Antimicrobial susceptibility testing

Antibiotic sensitivity testing or antibiotic susceptibility testing is the measurement of the susceptibility of bacteria to antibiotics. It is used because bacteria may have resistance to some antibiotics. Clinicians need to be sure if the available antibiotics can inhibit the growth of further multiplication of bacteria in question. 

In this era of antimicrobial resistance, it is recommended that clinical settings initiate antibiotic susceptibility testing to ensure that the prescribed antibiotic is the right one and the causative germ is susceptible to it. 

How does AST work?

The above picture shows a culture medium on which a specific bacterium is grown. Several types of antibiotics are then introduced to the medium to see which ones will affect the bacteria. Each small white circle contains a different antibiotic.

After a given time, the antibiotics slowly dissolve in the culture medium and start to kill bacteria around them, clearing the surrounding area. Comparing the left and right plates, it is clear that the antibiotics in the left plate were more effective in killing bacteria around them, while only two antibiotics on the right plate were able to produce the same effect. From this, clinicians can determine which antibiotics are effective for treating a specific infection based on the results of antimicrobial susceptibility testing (AST).

Role of AST in Containing AMR

Knowledge of bacterial antimicrobial susceptibility testing (AST) patterns is the cornerstone of an effective clinical and public health response to antimicrobial resistance (AMR). Reliable AST results are decision-making support tools that enable clinicians to prescribe appropriate antibiotics for patients. As a public health tool, AST data describing the prevalence, geographic distribution, and temporal trends of resistant pathogens should inform the standard treatment guidelines developed in the Pacific region.

AST not only serves as a public health tool but also improves patient outcomes by ensuring that the prescribed antibiotic is the right one for the infecting bacteria. AST plays a critical role in preventing the overuse and misuse of antimicrobials. Some clinicians may prefer to prescribe empirically and use more than one antibiotic or broad-spectrum agents to increase the chances of covering the targeted pathogen. This practice unnecessarily exposes our normal microbiota to strong and multiple types of antibiotics, giving bacteria the opportunity to learn how these antibiotics work and develop resistance against them.