Is there a need to implement standardisation into in vitro antimicrobial evaluation systems? A European collaboration perspective
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Background
Time kill curve (TKC) assessments are an essential step in the study of an antimicrobials pharmacodynamic characteristics. Surprisingly TKCs have not be formally standardised, therefore there remain concerns that different testing centres/methodologies may produce different results. Six centres participating in Gram-negative-Antibiotics NOW (GNA-NOW) consortium measured a series of TKCs with meropenem against E. coli to establish: Same-day (SD) vs different-day (DD) replication per centre (intra-site), and centre to centre (inter-site) correlations.
Methods
Meropenem was tested against three strains of E. coli (ATCC 25922; ESBL producer C1.55; OXA-48 producer C1.62). An inoculum of 1.5×10 6 CFU was specified with meropenem concentrations of x0, x1 to x16 MIC; and sampling assessment of bacterial density was determined at 0-24h. Experiments were performed in triplicate, aerobically at 37°C. Centre-specific methodology was collected. Meropenem, media, bacterial strains, were shipped from one central laboratory to participating laboratories. ANOVA and Friedman tests were used to assess SD, DD and between centre replications.
Results
Assessment of the methodologies between centres revealed many differences, including bacterial inoculum, meropenem preparation, volume of TKC vessel, vessel materials, agitation vs static cultures and sampling volumes. Intra-centre SD and DD analysis for all strains were generally associated with P >0.05 suggesting consistency. Inter-centre SD and DD comparisons resulted in P <0.05, indicating variable total bacterial load measurement between centres.
Conclusions
TKC methodologies varied between different centres, and while intra-centre comparison of SD and DD were generally consistent, inter-centre comparisons were not. Standardisation of TKC methodologies is required.
