Antimicrobial susceptibility testing

Antimicrobial susceptibility testing (AST) is a key process in microbiological studies to understand the effectiveness of antibiotics or other chemotherapeutic agents on inhibiting bacterial growth.
The aim of AST is to predict the treatment success of an antimicrobial substance in a patient suffering from a bacterial infection.
It involves the exposure of a bacterial isolate (a single type of bacteria separated from a mixed culture) to a range of concentrations of an antimicrobial substance.

Mechanisms of Action of Antimicrobials

Antimicrobials can inhibit bacterial growth by a variety of mechanisms such as by damaging the bacterial cell wall, inhibiting protein synthesis, or disrupting DNA replication.
Bactericidal substances result in the death of the bacteria, whereas bacteriostatic substances prevent or slow down bacterial growth.
Antibiotics vary in their spectrum of activity; some are broad-spectrum effective against a wide variety of bacteria, while others have a narrow spectrum, effective against few bacterial species.

The disk diffusion method is a commonly used technique in AST.
It involves placing paper disks impregnated with antibiotics on an agar plate spread with the bacteria. The plate is incubated to allow bacterial growth and diffusion of the antibiotic.
The area where bacterial growth has been prevented (the zone of inhibition) is measured to assess the effectiveness of the antibiotic.
The larger the zone of inhibition, the more the bacteria are susceptible to the antibiotic; however, the interpretation needs to incorporate other factors like antibiotic diffusion and bacteria growth rate.

The broth dilution method measures the Minimum Inhibitory Concentration (MIC) or Minimum Bactericidal Concentration (MBC) of an antimicrobial.
In this method, a serial dilution of the antibiotic is prepared in broth medium, each containing the test bacteria.
MIC is the lowest concentration of antibiotic that visibly prevents bacterial growth, while the MBC is the lowest concentration that results in bacterial death.
The broth dilution method is more accurate and informative than disk diffusion, but also more time-consuming and resource-intensive.

Antimicrobial resistance is the ability of bacteria to withstand the effects of an antimicrobial that was previously effective against them.
Resistance can occur due to genetic mutations or by acquiring resistance genes from other bacteria.
Conducting AST regularly can help identify and monitor the pattern of resistance, supporting clinicians to prescribe the most effective treatment.