The Inoculum Effect in the Era of Multidrug Resistance: Minor Differences in Inoculum Have Dramatic Effect on Minimal Inhibitory Concentration Determination.

The Inoculum Effect in the Era of Multidrug Resistance: Minor Differences in Inoculum Have Dramatic Effect on Minimal Inhibitory Concentration Determination. Antimicrob Agents Chemother. 2018 May 21;: Authors: Smith KP, Kirby JE Abstract The observed MIC may depend on the number of bacteria initially inoculated into the assay. This phenomenon is termed the inoculum effect (IE) and is often most pronounced for β-lactams in strains expressing β-lactamase enzymes. The Clinical and Laboratory Standards Institute (CLSI) recommended inoculum is 5 x 105 CFU mL-1 with an acceptable range of 2-8 x 105 CFU mL-1 IE testing is typically performed using an inoculum 100-fold greater than the CLSI recommended inoculum. Therefore, it remains unknown whether the IE influences MICs during testing performed according to CLSI guidelines. Here, we utilized inkjet printing technology to test the IE on cefepime, meropenem, and ceftazidime-avibactam. First, we determined that inkjet dispense volume correlated well with the number of bacteria delivered to microwells in two-fold (R2 = 0.99) or 1.1-fold (R2 = 0.98) serial dilutions. We then quantified the IE by dispensing orthogonal titrations of bacterial cells and antibiotics. For cefepime resistant and susceptible dose-dependent strains, a 2-fold increase in inoculum resulted in a 1.6 Log2-fold increase in MIC. For carbapenemase-producing strains, each 2-fold reduction in inoculum resulted in a 1.26 Log2-...
Source: Antimicrobial Agents and Chemotherapy - Category: Microbiology Authors: Tags: Antimicrob Agents Chemother Source Type: research