Permeabilizing effects of sub-inhibitory concentrations of microcystin on the growth of Escherichia coli.

Microcystin, a cyanotoxin produced by Microcystis aeruginosa, lacks potent antibacterial activity. When tested in combination, in vitro, inhibitory values for a range of hydrophobic antibiotics were significantly reduced in the presence of at least 1/20 x the minimum inhibitory concentration of microcystin. The degree of inhibition was equivalent to that of a well-characterised permeabilizing agent, polymyxin B nonapeptide. The permeabilizing ability of sub-inhibitory concentrations of microcystin to affect the envelope of Escherichia coli was demonstrated by a rapid and sustained reduction in absorbance values of lysozyme-treated cells and by enhanced uptake of crystal violet in microcystin-treated cultures. Direct effects of appropriate concentrations of microcystin on the integrity of bacterial outer and inner membranes were measured by release of specific enzyme markers. Although the exact mechanism for permeabilizing E. coli with microcystin has not been elucidated, the effects were consistent with permeability changes to the enterobacterial outer membrane caused by polymyxin B nonapeptide.