Radiochemical method for evaluating the effect of antibiotics on Escherichia coli biofilms

A simple radiochemical method for evaluating the action of antibiotics on Escherichia coli cells in biofilms is reported. After growth, biofilms of E. coli ATCC 25922 on disks of urinary catheter material were suspended in fresh medium containing or lacking an antibiotic, incubated for 4 h at 37 degrees C, and pulse-labeled with [3H]leucine for 5 min. Radioactivity in trichloracetic acid-precipitable material in the biofilm and in the surrounding medium (planktonic E. coli) was then measured. Antibiotic-induced inhibition of incorporation of [3H]leucine into the cells in the biofilm was far less pronounced than incorporation into planktonic cells and, furthermore, correlated well with loss in viable counts. The method is simple, inexpensive, and extremely timesaving.