Mycobacterium tuberculosis-the correct answer is d.mycobacterium tuberculosis is an important obligate aerobe to remember for usmle
step 1. (other obligate aerobes include all other mycobacteria, bordetella pertussis, and francisella
tularensis.). this organism can obtain energy only from respiration and must use oxygen as the terminal
electron acceptor. primary infection occurs in the upper lobes of the lung.
bacteroides fragilis(choice a) is an obligate anaerobic gram-negative bacillus that constitutes the primary
organism found in the colon. it is the most common cause of anaerobic infections, including abdominal
abscesses and peritonitis.
clostridium perfringens(choice b), along with all the other clostridium species, is an obligate anaerobic,
spore-forming, gram-positive bacillus. c. perfringens is an important cause of infection following trauma and
surgery and can cause gas gangrene (which can be life-threatening if not treated). c. perfringens can also
cause food poisoning.
e. coli(choice c) and p. aeruginosa(choice e) are both facultative organisms, meaning that they can grow both
in the presence or absence of oxygen. e. coli can ferment under anaerobic conditions or respire using oxygen
as the terminal electron acceptor. p. aeruginosa cannot ferment. it respires only, using nitrate as the terminal
acceptor under anaerobic conditions and oxygen as the terminal electron acceptor under aerobic conditions
