Effect of filament formation of gram-negative rods on MIC measurement of aztreonam by liquid culture method

Abstract

When aztreonam (AZT) acted on gram-negative rods in liquid culture (1 ml macrodilution method and 0.1ml microdilution method), remarkable filament formation was observed. This filament formation significantly affected the minimum inhibitory concentration (MIC) results obtained by a broth method. The strains used were Escherichia coli, Klebsiella pneumoniae, Enterobacter spp., Serratia marcescens and Pseudomonas aeruginosa, and the following results were obtained.
(1) Assuming that the precipitates formed in the test tubes by the broth dilution method were due to an increase in CFUs, the MICs were markedly higher than those obtained by the agar method. The precipitates were observed in a significant proportion of the strains.
(2) When the precipitates at the bottom of the test tube were microscopically examined, prominent filament formation was found.
(3) Cell counting in the broth was periodically performed, and it was found that the test tube in which precipitates were observed at the bottom but on turbidity was seen in the upper broth showed no increase in cell count. Thus, it was determined that the precipitates were not due to an increase in cell number.
(4) After confirming that the precipitates at the bottom of the test tube were filaments, MICs were measured by the broth method using only the turbidity of the supernatant as a marker. The value obtained was lower than that obtained including the precipitates, which was about the same as the MIC obtained by the agar method.
From the above results, it was found that when the MIC of AZT is measured by the broth method (macrodilution and microdilution method), the precipitates formed by filament formation should be excluded.