Quantitation of bacteria in water delivered by dental units as determined by plate-count and direct-count methods

The amount of bacteria in the water supply delivered by dental units was quantitated by direct-count and plate-count methods. Water samples collected from dental units following various intervals during which the units were not used were serially diluted 10 times. Various dilutions were plated onto trypticase soy agar. Samples were prepared for quantitation by the total direct-count and the total viable-count methods. The number of bacteria recovered from the dental unit water samples as quantitated by the total direct-count and total viable-count methods was 0.5 to 1.0 log greater than that revealed by the plate-count method. Direct-count methods are more rapid and more accurate than the plate count. Direct-count methods offer potential as a routine, accurate, and rapid method of standardizing the enumeration of bacteria in the water supply of dental units. The plate count remains a valuable method for characterizing bacterial isolates.