The Pseudomonas Infections of the Skin

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a) photo of acne b) photo of swollen ear.
(a) Hot tub folliculitis presents as an itchy red rash. It is typically caused by P. aeruginosa, a bacterium that thrives in wet, warm environments such as hot tubs. (b) Otitis externa (swimmer’s ear) may also be caused by P. aeruginosa or other bacteria commonly found in water. Inflammation of the outer ear and ear canal can lead to painful swelling. (credit b: modification of work by Klaus D. Peter)

OpenStax Microbiology

Another important skin pathogen is Pseudomonas aeruginosa, a gram-negative, oxidase-positive, aerobic bacillus that is commonly found in water and soil as well as on human skin. P. aeruginosa is a common cause of opportunistic infections of wounds and burns. It can also cause hot tub rash, a condition characterized by folliculitis that frequently afflicts users of pools and hot tubs. P. aeruginosa is also the cause of otitis externa (swimmer’s ear), an infection of the ear canal that causes itching, redness, and discomfort, and can progress to fever, pain, and swelling.

Wounds infected with P. aeruginosa have a distinctive odor resembling grape soda or fresh corn tortillas. This odor is caused by the 2-aminoacetophenone that is used by P. aeruginosa in quorum sensing and contributes to its pathogenicity. Wounds infected with certain strains of P. aeruginosa also produce a blue-green pus due to the pigments pyocyanin and pyoverdin, which also contribute to its virulence. Pyocyanin and pyoverdin are siderophores that help P. aeruginosa survive in low-iron environments by enhancing iron uptake. P. aeruginosa also produces several other virulence factors, including phospholipase C (a hemolysin capable of breaking down red blood cells), exoenzyme S (involved in adherence to epithelial cells), and exotoxin A (capable of causing tissue necrosis). Other virulence factors include a slime that allows the bacterium to avoid being phagocytized, fimbriae for adherence, and proteases that cause tissue damage. P. aeruginosa can be detected through the use of cetrimide agar, which is selective for Pseudomonas species.

Pseudomonas spp. tend to be resistant to most antibiotics. They often produce β-lactamases, may have mutations affecting porins (small cell wall channels) that affect antibiotic uptake, and may pump some antibiotics out of the cell, contributing to this resistance. Polymyxin B and gentamicin are effective, as are some fluoroquinolones. Otitis externa is typically treated with ear drops containing acetic acid, antibacterials, and/or steroids to reduce inflammation; ear drops may also include antifungals because fungi can sometimes cause or contribute to otitis externa. Wound infections caused by Pseudomonas spp. may be treated with topical antibiofilm agents that disrupt the formation of biofilms.

a) A red plate with white colonies. B) A clear plate with green colonies; the green extends past the colony. C) a dark plate with glowing colonies.
(a) These P. aeruginosa colonies are growing on xylose lysine sodium deoxycholate (XLD) agar. (b) Pseudomonas spp. can produce a variety of blue-green pigments. (c) Pseudomonas spp. may produce fluorescein, which fluoresces green under ultraviolet light under the right conditions. (credit a: modification of work by Centers for Disease Control and Prevention)


Parker, N., Schneegurt, M., Thi Tu, A.-H., Forster, B. M., & Lister, P. (n.d.). Microbiology. Houston, Texas: OpenStax. Access for free at: