Dental units: an environmental study of sources of potentially pathogenic mycobacteria.

SETTING: Infections caused by non-tuberculous mycobacteria (NTM) are generally thought to be acquired from environmental sources. However, little is known about the situations in which transmission occurs. OBJECTIVE: In an attempt to identify situations of relevant contact with NTM we investigated the water to which patients are exposed during dental treatment. DESIGN: The concentration and species of NTM were determined in 43 cooling and spray water samples from 21 dental units in ten offices. In addition, mycobacterial colonization of 16 biofilm samples from the waterlines of two dental units was investigated. RESULTS: The mean NTM concentration in the water samples was 365 colony-forming units (cfu) per mL, exceeding the mean drinking water concentration by a factor of almost 400. In the biofilm samples the mean NTM density amounted to 1165 cfu/cm2. The species identified included Mycobacterium gordonae, M. flavescens, M. chelonae, 'M. chelonae-like organism' and M. simiae. CONCLUSION: High numbers of NTM may be swallowed, inhaled or inoculated into oral wounds during dental treatment, possibly resulting in colonization, sensitization or infection. Mycobacterial proliferation in biofilms forming within dental units may explain the extent of NTM contamination of dental spray and cooling water.

Resistance of bacterial strains to dry conditions: use of anhydrous silica gel in a desiccation model system.

The viability of 18 bacterial strains desiccated on anhydrous silica gel and stored at a temperature of 22 degrees C for at least 3 months was determined. According to their stability in the dried state, these strains could be classified into three typical groups. Group 1, containing Gram-positive strains and Salmonella serotypes, was marked by a very slow decrease of the concentration of culturable cells from day 14 on (respectively day 21 for Salmonella thompson). The rate of decrease expressed as regression coefficient (b) ranged from -0.000389 to -0.00521 log (cfp ml-1) per d. The Group 2 strains Enterobacter cloacae and Escherichia coli did not reach a comparable slow decrease in the dry material within the indicated time period. Regression coefficients were respectively -0.04406 and -0.03412 log (cfp ml-1) per d. The reciprocal values -(1/b) were respectively 23 d per log (cfp ml-1) and 29 d per log (cfp ml-1), indicating the time periods in which a reduction of 1 log unit of culturable cells occurred. Group 3 strains Pseudomonas aeruginosa, Aeromonas hydrophila and Aer. sobria were marked by a significant susceptibility to cell damage caused during desiccation and reconstitution. A high initial decrease (ID) of the concentration of culturable organisms seems to be a characteristic property of these bacterial strains: culturable organisms could not be detected after storage for 1 d (Aer. hydrophila, Aer. sobria) or 7 d (Ps. aeruginosa). The wide range of resistance of the different bacterial strains tested indicated that the silica gel model system is a suitable tool for microbiological challenge tests to investigate the survival of micro-organisms exposed to desiccation and their stability in dry materials.

Occurrence of mycobacteria in biofilm samples.

The occurrence of mycobacteria was studied in 50 biofilm samples from water treatment plants, domestic water supply systems and aquaria. Mycobacteria were found in 90% of the samples and their densities usually ranged between 10(3) and 10(4) cfu/cm2 (maximum density 5.6 x 10(6) cfu/cm2). Organic substances such as plastics and rubber were usually colonized by larger numbers of mycobacteria than inorganic substances such as copper and glass. The highest mycobacterial densities were found on plastic surfaces which were continuously perfused with water at temperatures between 22 and 30 degrees C. The species identified include Mycobacterium chelonae, M. flavescens, M. fortuitum, M. gordonae, M. kansasii, and M. terrae/nonchromogenicum. The occurrence in microcolonies indicate that biofilms may be an important replication site of aquatic mycobacteria.