Serum bactericidal testing with the Autobac system

Current methodology for the serum bactericidal test requires a minimum of 48 h. A procedure was devised for performing this test with the Autobac system (General Diagnostics, Div. Organon Inc., Raleigh, N.C.) in a shortened time span. All titers obtained with the Autobac were compared against results obtained with a standardized tube dilution procedure. The Autobac low-thymidine eugonic broth performed comparably to the tube dilution diluent, a 1:1 ratio of pooled human serum and cation-supplemented Mueller-Hinton broth (99.2% correlation between bactericidal endpoints). Over 300 tests were conducted by using stock reference bacterial strains, clinical isolates, pooled human serum seeded with antimicrobial agents, and serum from patients on antimicrobial therapy. With the Autobac procedure, serum inhibitory titers can be reported in 3 to 4 h (93.4% correlation with the tube dilution procedure). Serum bactericidal titers can be obtained in 24 h without the necessity of subculturing (95.6% correlation). With the exception of staphylococci tested against penicillin, serum bactericidal titers can be obtained in 3 to 4 h (88.4% correlation). The Autobac procedure can provide the clinical laboratory with a rapid, reliable method for performing the serum bactericidal test.