Development and evaluation of the spiral gradient endpoint method for susceptibility testing of anaerobic gram-negative bacilli.

The spiral gradient endpoint (SGE) method for antimicrobial susceptibility testing was evaluated as an alternative agar-dilution procedure that would require less time and materials than the reference standard agar-dilution (SAD) susceptibility test for anaerobic bacteria. For the SGE test a spiral plater produces a drug concentration gradient equivalent to up to eight twofold dilutions in a single agar plate. Bacteria are streaked in radial lines across this gradient, and the drug concentration at the endpoint location where growth ceases can be calculated. Early results demonstrated the need to develop a standardized procedure, various technical improvements, and revised SGE formulas that correct for drug diffusion in calculating endpoint concentrations for tests on aerobes and anaerobes. The revised SGE method demonstrated an overall 90.7% agreement (within +/- 1 twofold dilution) of the minimal inhibitory concentrations with those determined by the SAD method tested in parallel for 161 strains of a wide variety of anaerobic gram-negative bacilli and eight antimicrobial agents. The reproducibility, sensitivity, and significantly increased efficiency warrant further evaluation of the revised SGE method.