Abstract:
The rapid development and spread of bacterial resistance to antimicrobial drugs is an increasing threat to human and animal health. Information on the prevalence of bacterial resistance to specific antimicrobial agents in both humans and animals together with changes occurring over time, is required to understand the magnitude of the problem, to make decisions and to take actions that are based on risk assessment (3). The ultimate goal is to preserve the effectiveness of available antimicrobial drugs for the benefit of future generations of animals and humans. The emergence of resistance and the spread of resistant bacteria can be limited by implementing a veterinary antimicrobial drug policy, in which inter alia systematic monitoring and prudent use play essential parts (1). Testing the susceptibility of bacteria to antimicrobial drugs is fundamental to the study of resistance. Sensitivity testing serves two purposes: to provide reliable results to the prescriber of antimicrobial drugs and to monitor changes in susceptibility of microbial populations. Standardized methods are needed for surveillance in the national and international context and harmonization of methods to provide meaningful comparisons between individual centres and countries (1, 3). The objective of this study was to establish a repeatable, standardized laboratory procedure for monitoring the development of antimicrobial resistance in bacteria isolated from animals and food of animal origin in South Africa. The bacteria included in this study represented three different groups, namely zoonotic bacteria (Salmonella), indicator bacteria (Escherichia coli, Enterococcus faecalis, E. faecium) and veterinary pathogens (Mannheimia haemolytica). Thirty isolates of each organism were collected with the aim of standardizing the laboratory methodology for a future national veterinary surveillance programme. Susceptibility to 10 antimicrobial agents was determined by means of minimum inhibitory concentrations (MIC’s) using the microdilution method. The method according to the National Committee for Clinical Laboratory Standards was used as the standard. Susceptibility tests were repeated once for each individual organism. Quality control measures were included to ensure that accurate results were obtained. Repeatability was satisfactory as results from duplicate tests differed by only one twofold dilution. Multi-well plates prepared in-house for MIC determinations also yielded repeatable results after two months of storage at -70°C. Within this limited sample of bacteria, MIC results did not indicate meaningful resistance against any of the 10 selected antimicrobials.
