Table 1.

Susceptibility testing results and hydrolytic activity rates for ceftolozane alone and in combination with tazobactam at 4 μg/ml against E. coli strains producing different levels of CTX-M-15^a

E. coli strain	MIC (μg/ml)		Hydrolytic activityc	qRT-PCRd
	Ceftolozane alone	Ceftolozane + TAZb (4 μg/ml)
Control	0.25	0.25	−3	ND
Low producer	4	0.25	36	1
Moderate producer	16	0.25	120	8.3
High producer	64	0.25	580	43.9

^aThe transcription levels of bla_CTX-M-15 are also shown.

^bTAZ, tazobactam.

^cHydrolytic activity rates expressed as the amount (mg) of ceftolozane hydrolyzed per minute per milligram of protein.

^dExpression of bla_CTX-M-15 relative to the E. coli strain demonstrating the lowest CTX-M-15 production based upon MIC results and hydrolysis assays for β-lactams. ND, not detected.