The Measurement of Chlorine Activity in Biofilm Contaminated Dental Unit Water Lines

Potentially pathogenic microorganisms may participate in the formation of microbial biofilms on the interior walls of the plastic tubing used to supply coolant and irrigating water in dental units. A program was designed to test the effect of diluted commercial bleach (5.25% sodium hypochlorite) on bacteria colonizing dental unit water lines. A baseline bacteriological assay was performed on 24 dental units located at four separate institutional dental clinics located in the same metropolitan area. Twelve units at three clinics (designated Clinics I, II, and III) had been treated once weekly with 1:10 NaOCl for periods ranging from 6 mo to 6 yr. Twelve units at Clinic IV, which had never been treated with hypochlorite, were selected for a 28-day study of the effectiveness of two dilutions (1:10, 1:100) of 5.25% sodium hypochlorite for control of biofilm growth in dental water lines. Eight units were treated weekly with either 1:10 or 1:100 solutions of commercial bleach. The four control units were treated only with sterile deionized water. The eight units at Dental Clinics I and II had relatively low levels of planktonic bacterial colony forming units/ml (CFU/ml) ranging from 0 to 8.8 × 10 3CFU/ml with a mean of 1.1 × 10 3CFU/ml (5 of 8 units had 0 CFU/ml and two had only 30 CFU/ml). Characteristic profile curves were produced when chlorine concentration (ppm) was plotted versus a cumulative volume of solution flushed through the units. The chlorine concentration showed very little initial change, then rapidly fell to zero as the solutions were flushed from the units with sterile deionized water. The total chlorine mass recovered at these clinics varied from 94 to 101.7%, with the average being approximately 96%. When chlorine concentration was plotted against the total volume of effluent recovered (including diluent), a characteristic chlorine profile curves resulted which was dubbed a Type I Curve. Planktonic bacteria counts at Clinics III and IV were much higher (Clinic III range: 1 × 10 2CFU/ml to 4.8 × 10 5CFU/ml, median: 1.0 × 10 4CFU/ml; Clinic IV range: 5.6 × 10 4CFU/ml to 1.1 × 10 6CFU/ml, mean: 4.7 × 10 5CFU/ml), suggesting high levels of biofilm contamination. The chlorine recovered from Clinics III and IV varied from 40 to 88% with the average being 80%. The chlorine profile curves for the units at Clinics III and IV initially fell very rapidly, then more slowly as sterile distilled water was used to flush residual hypochlorite from the lines