Antibiotic Resistance and Susceptibility

Introduction to Antibiograms

• The American Society for Microbiology defines susceptible (S) as indicating that the pathogen is likely to be inhibited if the antimicrobial compound in the blood reaches concentrations usually achievable, intermediate (I) as implying possible clinical applicability in body sites where the drug is physiologically concentrated, and resistant (R) as indicating that the pathogen is not likely to be inhibited even at maximum antibiotic doses, with alternative therapy to be selected 1
• The Minimum Inhibitory Concentration (MIC) is the lowest concentration of an antibiotic that prevents bacterial growth under defined laboratory conditions, reported in μg/mL, with lower MIC values indicating greater susceptibility 1

Interpreting Antibiograms

• Hospital-wide antibiograms may mask important susceptibility differences across units, and it is recommended to look for antibiograms stratified by hospital unit, specimen type, patient population, and community vs. healthcare-acquired infections, with the Infectious Diseases Society of America suggesting consideration of these factors 2
• The Centers for Disease Control and Prevention (CDC) recommends comparing antibiograms with previous years' data to identify emerging resistance patterns, with decreasing susceptibility rates over time prompting reassessment of empiric therapy choices 2
• Antibiograms should be interpreted considering the site of infection, as the concentration of the antibiotic can vary by tissue, which is a critical factor in ensuring effective treatment and minimizing resistance 3

Limitations and Considerations

• Previous antibiotic exposure is a major risk factor for resistance, according to the World Health Organization (WHO), and susceptibility varies based on healthcare facility residence and recent hospitalization, with the WHO emphasizing the importance of considering these factors 4
• Standard antibiograms may not reflect susceptibility in mixed infections or unusual infection sites where antibiotic concentrations differ, with the Clinical and Laboratory Standards Institute (CLSI) noting that antibiograms cannot account for exceptions or unforeseen mutations 5
• Understanding the limitations of antibiogram data is essential for proper interpretation and application, including recognizing that antibiograms are specific to each antibiotic and that resistance patterns can evolve differently across bacterial species 3

Clinical Application

• The Infectious Diseases Society of America recommends using the most specific antibiogram available for the patient population and infection site, considering patient-specific risk factors that may increase the likelihood of resistant organisms, and remembering that antibiograms guide empiric therapy but should be adjusted once culture and susceptibility results are available 2
• For critically ill patients, the Society of Critical Care Medicine suggests considering broader initial coverage with subsequent de-escalation based on culture results, and using antibiograms in conjunction with clinical guidelines for specific infection types to optimize therapy selection 5
• All successful antimicrobial therapy should be susceptibility-based to achieve high cure rates (≥95%), as recommended by the Annual Review of Medicine 6
• Therapy selection can be guided by direct susceptibility testing of the patient's isolate or local population data showing which empiric therapies remain effective, according to the Annual Review of Medicine 6
• Patient history of prior antibiotic exposure is crucial in predicting potential resistance, as stated by the Annual Review of Medicine 6
• Examples of antibiogram-guided therapy include:
• The terms "susceptibility" and "sensitivity" are synonymous, referring to the likelihood of a bacterial infection responding to a particular antibiotic at achievable drug concentrations, while "resistance" indicates that the infection is unlikely to respond even to maximum antibiotic doses, as defined by Clinical Microbiology and Infection guidelines 1
• Breakpoints are established based on pharmacokinetic properties of the antibiotic, achievable drug concentrations at infection sites, and clinical response data, to define the categories of susceptible, intermediate, and resistant, according to Clinical Microbiology and Infection guidelines 1

Best Practices for Antimicrobial Therapy

• Obtain thorough history of prior antibiotic use before selecting therapy, as recommended by the Annual Review of Medicine 6
• Consider susceptibility testing before initiating treatment for serious infections, according to the Annual Review of Medicine 6
• Confirm treatment success with appropriate follow-up testing, as recommended by the Annual Review of Medicine 6
• Share local resistance data to guide community prescribing practices, according to the Annual Review of Medicine 6