Determining vancomycin Etest MICs in patients with MRSA bloodstream infection does not support switching antimicrobials

OBJECTIVES

Elevated vancomycin minimum inhibitory concentrations (MIC) have been reported to adversely affect clinical outcome in methicillin-resistant Staphylococcus aureus (MRSA) bloodstream infection (BSI). We therefore examined the association between vancomycin MIC and outcome considering various potential confounders.

METHODS

Clinical data and bacterial isolates were prospectively collected from patients with MRSA BSI from 2006 to 2012 as part of the Invasive Staphylococcus aureus Infection Cohort (INSTINCT) study. Antimicrobial susceptibility was assessed by Etest, broth microdilution (BMD) and VITEK 2. Bacterial genotypes were determined by spa typing. Using univariate and Cox regression analyses, we investigated the impact of low (≤1.0 mg/L) and high (≥1.5 mg/L) vancomycin Etest MIC on clinical outcomes.

RESULTS

Ninety-one MRSA BSI episodes were included, of which 79 (86.8%) were caused by spa types t003, t032 and t045. High vancomycin MICs were seen only if using Etest but not confirmed using standard reference BMD. When episodes were stratified into low and high vancomycin Etest MIC groups, 30-day overall mortality was 34.5% and 27.3%, respectively (P = 0.64, OR 0.71; 95% confidence interval [CI] 0.27-1.79). Variables significantly associated with all-cause mortality in the Cox model were age (P = 0.003), acute physiology score (P = 0.0006), and Charlson comorbidity index (P = 0.018).

CONCLUSIONS

Vancomycin MICs may vary dependent on testing methodologies and local MRSA epidemiology. The patients' underlying disease and individual comorbidities rather than elevated vancomycin MICs determine adverse clinical outcomes in MRSA BSI. Routine Etest MIC testing of MRSA isolates is of limited value for treatment decisions.

Polymorphisms in Fibronectin Binding Proteins A and B among Staphylococcus aureus Bloodstream Isolates Are Not Associated with Arthroplasty Infection

Background

Nonsynonymous single nucleotide polymorphisms (SNPs) in fibronectin binding protein A (fnbA) of Staphylococcus aureus are associated with cardiac device infections. However, the role of fnbA SNPs in S. aureus arthroplasty infection is unknown.

Methods

Bloodstream S. aureus isolates from a derivation cohort of patients at a single U.S. medical center with S. aureus bacteremia (SAB) and prosthetic hip or knee arthroplasties that were infected (PJI, n = 27) or uninfected (PJU, n = 43) underwent sequencing of fnbA and fnbB. A validation cohort of S. aureus bloodstream PJI (n = 12) and PJU (n = 58) isolates from Germany also underwent fnbA and fnbB sequencing.

Results

Overall, none of the individual fnbA or fnbB SNPs were significantly associated with the PJI or PJU clinical groups within the derivation cohort. Similarly, none of the individual fnbA or fnbB SNPs were associated with PJI or PJU when the analysis was restricted to patients with either early SAB (i.e., bacteremia occurring <1 year after placement or manipulation of prostheses) or late SAB (i.e., bacteremia >1 year after placement or manipulation of prostheses).

Conclusions

In contrast to cardiac device infections, there is no association between nonsynonymous SNPs in fnbA or fnbB of bloodstream S. aureus isolates and arthroplasty infection. These results suggest that initial steps leading to S. aureus infection of cardiovascular and orthopedic prostheses may arise by distinct processes.

Autoinfection as a cause of postpartum subdural empyema due to Mycoplasma hominis

Mycoplasma hominis is a commensal of the genitourinary tract, which is infrequently associated with urogenital infections. Extra-urogenital infections due to M. hominis are rare. Here, we report an unusual case of M. hominis subdural empyema in a woman occurring shortly after delivery. The patient presented with symptoms suggestive of bacterial meningitis. Spinal imaging revealed a subdural empyema that required neurosurgical intervention. Cultures from intraoperatively obtained biopsies identified M. hominis as the causative pathogen. The patient was treated with oral moxifloxacin for 4 weeks resulting in the resolution of the spinal lesion. The subdural empyema was presumably caused by a contaminated epidural blood patch performed with the patient's own blood during an episode of transient M. hominis bacteremia after delivery. The blood patch was indicated for the treatment of cerebrospinal fluid leakage, which had occurred after epidural anesthesia. Our findings highlight the significance of transient M. hominis bacteremia after delivery and implicate that M. hominis should be considered as a causative agent of extra-genitourinary tract infections particularly during the postpartum period or after genitourinary manipulation.

Amino acid alterations in fibronectin binding protein A (FnBPA) and bacterial genotype are associated with cardiac device related infection in Staphylococcus aureus bacteraemia

OBJECTIVES

Staphylococcus aureus initiates cardiac device-related infection (CDI) by binding of fibronectin binding protein A (FnBPA) to the device's surface. In FnBPA, specific "binding enhancing" amino acid alterations are associated with CDI. However, no study has investigated whether these mutations also occur in geographically different regions and whether they arise during infection or are inherent properties of the infecting isolate.

METHODS

We analysed bacterial isolates from 34 patients with S. aureus bacteraemia and implanted cardiac devices for association with CDI, FnBPA sequence, classification into a clonal complex (CC), and binding to fibronectin (Fn).

RESULTS

We confirmed that amino acid alterations at positions 652, 782, and 786 in FnBPA were associated with CDI (p = 0.005). Furthermore, CC15 and CC45 isolates were associated with CDI (p = 0.004). All isolates within a CC exhibited a characteristic mutation pattern, with major changes occurring in CC45 including a duplication of D1 and an altered immunogenic epitope in the D3 repeat. Isolates harbouring the "binding enhancing" mutations showed a slightly increased Fn binding capability, whereas Fn binding was decreased in CC45 isolates, according to a microtiter plate assay.

CONCLUSIONS

FnBPA sequence variations are lineage specific and display inherent properties of the infecting isolate. Sequence analysis of FnBPA, as well as the bacterial genotype, may be used to predict the risk for device-related infection.