DR.T.V.RAO MD 1

Dr.T.V.Rao MD

KIRBY-BAUER DISK DIFFUSION TESTING

SKILL BASED LEARNING

DR.T.V.RAO MD 2

PRINCIPLES OF SUSCEPTIBILITY TESTING

DR.T.V.RAO MD 3

• Kirby – Bauer Agar disk diffusion method provides qualitative interpretive category results of susceptible, intermediate, and resistant bacterial isolates.

KIRBY-BAUER TEST IS A AGAR DISK DIFFUSION METHOD

DR.T.V.RAO MD 4

KIRBY-BAUER ANTIBIOTIC TESTING• Kirby-Bauer antibiotic testing (KB testing or disk

diffusion antibiotic sensitivity testing) is a test which uses antibiotic-impregnated paper disks to test whether particular bacteria are susceptible to specific antibiotics. A known quantity of bacteria are grown on agar plates in the presence of thin filter paper discs containing relevant antibiotics. If the bacteria are susceptible to a particular antibiotic, an area of clearing surrounds the wafer where bacteria are not capable of growing (called a zone of inhibition).

5

5

AGAR DISK DIFFUSION METHOD• Medium Mueller Hinton 4 mm thickness

pH 7.2 to 7.4

• Antibiotic storage -20oC minimum

disks temperature

• Inoculum McFarland 0.5 (108 bacteria/mL)

• Incubator temperature 35oC

• atmosphere ambient air

DR.T.V.RAO MD 6

PROCEDURE• Prepare a pure culture (18-24 hrs) of the sample on a non-

selective medium

• Adjust turbidity until it is equivalent to the 0.5 McFarland Turbidity Standard

0.5 McFarland StandardSample

DR.T.V.RAO MD 7

Select at least 4 to 5 well-isolated colonies of the same morphological type from an agar plate. Touch the top of each colony with a wire loop and transfer the growth to a tube containing 4 to 5 mL of a suitable broth medium, such as tryptic-soy broth. Allow the broth culture to incubate at 35°C until it achieves or exceeds the turbidity of 0.5 McFarland standard.

ASEPTIC TRANSFER OF COLONIES FOR PROPAGATION OF BACTERIA

DR.T.V.RAO MD 8

CHOOSING A COLONY FOR TESTING

DR.T.V.RAO MD 9

MCFARLAND 0.5 AND ADJUSTED TEST ORGANISM

DR.T.V.RAO MD 10

• Within 15 minutes after adjusting the turbidity of the inoculum suspension, dip a sterile non-toxic swab on an applicator into the adjusted suspension. Rotate the swab several times, pressing firmly on the inside wall of the tube above the fluid level to remove excess inoculum from the swab.

INOCULATE THE PLATE WITH UNIFORMITY

DR.T.V.RAO MD 11

• Inoculate the dried surface of a Muller-Hinton agar plate by streaking the swab over the entire sterile agar surface. Repeat this procedure two more times, and rotate the plate 60° each time to ensure an even distribution of inoculum. Replace the plate top and allow 3 to 5 minutes, but no longer than 15 minutes, for any excess surface moisture to be absorbed before applying the antibiotic disks.

UNIFORM INOCULUM ON PLATES IS ESSENTIAL FOR EFFECTIVE DETECTION

DR.T.V.RAO MD 12

• Within 15 minutes of adjusting the turbidity

• dip a sterile cotton swab into the sample

• streak a lawn of bacteria on Mueller-Hinton agar

Leave the lid agar for 3-5 minutes (no more than 15 minutes) to allow plate to dry

PROCEDURE

DR.T.V.RAO MD 13

• The agar medium should have pH 7.2 to 7.4 at room temperature. The surface should be moist but without droplet of moisture. The antibiotic disks should be maintained at 8°C or lower or freeze at -14°C or below until needed, according to the manufacturer's recom mendations. Allow the disks to warm to room temperature before use. Don't use expired disks.

BE CAUTIOUS

DR.T.V.RAO MD 14

• There should be an almost confluent lawn of growth when done properly. If only isolated colonies grow, the inoculum was too light and the test should be repeated. To avoid extremes in inoculum density, never use undiluted overnight broth cultures for streaking plates./li>

GETTING IDEAL RESULTS DEPEND ON RIGHT INOCULUM

DR.T.V.RAO MD 15

PROCEDURE• Apply antibiotic impregnated disks on the bacterial lawn

• Important: where the disk drops is where it stays

• Incubate for 16-18 hours at 33 ± 2oC unless otherwise instructed

DR.T.V.RAO MD 16

• Place the appropriate disks evenly (no closer than 24 mm from center to center) on the surface of the agar plate either by using a sterile forceps or the dispensing apparatus. No more than 12 disks should be placed on one 150 mm plate or more than 5 disks on a 100 mm plate. A disk is not to be moved once it has come in contact with the agar surface since some of the compound diffuses almost instan taneously.

