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Background The inoculum effect (InE) refers to an increase in the MIC of an antibiotic when a large burden of bacteria is present. MSSA producing type A or C β-lactamase (β-lac) that display this effect may be at risk of clinical failure when treated with cefazolin (CFZ) for a deep-seated infection. We have previously shown that CFZ plus clavulanic acid (CL) abolished the InE in vitro. The aim of this study was to evaluate the effectiveness of the combination in vivo at clinically achievable concentrations of both CFZ and CL. Methods S. aureus TX0117, a type A Bla+ clinical isolate from a patient who failed CFZ therapy and TX0117-cured (TX0117c), a derivative of TX0117 which lacks β-lac activity, were used in a rat model of endocarditis. One animal per strain, in addition to historical controls (n = 22), was sacrificed at the start of therapy to assess colony forming units (CFU) per gram of vegetation at T = 0. CFZ 50mg/kg alone (n = 11) or CFZ 50mg/kg plus CL 4mg/kg (n = 7) was given IM every 8 hours for 72 hours. Doses were selected to mimic mean serum concentration of standard doses (given IM (CFZ) or PO (CL)) in humans. Rats were sacrificed 16 hours after the last antibiotic dose, aortic valves were aseptically excised, weighed, homogenized in 1ml of saline and the entire volume was plated in serial 10-fold dilutions on mannitol salt and/or brain-heart infusion agar. Representative recovered colonies were tested for β-lac activity using nitrocefin. Comparisons of CFU between groups were done by the Mann–Whitney, Wilcoxon unpaired test with significance at p < 0.05. Results At baseline, there was no significant difference between the CFU/g of control animals infected with the two strains (TX0117 7.3±1.3 and TX0117c 7.89±0.83, mean log10 ± SD). Compared with untreated controls, the TX0117 group treated with CFZ alone had a reduction of 2±0.6 CFU/g, while the CFZ plus CL arm had a 7.1±0.5 CFU/g reduction, a statistically significant difference between the two arms (P = 0.0002). CFZ treatment of the TX0117c strain lacking blaZ activity was similar to CFZ+CL (6.5±0.6 log10 CFU/g reduction, P < 0.0001). Conclusion Against Bla+ TX0117, the addition of CL, at a dose mimicking human PO kinetics, restored the efficacy of CFZ and overcame the InE. This provides a proof-of-concept for the use of oral CL with CFZ when there is a concern for the InE. Disclosures All authors: No reported disclosures.

Cefazolin plus Clavulanic Acid Overcomes the Inoculum Effect in a Methicillin-Susceptible Staphylococcus aureus (MSSA) Rat Endocarditis Model