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Inoculum Effect

Description

The inoculum effect refers to an increase in the MIC for an isolate to an antibiotic associated with a higher inoculum into the liquid test media. This is classically seen with a Staphyloccus species tested against a beta-lactam antibiotic, but it has been reported with many bacteria against many antibiotics.


Criteria - both of the following:

(1) An increase in the inoculum size >= 0.5 LOG10 is associated with

(2) A significant increase in the MIC (at least 2 dilutions, or a 4-fold increase in the antibiotic concentration).

 

where:

• A 0.5 LOG10 increase is about 3.2 times.

 

Theories for its occurrence:

(1) inactivation by drug-targeting enzymes

(2) selection for a drug-resistant strain

 

Limitations:

• It is essential to control the inoculum size, incubation time, incubation period, reagent vessel and other testing factors.


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Specialty

ICD-10