Polymyxins antimicrobial action

Synergistic action of colistin and polymyxin in combination with other antibiotics in vitro and in vivo has been reported by several investigators " . In clinical application the effect of these combinations seems to be more a broadening of the antimicrobial spectrum than a synergistic interaction. Several salts and derivatives of both colistin and polymyxin have been reported. Commercially available are the sulphates and also recently the sodium sulphomethyl derivatives which are prepared by treatment of the free bases of polymyxin and colistin with formaldehyde and sodium bisulphite . These derivatives have a lower toxicity than the sulphates, however, and their in vitro antimicrobial activity is also reduced. 

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