The CLSI recommended quality control strain ATCC 25923 (#2) was included each time and gave zone of inhibition diameter within the expected range (29-35 mm) . &The zone edge test was also applied and the edge of the zone of inhibition was observed. S. aureus ATCC 29213 (#1) was used as a positive control for the zone edge test (sharp edge), and ATCC 25923 (#2) as a negative control (fuzzy edge). ‘β’ denotes β-lactamase producing strain. To ascertain whether isolates producing detectable amounts
of β-lactamases would show altered disk diffusion results, we performed disk-diffusion assays for the predicted ‘cefazolin selleck kinase inhibitor less active’ isolates (#1, #6, #18, #19, #20) (Figure 2)
of the β-LEAF assay using both ‘induced’ and ‘un-induced’ growth cultures as inoculum respectively (conventional AST is usually performed using ‘un-induced’ inoculums). This would also verify if observed discrepancy in antibiotic activity/susceptibility prediction between the β-LEAF assay and disk-diffusion was caused Talazoparib by the different induction statuses (β-LEAF assay = induced growth cultures, disk diffusion assays = standard growth, see Methods). Using induced cultures as starting inoculum, however, did not change the results of cefazolin AST, compared to using standard (un-induced) inoculum (Additional file 3: Table S1). β-lactamase detection is an important screening test, and the zone edge test (using penicillin) has recently been included in the CLSI guidelines for this purpose. [41, 42]. A sharply demarcated zone edge in disk diffusion assays correlates
well with β-lactamase production [41, 42, 55]. Based on this criterion, a sharp zone edge for isolates #1, #6, #18, #19, and #20 was seen, designating them lactamase producers (Table 3, Additional file 2: Figure S2). The same set of isolates was predicted to be ‘cefazolin less active’ and lactamase producers using the β-LEAF assay and nitrocefin tests (Figure 2, Table 1 (nitrocefin test results), Table 2). Thus, the disk-diffusion test results on the whole, with results from cefazolin susceptibility and zone edge tests taken together, corresponded with the β-LEAF assay predictions, O-methylated flavonoid as by virtue of β-lactamase production respective isolates may show some degree of resistance to cefazolin. Table 2 summarises comparison of results for β-lactamase production (columns 2–4) and cefazolin susceptibility/activity (columns 5–6), along with the β-lactamase genotypes (column 1) for all isolates in the study. Overall, the results from the rapid β-LEAF assay were consistent with results from the standard methods, validating the methodology. However, the presence of the blaZ gene did not always correlate with a lactamase positive phenotype.