A 26-year-old man comes to the office due to a 3-day-history of dysuria and urethral discharge. The symptoms developed about 2 weeks after he had unprotected sexual intercourse with a new partner. His temperature is 37.1 C (98.8 F). On physical examination, a mucoid discharge is expressed with gentle milking of the penis. Gram stain of the discharge reveals numerous neutrophils with intracellular diplococci. A sample of the discharge is placed on an antibiotic-containing medium, and bacterial colonies are cultured. Which of the following terms best describes the medium?

Question

A 26-year-old man comes to the office due to a 3-day-history of dysuria and urethral discharge.  The symptoms developed about 2 weeks after he had unprotected sexual intercourse with a new partner.  His temperature is 37.1 C (98.8 F).  On physical examination, a mucoid discharge is expressed with gentle milking of the penis.  Gram stain of the discharge reveals numerous neutrophils with intracellular diplococci.  A sample of the discharge is placed on an antibiotic-containing medium, and bacterial colonies are cultured.  Which of the following terms best describes the medium?

Quizplus · Accepted Answer

This patient with dysuria and urethral discharge has gonococcal urethritis as indicated by the Gram stain showing gram-negative diplococci within leukocytes. Neisseria gonorrhoeae can be cultured on Thayer-Martin VCN selective medium, which contains vancomycin, colistin, nystatin, and trimethoprim. These antibiotics kill potential contaminants such as gram-positive organisms (vancomycin), gram-negative organisms other than Neisseria (colistin and trimethoprim), and fungi (nystatin). (Choice A) Differential media help identify cultured organisms based on their metabolic and biochemical properties. Examples of differential media include the MacConkey and eosin methylene blue (EMB) agars used to culture enteric organisms. Organisms that ferment lactose will appear pink on MacConkey agar and black on EMB agar. (Choice B) Enrichment media contain special growth factors required for some organisms. Examples include the X and V factors required by Haemophilus or the anaerobic conditions needed by Clostridium species. (Choice C) Reducing media (eg, thioglycolate broth) remove oxygen and are used to culture anaerobic organisms. (Choice E) A synthetic medium is any chemically-defined medium for which all of the chemical contents are known. Educational objective: Neisseria can be isolated by culture on selective media such as the Thayer-Martin VCN (vancomycin/colistin/nystatin) medium, which inhibits growth of contaminants such as gram-positive organisms, gram-negative organisms other than Neisseria, and fungi.

A 26-Year-Old Man Comes to the Office Due to a 3-Day-History