In vitro activity of tedizolid against linezolid-resistant staphylococci and enterococci

The tedizolid MIC of 27 clinical isolates of linezolid-resistant staphylococci and enterococci was determined. Tedizolid MICs were ≥1μg/mL and were 4- to 32-fold lower than those of linezolid. Linezolid resistance mechanisms included G2576T 23S rRNA gene and rplC and rplD mutations.

In vitro activity of ceftaroline against staphylococci from prosthetic joint infection

We tested the in vitro activity of ceftaroline by Etest against staphylococci recovered from patients with prosthetic joint infection, including 97 Staphylococcus aureus isolates (36%, oxacillin resistant) and 74 Staphylococcus epidermidis isolates (74%, oxacillin resistant). Ceftaroline inhibited all staphylococci at ≤0.5 μg/mL. The ceftaroline MIC(90/50) values for methicillin-susceptible S. aureus, methicillin-susceptible S. epidermidis, methicillin-resistant S. aureus, and methicillin-resistant S. epidermidis were 0.19/0.125, 0.094/0.047, 0.5/0.38, and 0.38/0.19 μg/mL, respectively. Based on these in vitro findings, ceftaroline should be further evaluated as a potential therapeutic option for the treatment of prosthetic joint infection caused by methicillin-susceptible and methicillin-resistant S. aureus and S. epidermidis.

Elution of High Dose Amphotericin B Deoxycholate From Polymethylmethacrylate

Fungal periprosthetic joint infections are rare, devastating complications of arthroplasty. There is conflicting evidence as to the efficacy of amphotericin B elution from cement spacers. The purpose of this study was to determine whether concentrations of amphotericin B released from bone cement over time would be efficacious in treating a periprosthetic infection. A continuous flow chamber was used to evaluate the in vitro release of amphotericin from cement beads containing 7.5% amphotericin. Following polymerization, 3.3% of the initially loaded amphotericin B was detected. The peak mean concentration eluted from the bone cement was 0.33 μg/mL at 8 hours. The AUC0-24 was 2.79 μg/mL/h; 0.20% of the amphotericin B was released. In conclusion, amphotericin B is released from bone cement at a clinically useful concentration.

Superantigens produced by catheter-associated Staphylococcus aureus elicit systemic inflammatory disease in the absence of bacteremia

SAgs, produced by Staphylococcus aureus, play a major role in the pathogenesis of invasive staphylococcal diseases by inducing potent activation of the immune system. However, the role of SAgs, produced by S. aureus, associated with indwelling devices or tissues, are not known. Given the prevalence of device-associated infection with toxigenic S. aureus in clinical settings and the potency of SAgs, we hypothesized that continuous exposure to SAgs produced by catheter-associated S. aureus could have systemic consequences. To investigate these effects, we established a murine in vivo catheter colonization model. One centimeter long intravenous catheters were colonized with a clinical S. aureus isolate producing SAgs or isogenic S. aureus strains, capable or incapable of producing SAg. Catheters were subcutaneously implanted in age-matched HLA-DR3, B6, and AE(o) mice lacking MHC class II molecules and euthanized 7 d later. There was no evidence of systemic infection. However, in HLA-DR3 transgenic mice, which respond robustly to SSAgs, the SSAg-producing, but not the nonproducing strains, caused a transient increase in serum cytokine levels and a protracted expansion of splenic CD4(+) T cells expressing SSAg-reactive TCR Vβ8. Lungs, livers, and kidneys from these mice showed infiltration with CD4(+) and CD11b(+) cells. These findings were absent in B6 and AE(o) mice, which are known to respond poorly to SSAgs. Overall, our novel findings suggest that systemic immune activation elicited by SAgs, produced by S. aureus colonizing foreign bodies, could have clinical consequences in humans.

