Microbiological control of pharmaceutical products containing preservatives: drawbacks of the method of membrane filtration.

Microbiological assays carried out on membranes employed for the filtration of solutions of preservatives with concentration used in pharmaceuticals, demonstrated that such membranes, even after washing with 300 ml (100 ml X 3) of peptone water according to the guidelines of the Pharmacopoeia, maintain very frequently a remarkable antimicrobial activity revealed by inhibition zones and by disappearance or decrease of the number of colonies forming units. Such drawbacks can interfere negatively with the microbiological controls of pharmaceuticals containing preservatives.