis where the streak meets the staphyloccocal inoculum. The hemolysis pattern is indicative of the presence of CAMP factor, produced by GBS, which acts synergistically with staphylococcal beta-hemolysin. |
| Approved | Approved::Yes |
| Keyword | WBRKeyword::Streptococcus agalactiae, WBRKeyword::Group B streptococcus, WBRKeyword::CAMP test, WBRKeyword::Beta-hemolysis, WBRKeyword::Identification tests, WBRKeyword::GBS, WBRKeyword::toxins |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |
