Biocides and Fungi

BIOCIDES  AND  FUNGI

Fungi and their spores are a major potential source of contamination in pharmaceutical product preparation and aseptic processing as they are ubiquitous in the environment. The activity of biocides against fungi has not been widely documented (Table 20.6). It is often assumed that the interactions of biocides with fungal and yeast cells can be extrapolated from what is known of the interactions of these agents against bacteria. However, the fungal cell wall is fundamentally different from that of the bacteria and little is known about its capacity to impede the penetration of biocides. The interactions of QACs and biguanides with the fungal cell have been studied to some extent. Available information tentatively links cell wall glucan, wall thickness and consequent relative porosity to the sensitivity of Saccharomyces cerevisiae to chlorhexidine. Moulds tend to be less susceptible to biocides than yeasts, although more evidence is needed since only a limited number of fungal genera have been investigated.

Fungi have been shown to possess additional mechanisms of resistance to biocide intervention. The expression of degradative enzymes notably against metallic salts (e.g. copper, mercury) has been documented. In S. cerevisiae the production of hydrogen sulphide combining with heavy metal results in insoluble sulphides which are better tolerated by the microorganism. The expression of formaldehyde dehydrogenase to decrease the effect of formaldehyde has also been reported in Penicillium species. The presence of efflux pumps has now been widely reported in fungi, although their role in biocide resistance has been little investigated.