WBR0301
| Author | [[PageAuthor::Rim Halaby, M.D. [1], Alison Leibowitz [2] (Reviewed by Alison Leibowitz)]] |
|---|---|
| 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 spinal tap. 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) typically manifests with a 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 the GBS, which acts synergistically with staphylococcal beta-hemolysin.]] |
| 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, which is resistant to bacitracin, and has the ability to hydrolyze sodium hippurate. Clinically, it is the most common cause of neonatal meningitis and septicemia.
Specimens of isolated GBS 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 the presence of CAMP factor, produced by the 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 |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |
