Spiral gradient endpoint method compared to standard agar dilution for susceptibility testing of anaerobic gram-negative bacilli

Antimicrobial susceptibility testing of Shiga toxin-producing Escherichia coli from various samples by using a spiral gradient endpoint technique

Shiga toxin-producing Escherichia coli (STEC) remains a major public health concern. Microbial resistance may be due to use of antimicrobial agents (AAs) as a growth promoter in food animals or overuse of AAs in humans. The objective of the current study was to determine the antimicrobial susceptibility patterns of STEC strains isolated from food, veterinary, and clinical sources against 14 AAs by using the spiral gradient endpoint method. One hundred ten isolates from three sources were characterized. Results of the current study showed that all strains were resistant to the folate pathway inhibiting AAs including tylosin tartrate (gradient minimum inhibitory concentration [GMIC] ranges from ≥180.00 to 256.00 μg/mL; end concentration [EC] ranges from ≥130.00 to 151.22 μg/mL; and tail-end concentration [TEC] ≥145.00 μg/mL). All the strains isolated from three sources were susceptible to the fluoroquinolone class of AAs (GMIC ranges from ≤1.00 to 64.30 μg/mL; EC ranges from ≤3.33 to 72.00 μg/mL; and TEC ranges from ≤12.13 to 45.00 μg/mL). Among the food isolates, less resistance was found within the aminoglycoside and amphenicol group (GMIC ≥256.00 μg/mL; EC=161.00 μg/mL). Eight strains were resistant to one to three, 44 strains were resistant to four to six, and two strains were resistant to seven or more AAs. All the clinical isolates (100%) were susceptible to the fluoroquinolones and gentamycin. Results also showed that antimicrobial resistance was observed between four and six AAs among the isolates. Some veterinary isolates were resistant to five AAs. Least AAs resistance was shown by 3.7% of isolates to gentamycin and 7.45% to chloramphenicol. This study showed an increasing trend of antimicrobial resistant strains of STEC, and we suggest that periodic surveillance of the antimicrobial susceptibility may be a useful measure to detect the antimicrobial resistant pathogens.

Synergy assessed by checkerboard. A critical analysis

The checkerboard dilution test is widely used for evaluation of in vitro synergy for multiple drugs, although problems in performance, standardization, and interpretation have been noted. A major problem inherent in this commonly used method is the use of twofold dilutions for the antibiotic concentrations. We evaluated an alternative method proposed by Horrevorts and colleagues that preserved the twofold dilution scheme. Giant checkerboards were constructed from a series of component checkerboards using rifampin and minocycline against Staphylococcus aureus. We found that this method improved the stability of the fractional inhibitory concentration (FIC) indices, but required substantially more labor and generated other problems. FIC interpretation and calculation remained compromised by the twofold dilution scheme. We have analyzed the theoretical basis of the checkerboard and its FIC calculation and conclude that the twofold dilution with its exponential increase in dilutions makes this method of synergy evaluation inherently unstable. The principle of examining growth at multiple dilutions of combined antibiotics is valid for assessment of synergy, but newer methods need to be devised.