Antimicrobial activity of fluoroquinolone photodegradation products determined by parallel-line bioassay and high performance liquid chromatography

The fluoroquinolones produce multiple photodegradation products. Little is known about these products, particularly whether any possess antimicrobial activity. To investigate this, we used the parallel-line bioassay to investigate discrepancies in zone of inhibition size in conjunction with high performance liquid chromatography (HPLC) analysis. A continuous flow photochemical reaction unit ('Beam-Boost') was used to partially photodegrade the fluoroquinolones ofloxacin, levofloxacin, ciprofloxacin and moxifloxacin (0.02 mM) by between 15 and 89%, as confirmed by HPLC. The concentration of residual parent fluoroquinolone in each irradiated sample was measured by HPLC and a non-irradiated control solution was prepared at the same concentration. These were compared by parallel-line bioassays using Escherichia coli, Enterobacter cloacae and Klebsiella oxytoca. With ofloxacin and levofloxacin, the zone size for the control solution was significantly less than that of the irradiated solutions, with >15% photodegradation in at least two of the indicator organisms, indicating that the photodegradation products possess antimicrobial activity. No difference was seen with ciprofloxacin at any level of photodegradation with any of the indicator organisms, nor with moxifloxacin at 30 and 54% photodegradation. A significant difference was observed with E. cloacae only, at 83% photodegradation.