Decrease in antibiotic susceptibility or increase in resistance?

In recent years, the subject of the emergence and subsequent increase in the incidence of resistance to anti microbial agents has become a ‘hot topic’ amongst the scientific and lay communities. Reports from both arenas suggest that antibiotics are rapidly losing their effectiveness, with some authors going so far as to suggest that we are approaching a post-antibioti c era. 1,2 On the other hand, there are those who take a more conservative view and point out that, while some strains of bacteria are resistant to many, or even most, antibiotics, the number of these multi-resistant organisms is extremely small compared with the overwhelming number of bacterial pathogens that are still amenable to therapy with currently available antimicrobial agents. 3,4 From our perspective, there are two basic reasons for this divergent point of view. Firstly, there is the lack of a national or international monitoring system for antibiotic susceptibility testing that subscribes to a single set of standard operating procedures with the appropriate quality controls. Secondly, there is confusion surrounding terminology amongst those who report the results of susceptibilit y testing. Our purpose here is to expand on these issues and to propose a solution to at least some of the problems. At f irst glance, the lack of uniformity of susceptibilit y testing procedures on a national or international level appears to be an insurmountabl e obstacle. However, the existence of two programmes gives reason to hope that this may not be the case. One is the Interagency Working Group on Antimicrobial Susceptibility Monitoring initiated by the Food and Drug Administration (FDA) in the USA. This programme involves determining the susceptibilities of Salmonella spp. isolated from humans and animals at the Centers for Disease Control and Prevention (CDC) and the United States Department of Agriculture (USDA), respectively. The two laboratories have adopted the same procedure, including the use of microbroth dilution plates obtained from a common source, and the same antimicrobial agents and dilution schemes. In order to ensure the uniformity of the test results, each laboratory evaluates a specified number of isolates from the other. The second programme is a monitoring system instituted by Microbiology Reference Laboratory (MRL) Pharmaceutical Services which requires each participating laboratory to adhere to accepted standards of suscepti bility testing, as well as MRL’s guidelines and extensive quality control programme, for the purpose of ensuring the accuracy of the data obtained for a large number of bacterial species. The success of these programmes illustrates that there are indeed mechanisms in place that are capable of facilitating uniformity of the results generated by surveillance systems. The confusion in the terminology used in susceptibility testing is even more contentious. In general, most publications concerned with bacterial resistance have expressed their findings in terms of the numbers and/or percentages of bacteria that are susceptible or resistant to specific antibacterial agents. This is illustrated by a recent study of the susceptibilities of 128 different bacterial species (all clinical isolates) to ciprofloxacin: 5 in the report based on this study, 95% of 99,040 strains of Escherichia coli were shown to be susceptible to the fluoroquinolone. There are two problems associated with reporting data in this way. Firstly, there is always the possibility that the