Standardization of inoculum size for disc susceptibility testing: a preliminary report of a spectrophotometric method

Many methods of disc susceptibility testing aim at an inoculum to yield a semi-confluent growth. We have improved the means of standardizing inoculum by measuring the turbidity with a spectrophotometer. Absorbance measurements were made for various bacterial suspensions and dilutions thereof, from 1 in 2 to 1 in 1000. Volumes of 0.1 ml of each dilution were spread on to Diagnostic Sensitivity Test agar. Viable counts of bacteria were made for each absorbance reading and for what constituted semi-confluent growth. Suspensions of bacteria having similar absorbance readings contained variable numbers of viable bacteria depending on the species. The number of bacterial cells that yielded semi-confluent growth also varied with different bacterial species. A chart was prepared to indicate the appropriate dilutions of bacterial suspensions with different absorbance readings to produce semi-confluent growth in sensitivity testing of each species.