Antimicrobial Susceptibility Testing in Veterinary Medicine
In Vitro Antimicrobial Susceptibility Testing and Resistance Determination in Veterinary Microbiology · Also known as: antibiotic sensitivity testing, MIC determination, resistance profiling
Antimicrobial susceptibility testing (AST) is a systematic in vitro laboratory method that determines which antimicrobial agents are effective against an isolated bacterial or fungal pathogen. Standardized by the Clinical and Laboratory Standards Institute (CLSI) and other regulatory bodies since the 1960s, AST guides targeted therapeutic decisions, supports infection control, and generates epidemiological data on resistance patterns essential for combating antimicrobial resistance in animal populations.
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When to use it
Perform AST for all clinically significant bacterial isolates from serious infections (systemic, sepsis, surgical site infections, nosocomial), when empiric therapy fails, to guide targeted treatment selection, for surveillance of resistance patterns in institutional or population-level settings, and before implementing antimicrobial stewardship programs. Critical in high-consequence infections and when organism identification alone does not predict susceptibility.
Strengths & limitations
- Provides targeted treatment selection based on organism-specific susceptibility, improving clinical outcomes
- Identifies resistance determinants and supports surveillance for emerging resistance threats
- Reduces unnecessary exposure to broad-spectrum agents, supporting stewardship and limiting collateral damage to normal microbiota
- Multiple standardized methods (disk diffusion, broth microdilution, gradient strips) allow flexibility for different settings
- Quantitative MIC data enable pharmacokinetic/pharmacodynamic optimization of dosing and selection among agents
- Culture and testing require 24-48 hours, delaying results and necessitating empiric therapy initiation in acutely ill animals
- Some organisms are fastidious or difficult to culture, leading to failures or delayed identification
- In vitro results may not perfectly predict clinical efficacy, which also depends on drug penetration, immune factors, and biofilm formation
- Resistance mechanisms may be rapid (e.g., ESBL production, carbapenemase production) and emerge during therapy
- Multiple resistance genes or plasmids complicate interpretation and predictability
Frequently asked
Why are veterinary breakpoints different from human clinical breakpoints?
Veterinary breakpoints account for differences in drug metabolism, dosing regimens, body temperatures, and disease pathophysiology in various animal species. Extrapolating human breakpoints to animals can lead to incorrect susceptibility interpretations.
What does 'intermediate' susceptibility mean, and should I use that antibiotic?
Intermediate means the organism is less reliably susceptible. Use intermediate-categorized drugs only if they achieve high concentrations at the infection site and higher doses are warranted; consult pharmacokinetic data and clinical experience.
How do I interpret multidrug-resistant organisms, and what are my treatment options?
Multidrug resistance limits choices; consult AST results carefully, consider combination therapy, refer to stewardship guidelines or specialists, and ensure source control (drainage, debridement) is optimized alongside antimicrobial therapy.
Can molecular testing replace culture and AST?
Molecular methods can rapidly identify organisms and detect common resistance genes, but they do not provide quantitative MIC data. Molecular testing is complementary; culture and AST remain essential for comprehensive susceptibility profiling.
Sources
- Clinical and Laboratory Standards Institute (CLSI). (2023). Performance Standards for Antimicrobial Susceptibility Testing of Bacteria Isolated from Animals (CLSI M100, 4th ed., Veterinary Supplement). Wayne, PA: CLSI. link ↗
- Weese, J. S., Giguère, S., Guardabassi, L., et al. (2011). Guidelines for antimicrobial use in horses. AAFP Clinical Microbiology. Journal of Equine Veterinary Science, 31(5), 180-187. link ↗
- Guardabassi, L., Prescott, J. F., Weese, J. S. (2018). Antimicrobial Therapy in Veterinary Medicine (5th ed.). Ames, IA: Wiley-Blackwell. link ↗
How to cite this page
ScholarGate. (2026, June 3). In Vitro Antimicrobial Susceptibility Testing and Resistance Determination in Veterinary Microbiology. ScholarGate. https://scholargate.app/en/veterinary-medicine/antimicrobial-susceptibility-vet
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