P-742. Characterization of the Inoculum Effect in Battlefield Trauma Patients with Methicillin-Susceptible <i>Staphylococcus aureus</i> Infections

Abstract Background The clinical relevance of the cefazolin inoculum effect (CIE) in methicillin-susceptible Staphylococcus aureus (MSSA) isolates has been debated, with some studies showing poor outcomes in CIE(+) MSSA patients treated with β-lactams and others finding no effect. As less is known about CIE in trauma patients, we examined characteristics of US wounded military personnel with MSSA infections with and without CIE. Methods Data and initial / serial (defined as isolation ≥ 7 days from prior isolate) MSSA isolates were collected from the Trauma Infectious Disease Outcomes Study (6/09-12/14). Isolates collected for surveillance were excluded. Antimicrobial susceptibility testing was performed via BD Phoenix Automated Microbiology System using CLSI criteria. Isolates were tested for CIE by broth microdilution utilizing standard (∼5x105 cfu/mL) and high (HI) inocula (∼5x107 cfu/mL). CIE(+) was defined as a MIC increase to ≥ 16 μg/mL with HI. Results Among 112 MSSA patients, 6 had CEI(+) and 106 CIE(-) isolates. The patients were primarily males (99%) who sustained critical injuries (mean injury severity score 36) via blasts (65%), requiring critical care (80%) and prolonged hospitalization (mean 45 days, Table 1). All patients received 1st generation cephalosporin or β-lactam with Staphylococcus coverage. There were no significant differences between the CIE(+) and CIE(-) patients. No CIE(+) patients had serial isolates, and no patients in either group died. A total of 179 MSSA isolates were assessed: 9 (5%) CIE(+) and 170 (95%) CIE(-). The majority of MSSA isolates were from respiratory (N=87, 49%), wound (N=72, 40%), and blood cultures (N=12, 7%). All isolates were uniformly susceptible to cefazolin, ampicillin-sulbactam, and ceftaroline, and had reduced susceptibility to doxycycline and tetracycline (Table 2). When compared to CIE(-) isolates, CIE(+) isolates had decreased susceptibility to clindamycin and erythromycin. Conclusion CIE was uncommon with battlefield MSSA infections. Evaluation of clinical outcomes was limited (i.e., small numbers), but serial CIE(+) isolation was not observed. Although CIE(+) isolates were more resistant to clindamycin and erythromycin, isolates remained susceptible to other first-line antimicrobials. Disclosures All Authors: No reported disclosures