In Vivo Effects of Cefazolin, Daptomycin, and Nafcillin in Experimental Endocarditis with a Methicillin-Susceptible Staphylococcus aureus Strain Showing an Inoculum Effect against Cefazolin

Abstract

Several reports have implicated the inoculum effect that some strains of type A beta-lactamase (Bla)-producing, methicillin-susceptible Staphylococcus aureus (MSSA) show against cefazolin as the cause for clinical failures in certain serious deep-seated infections. Here, using a previously reported MSSA strain displaying this phenotype (TX0117), we obtained a Bla-cured derivative (TX0117c) with a combination of novobiocin and high temperature. Both isolates were then used in a rat endocarditis model and treated with cefazolin, nafcillin, and daptomycin, given to simulate human dosing. Animals were treated for 3 days and either sacrificed at 24 h after the last antibiotic dose (standard group) or left untreated for an additional 3 days (relapse group). With TX0117 in the standard treatment group, daptomycin and nafcillin were both significantly better than cefazolin in reducing CFU/g of vegetations, achieving mean log 10 reductions compared to levels in untreated rats of 7.1, 5.3, and 1.8, respectively (cefazolin versus daptomycin, P < 0.0001; cefazolin versus nafcillin, P ‫؍‬ 0.005; daptomycin versus nafcillin, P ‫؍‬ 0.053). In addition, cefazolin was significantly more effective in reducing vegetation titers of TX0117c than of TX0117 (mean log 10 reduction of 1.4 versus 5.5, respectively; P ‫؍‬ 0.0001). Similar results were observed with animals in the relapse group. Thus, these data show that there can be an in vivo consequence of the in vitro inoculum effect that some MSSA strains display against cefazolin and indicate a specific role for Bla production using a Bla-cured derivative strain against which cefazolin regained both in vitro and in vivo activity. Staphylococcus aureus continues to be an important pathogen causing invasive infections associated with significant morbidity and mortality. These infections include endocarditis, pneumonia, bacteremia, infections of prosthetic devices and catheters, complicated abscesses, osteomyelitis, and other deep-seeded infections, and methicillin-susceptible S. aureus (MSSA) strains still cause a significant proportion of these infections. Currently, more than 90% of S. aureus isolates produce ␤-lactamase (Bla), and four different variants of this enzyme (A, B, C, and D) have been identified by different methods (1-4). The type A Bla was shown to have a higher rate of hydrolysis of cefazolin (5), and several cases of cefazolin clinical failures have been reported in patients with endocarditis caused by MSSA strains showing an inoculum effect against this cephalosporin (6-9). These strains typically display an MIC of cefazolin of greater than 50 g/ml when tested at a high inoculum, associated with cefazolin degradation (7,8). The clinical impact of S. aureus strains showing an inoculum effect against cefazolin on treatment outcome is of potential concern since (i) cefazolin is an option for the treatment of MSSA endocarditis in patients with non-immediate-type hypersensitivity to penicillin (10, 11), (ii) many physicians discharge patients to complete a course of home...

Efficacy of Ceftaroline against Methicillin-Susceptible Staphylococcus aureus Exhibiting the Cefazolin High-Inoculum Effect in a Rat Model of Endocarditis

“…Indeed, a significant difference in bacterial counts in vegetations was observed in CFZ-treated animals compared with daptomycin-and NAF-treated animals. The efficacy of CFZ in vivo was restored when the strain was cured of ␤-lactamase (TX0117c) (21). The results of the current work reiterate that the CFZ InE can influence the efficacy of CFZ in vivo (4,5).…”

“…We utilized doses of NAF and CFZ that, although shown to be effective in previous experimental IE models, were chosen without guidance from PK evaluation or serum concentrations (21,44,45). However, the efficacy of these agents (NAF and CFZ) was confirmed in our experiments despite the limitation.…”

“…Aortic valve endocarditis was produced in male Sprague-Dawley rats weighing ϳ200 g following our previously published methods (21,56). In brief, the animals were anesthetized with isoflurane for intravascular catheter placement.…”

“…Fifteen MSSA (␤la type A) strains from a previously published study (5) obtained from patients included in clinical studies of acute bacterial skin/soft tissue infections (ATLAS phase III trials), hospital-associated pneumonia (ATTAIN phase III trials), and endocarditis (ICE cohort) (5,(50)(51)(52)(53) were included to determine CPT MICs. The previously described strains S. aureus TX0117 (harboring ␤la type A and exhibiting the CFZ InE) and its ␤la-cured derivative, TX0117c (21,22), were used in the rat IE model. The sequences of blaZ genes from all MSSA strains were already available from a previous study (5), confirming that all the strains harbored ␤la type A exhibiting Thr128 and Ser216 substitutions, which distinguishes it from other ␤la types (12).…”

“…The goal of the present study was to assess the in vivo efficacy of CPT against a previously characterized MSSA strain that exhibits the CFZ InE (TX0117) and its ␤la-cured derivative (TX0117c) (21,22) using a rat model of infective endocarditis (IE). Moreover, using the same model, we compared the in vivo efficacy of CPT with those of CFZ and NAF.…”