A 22-year-old woman participates in a research study. She is asymptomatic and has not had a respiratory illness in the last 8 weeks. She reports no chronic medical problems and takes no medications. A nasopharyngeal specimen is obtained and cultured on a selective medium containing vancomycin, colistin, nystatin, and trimethoprim. The organism most likely to be isolated is typically associated with which of the following conditions?

Question

A 22-year-old woman participates in a research study.  She is asymptomatic and has not had a respiratory illness in the last 8 weeks.  She reports no chronic medical problems and takes no medications.  A nasopharyngeal specimen is obtained and cultured on a selective medium containing vancomycin, colistin, nystatin, and trimethoprim.  The organism most likely to be isolated is typically associated with which of the following conditions?

Quizplus · Accepted Answer

The nasopharynx is colonized with a wide range of gram-positive and gram-negative bacteria. Therefore, nasopharyngeal swabs are often cultured on media containing antibiotics to isolate a specific organism. The agar described in this case is the Thayer-Martin medium, used to identify pathogenic Neisseria organisms such as N meningitidis (a common cause of meningitis) and N gonorrhoeae (a common cause of urethritis). The Thayer-Martin medium is a chocolate sheep blood agar infused with vancomycin to inhibit gram-positive bacteria; with colistin and trimethoprim to inhibit gram-negative bacteria (other than pathogenic Neisseria species); and with nystatin to inhibit yeast. (Choice A) The most common cause of cystitis is Escherichia coli, a gram-negative rod inhibited by colistin and trimethoprim. E coli is not a common nasopharyngeal organism. (Choice B) Dental carries are caused primarily by Streptococcus (eg, S mutans) and Lactobacillus species, gram-positive organisms inhibited by vancomycin. (Choice C) Corynebacterium diphtheriae is the cause of diphtheria. It usually presents with slow-onset sore throat, malaise, and low-grade fever. C diphtheriae is a gram-positive organism inhibited by vancomycin. (Choices E and F) The most common cause of both otitis media and community-acquired pneumonia in adults is Streptococcus pneumoniae. Although S pneumoniae can colonize the nasopharynx, this gram-positive organism is inhibited by vancomycin. (Choice G) Rheumatic heart disease is caused by Streptococcus pyogenes (group A streptococci), a common gram-positive pharyngeal pathogen. S pyogenes is inhibited by vancomycin. Educational objective: The Thayer-Martin medium is used to isolate pathogenic Neisseria species such as N meningitidis and N gonorrhoeae. It is a chocolate sheep blood agar that contains vancomycin to inhibit the growth of gram-positive organisms; colistin and trimethoprim to inhibit gram-negative bacteria (other than pathogenic Neisseria); and nystatin to inhibit yeasts. __________ References: Cellular and molecular biology of Neisseria meningitidis colonization and invasive disease. (http://www.ncbi.nlm.nih.gov/pubmed/20132098)

A 22-Year-Old Woman Participates in a Research Study