WBR0846
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| Author | [[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian)]] |
|---|---|
| Exam Type | USMLE Step 1 |
| Main Category | Microbiology |
| Sub Category | General Principles |
| Prompt | [[Prompt::In an experiment, a researcher obtains swab samples from 50 asymptomatic volunteers to detect the possible bacterial species that colonize uninfected individuals. He isolates several species and records his findings. Despite a significant similarily between recovered species, the growth of the bacteria shown in the image below was noted in approximately 35% of his subjects. Which of the following is most likely the site of colonization of the cultured organism?
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| Answer A | Rectum |
| Answer A Explanation | Several organisms, such as Lactobacilli, can colonize the rectum. |
| Answer B | Oral cavity |
| Answer B Explanation | Anaerobes, such as Actinomyces israelii colonize the oral cavity. |
| Answer C | Vagina |
| Answer C Explanation | Several organisms, such as Lactobacilli, can colonize the vagina. |
| Answer D | Anterior nares |
| Answer D Explanation | Staphylococcus aureus colonizes the anterior nares. |
| Answer E | Nasopharynx |
| Answer E Explanation | Several bacteria can colonize the nasopharynx, such as Neisseria species, H. influenzae type B, M. catarrhalis, and S. pneumoniae. |
| Right Answer | D |
| Explanation | The image shown above is that of Staphylococcus aureus. S. aureus is a gram-positive (violet color on gram-stain) coccus (circular shape) organized in clusters that resemble grapes. In fact, the Greek word "staphyle" means "grape". Staphylococcus aureus classically colonizes the skin transiently and anterior nares more persistently. In approximately 30% of individuals, S. aureus can be found in the anterior nares with absolutely no symptoms. Other sites of S. aureus colonization are also possible, such as in the throat. Nasal carriage of S. aureus has become a way of spread of multiresistant staphylococci, such as methicillin-resistant S. aureus (MRSA). S. aureus is rather virulent due to several factors including its polysaccharide capsule, surface adhesins, exotoxins (hemolysins, superantigens, & Panton-Valentine toxin), as well as the ability to form biofilms most often detected in S. aureus species that are able to persist in the anterior nares and colonize mucosal surfaces. Educational Objective: Staphylococcus aureus are gram-positive cocci that are arranged in clusters. The classical site of primary colonization is the anterior nares. |
| Approved | Yes |
| Keyword | Staphylococcus, Staphylococcal, Anterior nares, Colonization, Culture, Swab, Gram-positive, Cocci, Coccus |
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