Staphylococcus aureus resistance to topical antimicrobials in atopic dermatitis

Abstract

Background:: Topical antimicrobial drugs are indicated for limited superficial pyodermitis treatment, although they are largely used as self-prescribed medication for a variety of inflammatory dermatoses, including atopic dermatitis. Monitoring bacterial susceptibility to these drugs is difficult, given the paucity of laboratory standardization.

Objective:: To evaluate the prevalence of Staphylococcus aureus topical antimicrobial drug resistance in atopic dermatitis patients.

Methods:: We conducted a cross-sectional study of children and adults diagnosed with atopic dermatitis and S. aureus colonization. We used miscellaneous literature reported breakpoints to define S. aureus resistance to mupirocin, fusidic acid, gentamicin, neomycin and bacitracin.

Results:: A total of 91 patients were included and 100 S. aureus isolates were analyzed. All strains were methicillin-susceptible S. aureus. We found a low prevalence of mupirocin and fusidic acid resistance (1.1% and 5.9%, respectively), but high levels of neomycin and bacitracin resistance (42.6% and 100%, respectively). Fusidic acid resistance was associated with more severe atopic dermatitis, demonstrated by higher EASI scores (median 17.8 vs 5.7, p=.009). Our results also corroborate the literature on the absence of cross-resistance between the aminoglycosides neomycin and gentamicin.

Conclusions:: Our data, in a southern Brazilian sample of AD patients, revealed a low prevalence of mupirocin and fusidic acid resistance of S. aureus atopic eczema colonizer strains. However, for neomycin and bacitracin, which are commonly used topical antimicrobial drugs in Brazil, high levels of resistance were identified. Further restrictions on the use of these antimicrobials seem necessary to keep resistance as low as possible.

Graph 1

Relative frequencies of S. aureus isolates resistant to topical antimibrobials, cultured from the nose and cutaneous lesions of AD patients (n=68) Resistance was defined by: * historical MIC breakpoints; ** CLSI standard 10µg disk diffusion assay; § BSAC breakpoint on 5µg disk diffusion assay; ¶ EUCAST MIC breakpoints; ‡ BSAC breakpoint on 10µg disk diffusion assay; £ EUCAST breakpoint on 10µg disk diffusion assay.

Graph 2

ROC curve analysis to define best inhibition zone size to define susceptibility to neomycin (according to MIC reference) A value of 16.5mm of inhibition zone diameter corresponded to the best breakpoint regarding sensitivity and specificity to define S. aureus susceptibility to neomycin by disk diffusion technique, using a 30µg neomycin disk. ** the Area Under Curve (AUC) was calculated as 0.942 (CI 95% 0.895 to 0.989. p<.0001)