A Semiautomated Microcalorimetric Method of Antibiotic Sensitivity Testing

Abstract

Fifteen clinically important bacteria have been isolated from urine samples and shown to produce metabolic heat in a flow modification of the Beckman Model 190B Microcalorimeter. The bacteria produce heat at a faster rate under aerobic conditions than under anaerobic conditions, and the ratio is characteristic of the particular bacteria for those tested under both conditions (Escherichia coli, γ-Streptococcus, and Pseudomonas). The sensitivity of the bacteria to eight antibiotics has been tested on the basis of continued increase in metabolic heat (+), unchanging metabolic heat (O), or decreasing metabolic heat (−). Correlations have been made with the disk agar diffusion method and the percentage agreement for 291 tests is 77% under anaerobic conditions. Some tests have been performed under aerobic conditions, which increased the percentage agreement to 87%. The distinctive features of the calorimetric method are: it distinguishes between bacteriostatic and bactericidal drugs; it gives results 12 to 24 hr. sooner than the disk agar diffusion method; it can be fully automated.