TABLE 1.

Activity of polymyxin B in different concentrations in combination with imipenem, meropenem, or tigecycline in time-kill assays

Antimicrobials and concns (mg/liter)a	Log ΔCFUb in strainc:
	EC1	EC2	KP1	KP2	SM1	SM2
PMB+IPM
PMB (0.5) + IPM (4)	−6.47	−4.74	−5.24	−4.70	−2.88	−2.51
PMB (1) + IPM (4)	−9.80	−5.64	−9.17	−4.60	−3.26	−2.62
PMB (2) + IPM (4)	−9.62	−4.54	−7.81	−4.20	−3.39	−2.65
PMB+MEM
PMB (0.5) + MEM (4)	−11.60	−4.24	−3.47	−5.17	−4.98	−3.55
PMB (1) + MEM (4)	−10.14	−3.78	−8.38	−5.08	−5.78	−3.58
PMB (2) + MEM (4)	−3.07	−5.62	−8.14	−4.87	−7.93	−3.77
PMB+TGC
PMB (0.5) + TGC (1)	−1.20	−0.94	−5.78	−4.69	−2.90	−1.04
PMB (1) + TGC (1)	−1.32	−2.20	−6.48	−4.60	−2.97	−1.18
PMB (2) + TGC (1)	−2.71	−4.54	−6.57	−4.20	−3.07	−2.51

^aPolymyxin B (PMB) in different concentrations in combination with imipenem (IPM), meropenem (MEM), or tigecycline (TGC).

^bLog ΔCFU was calculated as follows: final inoculum of the combined drugs − final inoculum of the most active drug in combination (log₁₀ CFU/ml). Synergy highlighted in bold type was defined as a decrease in colony count of ≥2 log₁₀ units after 24 h by the combination compared with the most active single agent. Bactericidal activity was defined as a decrease in colony count of ≥3 log₁₀ units after 24 h.

^cThe two E. cloacae strains are EC1 and EC2. The two K. pneumoniae strains are KP1 and KP2. The two S. marcescens strains are SM1 and SM2. The MICs (in milligrams per liter) of PMB, IPM, MEM, and TGC, respectively, of each strain follow (MICs highlighted in bold indicate resistance): for strain EC1, 2, 64, 128, and 1; for EC2, 2, 64, 32, and 1; for KP1, 2, 32, 32, and 4; for KP2, 2, 8, 32, and 1; for SM1, 64, 128, 32, and 1; and for SM2, >64, 256, 64, and 4.