COLISTIN ANTIMICROBIAL SUSCEPTIBILITY TESTING (AST) - WHICH METHODS ARE AVAILABLE?

The rise in carbapenem resistance (CR) among Gram-negative bacteria has renewed interest in colistin. Recently, the EUCAST-CLSI Polymyxin Breakpoints Working Group declared that broth microdilution (BMD) is the only valid method for colistin susceptibility testing. Prompt and accurate reporting of colistin susceptibility is of great importance in settings where CR Gram-negative bacteria are prevalent. This is a challenging task for many clinical laboratories. BMD is not easily incorporated into the routine work of clinical laboratories, and usually this test is incorporated serially, resulting in delayed susceptibility reporting. Clinical microbiology laboratories only rarely perform reference broth microdilution, which requires freshly prepared or frozen antibiotic solutions. However, a number of more user-friendly commercial products for colistin BMD have recently become available. Methods widely used for AST at clinical laboratories are gradient tests, disk diffusion, and semi-automated devices. Gradient tests and semi-automated AST devices have been extensively used at clinical laboratories, despite the problems reported with colistin AST on these systems. Currently available techniques including genotupic and phenotypic are evaluated in this study to provide relevant information for diagnostic laboratories. This study also presents the use of a neutropenic mouse thigh model of infection in relation to recently identified mechanisms of polymyxin resistance, bacterial fitness and the corelation to colistin susceptibility.