Preparing inoculum for susceptibility testing of anaerobes

We studied six methods for preparation of inoculum to use in susceptibility tests of anaerobic bacteria to determine viable counts of 15 different species of anaerobes. We counted viable bacteria for each method-organism combination. Methods studied included those used for the more routine tests (broth microdilution and broth-disk elution) and for the National Committee for Clinical Laboratory Standards standard reference agar method. Inocula prepared from broth suspensions of organisms from 24- and 48-h anaerobe blood agar plates and adjusted to the turbidity of a 0.5 McFarland standard gave the most consistent counts for all organisms and also the highest numbers. Counts from these suspensions were higher than those from overnight growth in thioglycolate or Schaedler broth when all were adjusted against the same turbidity standard. Preparing inoculum directly from agar plates may also speed up the sometimes lengthy process of susceptibility testing of anaerobes and thus make results more clinically useful.