Susceptibility testing: inoculum size dependency of inhibition using the Colworth MIC technique

The minimum inhibitory concentration, MIC, is an accepted and well used cri terion for measuring the susceptibility of organisms to inhibitors. Many fa ctors influence the MIC value obtained, including temperature, inoculum siz e and type of organism. A modification of the method developed in this labo ratory to obtain inhibition profiles of antimicrobials was used to examine the effect of inoculum size on the degree of inhibition observed with respe ct to inhibitor concentration. The data obtained enabled the production of an empirical model of inhibition, based on a Gompertz function, relating th e level of growth observed to both the inoculum size and concentration of t he inhibitor. The inoculum size dependencies of phenethyl alcohol, phenoxye thanol, p-chloro-m-cresol, trichloro-phenol, thymol and dodecyltrimethylamm monium bromide against Staphylococcus aureus were obtained.