Diagnostic Laboratory Section

Procedure

PROCEDURE FOR ANTIBIOGRAM

• Keep the required number of MH agar plates, properly labeled after checking for the absence of any visible growth.
• A sterile swab is dipped into the standardized suspension (prepared test sample ) of bacteria and excess fluid is removed by pressing the swab on the sides of the test tube above the fluid level.
• The swab is streaked over the entire surface of the agar with an aim of getting uniform inoculations. A final sweep with the swab can be made around the rim of the petridish.
• In the case of testing Streptococci 5-10% sheep blood agar is used for inoculation.
• The inoculated plates are allowed to stand for 3-5 minutes but no longer than 15 minutes for any excess of moisture from the inoculum to be absorbed by the agar before applying antimicrobial discs.
• The discs are placed on to the agar surface using disc dispenser or sterile forceps, not closer than 24 mm (center to center), 14 mm (periphery).
• Each disc is gently pressed on the agar with the help of forceps to get complete contact with the agar.
• The plates are to be kept in the incubator within 15 minutes of applying the disc and incubate for 16-18 hours. Plates must not be incubated under increased concentration of CO2, as this will alter the zone of inhibition.