Beard folliculitis by Klebsiella aerogenes in a young man: An emergent sexually transmitted infection?

Background

Beard folliculitis is a frequent dermatologic condition, yet chronic, treatment-refractory, or atypical cases may require evaluation for less common or emerging pathogens. Klebsiella aerogenes, a Gram-negative bacillus primarily linked to healthcare-associated infections, ¹ has recently been implicated in cases of beard folliculitis among men who have sex with men (MSM).^2,3 Environmental exposures—particularly communal hot tubs—and oro-anal sexual practices may play a role in transmission. ² Recognizing K. aerogenes–associated folliculitis as a potential sexually transmitted infection (STI) is essential for accurate diagnosis, effective treatment and prevention.

Presentation

A 23-year-old MSM presented with a three-year history of treatment-resistant beard folliculitis. The patient denied systemic symptoms, prior STIs, or use of HIV pre-exposure (PrEP) or post-exposure prophylaxis (PEP), as well as animal contact. He shaved exclusively with a personal electric razor and reported occasional use of communal hot tubs. Previous treatments with oral doxycycline, topical clindamycin, and intranasal mupirocin yielded no significant improvement. Partial clinical response was noted following the initiation of oral isotretinoin. Examination revealed follicular erythema on the upper lip, the only area of the beard not shaved, accompanied by a solitary millimetric pustule at the right lateral margin and an inflammatory papule on the right lateral aspect of the chin (Figure 1(a)).OPEN IN VIEWER

Investigations

A pustular swab was obtained for microscopy and culture. No fungal elements were seen. Culture identified wild-type K. aerogenes. Routine blood tests, including metabolic panel, liver function, and lipid profile, were within normal limits except for a mildly elevated total cholesterol (204 mg/dL). Complete blood count and differential were unremarkable. Immunological evaluation showed normal levels and distributions of T (CD4+, CD8+), B, and NK cells, with appropriate representation of B-cell subpopulations. Serum immunoglobulins (IgG, IgA, IgM) were also within reference ranges.

Differential diagnosis

The differential included tinea barbae due to Trichophyton mentagrophytes ITS genotype VII (TMVII), a dermatophyte that has been increasingly reported in MSM and sex workers with facial, genital or buttock lesions. TMVII infections can be very inflammatory and purulent, often mimicking bacterial folliculitis and causing delays in diagnosis and treatment.^4–7 Notably, bacterial superinfection of TMVII with K. aerogenes has also been reported, ⁷ underscoring the importance of thorough microbiological and mycological assessment. In this case, negative microscopy and complete response to antibiotic therapy favored a bacterial etiology.

Treatment

At presentation, the patient had been receiving isotretinoin 20 mg daily for approximately 1 year, with only partial clinical improvement. This regimen was maintained, and antibiotic treatment was initiated with ciprofloxacin 500 mg twice daily and topical gentamicin sulfate 0.3% in an oil-in-water emulsion applied twice daily to the beard, mustache, and nasal vestibules. Antibiotic selection was guided by antimicrobial susceptibility testing and supported by previous case reports. ² Communal hot tub use was also discouraged. After 21 days of combined therapy, complete resolution was achieved, with minimal residual erythema on the upper lip, which the patient attributed to local irritation from topical gentamicin (Figure 1(b)). At this point, antibiotics were discontinued and isotretinoin was tapered to 20 mg every 48 hours for an additional 6 weeks before cessation. No clinical relapse was observed after 4 months of follow-up.

Discussion

K. aerogenes, formerly known as Enterobacter aerogenes, is an opportunistic Gram-negative enteric bacterium widely present in environmental and healthcare settings, primarily linked to respiratory, urinary, and bloodstream infections. ¹ Its role in cutaneous infections has recently gained attention, particularly among MSM.

A recent French case series ² described seven MSM patients with beard folliculitis associated with K. aerogenes. Most achieved remission with extended antibiotic regimens, based on quinolones alone or combined with cotrimoxazole for two to 6 weeks, while others relapsed after treatment cessation. Notably, hot tub exposure was a recurring factor, while STI history was uncommon, as in our patient. A separate Belgian case series ³ reported similar findings on four MSM patients. They were treated with cotrimoxazole for one to 2 weeks and all relapsed within 10 to 30 days. Only one of them reported hot tub exposure and none were living with HIV, with no mention to other STIs. In both series,^2,3 some patients had a regular partner without facial lesions.

K. aerogenes has been linked to Gram-negative folliculitis in acne patients since the 1980s. ⁸ Oral isotretinoin therapy with 0.5–1.0 mg/kg/day for two to 5 months has been used as an effective long-lasting treatment that results in the elimination of Gram-negative bacteria from the face and nasal vestibules.^8,9 Although not directly antibacterial, isotretinoin reduces sebum production, creating a drier skin surface less conducive to Gram-negative proliferation.^8–10 Additionally, it modulates the innate immune response by reducing neutrophil migration via downregulation of matrix metalloproteinases and by attenuating Toll-like receptor 2-mediated pathways. ¹⁰ Given that relapses were frequent in prior reports lacking isotretinoin use,^2,3 our case raises the hypothesis that concurrent isotretinoin may help prevent recurrence when added to antibiotic treatments.

A genomic study ¹ reported a high nasal carriage of K. aerogenes in MSM, supporting the hypothesis of sexual transmission. Oro-anal contact may facilitate cephalic translocation of enteric bacteria, explaining colonization sites such as the beard or nasal area. Although asymptomatic nasal carriage appears common in MSM, the reasons why only some individuals develop overt folliculitis remain unclear. In our case, the patient had a normal immunological profile, suggesting that host immune deficiency is unlikely to explain susceptibility in this context.

Our case aligns with the findings of Bérot et al. ² and Yin et al., ³ reinforcing the possible sexual transmission pathway and the environmental contribution to persistence or recurrence. We suggest that K. aerogenes be considered in the differential diagnosis of folliculitis in MSM. Bacterial cultures should be performed, prolonged treatment considered and potential sources such as communal hot tubs avoided to prevent recurrence.

Tinea barbae due to TMVII remains a critical differential in this population, as its clinical features can closely mimic bacterial folliculitis.^4–7 Notably, K. aerogenes superinfection of TMVII lesions has also been reported, ⁷ underscoring the need for comprehensive mycological and microbiological evaluation.