Question

The advantages and disadvantages of using disk-diffusion assay in screening extracts, fractions or pure substances for their antimicrobial potency or to investigate the antagonism between microorganisms. (essay form)

Answer 1

ADVANTAGES OF DISC DIFFUSION ASSAY -

1.screening substances or extracts for their antimicrobial potency is important to develop new therapeutics. With increasing anti microbial resistance , new discoveries are must.
Disc diffusion assay is commonly used for this. IT HAS THE ABILITY TO TEST MULTIPLE MICROBES AND ANTIMICROBIAL AGENTS AT THE SAME TIME. More testing can be done in lesser time.
it is widely used to screen plant extracts, oils,etc for antibiotic use .

2.IT IS VERY EASY TO PERFORM.
✓ spread a bacterial inoculum to a 150 mm diameter Meuller Hinton agar plate.
✓ upto 12antibiotic discs can be placed on the agar surface.
✓ incubate the plate.
✓ measure zone of growth inhibition around each disc.
•no special equipment needed.
•no highly specialised professional needed. It can easily be performed by a clinician.

RESULTS ARE ALSO EASY TO INTERPRET.

3.THIS TEST IS EXTREMELY CHEAP. it costs only 2-5 dollars per test.

4.has low variability coefficient.

5.REQUIRES NO SPECIAL AND COSTLY EQUIPMENTS - it can be performed by routine materials present in microbiology laboratory . So, it can be performed in flexible conditions.

6.IS USEFUL TO STUDY ANTAGONISM BETWEEN MICROBES . since this test allows many microorganisms to grow at once, it can be used to check antagonism between different microbes. Studying the zone of growth inhibition around test microbes can help you determine their relation.

DISADVANTAGES
1. HIGH CHANCES OF ERROR
•size of zone of inhibition depends upon the disc size. Wider disc produce wider zones.
•type of paper also affects the rate of diffusion.
•error may occur in the potency of drug in different discs.
•time interval between inouculation and intubation is very specific. If exceeded, it affects zone of inhibition.
•shape of zone of inhibition is not smooth. Measuring these rough zones can be an issue.

2.CANNOT BE USED TO GROW ALL TYPES OF BACTERIA. this test is limited in its application. It cannot be used to test all types of bacteria. However , it has been standardized for bacteria like hemophilia influenzae via use of specialised media.

3.IT CANT TELL ANTIBIOTIC PERFORMANCE IN INVIVO SITUATIONS. Antibiotics differ in their ability to diffuse in the agar media of this method. The media does alter the potential of antibiotic.

4. Relative antibiotic susceptibility can't be accurately interpreted.