Antimicrobial Susceptibility Testing

Introduction

Antimicrobial susceptibility testing (AST) is a crucial laboratory procedure used to determine the effectiveness of antibiotics against specific bacterial and fungal infections. This process helps clinicians select the most appropriate antimicrobial treatment, minimizing resistance development.

Factors Influencing Antimicrobial Susceptibility

Resistance Mechanisms

• Genetic Mutations: Bacteria can evolve through spontaneous mutations, altering target sites to evade antibiotic action.
• Efflux Pumps: Some microorganisms expel antibiotics before they can exert their effect.
• Enzyme Production: Beta-lactamases degrade beta-lactam antibiotics, rendering them ineffective.

Drug Pharmacokinetics

• Absorption: The bioavailability of oral antibiotics determines their therapeutic impact.
• Distribution: Tissue penetration influences efficacy—some drugs concentrate in specific body sites.
• Metabolism: Enzyme activity in the liver affects drug breakdown and duration of action.

Methods of Antimicrobial Susceptibility Testing

Disk Diffusion Method (Kirby-Bauer Test)

A simple yet effective method using antibiotic-impregnated discs placed on agar plates inoculated with bacteria. Zones of inhibition indicate susceptibility levels.

Broth Microdilution

A quantitative method determining the Minimum Inhibitory Concentration (MIC), the lowest concentration of an antibiotic that prevents visible bacterial growth.

Clinical Implications and Future Directions

With antibiotic resistance on the rise, AST plays a vital role in ensuring targeted therapy, reducing inappropriate antibiotic use, and guiding the development of new antimicrobials.