Comparison of Three Different in Vitro Methods of Detecting Synergy: Time-kill, Checkerboard, and E Test

An in vitro method of detecting synergy which is simple to perform, accurate, and reproducible and has the potential for clinical extrapolation is desirable. Time-kill and checkerboard methods are the most widely used techniques to assess synergy but are time-consuming and labor-intensive. The Epsilometer test (E test), a less technically demanding test, has not been well studied for synergy testing. We performed synergy testing of Escherichia coli ATCC 35218, Enterobacter cloacae ATCC 23355, Pseudomonas aeruginosa ATCC 27853, and Staphylococcus aureus ATCC 29213 with various combinations of cefepime or ceftazidime with tobramycin or ciprofloxacin using time-kill, checkerboard, and E test techniques. Time-kill testing was performed against each organism alone and in combinations at one-fourth times the MIC (1/4 x MIC) and 2 x MIC. With checkerboard tests, the same combinations were studied at concentrations ranging from 1/32 x to 4 x MIC. Standard definitions for synergy, indifference, and antagonism were utilized. E test strips were crossed at a 90 degree angle so the scales met at the MIC of each drug alone, and the fractional inhibitory concentrations index was calculated on the basis of the resultant zone on inhibition. All antimicrobial combinations demonstrated some degree of synergy against the test organisms, and antagonism was infrequent. Agreement with time-kill testing ranged from 44 to 88% and 63 to 75% by the checkerboard and E test synergy methods, respectively. Despite each of these methods utilizing different conditions and endpoints, there was frequent agreement among the methods. Further comparisons of the E test synergy technique with the checkerboard and time-kill methods are warranted.