Table 3 Checkerboard assays between erythromycin and the cpl-1 endolysin with the erythromycin-resistant pneumococcal isolates. The table indicates minimal inhibitory concentrations observed for erythromycin, endolysin cpl-1 or the different combinations thereof. FIC is the fractional inhibitory concentration, which indicates additive effects if values are between 0.50 and 4.00. The EUCAST breakpoint for erythromycin resistance is set at 0.25 µg/mL