[Value of bead-type cassettes for antibiograms of Streptococci in Autobac units]

Antimicrobial susceptibility testing of Streptococci in automated units (Autobac) is not always satisfactory: aggregates formed spontaneously by Streptococci in the medium used can reduce the reliability of tests by interfering with correct reading, and, in many cases, bacterial growth is very slow, necessitating long periods for analysis (up to 6 hours). Owing to the fact that use of "bead type" cassettes for antifungal susceptibility testing in the Autobac improved results, and in particular prevented the formation of aggregates, we decided to adopt the same system for tests of streptococci. Each compartment in these cassettes contains glass microbeads which prevent aggregate formation thanks to an agitation mechanism. Rapid sedimentation of these beads allows normal reading with the Autobac. A comparative study of 66 strains of streptococci with various antibiotics was performed. Three techniques were used: classical diffusion technique, susceptibility testing with the standard cassette in the Autobac, and test with the bead type cassette in the Autobac. Parameters evaluated included the effective duration of each test, and comparison of the results (Autobac with and without beads, diffusion method). Results are detailed and discussed.