This paper presents the finding of the possible cause of the high false-positive rate in acid-fast staining in histological examinations. Using acid-fast staining, culture, and PCR, acid-fast bacilli were detected in 83.7% of 49 hospital tap water samples and nontuberculous mycobacteria (NTM) were detected in 20.4% of the same 49 samples. The 10 NTM isolates were also identified to the species level using PCR-restriction fragment length polymorphism. Our findings indicate that NTM in hospital tap water are the possible cause of false positives in acid-fast staining and of nosocomial infection in immunocompromised patients.Nontuberculous mycobacteria (NTM) are the common organisms which cause diseases in immunocompromised patients. Mycobacterium genavense from hospital tap water has been demonstrated to cause nontuberculous mycobacteriosis in human immunodeficiency virus-infected patients (5). In addition, several kinds of NTM have been identified in ice samples and public drinking water samples (4). Molecular techniques, including PCR and restriction fragment length polymorphism (RFLP), have been applied for the detection and identification of mycobacteria, including M. tuberculosis and NTM. Use of the genus-specific 16S rRNA and the rpoB gene of mycobacteria for the identification of NTM has also been documented (2, 6). In addition, other genes, such as those encoding the conserved and nonconserved regions in 65-kDa heat shock proteins of mycobacteria, have been found to be suitable for the identification of NTM (9).In our clinical laboratory, the contamination of tap water by NTM usually causes false positives in the acid-fast staining of histological sections. This causes a major problem in the quality control of clinical examinations, especially for histological examinations. Thus, the detection of mycobacteria in tap water was performed. The tap water samples were subjected to acidfast staining, Lowenstein-Jensen (LJ) medium culture, and the PCR-RFLP method to detect NTM. The gene for heat shock protein 65 was chosen for the detection of NTM by PCR in this study. The positive result of PCR amplification generated a 439-bp fragment, which was further digested by restriction enzymes for the identification of NTM to the species level. Our results should be important for quality control of acid-fast staining in histological examinations and for the investigation of nosocomial infection in immunocompromised patients.Tap water samples (200-ml volume) were collected from various sources in the clinical laboratory at Yuan's General Hospital. After centrifugation at 4,500 ϫ g for 30 min, the pellet was resuspended in 60 l of Tris-EDTA (TE) buffer. All water samples were subsequently examined by acid-fast staining, LJ medium culture, and direct-PCR analysis.Acid-fast staining was performed by the traditional Kinyoun method (7). The glass slide was cleaned with 95% ethanol and then coated with bovine serum albumin (fraction V; Sigma Chemical Co., St. Louis, Mo.). Each water sample (10 l) was applied. The acid-fast sta...
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