Effective Date: 
Thu, 08/01/2013
Fri, 01/05/2018
Wed, 04/12/2017


Purpose and Scope:


The Kirby-Bauer test, known as the disk-diffusion method, is the most widely used antibiotic susceptibility test in determining what choice of antibiotics should be used when treating an infection. This method relies on the inhibition of bacterial growth measured under standard conditions. For this test, a culture medium, specifically the Mueller-Hinton agar, is uniformly and aseptically inoculated with the test organism and then filter paper discs, which are impregnated with a specific concentration of a particular antibiotic, are placed on the medium. The organism will grow on the agar plate while the antibiotic “works” to inhibit the growth.  If the organism is susceptible to a specific antibiotic, there will be no growth around the disc containing the antibiotic. Thus, a “zone of inhibition” can be observed and measured to determine the susceptibility to an antibiotic for that particular organism. The measurement is compared to the criteria set by the National Committee for Clinical Laboratory Studies (NCCLS). Based on the criteria, the organism can be classified as being Resistant (R), Intermediate (I) or Susceptible (S).


Principle of the method:


The media used in this test has to be the Mueller-Hinton agar because it is an agar that is thoroughly tested for its composition and its pH level. Also, using this agar ensures that zones of inhibitions can be reproduced from the same organism, and this agar does not inhibit sulfonamides.  The agar itself must also only be 4mm deep.  This further ensures standardization and reproducibility.

The size of the inoculated organism must also be standardized (using the BBD Prompt system).  The reasons are because if the size of the inoculum is too small, the zone of inhibition will be larger than what it is supposed to be (“the antibiotics will have a distinct advantage”) and if the inoculum is too large, the zone of inhibition will be smaller.



Specimen Requirement:

Well isolated colonies from BAP agar plate only.


 Quality Control:

Perform weekly Quality Control for the antimicrobial disks by setting up the stock culture of E. coli (ATCC 25922) for Kirby Bauer sensitivities. The zones sizes should be as follows:


E. Coli (ATCC 25922) S. Aureus (ATCC 25923)

Ampicillin 10  16-22mm

        Cefazolin 29-35  mm

Cephalothin 30  15-21 mm          Clindamycin 24-30  mm

Ciprofloxacin 5  30-40 mm

Oxacillin (Cefoxitin) 23-29 mm
Nitrofurantoin 300  20-25 mm Penicillin 26-37 mm

Sulfisoxazole 250  15-23 mm

 Tetracycline 24-30 mm

Trimeth/Sulfa 23-29 mm
Trimeth/Sulfa 24-32 mm




Reagents and Supplies:


Mueller Hinton agar plates

BBL BD Sensi Disks (various antimicrobials as in QC above)

37 C Incubator

Sterile polyester or cotton swabs

Hardy Diagnostic Saline tubes

McFarland Latex 0.5 Standard and Wickerham Card

Calipers, ruler, or template for measuring the diameters of inhibitory zones.




  • Preparation of Bacterial Suspension


  1. Remove a Hardy Diagnostic Saline 0.85%, 1.8mL tube from the box, label with the patient name and place in a test tube rack.
  2. Using a 1ul loop, pick several isolated colonies from the agar surface.
  3. Immerse the loop in a labeled saline tube. Vortex.
  4. Using a Wickerham Card and a vortexed McFarland Latex 0.5 standard, compare the turbidity of the inoculated saline tube with the Standard.  If the turbidity is comparable, proceed with the inoculation of the Mueller Hinton Plate. If not, adjust the turbidity by adding more isolated colonies in the same manner if the turbidity is less than the standard or more saline if the turbidity is greater.  Once the turbidity is comparable to the standard, proceed with the inoculation of the labeled Mueller Hinton plate.
  5. The bacterial suspension should be used within 6 h of preparation. If not used immediately after preparation, shake vigorously to resuspend the   bacteria just prior to use.
  • B. Inoculation of Mueller Hinton Agar

 Allow plates to come to room temperature before use.

  1. Dip a sterile cotton swab into the bacterial suspension. To remove excess liquid, rotate the swab several times with a firm pressure on the inside wall of the tube above the fluid level.
  2. Using the swab, streak the Mueller-Hinton agar plate to form a bacterial lawn.
  3. To obtain uniform growth, streak the plate with the swab in one direction, rotate the plate 90° and streak the plate again in that direction.
  4. Repeat this rotation 3 times.
  5. Allow the plate to dry for approximately 5 minutes.
  6. Use an Antibiotic Disc Dispenser to dispense disks containing specific antibiotics onto the plate.
  7. Using sterile sticks or loops, gently press each disc to the agar to ensure that the disc is attached to the agar.
  8. Plates should be incubated overnight at an incubation temperature of 37°C.


C. Reading and Interpreting Zone Sizes


  1. After overnight incubation measure the zone sizes (area of no growth around the disk) in millimeters using a ruler or template.
  2. Enter the zone sizes into the Kirby Bauer sensitivities log along with the patients information.
  3. Interpret the results as Resistant, Intermediate or Sensitive for each antimicrobial according to the ranges listed on the log for Enteric gram negative rods.
  4. Enter the results (R, I or S) into the LIS.
  5. For Staph species that are Cefoxitin (Oxacillin) Resistant, Penicillin and Cefazolin will be reported as "not effective for MRSA".




This system is only set up for Enteric gram negative rods or a Staph aureus gram positive cocci. Therefore make sure the gram negative organism is lactose fermenting,and oxidase negative before setting up.

Non-enterics, streptococci/enterococci, gram positive rods and gram negative cocci must be sent out to Quest if sensitivities are needed.




BD BBL Prompt Inoculation System Pkg insert #5308-10  Rev 06/2010

Bauer, A.W., W.M.M. Kirby, J.C. Sherris, and M. Turck. 1966. Antibiotic

susceptibility testing by a standardized single disk method. Am. J. Clin.

Pathol. 45:493 496.


National Committee for Clinical Laboratory Standards. 2003. Approved

standard: M2-A8. Performance standards for antimicrobial disk susceptibility

tests, 8th ed. National Committee for Clinical Laboratory Standards, Wayne,