WBR0846
| Author | [[PageAuthor::Rim Halaby, M.D. [1]]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Microbiology |
| Sub Category | SubCategory::General Principles |
| Prompt | [[Prompt::In an experiment, a researcher grows bacteria in cultures that are colonized in uninfected asymptomatic 50 volunteers. Using appropriate technique, he notes the growth of the bacteria shown in the image below in approximately 30% of his subjects. Which of the following is most likely the site of colonization of the cultured organism?
|
| Answer A | AnswerA::Rectum |
| Answer A Explanation | AnswerAExp::Several organisms, such as lactobacilli, can colonize the rectum |
| Answer B | AnswerB::Oral cavity |
| Answer B Explanation | AnswerBExp::Anaerobes, such as Actinomyces israelii colonize the oral cavity. |
| Answer C | AnswerC::Vagina |
| Answer C Explanation | AnswerCExp::Several organisms, such as lactobacilli, can colonize the vagina. |
| Answer D | AnswerD::Anterior nares |
| Answer D Explanation | AnswerDExp::Staphylococcus aureus colonizes the anterior nares. |
| Answer E | AnswerE::Nasopharynx |
| Answer E Explanation | AnswerEExp::Several bacteria can colonize the nasopharynx, such as Neisseria species, H. influenzae type B, M. catarrhalis, and S. pneumoniae |
| Right Answer | RightAnswer::D |
| Explanation | [[Explanation::The image shown above is that of Staphylococcus aureus. S. aureus appears as gram-positive (violet color on gram-stain) cocci (circular shape) that are organized in clusters that resemble grapes. In fact, the Greek word "staphyle" means "grape". Staphylococcus aureus classically colonizes the anterior nares. Approximately 30-50% of individuals carry S. aureus in the anterior nares and are completely healthy. Other sites of S. aureus are also possible, such as in the throat.
Educational Objective: Staphylococcus aureus are gram-positive cocci that are arranged in clusters. The classical site of primary colonization is the anterior nares. Reference:
Hu L, Umeda A, Kondo S, et al. Typing of Staphylococcus aureus colonising human nasal carriers by pulsed-field gel electrophoresis. J Med Microbiol. 1995; 42(2):127-32. |
| Approved | Approved::No |
| Keyword | WBRKeyword::staphylococcus, WBRKeyword::staphylococcal, WBRKeyword::anterior, WBRKeyword::nares, WBRKeyword::colonization, WBRKeyword::primary, WBRKeyword::site, WBRKeyword::culture, WBRKeyword::swab, WBRKeyword::grape, WBRKeyword::cluster, WBRKeyword::gram, WBRKeyword::positive, WBRKeyword::violet, WBRKeyword::stain, WBRKeyword::cocci, WBRKeyword::coccus |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |
