TABLE 4.

Antimicrobial resistance of E. coli isolates from the irrigation return flows (IRFs) and background canal site (MLC).

Antimicrobial	Breakpoint (μg mL–1)	No. of resistant (%)
		IRFs	MLC
		(n = 171)	(n = 16)
Amoxicillin/clavulanic acid	≥32/16	10 (5.8)	2 (12.5)
Ampicillin	≥32	13 (7.6)	3 (18.8)
Azithromycin	≥16	1 (0.6)	0 (0)
Cefoxitin	≥32	8 (4.7)	2 (12.5)
Ceftiofur	≥8	1 (0.6)	2 (12.5)
Ceftriaxone	≥4	2 (1.2)	2 (12.5)
Chloramphenicol	≥32	4 (2.3)	3 (18.8)
Ciprofloxacin	≥1	4 (2.3)	5 (31.3)
Gentamicin	≥16	0 (0)	0 (0)
Nalidixic Acid	≥32	4 (2.3)	5 (31.3)
Streptomycin	≥32	9 (5.3)	3 (18.8)
Sulfisoxazole	≥256	12 (7)	5 (31.3)
Tetracycline	≥16	21 (12.3)	4 (25)
Trimethoprim/sulfamethoxazole	≥4/76	2 (1.2)	4 (25)