Onychomycosis is a fungal infection of the nail that is often recalcitrant to treatment and prone to relapse. Traditional potassium hydroxide and culture diagnosis is costly and time-consuming. Therefore, molecular methods were investigated to demonstrate effectiveness in diagnosis and to quantify the microbial flora present that may be contributing to disease.
A total of 8,816 clinically suspicious toenail samples were collected by podiatric physicians across the United States from patients aged 0 to 103 years and compared with a control population (N = 20). Next-generation sequencing and quantitative polymerase chain reaction were used to identify and quantify dermatophytes, nondermatophyte molds, and bacteria.
Approximately 50% of suspicious toenails contained both fungi and bacteria, with the dermatophyte Trichophyton rubrum contributing the highest relative abundance and presence in 40% of these samples. Of the remaining 50% of samples, 34% had bacterial species present and 16% had neither. Fungi only were present in less than 1% of samples. Nondermatophyte molds contributed to 11.0% of occurrences in fungus-positive samples. All of the control samples were negative for fungi, with commensal bacterial species composing most of the flora population.
Molecular methods were successful in efficiently quantifying microbial and mycologic presence in the nail. Contributions from dermatophytes were lower than expected, whereas the opposite was true for nondermatophyte molds. The clinical significance of these results is currently unknown.
Historically recalcitrant to treatment, infection of the nail unit is a pervasive clinical condition affecting about 10%-20% of the U.S. population; patients present with both cosmetic symptomatology and pain, with subsequent dystrophic morphology. To date, the presumptive infectious etiologies include classically-reported fungal dermatophytes, non-dermatophyte molds, and yeasts. Until now, the prevalence and potential contribution of bacteria to the clinical course of dystrophic nails had been relatively overlooked, if not dismissed. Previously, diagnosis had been largely made via clinical presentation, although microscopic examinations (KOH) of nail scrapings to identify fungal agents, and more recently, panel-specific PCR assays have been employed to elucidate causative infectious agents. Each of these tools suffers from test-specific limitations. However, molecular-age medicine now includes DNA-based tools to universally assess any microbe or pathogen with a known DNA sequence. This affords clinicians with rapid DNA sequencing technologies at their disposal. These sequencing-based diagnostic tools confer the accuracy of DNA level certainty, while concurrently obviating cultivation or microbial phenotypical biases. Using DNA sequencing-based diagnostics, the results herein document the first identification and quantification of significant bacterial, rather than mycotic, pathogens to the clinical manifestation of dystrophic nails. In direct opposition to the prevailing and presumptive mycotic-based etiologies, the results herein invoke questions about the very basis for our current standards of care, including effective treatment regimens.