WBR0344

Author	[[PageAuthor::Serge Korjian M.D. (Reviewed by Alison Leibowitz) (Reviewed by Serge Korjian)]]
Exam Type	USMLE Step 1
Main Category	Microbiology
Sub Category	General Principles
Prompt	A student is conducting an experiment to test optimal bacterial survival environments. She begins by plating a mixture of bacterial organisms on blood agar and allowing them to grow for several days in an incubator at 37 °C with ample oxygen. She 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?
Answer A	Legionella pneumophila
Answer A Explanation	Legionella pneumophila is an obligate aerobe.
Answer B	Pseudomonas aeruginosa
Answer B Explanation	Pseudomonas aeruginosa is an obligate aerobe.
Answer C	Bacteroides fragilis
Answer C Explanation	Bacteroides fragilis is an obligate anaerobe.
Answer D	Actinomyces israelii
Answer D Explanation	Actinomyces israelii is an obligate anaerobe.
Answer E	Escherichia coli
Answer E Explanation	Escherichia coli is facultatively anaerobic.
Right Answer	E
Explanation	Facultatively anaerobic organisms are capable of surviving independent of the presence or absence of oxygen. 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 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, Legionella spp., and Mycobacterium tuberculosis. Obligate anaerobes that cannot tolerate oxygen as a result of oxidative damage include: Actinomyces, Bacteroides, and Clostridium. Educational Objective: Facultatively anaerobic organisms such as Escherichia coli and 'Staphylococcus spp. can survive independent of the presence or absence 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/
Approved	Yes
Keyword	Faculatitive anaerobes, Obligate anaerobes, Obligate aerobes, Escherichia coli
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