Five different antibiotics were selected for testing according
to their modes of action. Ampicillin (50 g ml−1), erythromycin
(50 g ml−1), kanamycin (50 g ml−1), novobiocin (50 g ml−1),
and tetracycline (15 g ml−1) were added separately to 5 ml TSB,
before 20 l of an overnight test culture was added and then incubated
at 30 ◦C in a shaking water bath for 24–48 h. The absorbance
at 600 nm of all samples was measured using uninoculated TSB as
a blank.