Ceftriaxone susceptibility of oxacillin-susceptible Staphylococcus aureus from patients with prosthetic joint infection

Ceftriaxone is used to treat oxacillin-susceptible S. aureus (OSSA) prosthetic joint infection (PJI). Susceptibility of ceftriaxone against OSSA has been questioned. Ceftriaxone susceptibility was determined against 100 PJI OSSA isolates. Ceftriaxone MIC90/MIC50 were 8/4 and 4/3μg/mL by broth microdilution and Etest, respectively. Ceftriaxone susceptibility is inferable by oxacillin susceptibility.

Superantigen profiling of Staphylococcus aureus infective endocarditis isolates

The frequency of superantigen production among Staphylococcus aureus isolates associated with endocarditis is not well defined. We tested 154 S. aureus isolates from definite infective endocarditis cases for the presence of staphylococcal enterotoxins A-E, H, and TSST-1 by PCR, enzyme-linked immunosorbent assay, and using an HLA-DR3 transgenic mouse splenocyte proliferation assay. Sixty-three isolates (50.8%) tested positive for at least 1 superantigen gene, with 21 (16.9%) testing positive for more than 2. tst (28.6%) was most common, followed by seb (27%), sea (22.2%), sed (20.6%), see (17.5%), and sec (11.1%). Of 41 methicillin-resistant S. aureus, 21 had superantigen genes, with sed being more frequently detected in this group compared to methicillin-susceptible S. aureus (P < 0.05). Superantigen genes were not associated with mortality (P = 0.81). 75% of PCR-positive isolates induced robust splenocyte proliferation. Overall, more than half of S. aureus isolates causing endocarditis carry superantigen genes, of which most are functional.

Treatment of Methicillin-resistant Staphylococcus aureus experimental Osteomyelitis with bone-targeted Vancomycin

INTRODUCTION

Methicillin-resistant S. aureus (MRSA) is a common cause of bone and joint infection. BT2-peg2-vancomycin is an investigational bone-targeted formulation of vancomycin which we hypothesized would have increased antimicrobial activity compared to conventional vancomycin in a chronic experimental MRSA osteomyelitis model.

METHODS

We tested bone affinity using an hydroxyapatite (HA) binding assay, assessed the in vitro antimicrobial susceptibility of 30 MRSA isolates, and compared vancomycin and BT2-peg2-vancomycin in a rat experimental osteomyelitis model.

RESULTS

Vancomycin did not bind to hydroxyapatite (HA binding index = 0), whereas BT2-peg2-vancomycin showed appreciable binding (HA binding index = 57). The MIC50 was 1 μg/ml and the MIC90 was 2 μg/ml for both vancomycin and BT2-peg2-vancomycin. The MBC90 was 16 and 4 μg/ml for vancomycin and BT2-peg2-vancomycin, respectively. Treatment with 50 mg/kg of vancomycin every 12 hours (median, 4.73 log10 cfu/g), 63.85 mg/kg (equivalent to 50 mg/kg vancomycin) of BT2-peg2-vancomycin every 12 hours (median, 3.93 log10 cfu/g) or 63.85 mg/kg of BT2-peg2-vancomycin once per week (median, 5.00 log10 cfu/g) was more active than no treatment (median, 5.22 log10 cfu/g) (P =0.0481). Treatment with 63.85 mg/kg of BT2-peg2-vancomycin every 12 hours was more active than all other treatment regimens evaluated (P≤0.0150), but was associated with high plasma BT2-peg2-vancomycin levels, decreased animal weight, increased kidney size, creatinine and BUN, and leukocytosis with tubulointerstitial nephritis.

CONCLUSION

With optimization of pharmacokinetic parameters to prevent toxicity, BT2-peg2-vancomycin may be useful in the treatment of MRSA osteomyelitis.

