Using several-fold higher concentrations of the test β-lactam antibiotic, compared to the probe, enhances the likelihood that the antibiotic will be the preferred substrate of the lactamase in the competition reaction in the assay. The reduced fluorescence indirectly reflects the ability of the β-lactamase to bind and cleave the tested antibiotic (large difference = antibiotic can be readily
bound and hence cleaved and inactivated). Notably, unlike growth based conventional AST methods, the end-point SCH727965 cell line of the β-LEAF assay is not bacterial viability or differences in growth pattern. The read-out of the assay is fluorescence, which reflects probe cleavage due to the enzymatic activity of the β-lactamase. Importantly, the β-LEAF assay is rapid compared to the conventional growth based AST methods (1 h versus 20–24 h for disk diffusion/MIC conventionally or ~8 h with automated instruments). The observation in Figure 2 of low to negligible fluorescence in β-LEAF + cefazolin reactions with all β-lactamase ‘positives’ (#1, #6, #18, #19, #20) suggests that cefazolin can be readily targeted and inactivated by the respective lactamases, and would be anticipated to be a less effective treatment option for these bacteria. An expectation of this assay is that the reduction in probe fluorescence in the presence of an antibiotic will be inversely proportional to its predicted activity
against the pathogen. If fluorescence is completely reduced in the presence of an antibiotic, then the respective antibiotic can be readily cleaved and inactivated by β-lactamase. Nepicastat chemical structure However, if despite the ‘saturating’ amount of antibiotic, some fluorescence selleck chemical increase reflecting probe cleavage is still observed (e.g. cefepime reactions in Figure 3), the lactamase may not be capable of effectively destroying the antibiotic, and the antibiotic predicted as likely to be active. In experiments with multiple antibiotics (Figure 3) a ratio of the cleavage rate of β-LEAF in presence of an antibiotic to the cleavage rate of β-LEAF alone, for each antibiotic tested, is shown in Table 4. For β-lactamase based resistance, the ratio of cleavage
rates closer to 1 (Table 4) would indicate greater β-lactam antibiotic efficacy. With more rigorous testing Sclareol from multiple data sets on a large number of isolates, cut-offs could be set up to develop the ratios as a ‘β-lactamase-based antibiotic activity/susceptibility index’ within specific limits. We recognize that there are a wide variety of lactamases, and note that with appropriate kinetic analysis (such as building on our previous study ), the approach presented here has the potential of characterizing the different lactamases. The motivation for the choice of antibiotics used in this initial study was to test three different generations of cephalosporin antibiotics. Cephalosporins are a standard treatment for skin and soft-tissue infections [58, 59].