Question

What is the difference between a qualitative and quantitative test. How would this be related to bacterial susceptibility?

Answer 1

The susceptibility of bacteria to certain antibacterial drugs is performed by both the qualitative and quatitative tests. The qualitative tests enable to know whether the bacteria are susceptible or inhibited by certain antibacterial drug or not (or resistant to drugs). The qualitative tests categorize the bacteria into three categories such as susceptible (S), intermediate (I) and resistant (R).

The susceptible category bacteria do not grow in presence of specific antibiotic drug. The intermediate category bacteria show intermediate growth in presence of specific antibiotic. The resistant category bacteria exhibit extensive growth in presence of particular antibiotic drug. The commonly used qualitative method is the disk diffusion method or Kirby - Baur test. In this test, antibiotic disks are impreganted in growth plates of the bacteria or test culture and incubated. The zone diameter is measured after incubation allow the bacteria to categorize into S, I or R strains.

The quantitive methods involve the determination of minimum concentration of drug that inhibits the growth of specific bacteria. This method is known as minimum inhibitory concentration and it results in numeric values. The test results categorize the bacteria into sensitive (S), intermediate (I), resistant (R) and non-susceptible (NS). The minimum inhibiotry concentration test can be done using agar and broth cultures. Therefore, qualitative tests determine the activity of drug on bacteria whether it inhibits growth or not. The quantitative tests determine the concentration of the drug that can inhibit the growth of bacteria.