What is the role of lab tests in the workup of gram-negative folliculitis?

Updated: Aug 07, 2019
  • Author: Mordechai M Tarlow, MD; Chief Editor: William D James, MD  more...
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Answer

The diagnosis of gram-negative folliculitis can often be made based on the history and the physical examination findings alone. However, confirmation with Gram stain and culture is recommended.

In confirming the diagnosis with Gram stain and culture, use special care in culturing. Gram-negative organisms are sensitive to desiccation; samples must be taken quickly and cultured as soon as possible. The pustule that is sampled should also be fresh. A small pustule on an erythematous base is preferable for culturing purposes.

Culture pustules in any patient with acne who is in their late teens or older and has been on long-term antibiotics and develops a pustular form of the disease.

Gram-negative organisms cannot be recovered from every pustule but will be found on cultures of the anterior nares.

Selective medium-containing dyes, such as methylene blue, allow selective growth of gram-negative organisms while inhibiting growth of gram-positive organisms.

The organisms that produce colonies on eosin-methylene blue agar are classified as either lactose-fermenting gram-negative rods or Proteus species by their cultural characteristics and their ability to ferment lactose. Lactose-fermenting, gram-negative rods produce small, dark, discreet, metallic colonies. Proteus species produce rapidly spreading, translucent, and odorous colonies.

In patients with facial folliculitis that presents a diagnostic challenge, a potassium hydroxide mount (10-20% potassium hydroxide is used to stain a sample on a slide and look for possible fungal elements) and a skin biopsy specimen may be of value.


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