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A 14-year-old patient is brought in by his parents because of a sore throat. On physical
examination, he is febrile, and has pharyngeal erythema with a tonsillar abscess. A throat culture
on sheep blood agar yields colonies of gram-positive cocci that are surrounded by a zone of
complete hemolysis. The organism was also plated on mannitol salt agar; it grew well and caused
the medium to turn yellow. Which of the following microorganisms is the most likely cause of
the patient's illness?
A. Corynebacterium diphtheriae
B. Haemophilus influenzae
C. Staphylococcus aureus
D. Streptococcus salivarius
E. Streptococcus pyogenes
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cccc staph aureus yellow colonies on BA
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The correct answer is C. The description of the agent is consistent with a staphylococcal
organism (catalase-positive, gram-positive cocci that grows on mannitol salt agar. The organism
is most likely S. aureus, as it was able to ferment mannitol (as evidenced by the color change in
the mannitol salt agar after incubation) and was beta-hemolytic. Other characteristics of this
organism are coagulase production and excretion of DNAse from colonies. Staphylococcal
organisms are also notorious for formation of abscesses; the patient had an abscess in the
tonsillar region.
The diphtheria bacilli (Corynebacterium diphtheriae; choice A) are gram-positive, pleomorphic
rods that are arranged in palisades. They are non-hemolytic and would not grow on mannitol salt
agar. These organisms produce a whitish-gray pseudomembrane on the pharynx or the larynx;
constitutional symptoms from toxemia are the major features of diphtheria.
Epiglottitis is the most common disease of the upper respiratory tract produced by Haemophilus
influenzae(choice B), a gram-negative, encapsulated rod. It is also a common cause of otitis
media in children and may cause bronchitis, bronchiolitis, and pneumonia in adults.
Streptococcus salivarius(choice D) is a gram-positive coccus that is usually alpha-hemolytic. It
is normal flora of the oral cavity and is sometimes implicated in subacute bacterial endocarditis,
but is not associated with tonsillar abscesses.
Streptococcus pyogenes(choice E) is a beta-hemolytic, gram-positive coccus that grows in
chains, as opposed to the random, grape-like clusters of the staphylococci. These organisms are
the most common cause of pharyngitis, which is usually manifested by severe sore throat, fever,
a beefy red pharynx, and a tonsillar exudate.