Combinations of cefoxitin plus other β-lactams are synergistic in vitro against community associated methicillin-resistant Staphylococcus aureus

In vitro studies demonstrate that oxacillin minimal inhibitory concentrations (MICs) of methicillin-resistant S. aureus (MRSA) strains USA300 and 400 decrease in the presence of cefoxitin. The aim of this study was to characterize the activity of cefoxitin plus β-lactams against a collection of MRSA isolates. We assessed the in vitro antimicrobial activity of a selection of β-lactams alone and together with subinhibitory concentrations of cefoxitin against a collection of MRSA, methicillin-susceptible S. aureus (MSSA), and vancomycin-intermediate S. aureus (VISA) isolates using MICs and time kill assays. For community-associated (CA) MRSA strains USA300 and USA400, MICs of nafcillin, cefazolin, cephalexin, cefuroxime, ceftriaxone and cefotaxime decreased by 8- to 64-times in the presence of 10 μg/ml cefoxitin. In contrast, for hospital-associated (HA) strains COLn, N315, and Mu50, there was no change in any β-lactam MIC in the presence of cefoxitin. When combined with cefoxitin, the cephalexin MIC decreased for eight CA-MRSA and five MSSA sequence types but did not change for seven HA-MRSA sequence types. β-lactam/cefoxitin combinations were synergistic against CA- but not HA-MRSA strains in time kill assays. Cefoxitin combined with a variety of β-lactams enhances their activity against CA-MRSA strains in vitro. Further studies of combination β-lactam therapy may provide insight into β-lactam biology, penicillin binding protein cooperativity, and novel therapeutic strategies against MRSA.

Antimicrobial susceptibility and biofilm formation of Staphylococcus epidermidis small colony variants associated with prosthetic joint infection

We determined the frequency of isolation of non-aureus staphylococcal small colony variants (SCVs) from 31 patients with staphylococcal prosthetic joint infection (PJI) and described the antimicrobial susceptibility, auxotrophy, and biofilm-forming capacity of these SCVs. Eleven non-aureus SCVs were recovered, all of which were Staphylococcus epidermidis, and none of which was auxotrophic for hemin, menadione, or thymidine. Aminoglycoside resistance was detected in 5. Two were proficient, and 7 were poor, biofilm formers. With passage on antimicrobial free media, we observed a fluctuating phenotype in 3 isolates. We also noted a difference in antimicrobial susceptibility of different morphology isolates recovered from the same joints despite similar pulsed-field gel electrophoresis patterns. Our findings suggest S. epidermidis SCVs are common in PJI, and while they have a similar appearance to S. aureus SCVs, they do not necessarily share such characteristics as aminoglycoside resistance; auxotrophy for hemin, menadione, or thymidine; or enhanced biofilm formation. We also underscore the importance of antimicrobial susceptibility testing of all morphologies of isolates recovered from PJI.

Implant sonication for the diagnosis of prosthetic elbow infection

BACKGROUND

Periprosthetic infection is a potentially devastating complication of elbow arthroplasty, associated with formation of microbial biofilm on the implant surface. The definitive microbiologic diagnosis of periprosthetic infection after elbow arthroplasty may be difficult to establish. Our study aim was to compare the diagnostic accuracy of conventional periprosthetic tissue culture and culture of fluid derived from vortexing and bath sonication of the explanted hardware (a biofilm-sampling strategy).

MATERIALS AND METHODS

Patients undergoing revision elbow arthroplasty at our institution between July 2007 and July 2010, from each of whom 2 or more periprosthetic tissue cultures and 1 implant sonicate culture were obtained, were studied. A standardized definition of orthopedic implant-associated infection was applied.

RESULTS

We identified 27 subjects with aseptic failure and 9 with prosthetic elbow infection. Rheumatoid arthritis was the most common underlying disorder. The Coonrad-Morrey prosthesis was the most common type of implant used. The sensitivities of implant sonicate and periprosthetic tissue culture were 89% and 55%, respectively (P = .18), and the specificities were 100% and 93%, respectively (P = .16). Coagulase-negative staphylococci (n = 7) and Staphylococcus aureus (n = 2) were isolated in cases of infection.

CONCLUSION

Culture of the implant by sonication is at least as sensitive as periprosthetic tissue culture to detect prosthetic elbow infection.