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 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, Mycobacterium tuberculosis, and Bacillus spp. Obligate anaerobes that cannot tolerate oxygen due to oxidative damage include: Actinomyces, Bacteroides, Clostridium. Educational Objective: Facultatively anaerobic organisms such as E. coli can survive regardless of the presence of oxygen. Reference: 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/ Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::Faculatitive anaerobes, WBRKeyword::obligate anaerobes, WBRKeyword::obligate aerobes, WBRKeyword::Escherichia coli
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