WBR0344

Author	[[PageAuthor::Rim Halaby, M.D. [1], Alison Leibowitz [2] (Reviewed by Alison Leibowitz)]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Microbiology
Sub Category	SubCategory::General Principles
Prompt	[[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?]]
Answer A	AnswerA::''Legionella pneumophila''
Answer A Explanation	AnswerAExp::''Legionella pneumophila'' is an obligate aerobe.
Answer B	AnswerB::''Pseudomonas aeruginosa''
Answer B Explanation	AnswerBExp::''Pseudomonas aeruginosa'' is an obligate aerobe.
Answer C	AnswerC::''Bacteroides fragilis''
Answer C Explanation	AnswerCExp::''Bacteroides fragilis'' is an obligate anaerobe.
Answer D	AnswerD::''Actinomyces israelii''
Answer D Explanation	AnswerDExp::''Actinomyces israelii'' is an obligate anaerobe.
Answer E	AnswerE::''Escherichia coli''
Answer E Explanation	AnswerEExp::''Escherichia coli'' is facultatively anaerobic.
Right Answer	RightAnswer::E
Explanation	[[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. Educational Objective: 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/]]
Approved	Approved::Yes
Keyword	WBRKeyword::Faculatitive anaerobes, WBRKeyword::obligate anaerobes, WBRKeyword::obligate aerobes, WBRKeyword::Escherichia coli
Linked Question	Linked::
Order in Linked Questions	LinkedOrder::