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. 1996 Aug;40(8):1914–1918. doi: 10.1128/aac.40.8.1914

Comparison of three different in vitro methods of detecting synergy: time-kill, checkerboard, and E test.

R L White 1, D S Burgess 1, M Manduru 1, J A Bosso 1
PMCID: PMC163439  PMID: 8843303

Abstract

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.

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Selected References

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  1. Anderson E. T., Young L. S., Hewitt W. L. Antimicrobial synergism in the therapy of gram-negative rod bacteremia. Chemotherapy. 1978;24(1):45–54. doi: 10.1159/000237759. [DOI] [PubMed] [Google Scholar]
  2. Bayer A. S., Morrison J. O. Disparity between timed-kill and checkerboard methods for determination of in vitro bactericidal interactions of vancomycin plus rifampin versus methicillin-susceptible and -resistant Staphylococcus aureus. Antimicrob Agents Chemother. 1984 Aug;26(2):220–223. doi: 10.1128/aac.26.2.220. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Berenbaum M. C. Correlations between methods for measurement of synergy. J Infect Dis. 1980 Sep;142(3):476–480. doi: 10.1093/infdis/142.3.476. [DOI] [PubMed] [Google Scholar]
  4. Berenbaum M. C. Synergy assessment with growth curves. J Infect Dis. 1984 Aug;150(2):304–304. doi: 10.1093/infdis/150.2.304. [DOI] [PubMed] [Google Scholar]
  5. Chadwick E. G., Shulman S. T., Yogev R. Correlation of antibiotic synergy in vitro and in vivo: use of an animal model of neutropenic gram-negative sepsis. J Infect Dis. 1986 Oct;154(4):670–675. doi: 10.1093/infdis/154.4.670. [DOI] [PubMed] [Google Scholar]
  6. Chan E. L., Zabransky R. J. Determination of synergy by two methods with eight antimicrobial combinations against tobramycin-susceptible and tobramycin-resistant strains of Pseudomonas. Diagn Microbiol Infect Dis. 1987 Feb;6(2):157–164. doi: 10.1016/0732-8893(87)90101-5. [DOI] [PubMed] [Google Scholar]
  7. De Jongh C. A., Joshi J. H., Newman K. A., Moody M. R., Wharton R., Standiford H. C., Schimpff S. C. Antibiotic synergism and response in gram-negative bacteremia in granulocytopenic cancer patients. Am J Med. 1986 May 30;80(5C):96–100. [PubMed] [Google Scholar]
  8. Fuchs P. C., Jones R. N., Barry A. L., Thornsberry C. Evaluation of the in vitro activity of BMY-28142, a new broad-spectrum cephalosporin. Antimicrob Agents Chemother. 1985 May;27(5):679–682. doi: 10.1128/aac.27.5.679. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. King T. C., Schlessinger D., Krogstad D. J. The assessment of antimicrobial combinations. Rev Infect Dis. 1981 May-Jun;3(3):627–633. doi: 10.1093/clinids/3.3.627. [DOI] [PubMed] [Google Scholar]
  10. Klastersky J., Cappel R., Daneau D. Clinical significance of in vitro synergism between antibiotics in gram-negative infections. Antimicrob Agents Chemother. 1972 Dec;2(6):470–475. doi: 10.1128/aac.2.6.470. [DOI] [PMC free article] [PubMed] [Google Scholar]
  11. Klastersky J., Meunier-Carpentier F., Prevost J. M., Staquet M. Synergism between amikacin and cefazolin against Klebsiella: in vitro studies and effect on the bactericidal activity of serum. J Infect Dis. 1976 Sep;134(3):271–276. doi: 10.1093/infdis/134.3.271. [DOI] [PubMed] [Google Scholar]
  12. Lau W. K., Young L. S., Black R. E., Winston D. J., Linne S. R., Weinstein R. J., Hewitt W. L. Comparative efficacy and toxicity of amikacin/carbenicillin versus gentamicin/carbenicillin in leukopenic patients: a randomized prospective trail. Am J Med. 1977 Jun;62(6):959–966. doi: 10.1016/0002-9343(77)90669-6. [DOI] [PubMed] [Google Scholar]
  13. Lerner S. A., Dudek E. J., Boisvert W. E., Berndt K. D. Effect of highly potent antipseudomonal beta-lactam agents alone and in combination with aminoglycosides against Pseudomonas aeruginosa. Rev Infect Dis. 1984 Sep-Oct;6 (Suppl 3):S678–S688. doi: 10.1093/clinids/6.supplement_3.s678. [DOI] [PubMed] [Google Scholar]
  14. Moellering R. C., Jr Antimicrobial synergism--an elusive concept. J Infect Dis. 1979 Oct;140(4):639–641. doi: 10.1093/infdis/140.4.639. [DOI] [PubMed] [Google Scholar]
  15. Moody J. A., Gerding D. N., Peterson L. R. Evaluation of ciprofloxacin's synergism with other agents by multiple in vitro methods. Am J Med. 1987 Apr 27;82(4A):44–54. [PubMed] [Google Scholar]
  16. Norden C. W., Wentzel H., Keleti E. Comparison of techniques for measurement of in vitro antibiotic synergism. J Infect Dis. 1979 Oct;140(4):629–633. doi: 10.1093/infdis/140.4.629. [DOI] [PubMed] [Google Scholar]
  17. Robinson A., Bartlett R. C., Mazens M. F. Antimicrobial synergy testing based on antibiotic levels, minimal bactericidal concentration, and serum bactericidal activity. Am J Clin Pathol. 1985 Sep;84(3):328–333. doi: 10.1093/ajcp/84.3.328. [DOI] [PubMed] [Google Scholar]
  18. Ryan R. W., Kwasnik I., Tilton R. C. Methodological variation in antibiotic synergy tests against enterococci. J Clin Microbiol. 1981 Jan;13(1):73–75. doi: 10.1128/jcm.13.1.73-75.1981. [DOI] [PMC free article] [PubMed] [Google Scholar]
  19. Weinstein A. J., Moellering R. C., Jr Penicillin and gentamicin therapy for enterococcal infections. JAMA. 1973 Feb 26;223(9):1030–1032. [PubMed] [Google Scholar]
  20. Weinstein R. J., Young L. S., Hewitt W. L. Comparison of methods for assessing in vitro antibiotic synergism against Pseudomonas and Serratia. J Lab Clin Med. 1975 Nov;86(5):853–862. [PubMed] [Google Scholar]

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