Bacillus thuringiensis: a causative agent of pitted keratolysis.

Abstract

A 43-year-old man presented with a prolonged history of malodour and burning irritation of his soles. He previously treated himself with an antifungal ointment without improvement. Small pits and crateriform erosions affecting the stratum corneum—typical symptoms of pitted keratolysis1—were observed on physical examination. Pits and craters were localised on both soles on the weight-bearing areas of the forefoot and heels, and deeper crateriform lesions were present on the metatarsal region of the right sole (Fig. 1). Tinea pedis was excluded with Wood light examination that clearly indicated the pure bacterial origin of the infection. Skin scrapings were taken from 6 pits from the ball of the left sole and 6 crateriform erosions from the ball of the right sole. The presence of fungus was excluded with native light microscopic investigation. After homogenisation, samples were spread on Luria Bertani agar (LBA), brain heart infusion agar (BHIA), blood agar (BA), anaerobic blood agar (ABA) and Sabouraud agar (SA) plates. LBA, BHIA, BA and SA plates were incubated for 24 h under aerobic and elevated (5%) CO2 conditions, whereas ABA plates were incubated for 48 h under anaerobic conditions. SA and anaerobic plates were declared as negative after 7 and 14 days, respectively. Ten colonies from each morphotypes were collected from the plates and identified with matrix-assisted laser desorption/ionisation–time of flight mass spectrometry (MALDI-TOF MS) and, in one case, with 16S rRNA sequence determination by using the universal prokaryotic 16S rRNA primers, Uni16S27_F and Uni16S1492_R.2 Altogether six bacterium species were identified from the lesions with MALDI-TOF MS. Bacillus thuringiensis could be detected in each lesion, and the presence of Staphylococcus simulans, Staphylococcus haemolyticus, Streptococcus parasanguinis and Streptococcus mitis was limited to up to three lesions (Table S1). Sequence determination of the 16S rRNA coding DNA region of the sixth isolate revealed a 99% homology to different Macrococcus species (GenBank: MZ577562). B. thuringiensis was the only species that could be detected from every lesion and showed the strongest proteolytic activity (Table S2), a characteristic feature in pitted keratolysis progression. Pitted keratolysis is a superficial skin infection affecting the stratum corneum.3 Although the roles of predisposing factors like hyperhydrosis, elevated skin pH, poor foot hygiene, obesity and immunodeficiency were recently demonstrated, pitted keratolysis can affect healthy patients of different socio-economic statuses and ages, typically in people wearing occlusive shoes, including athletes, sailors, soldiers or patients living in humid, tropical countries.4, 5 Today, therapy for pitted keratolysis is empiric and based on topical or oral antibiotics, primarily erythromycin and clindamycin. The remarkable number of registered therapeutic failures and recurrences5 justifies the need for a more thorough investigation of the aetiologic agents of pitted keratolysis, knowledge of which is limited and relies on the few early observations where the roles of Actinomyces keratolytica, Streptomyces, Corynebacterium species, Dermatophilus congolensis and Kytococcus sedentarius were reported.4 In this case report, we have identified B. thuringiensis, a Gram-positive, spore-forming bacterium, from the lesions of pitted keratolysis case. Its presence in all the investigated lesions and its strong keratolytic activity (Fig. 2) suggest its flagship role in this skin disease, and others were either normal skin microbiota or opportunistic. After identification, a 7-day topical erythromycin (2%) therapy was successfully performed. As a result unpleasant symptoms were eliminated in three days, but crateriform lesions disappeared only in months. None Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.