WBR0344
| Author | [[PageAuthor::Rim Halaby, M.D. [1]]] |
|---|---|
| Exam Type | ExamType::USMLE Step 1 |
| Main Category | MainCategory::Microbiology |
| Sub Category | SubCategory::General Principles |
| Prompt | [[Prompt::A student is conducting an experiment to test optimal bacterial survival environments. In one set-up, he plates a mixture of bacterial organisms on blood agar and allows them to grow for several days in an incubator at 37C with ample oxygen. He then places the plate in an airtight container with an alkaline solution containing pyrogallic acid that absorbs any oxygen from the container and allows the plate to stand for several days. Which of the following organisms is likely to survive both steps?]] |
| Answer A | AnswerA::Bacillus cereus |
| Answer A Explanation | AnswerAExp::Bacillus spp. are obligate aerobes. |
| Answer B | AnswerB::Nocardia spp. |
| Answer B Explanation | AnswerBExp::Nocardia spp. are obligate aerobes. |
| Answer C | AnswerC::Bacteroides fragilis |
| Answer C Explanation | AnswerCExp::Bacteroides fragilis is an obligate anaerobe. |
| Answer D | AnswerD::Actinomyces spp. |
| Answer D Explanation | AnswerDExp::Actinomyces spp. are obligate anaerobes. |
| Answer E | AnswerE::Escherichia coli |
| Answer E Explanation | AnswerEExp::Escherichia coli is the only organism among the presented to be facultatively anaerobic and can survive in conditions that lack oxygen. |
| Right Answer | RightAnswer::E |
| Explanation | [[Explanation::Facultatively anaerobic organisms are capable of surviving regardless of the presence of oxygen. The produce energy by aerobic respiration in the presence of oxygen, but are capable of using fermentation as a source of energy production when oxygen is not available. Examples include Staphylococcus spp. and E. coli. Escherichia coli is a gram-negative, facultative anaerobic rod-shaped organism that can produce energy using a variety of substrates. In anaerobic conditions, it uses mixed-acid fermentation producing ethanol, lactate, acetate, succinate, and carbon dioxide.
Obligate aerobes that depend on oxygen for energy production include: Nocardia, Pseudomonas aeruginosa, Mycobacterium tuberculosis, and Bacillus. Obligate anaerobes that cannot tolerate oxygen due to oxidative damage include: Actinomyces, Bacteroides, Clostridium.
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| Approved | Approved::No |
| Keyword | WBRKeyword::Faculatitive anaerobes, WBRKeyword::obligate anaerobes, WBRKeyword::obligate aerobes, WBRKeyword::Escherichia coli |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |