Polymerase chain reaction and reverse cross blot hybridization assay for detection of mycobacterial DNA in lupus vulgaris

For 5 years, an 83-year-old man had been suffering from slightly itchy erythematous plaques with clearcut margins, located on his left thigh and on his right arm; in addition, on his right auricle there was an erythematous patch with yellowish shadings that had appeared about 3 years before and had progressively spread to the temporal-zygomatic region, the chin and the mandibular arch. These lesions were strongly suggestive of lupus vulgaris; however the conventional bacteriological examinations performed on the biopsy specimen from lesional skin were negative. A diagnosis of lupus vulgaris was achieved through the detection of the 16S rRNA gene of Mycobacterium tuberculosis in a skin biopsy of the patient by means of a polymerase chain reaction followed by a reverse cross blot hybridization, a method which allows the identification of different mycobacterial species in a single hybridization procedure.