Abstract: A method for evaluating the comparative effect of growth-affecting substances, such as antimicrobial drugs or antibiotics, on different cultures of microorganisms is disclosed. The cultures are plated on the surface of a culture medium (12), such as agar, in adjacent at least partially separated tracks, such as tracks formed as Archimedes spirals. The cultures may be different concentrations of the same microbe or a test culture and a reference culture. The growth-affecting substances (22) are placed in contact with the culture medium on which the cultures have been plated preferably in the form of disks containing a powdered growth-affecting substance which dissolves and diffuses differentially into the culture medium to produce zones (24) of inhibition of growth extending radially outward from the disk.

Abstract: A method for determining an interacting culture medium concentration IAC of a growth-affecting substance in a volume of the culture medium related to growth of a microbial population deposited on a surface of the culture medium in accordance with the invention includes the steps of depositing the growth-affecting substance at a selected stock concentration SC in a programmed deposition on the surface of the culture medium such that the volume of the stock concentration at any deposited location on the surface of the culture medium is determinable; determining a transformation function TF which accounts for movement of the deposited growth-affecting substance through the culture medium; and calculating IAC as a function of SC and TF.

Abstract: The present invention is an improved system for counting of bacterial colonies on a plate. The system permits the counting of colonies under adverse colony counting conditions where there is overlap of colonies and analyzes the plate in a plurality of segments. The colony counts for each segment are subjected to count correction, size qualification and distribution qualification to correct for missed counts and to eliminate data from the system data base when the count for a segment will not produce reliable data. The entire plate is also analyzed for subsample qualification which provides the best estimate of the bacteria containing liquid which was used to produce the colonies as represented by a coefficient of variation of the bacterial density in the liquid.

Abstract: A method and apparatus for calculating the growth interacting substance potency and growth ratio for the intersections of a scanning spot with a track of visible microbial colonies on an interaction plate. The potency is a measure of the weight of a growth interacting substance which is deposited on the interaction plate per unit volume of the growth culture medium. The growth ratio is a measure of the growth interacting substance effect on growth of colonies of microbes on the interaction plate along the path of interaction of the scanning spot with visible microbial colonies.

Abstract: A method for testing microbial interaction with growth affecting substances. A pattern of a microbe containing solution is applied on an interaction plate in a programmed concentration. A solution of a growth interacting substance is applied on the interaction plate in a programmed potency in a pattern which contacts the microbe containing solution to cause contact of the microbes with the growth interacting substance. The resultant plate is incubated for a time sufficient to produce visible microbial colonies on the interaction plate. The potency of a growth interacting substance is determined at any point of interest on the incubated interaction plate by correlating the position of the point of interest with the programmed volume of the growth interacting substance deposited at that point.