Sensitivity testing of veterinary pathogens with a semi-automatic image analysis system compared with tablet diffusion and agar dilution tests

Recently a commercial computer-controlled image analysis system (IAS) was i ntroduced to measure automatically the diameters of inhibition zones in the agar diffusion test. However, there is little information on the precision of this method. In the present study clinical isolates of Salmonella spp, (N = 104), Escherichia coli (N = 100), Pasteurella spp, (N = 99), Actinobac illus pleuropneumoniae (N = 85), porcine streptococci (N = 100), and Staphy lococcus aureus (N = 95) were tested in the agar diffusion test, using nine teen different antibiotics in tablets. All inhibition zone diameters were f irst measured by a laboratory technician and then by the IAS, Although the zone diameters of all bacteria-antibiotic combinations measured by the IAS and those measured by the laboratory technician showed a significant positi ve correlation, the size of the inhibition zone diameters measured by the t echnician and the IAS differed significantly in 59% of the combinations. Ho wever, these differences were very small and may have no clinical relevance . The AS was also used to calculate minimum inhibitory concentrations (MIC values) from the zone diameters, In 82% of the bacteria-antibiotic combinat ions MIC values calculated by the IAS showed a significant positive correla tion with MIC values obtained with the reference agar dilution test. Howeve r, in 92% of the bacteria-antibiotic combinations, the calculated MIC value s differed significantly from the reference values. In some cases these dif ferences were so large that they could be of clinical relevance, The IAS wa s unable to measure the diameter of inhibition zones of porcine streptococc i properly, due to poor contrast. We concluded that when tablets are used a s antibiotic carriers the IAS accurately measures the diameter of inhibitio n zones for bacteria species that give good contrast between the agar and b acterial growth. MIC values determined with the IAS were only indicative of those determined with the reference agar dilution test.