A rapid and sensitive colorimetric measurement of antibiotic efficacy against Escherichia coli in vitro.

A common dye of prussian blue (PB) as an indicator was used to develop a colorimetric method for detecting the efficacy of the antibiotics in vitro. Considering the electronic production capacity of microbial respiration, ferricyanide was employed in transferring electrons from target microorganism of Escherichia coli (E. coli) to produce ferrocyanide. Subsequently, ferrocyanide reacted with ferric ions to form PB. In view of relationship between the PB yield and the bacterial activity, the efficacy of the antibiotics on E. coli was directly detected at 700 nm of PB absorption. When the 5% activity of antibiotics on 20 isolates of E. coli was quantified as 5% efficacy, the applied concentrations of eight antibiotics, such as cefepime, ceftriaxone sodium, cefoperazone sodium, piperacillin sodium, amoxicillin, gentamicin, amikacin and levofloxacin were 2, 2, 4, 4, 10, 4, 8 and 8 μg mL⁻¹, respectively. To compare with minimum inhibitory concentration results obtained by Clinical and Laboratory Standards Institute broth macrodilution method, the results of PB methods showed good agreements except with gentamicin. Paired t-test result (P) also showed that difference between two methods was statistically significant (P = 0.006).