What is an antibiotic sensitivity test?

Antibiotic Sensitivity Testing

An antibiotic sensitivity test is a laboratory procedure that determines whether bacteria causing an infection will be killed or inhibited by specific antibiotics, guiding clinicians to select the most effective treatment for bacterial infections. 1, 2

Definition and Purpose

• Antibiotic sensitivity testing (also called antimicrobial susceptibility testing or AST) measures how effectively antibiotics inhibit bacterial growth in vitro to predict clinical response to treatment 3
• The test helps identify which antibiotics will be effective against a specific bacterial pathogen, allowing for targeted therapy rather than empirical treatment 4
• AST is particularly important for pathogens capable of demonstrating resistance to commonly prescribed antibiotics 4
• The results facilitate communication between the laboratory and clinician, and between laboratories nationally and internationally 1

Testing Methods

Minimum Inhibitory Concentration (MIC) Determination

• MIC represents the lowest concentration of an antibiotic that inhibits visible bacterial growth 5, 6
• MIC is considered the gold standard for antibiotic susceptibility testing in clinical practice 6
• Several methods are used to determine MIC:
  • Broth dilution: Serial dilutions of antibiotics in tubes of broth 3
  • Agar dilution: Serial dilutions of antibiotics incorporated into agar plates 3
  • Gradient diffusion (E-test): Paper strips with antibiotic gradient placed on agar 3

Disk Diffusion Method

• Involves placing antibiotic-containing disks on agar plates inoculated with bacteria 3
• Bacteria are classified as sensitive or resistant based on the size of growth inhibition zones around the disks 3
• This method is simpler but less precise than MIC determination 7

Interpretation of Results

Susceptibility Categories

• Susceptible (S): Bacteria likely to respond to treatment with the antibiotic at recommended dosages 2
• Intermediate (I): Bacteria with variable or indeterminate responses to standard antibiotic therapy 2
• Resistant (R): Bacteria unlikely to respond even to maximum doses of the given antibiotic 2

Clinical Implications

• Susceptible bacteria are expected to be effectively treated with standard doses of the antibiotic 2
• For bacteria with intermediate susceptibility, higher doses or alternative antibiotics should be considered 2
• Resistant bacteria possess specific resistance mechanisms that make the antibiotic ineffective 1
• The European Committee on Antimicrobial Susceptibility Testing (EUCAST) and Clinical and Laboratory Standards Institute (CLSI) provide standardized breakpoints to categorize bacteria 5, 8

Clinical Applications

• AST is indicated when susceptibility to antimicrobials cannot be reliably predicted based on the identity of the pathogen 4
• For serious infections, antibiotics with MICs well below the susceptibility breakpoint should be considered to ensure adequate tissue penetration 5
• MIC determination is particularly important for specific infected sites (e.g., endocarditis, bone infections) and potentially resistant bacterial species 1
• Rapid tests can detect specific pathogens like S. aureus and determine methicillin susceptibility, allowing for faster treatment decisions 1

Quality Control and Standardization

• To ensure reproducible results, laboratories must follow standardized methods developed by organizations like EUCAST or CLSI 3, 6
• Standardized factors include growth media, incubation temperature and environment, inoculum size, and quality control parameters 3
• Mass spectrometry can provide rapid bacterial identification (within minutes), enabling earlier adaptation of antibiotic therapy 1

Limitations and Considerations

• AST results may be misleading when cultures yield mixed growth or usual microbiota unrelated to the actual infection 4
• Laboratory factors can influence MIC values, emphasizing the need for standardized testing procedures 3
• For some organisms with predictable susceptibility (e.g., Streptococcus pyogenes to penicillin), AST may not be routinely required 4
• Appropriate culture and susceptibility tests should be performed before treatment to identify causative organisms and determine their susceptibility 8

Emerging Technologies

• Automated systems for liquid samples can provide bacterial identification and antibiotic susceptibility results in 3-6 and 7-13 hours, respectively 1
• Mass spectrometry enables bacterial identification directly from positive blood culture bottles in approximately 30 minutes 1
• Molecular methods are being developed for rapid assessment of resistance patterns, often directly from clinical specimens 4
• These emerging technologies aim to address the need for faster AST results, particularly in life-threatening conditions like sepsis 7