Antimicrobial Susceptability Testing

Kirby Bauer Method

Antimicrobial impregnated filter paper disk

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Susceptibility test against more than one antibiotics by measuring size of zone of inhibition

paper disk

1949: Bondi and colleagues

filter paper disk

1966: Kirby, Bauer, Sherris and Tuck

4-5

Direct method (commonly used method)

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__ colonies of same morphology, 16-24 hours old selected from agar and suspended in broth or 0.85% saline

16-24

Direct method (commonly used method)

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4-5 colonies of same morphology, ___ hours old selected from agar and suspended in broth or 0.85% saline

broth, 0.85%

Direct method (commonly used method)

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4-5 colonies of same morphology, 16-24 hours old selected from agar and suspended in __ or _ saline

0\.5 McFarland density standard

adjust organism suspension turbidity to match a _______ (equivalent to 1.5 X 108 CFU)

Mueller-Hinton (M-H) agar base

KIRBY BAUER DISK DIFFUSION METHOD Growth medium

7\.2 to 7.4

KIRBY BAUER DISK DIFFUSION METHOD pH

15

Antibiotic disks are placed on M-H agar surface within __ minute of inoculation

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Incubate under recommended conditions within __ min. of disk addition

“S”, “I”, “R”

interpretive categories assigned using CLSI criteria

18-20 hours

The concentration of each disc is specified and after __ of incubation, inhibition zones around the discs are noted.

susceptible

implies that an infection due to the strain may be appropriately treated with the dosage of antimicrobial agent recommended for the type of infection and infecting species, unless otherwise contraindicated.

intermediate

category includes isolates with antimicrobial agent MICs that approach usually attainable blood and tissue levels and for which response rates may be lower than for susceptible isolates

resistant

strain are not inhibited by the usually achievable systemic concentrations of the agent with normal dosage schedules and/or

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fall in the range where specific microbial resistance mechanism are likely (e.g., β-Lactamases) and clinical efficacy has not been reliable in treatment studies.

Group A

primary testing panel, routine reporting of results for specific organisms

Group B

comprises agents that are important clinically, particularly to nosocomial infections. However,reportedselectively such as :

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When the organism is (R) to agents of the same class as in Grp. A.

Indications for reporting might include a selected specimen source (ex., 3 rd gen Ceph. For enteric bacilli from CSF or Co-trimoxazole for urine)

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Polymicrobial infections

Infection involvingmultiple sites

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On request in case of allergy intolerance or failure to respond to an agent in Grp. A.

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Forreporting to infection control as an epidemiologic aid.

Group C

Alternative or supplemental antimicrobial agents that may:

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Require testing in those institution that harbor endemic or epidemic strains resistant to several of the primary drugs esp. in the same class (ex. Β- lactams or aminoglycosides)

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Treatment of patients allergic to primary drugs

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Treatment of unusual organisms (ex. C for extra-intestinal Salmonella spp., or some VRE)

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For reporting to infection control as an epidemiologic aid.

Group U

used primarily for treating UTI, should not be routinely reported against pathogens recovered from other sites of infections.

Group O (“Other”)

Includes agents that have clinical indications for the organism group but are generally not candidates for routine testing

Group Inv (investigational)

includes antimicrobial agents that are investigational for the organism group and have not yet been approved by the FDA for the use in United States