WBR0344

Author	[[PageAuthor::Rim Halaby, M.D. [1], {[AJL]]
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Explanation	[[Explanation:: Educational Objective: References: ]]
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(Reviewed by Alison Leibowitz)

|ExamType=USMLE Step 1 |MainCategory=Microbiology |SubCategory=General Principles |MainCategory=Microbiology |SubCategory=General Principles |MainCategory=Microbiology |SubCategory=General Principles |MainCategory=Microbiology |MainCategory=Microbiology |SubCategory=General Principles |MainCategory=Microbiology |SubCategory=General Principles |MainCategory=Microbiology |SubCategory=General Principles |MainCategory=Microbiology |SubCategory=General Principles |MainCategory=Microbiology |MainCategory=Microbiology |SubCategory=General Principles |Prompt=A student conducting an experiment to test optimal bacterial survival environments, plates a mixture of bacterial organisms on blood agar and allows them to grow for several days in an incubator at 37 °C with ample oxygen. He then places the plate in an airtight container with an alkaline solution containing pyrogallic acid, which absorbs any oxygen from the container, and allows the plate to stand for several days. Which of the following organisms is most likely to survive both steps? |Explanation=Facultatively anaerobic organisms are capable of surviving in anaerobic environments. They produce energy by aerobic respiration in the presence of oxygen, but are capable of using fermentation as a source of energy production in anaerobic situations. Examples of facultatively anaerobic organisms include Staphylococcus spp. and Escherichia coli. E. coli is a gram-negative, facultative anaerobic rod-shaped organism that can produce energy using a variety of substrates. In order to generate ATP in anaerobic conditions, E. coli uses mixed-acid fermentation, which produces ethanol, lactate, acetate, succinate, and carbon dioxide.

• Obligate aerobes that depend on oxygen for energy production include: Nocardia, Pseudomonas aeruginosa, Legionella spp., and Mycobacterium tuberculosis.

• Obligate anaerobes that cannot tolerate oxygen as a result of oxidative damage include: Actinomyces, Bacteroides, and Clostridium.

|EducationalObjectives= Facultatively anaerobic organisms such as E. coli can survive regardless of the presence of oxygen.

|References= Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Section 1, Bacteriology. Available from: http://www.ncbi.nlm.nih.gov/books/NBK7994/

|AnswerA=Legionella pneumophila |AnswerAExp=Legionella pneumophila is an obligate aerobe. |AnswerB=Pseudomonas aeruginosa |AnswerBExp=Pseudomonas aeruginosa is an obligate aerobe. |AnswerC=Bacteroides fragilis |AnswerCExp=Bacteroides fragilis is an obligate anaerobe. |AnswerD=Actinomyces israelii |AnswerDExp=Actinomyces israelii is an obligate anaerobe. |AnswerE=Escherichia coli |AnswerEExp=Escherichia coli is facultatively anaerobic. |RightAnswer=E |WBRKeyword=Faculatitive anaerobes, obligate anaerobes, obligate aerobes, Escherichia coli, |Approved=Yes }}