4.5 Control of Microbial Growths using Biocides
The term ‘biocide’ refers collectively to fungicides, algicides and bacterio-
cides. They may serve to either inhibit growth/metabolic activity while pres-
ent, with effective metabolism being restored upon removal (biostatic), or
cause a lethal effect, through irreparable damage to target cells leading to cell
death (biocidal). The latter effect may be due to action against cell proteins,
consequently biocides are potentially harmful to humans and their toxicolog-
ical properties should be understood so that appropriate risk assessments can
be carried out. Unfortunately most biocides have not been designed specifi-
cally for object conservation and therefore adverse reactions and compatibil-
ity have rarely been considered.
The ideal featuresof a biocide are that it will have a wide spectrum of activ-
ity, kill target organisms but pose no harm to humans, have a long effective
life and cause no changes to stone. Unfortunately no candidate biocide fulfils
these criteria. Consequently there are many factors to be considered when
choosing a biocide for use in stone conservation and restoration. Clearly the
important and obvious factors are the range of effectiveness against target
organisms and any possible unwanted toxicity to humans. The potency of
biocides may be influenced by the ability of microorganisms to oxidise or
modify them, possibly in conjunction with UV light. It must also be consid-
ered that, although vegetative growth of microorganisms may be sensitive to
biocides, very often their spores are resistant. Thus the elimination of obvi-
ous surface growths may be followed very quickly by recolonisation, as
spores germinate and grow on dead organic matter residues when the active
biocide concentration declines.
Biocides in use. There are 5 main types of biocides that are active against
microbial growths:
- Quaternary ammonium compounds
- Amines
- Chlorophenols
- Phenoxides
- Metals.
Biocidal action is best developed in the quaternary ammonium com-
pounds, which are surface-active and can be bacteriocidal or bacteriostatic
depending on concentration. They have a wide spectrum of activity against
microorganisms, although they are less effective against one important class
of bacteria, the Gram-negative bacteria. Biocide treatments containing cop-
per have been reported as having the greatest chance of success in controlling
lichen growth. Nevertheless, whenever using biocides for controlling microbial
234 Chapter 9