When assayed only in the presence of β-LEAF, a significant increase in fluorescence was observed with the β-lactamase producer strain #1. However, when the assay included both β-LEAF and cefazolin, a drastically lower β-LEAF cleavage rate (as measured by fluorescence change over time) was seen (Figure 2). Strain #2 does not encode β-lactamase and showed low fluorescence in both the β-LEAF alone and β-LEAF + cefazolin reactions (Figure 2). Figure 2 β-LEAF assays determine β-lactamase production and cefazolin activity in S. aureus clinical

ABT-888 chemical structure isolates. β-LEAF assays were performed with two ATCC S. aureus control strains (known β-lactamase producer #1 and non-producer #2) and 25 S. aureus clinical isolates, with cefazolin as a test antibiotic. The different bacterial isolates were incubated with β-LEAF (probe) alone and β-LEAF and cefazolin respectively, and fluorescence was monitored over 60 min. The Selleck Peptide 17 y-axis

represents the cleavage rate of β-LEAF (measured as fluorescence change rate – milliRFU/min) normalized by bacterial O.D. (optical density) at 600 nm. The black bars depict cleavage rate when β-LEAF alone is used, to show β-lactamase production. The white bars depict cleavage rate of probe when both the probe and cefazolin are included in the reactions. The horizontal line indicates a proposed cut-off value (upper limit of mean ± 3X Std. deviation for strain #2, β-LEAF probe reaction) to demarcate β-lactamase production. Where the black and white bars are significantly different, the antibiotic is predicted to be less active. Results are

presented as the average of three independent experiments (each experiment contained samples in triplicates) and error bars represent the standard error for all isolates, except #2. For #2, the error bar is 3X standard deviation. The various clinical isolates showed different patterns of fluorescence, and were categorized by comparing with the profile of the control strains. When assayed with β-LEAF alone, isolates #6, #18, #19 and #20 showed appreciable β-LEAF cleavage rates similar to that observed for #1 (Figure 2), and were designated as β-lactamase producing strains. These also showed significantly lower Fossariinae cleavage rates when the assay was performed with both β-LEAF and cefazolin (Figure 2). Testing with several-fold higher concentration of the antibiotic compared to probe concentration (as per assay design) increases chances of the antibiotic becoming the preferred substrate for the respective lactamase enzyme. The corresponding decrease in β-LEAF cleavage in the presence of the antibiotic, compared to when β-LEAF is present alone i.e., reduction in fluorescence due to competition (Figure 1), is used to predict activity of the antibiotic (reduction in fluorescence is inversely proportional to its predicted activity in presence of a lactamase).