Antimicrobial susceptibility testing (AST) is performed daily on bacterial isolates in clinical laboratories. The techniques employed are often taken for granted. This paper traces the history and development of some methods still in common use for routine AST, e.g. disc diffusion and agar dilution. It was quickly recognized by early investigators that there were many variables affecting the results of these tests. Consequently, there was recognition (as early as the late 1950s) that standardization of these techniques was required. This need has led to many organizations producing standardized AST methodologies. Although some disc diffusion techniques that generated results within 4-6 h were described, most relied on 18-24 h incubation before a result was available. The clinical and economic pressures for rapid methods with low labour input led to the development of semi-automated and automated AST methodologies in the 1970s. Until 10 years ago, AST techniques relied on phenotypically testing the bacteria isolated. However, to increase the speed and reliability of resistance testing, the use of a genotypic approach has been advocated. The limitations and benefits of this new approach are discussed.

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Summary. The susceptibility of 130 clinical isolates of gram-positive cocci to a wide range of antimicrobial agents was assessed by ATP bioluminescence in a 4-h test. ATP assays were performed on a novel luminometer, the Amerlite Analyser, which measures luminescence from microtitration trays. For most organisms tested, there was good correlation (>90%) with conventional MIC values estimated on 18-h cultures. However, a problem was found with detection of penicillin resistance in Staphylococcusaureus by the ATP method, 13% of strains showing major disagreement. Methicillin resistance of S. aureus was shown reliably for most strains (94%) by ATP assay, provided they were incubated at 30°C. The Amerlite Analyser offers the potential for the development of a semi-automated antimicrobial susceptibility test, with a significant reduction in reagent costs when compared with previously described bioluminescence protocols.

SUMMARY After a case of peritonitis caused by Pseudomonas maltophilia had occurred 20 strains of the organism were investigated and the minimum inhibitory concentrations of a variety of antibiotics determined at 30°C and 37°C. There was a significant difference in susceptibility between 30°C (most resistant) and 37°C (most susceptible) for aminoglycosides and polymyxin B. No difference was seen with the other agents or in strains of Ps aeruginosa and Enterobacteriaceae tested under similar conditions. The possible mechanisms of this phenomenon are discussed below.