The accuracy of extended-spectrum beta-lactamase detection in Escherichia coli and Klebsiella pneumoniae in South African laboratories using the Vitek 2 Gram-negative susceptibility card AST-N255

Background: Phenotypic detection of extended-spectrum beta-lactamases (ESBLs) is based on the inhibition of ESBL enzymes by β-lactamase inhibitors and on the comparison of cephalosporin activity with or without a β-lactamase inhibitor. Many South African diagnostic laboratories rely on the Vitek 2 for automated susceptibility testing and for ESBL detection. However, the Gram-negative susceptibility card currently used locally (AST-N255) has been modified and its accuracy for ESBL detection is not known.Methods: We randomly selected 50 isolates of Klebsiella pneumoniae and Escherichia coli from a collection of clinical bloodstream isolates from Groote Schuur Hospital from 2015 to 2016, including ESBL-producing and non-ESBL-producing strains. We used standardised phenotypic (disc diffusion and broth microdilution) and genotypic (conventional polymerase chain reaction (PCR) for blaCTX-M, blaSHV and blaTEM) methods for detection of ESBLs. We compared ESBL detection by Vitek 2 to a composite reference standard comprising ESBL detection either by both phenotypic methods or by one phenotypic method together with genotypic detection.Results: The sensitivity of Vitek 2 system for detection of ESBLs was 33/36 or 92% (78% – 97%) for E. coli, and 40/40 or 100% (91% – 100%) for K. pneumoniae, whilst specificity was 10/10 or 100% (72% – 100%) and 9/10 or 90% (60% – 98%), respectively. This is comparable with previous studies.Conclusion: Using a composite reference standard of the phenotypic and genotypic methods employed in this study, no Vitek-categorised ESBL E. coli or K. pneumoniae was found to be a non-ESBL with the exception of possible misinterpretation with K. pneumoniae SHV-hyper-producing isolates.

Antimicrobial Stewardship in the Microbiology Laboratory: Impact of Selective Susceptibility Reporting on Ciprofloxacin Utilization and Susceptibility of Gram-Negative Isolates to Ciprofloxacin in a Hospital Setting

The objective of this study was to determine the impact of selective susceptibility reporting on ciprofloxacin utilization and Gram-negative susceptibility to ciprofloxacin in a hospital setting. Historically at our institution, the microbiology laboratory practice was to report ciprofloxacin susceptibility for allEnterobacteriaceaeregardless of susceptibility to other agents. A selective reporting policy was implemented which involved the suppression of ciprofloxacin susceptibility toEnterobacteriaceaewhen there was lack of resistance to the antibiotics on the Gram-negative panel. Ciprofloxacin utilization (measured in defined daily doses [DDD] per 1,000 patient days) was collected before and after the intervention and compared to moxifloxacin, trimethoprim-sulfamethoxazole, nitrofurantoin, and amoxicillin-clavulanate. Monthly susceptibility ofPseudomonas aeruginosaandEscherichia colito ciprofloxacin was tabulated. An interrupted time series analysis using segmented regression was performed. The mean monthly ciprofloxacin utilization decreased from 87 (95% CI, 83.7 to 91.2) to 39 (95% CI, 35.0 to 44.0) DDD per 1,000 patient days before and after the implementation of selective reporting, respectively. There was an immediate and sustained reduction in ciprofloxacin usage at 1, 3, 6, 12, and 24 months postintervention (P< 0.001). A compensatory increase in amoxicillin-clavulanate use was noted starting at 6 months postintervention and persisted for the study period (P< 0.027). Susceptibility ofE. coli, but not that ofP. aeruginosa, to ciprofloxacin was higher than predicted starting 12 months after the intervention (P< 0.05). In conclusion, selective reporting of ciprofloxacin susceptibly may be a useful intervention to reduce targeted antimicrobial utilization and improve Gram-negative susceptibility to ciprofloxacin. This approach should be considered as part of a broader multimodal antimicrobial stewardship program.

Phase transitions of quasistationary states in the Hamiltonian Mean Field model

The out-of equilibrium dynamics of the Hamiltonian Mean Field (HMF) model is studied in presence of an externally imposed magnetic field h. Lynden-Bell’s theory of violent relaxation is revisited and shown to adequately capture the system dynamics, as revealed by direct Vlasov based numerical simulations in the limit of vanishing field. This includes the existence of an out-of-equilibrium phase transition separating magnetized and non magnetized phases. We also monitor the fluctuations in time of the magnetization, which allows us to elaborate on the choice of the correct order parameter when challenging the performance of Lynden-Bell’s theory. The presence of the field h removes the phase transition, as it happens at equilibrium. Moreover, regions with negative susceptibility are numerically found to occur, in agreement with the predictions of the theory.