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Antimicrobial resistance

February 26, 2019


Antimicrobial resistance is one of the most serious global public health concerns today. Approximately 1.7 million patients in the United States acquire an infection in the hospital each year, about 99,000 of whom will die as a result. Seventy percent of the bacteria causing such infections are resistant to at least one antimicrobial commonly used to treat these infections.1 Antimicrobial resistance occurs when microorganisms adapt so that they are no longer able to be treated with antimicrobial drugs that once were effective.

In some of the most difficult cases, organisms become resistant to nearly all available antimicrobials, leaving health care providers with few treatment options for their patients. This is called multidrug resistance (MDR). One of the most urgent MDR threats is carbapenem-resistant Enterobacteriaceae, or CRE. CRE is resistant to almost all available antimicrobials. According to the CDC’s 2013 Threats Report, CRE causes 600 deaths per year in the United States.2 CDC is planning to release an updated AR Threats Report in fall 2019 and CRE is expected to remain at or near the top of the list. Clostridioides difficile is another significant threat which can cause life threatening diarrhea and colitis and often occurs in people who have been recently treated with antimicrobials. Widespread antimicrobial use over many years has contributed to increasing numbers of infections due to both MDR and C. difficile.


Essential to the fight against antimicrobial resistance is the implementation of an antimicrobial stewardship program (ASP) which focuses on responsible use of antimicrobials in health care facilities and in the community. Effective ASPs reduce unnecessary and inappropriate antimicrobial usage which can lead to a reduction in the development and spread of resistant bacteria and ultimately improve patient outcomes. Clinical microbiologists are involved with developing and implementing ASPs and design their antimicrobial susceptibility testing (AST) and reporting protocols to meet the needs of the ASP. In doing so, it is essential for each laboratory to utilize the most current AST and reporting standards.


Standards for AST guide clinical microbiologists and others in using the best practices in their laboratories to assess the in vitro activity of antimicrobials. Standards organizations such as Clinical and Laboratory Standards Institute (CLSI) develop and publish standards based on the most currently available information and data provided to them by individuals involved in all aspects of antimicrobial development, testing, and use. Recommendations for selection of the most appropriate agents for testing and reporting and for interpretation of test results are updated and published regularly. In January of each year, CLSI publishes a new edition of its most widely used AST standard, Performance Standards for Antimicrobial Susceptibility Testing, also known as M100.3

When clinical microbiologists examine bacterial cultures, they first identify the organism or organisms likely contributing to a patient’s infection. If the organism has unpredictable susceptibility to the antimicrobials that might be used in treating the suspected infection, the laboratory then performs AST using one or more of several test methods available. Subsequently, results are interpreted based on criteria listed in M100, enabling the laboratory to inform health care providers of those drugs that might be effective in treating the patient’s infection. Organisms can acquire antimicrobial resistance rapidly, and a drug that was previously effective for a specific type of organism might no longer be, highlighting the importance of reevaluating and updating AST standards regularly.


CLSI’s M100 and other AST standards are written by volunteer experts who are involved with developing, approving, prescribing, and testing of antimicrobials, and/or reporting of bacterial culture susceptibility results. These experts make up the CLSI Antimicrobial Susceptibility Testing Subcommittee. Multiple working groups within this subcommittee address very specific aspects of AST, such as determining which antimicrobials to consider for testing and reporting for a specific type of bacterium, assigning breakpoints to newer antimicrobial agents, and defining appropriate quality control protocols. To ensure fairness, the subcommittee is comprised of an equal balance of experts from health care, industry, and government. Health care experts include clinical microbiologists, pharmacists, and infectious disease specialists. Government participants include representatives from the FDA and CDC. Industry members include pharmaceutical and in vitro device manufacturers and statisticians. All work together to ensure that the most up to date and useful information is available to help health care providers prescribe the most effective treatment for their patients.


The responsible use of antimicrobials can be achieved through the implementation of an antimicrobial stewardship program and the use of the most current AST standards in the clinical microbiology laboratory. Responsible use of antimicrobials improves patient care outcomes and contributes to the global fight against antimicrobial resistance.