[Susceptibility testing of anaerobes (author's transl)]

The methodology of susceptibility testing of anaerobes has not yet been standardized and many individual test methods are in use. Among the factors which may influence the outcome of susceptibility tests are test organisms, antimicrobials and methods of susceptibility testing. Comparative studies have shown that the density of the inoculum largely influences the MICs of sulphonamides and certain other antimicrobials. In testing susceptibility of anaerobes to doxycycline, the MICs determined by agar dilution tests were 2--4--8 times the MICs obtained by broth dilution technique. However, with certain cephalosporins and beta-lactamase-producing strains of Bacteroides fragilis and other Bacteroides species the MICs obtained by the two methods differed by four to five dilution steps. Judging by the broth dilution test, the majority of the beta-lactamase-producing Bacteroides strains would be classified as resistant, whereas with beta-lactamase-negative strains of Sphaerophorus varius the broth dilution test gave much lower MICs than the agar dilution technique. These findings pose the rather embarrassing problem as to whether strains are to be regarded as resistant or sensitive. As is demonstrated by lack of stochastic linearity and/or low correlation coefficients, the results of standardized anaerobic agar diffusion tests are for the most part not correlated to MICs determined by either method of quantitative susceptibility testing.