TURBIDIMETRIC AND PLATE ASSAYS

TURBIDIMETRIC AND PLATE assays follow different dose response curves. With turb assays mechanisms of inhibition are more complex and go beyond the typically observed increase in generation time. During time of incubation, test antibiotics will kill a fraction of the microbial population, while permitting the remainder cells to grow at the same rate i.e., with the same lag

as the zero antibiotic containing control tubes. Physical factors need to be considered as well when comparing turb and plate assays. Bias caused by differences in cell size (due to elongation and division during the asynchronous bacterial cell cycles) will affect standard and test samples to different extents. The results being inaccurate potency values. Because of these attributes

dose ratios should be in small increments so that the overall response falls within the linear range of the dose response curve doses which follow a geometric progression e.g., dose ratios

of 1.12:1 to 1.40:1 must be followed. Departure from the narrow, straight-line dose response region towards the extremities i.e.,no growth or/and the extended lag will result in grossly inaccurate assays. Hence the 0.64, 0.80, 1.00, 1.25 and 1.56 dose format utilized for plate assay is not suited for turbidimetric assay work.