Synergy and Mode of Action of Ceftazidime plus Quercetin or Luteolin on Streptococcus pyogenes

Table 1

Minimum inhibitory concentrations (MICs), fractional inhibitory concentration (FIC), and FIC index (FICI) determined by checkerboard assays of ceftazidime, amoxicillin, penicillin, nisin, luteolin, and quercetin either alone or in combination against S. pyogenes DMST 30653, 30654, and 30655.

Strains	MICs (g mL^−1)						FIC (g mL^−1)		FICI
	Cef	Amo	Pen	Nis	Lut	Que	Cef + Lut	Cef + Que	Cef + Lut	Cef + Que
S. pyogenes DMST 30653	0.50	N/D	N/D	1.0	128.0	128.0	0.12 + 16.0	0.12 + 4.0	0.37	0.27
S. pyogenes DMST 30654	0.50	N/D	N/D	1.0	128.0	128.0	0.12 + 16.0	0.12 + 4.0	0.37	0.27
S. pyogenes DMST 30655	0.50	N/D	N/D	1.0	128.0	128.0	0.12 + 16.0	0.12 + 4.0	0.37	0.27
S. aureus ATCC 29213	4.0	0.5	1.0	1.0	N/D	N/D	N/D	N/D	N/D	N/D

S. aureus ATCC 29213, amoxicillin, and penicillin were used as a positive control.
FIC (g mL⁻¹) value of Cef + Lut at 0.12 + 16.0 in each row below this column means MIC of ceftazidime at 0.12 plus luteolin at 16.0 g mL⁻¹ in the combination.
FICI value of Cef + Lut at 0.37 in each row below this column means FIC of ceftazidime plus luteolin in combination was 0.37, which exhibited synergistic interaction.
S = susceptible; = synergistic interaction; N/D, not determined.
Cef = ceftazidime; Amo = amoxicillin; Pen = penicillin; Nis = nisin; Lut = luteolin; Que = quercetin.
The MICs are presented as the median values measured from three independent experiments; each experiment was performed in triplicate.