SONIC OSCILLATION AS AN AID IN THE COUNTING OF GROUP A STREPTOCOCCI BY THE POUR-PLATE METHOD

Abstract

Suspensions of washed group A streptococci (4 serological types) were adjusted to an optical density equivalent to 25 x 109 cells/ml. The standard-ized suspensions were oscillated for 1, 2, 3 and 4 minutes. Samples were withdrawn for viable counts by the pour-plate method, total microscopic counts, and counts of the number of cells per chain. No reduction was found in the total microscopic count. Chains of 7-12 cells were broken into pairs and single cells in 1-4 minutes. All pour plate counts were 2-3.4 times higher after 1 and 2 minutes oscillation than the original suspension. The viable count was 0.5 to 3.1% of the total microscopic count. Results show that a more accurate standardization of viable streptococci can be attained by use of the sonic treatment.