Susceptibility testing of fastidious organisms in the API microdilution panel

It has become increasingly important to perform routine susceptibility tests on certain anaerobic bacteria, Haemophilus influenzae and Streptococcus pneumoniae, as a result of the decreasing predictability of their antimicrobial susceptibility patterns. The antimicrobial susceptibilities of 49 anaerobic isolates, 25 H. influenzae isolates, and 25 S. pneumoniae isolates were determined concurrently by API Uniscept MIC trays and a conventional broth microdilution method using Wilkins-Chalgren broth, 5% Fildes in Schaedler broth, or 5% lysed horse blood in Mueller-Hinton broth, respectively. Analysis of 490 anaerobic organism-antibiotic combinations, 144 H. influenzae-antibiotic combinations, and 125 S. pneumoniae-antibiotic combinations showed that 98.9%, 100%, and 99.2%, respectively, of the API results were within +/- 1 log2 dilution of the reference system. The API Uniscept MIC panel would be acceptable for use as a routine susceptibility system for anaerobic organisms in a clinical microbiology laboratory. To eliminate trailing endpoints, however, further studies need to be performed to evaluate additional broth media for the susceptibility testing of H. influenzae and S. pneumoniae in the API panels.