Comparison of three methods of susceptibility testing of bacteroidaceae, peptococcaceae and other anaerobes to doxycycline.

The in vitro activity of doxycycline against 147 strains of gram-positive and gram-negative anaerobes was determined by broth dilution, agar dilution and agar diffusion tests. The strains were isolated from clinical specimens in 1977. Saccharolytic Bacteroides strains (39 B. fragilis, 6 B. thetaiotaomicron, 4 B. vulgatus) showed broth dilution MICs of less than or equal to 0.0625-4.0 microgram/ml after 5 h incubation and MICs of less than or equal to 16 microgram/ml after 15 h incubation at 37 degrees C. With strains of B. oralis, B. melaninogenicus ss. intermedius, B. corrodens, Veillonella sp. and Megasphaera elsdenii the broth dilution MICs were less than or equal to 0.0625-32 microgram/ml after 15 h incubation. Peptostreptococcus spp., Propionibacterium acnes, Eubacterium sp., Bifidobacterium sp. and Clostridium sp. had broth dilution MICs of less than or equal to 2 microgram/ml, whereas Peptococcus spp. were inhibited by less than or equal to 0.0625 to greater than 32 microgram/ml. With the great majority of strains tested, MICs were 2 to 256 times higher in agar than in broth dilution tests. Clinical and experimental studies seem to be needed to determine which of the in vitro data are correlated to the outcome of doxycycline therapy. Several groups and species were separately considered for statistical analysis of the relationship between zone size and MIC. With gram-positive anaerobes, correlation was poor between broth dilution MIC and zone size (correlation coefficients r = - 0.168 for Peptococcaceae, and r = - 0.108 for P. acnes). When calculating the regression lines for agar dilution MICs and zone diameters, a higher correlation was found (r = - 0.9 for Peptococcaceae; r = - 0.397 for P. acnes). With B. fragilis and other Bacteroides species correlation coefficients were r = - 0.807 to r = - 0.891 for broth and agar dilution MICs and zone size but stochastic linearity was lacking.