DO NOT LOAD THE PLATES WITH TOO MANY ANTIBIOTIC DISKS

DR.T.V.RAO MD 17

PROCEED WITH INCUBATION OF PLATES

• Invert the plate and place them in an incubator at 35°C within 15 minutes after disks are applied. The plates should be incubated aerobically . After 16-18 hrs. of incubation, examine each plate and measure the diameters of the zones of complete inhibition, including the diameter of the disk. Measure the zones to the nearest millimeter using a ruler. Large colonies growing within a clear zone of inhibition should be subcultured, reidentified and retested to know whether they are contaminants or mutants

The main concept is the “clinical categorisation"

• Strains are sorted according to level of Minimal Inhibitory Concentration (MIC) versus reference breakpoints

• c and C are the minor and major breakpoints

Susceptible Intermediate Resistant

MIC < c ≤ MIC < C ≤ MIC

INTERPRETATION

UNDERSTANDING BREAKPOINTS Words of laboratory specialists

• It is not possible to work alone

• Breakpoints are the expression of a consensus among the scientific community at a given time in a country

Breakpoints are determined using two approaches

• Pharmacological concept

• Epidemiological concept

DR.T.V.RAO MD 20

RESULTS• Antibiotics diffuse out onto the agar

• Concentration of antibiotics decrease as they diffuse further away from the disks

• After incubation, observe for a clearing on the bacterial lawn (zone of inhibition)

Bacterial growth

Zone of inhibition

incubation

DR.T.V.RAO MD 21

• Interpret the zone sizes by referring to the manufacturer provided standard table and report the organism to be either susceptible, intermediate, or resistant. Never compare the zone sizes of two different antibiotics and judge their effectiveness accordingly.

FOLLOW THE GUIDELINES FOR MEASURING ZONES OF RESISTANCE AND SUSCEPTIBILITY

DR.T.V.RAO MD 22

RESULTS• Measure the diameters of the zone of inhibition

• Interpret the results as “resistant” or “susceptible” according to the guideline provided by the CLSI

• Interpretation of the zone of inhibition is different for each bacteria-antibiotic combination

DR.T.V.RAO MD 23

23

DISK SUSCEPTIBILITY TESTING PROBLEMSPROPER INOCULUM AND HEAVY INOCULUM

DR.T.V.RAO MD 24

DISK SUSCEPTIBILITY TESTING PROBLEMSDISK PROPERLY APPLIED DISK NOT PROPERLY APPLIED

DR.T.V.RAO MD 25

AN AGAR GEL THAT IS TOO THICK LEADS TO SMALLER ZONES

Precision and accuracy ensured through control strains• Known susceptibility to

antimicrobial agents

Standard strains include

• Staphylococcus aureus ATCC 25923

• Escherichia coli ATCC 25922

• Pseudomonas aeruginosa ATCC 27853

STANDARD STRAINS FOR QUALITY ASSURANCE

DR.T.V.RAO MD 27

E-TEST®

• Same principle as the Kirby-Bauer Test

• Uses a plastic strip with a predefined gradient of antibiotic concentration

• Results are read directly on the strip where the zone of inhibition intersects with the strip

E Zone of inhibition

Bacterial growth

DR.T.V.RAO MD 28

RESULTS• Interpret results as “resistant” or “susceptible” according

to the guidelines provided in the package insert

• For ambiguous results, refer to the provided reading guide for :

• Organism related effects

• Drug related effects

• Resistance mechanism related effects

• Technical and handling effects

29

PATIENT RESULTS MAY BE INCORRECT IF:

• The organism was misidentified

• A clerical error was made

• Inappropriate choice of antimicrobials were tested and reported

• The wrong patient’s sample was examined

• The wrong test was ordered

• The sample was not preserved properly

ANTIMICROBIAL SUSCEPTIBILITY TESTING GOALS

• When possible, don’t duplicate systems

• Meet regulatory and other guiding requirements

• Use automation when possible and technically sound

• Have flexibility in antimicrobial battery

• Focus technologist time on tasks that utilize their training and expertise

• Get accurate results out as soon as possible

DR.T.V.RAO MD 31

• Programme created by Dr.T.V.Rao MD for Microbiologists in the Developing

World• Email

• doctortvrao@gmail.com