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Gram-negative folliculitis

Author: Dr Amanda Oakley Dermatologist, Hamilton, New Zealand, 1999. Reviewed and updated by Vanessa Ngan, Staff Writer; Copy Editor: Clare Morrison, April 2014.

Table of contents

What is Gram-negative folliculitis?

Gram-negative folliculitis is an acne-like disorder caused by a bacterial infection. Gram-negative bacteria include Escherichia coli, Pseudomonas aeruginosa, Serratia marcescens, Klebsiella and Proteus species.

The term "Gram negative" refers to the staining pattern of the organisms in the laboratory. Certain bacteria do not take up a stain known as "Gram".

Gram-negative folliculitis may result from long-term treatment of acne with tetracycline or topical antibiotics.

Other skin conditions due to Gram-negative bacteria include:

  • Spa pool folliculitis, which is due to Pseudomonas infection. This organism can withstand the elevated temperatures, increased alkalinity, and high organic content of a hot pool
  • Folliculitis due to Aeromonas hydrophila, which arises within an area of skin injury exposed to contaminated fresh water.

What does Gram-negative folliculitis look like?

Gram-negative folliculitis is a pustular rash resembling acne. It is often mistaken as a worsening of acne as it usually occurs in patients with existing acne.

In about 80% of patients, Gram-negative folliculitis causes superficial pustules with relatively few papules and comedones. These lesions are usually caused by Klebsiella, Escherichia and Serratia species.

In about 20% of patients with Gram-negative folliculitis, lesions are deep, nodular and cyst-like. These are caused by Proteus species, which are able to invade deeply into the skin and create pus-filled abscesses and cysts.

Gram-negative folliculitis lesions are usually found around the area of the upper lip under the nose, to the chin and cheeks.

Folliculitis due to Gram-negative bacteria

How is the diagnosis of Gram-negative folliculitis made?

Poor response of a pustular rash to usual anti-acne antibiotics may give rise to suspicion that an unusual organism may be present. The diagnosis of Gram-negative folliculitis is made by microscopy / cytology. The bacteria appear red or pink with Gram stain.

Bacterial culture identifies which species is responsible for the infection. The bacteria are tested for antibiotic sensitivity, as this differs among them.

What is the treatment for Gram-negative folliculitis?

Gram-negative folliculitis usually clears promptly with:

  • Antibiotics to which the organisms are sensitive (eg, ampicillin, trimethoprim)
  • Isotretinoin, which appears to act by suppressing sebum production and drying out the mucous membranes, especially the nasal mucous membranes, which is where causative bacteria are found

Spa-pool folliculitis usually clears within a few days without treatment.



  • Durdu M, Ilkit M. First step in the differential diagnosis of folliculitis: cytology. Crit Rev Microbiol. 2013;39(1):9–25. doi:10.3109/1040841X.2012.682051. PubMed

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