Rapid optical determination of β-lactamase and antibiotic activity

Abstract

Background: The absence of rapid tests evaluating antibiotic susceptibility results in the empirical prescription of antibiotics. This can lead to treatment failures due to escalating antibiotic resistance, and also furthers the emergence of drug-resistant bacteria. This study reports a rapid optical method to detect β-lactamase and thereby assess activity of β-lactam antibiotics, which could provide an approach for targeted prescription of antibiotics. The methodology is centred on a fluorescence quenching based probe (β-LEAF--β-Lactamase Enzyme Activated Fluorophore) that mimics the structure of β-lactam antibiotics.

Results: The β-LEAF assay was performed for rapid determination of β-lactamase production and activity of β-lactam antibiotic (cefazolin) on a panel of Staphylococcus aureus ATCC strains and clinical isolates. Four of the clinical isolates were determined to be lactamase producers, with the capacity to inactivate cefazolin, out of the twenty-five isolates tested. These results were compared against gold standard methods, nitrocefin disk test for β-lactamase detection and disk diffusion for antibiotic susceptibility, showing results to be largely consistent. Furthermore, in the sub-set of β-lactamase producers, it was demonstrated and validated that multiple antibiotics (cefazolin, cefoxitin, cefepime) could be assessed simultaneously to predict the antibiotic that would be most active for a given bacterial isolate.

Conclusions: The study establishes the rapid β-LEAF assay for β-lactamase detection and prediction of antibiotic activity using S. aureus clinical isolates. Although the focus in the current study is β-lactamase-based resistance, the overall approach represents a broad diagnostic platform. In the long-term, these studies form the basis for the development of assays utilizing a broader variety of targets, pathogens and drugs.

Figure 1

Schematic showing the principle of the β-LEAF assay. A. The β-LEAF probe comprises a β-lactam core structure including the cleavable lactam ring (green), flanked by two fluorophores (encircled), which undergo static quenching when the probe is intact. Following cleavage by β-lactamase, the fluorophores move apart and show fluorescence. B. Assay profile for β-lactamase producing bacteria C. Assay profile for lactamase non-producing bacteria.

Figure 2

β-LEAF assays determine β-lactamase production and cefazolin activity in S. aureus clinical 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 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.

Figure 3

β-LEAF assays can be used to determine activity of multiple antibiotics simultaneously. β-LEAF assays were set up with multiple antibiotics (cefazolin, cefoxitin and cefepime) in selected S. aureus isolates. Antibiotic activity was assessed in positive control strain #1, negative control strain #2 and four S. aureus clinical isolates that showed substantial β-lactamase production (#6, #18, #19, #20). The different bacterial strains were incubated with β-LEAF alone and β-LEAF and cefazolin/cefoxitin/cefepime respectively. Fluorescence was monitored over 60 min. The y-axis represents cleavage rate of β-LEAF (measured as fluorescence change rate – milliRFU/min) normalized by bacterial O.D. (optical density) at 600 nm. Results are presented as the average of three independent experiments (each experiment contained samples in triplicates) and error bars represent the standard error.