The development of bacterial resistance through acquired mechanisms such as mutation
and the acquisition of resistant determinants are of concern since a bacterium that was
previously susceptible can become insusceptible to a compound or a group of compounds
(Russell 2002a). The acquisition of resistant genes has been well described in the
literature (Chapman 2003, Lyon and Skurray 1987, Silver et al. 1989, White and
McDermott 2001) and it is particularly important to consider this as it might confer crossor co-resistance on occasion (Bjorland et al. 2001, Chapman 2003, Kücken et al. 2000,
Poole 2004).
However, there is little information on the effect of biocides on the transfer of genetic
determinants. One study in particular highlighted that while some biocides at a subinhibitory
(residual) concentration could inhibit genetic transfer, others increased genetic
transfer efficiency (Pearce et al. 1999).
There have been some investigations on co-transfer of resistant markers in epidemic
methicillin-resistant S. aureus following antibiotic treatment to decolonise patients
(Cookson et al. 1991a). The authors reported that there was no evidence of increase in
chlorhexidine MICs six years after the first isolation of the epidemic strains, although the
strain carried a qac gene (Cookson 2005). However, this was not the case with triclosan,
where clinical isolates of S. aureus showed high-level mupirocin resistance and low-level
triclosan resistance (MIC 2-4 mg/L) (Cookson et al. 1991b). The authors described that
resistance to both chemically unrelated compounds was transferred and cured together
(Cookson 2005). Table 8 summarises the main bacterial mechanisms of resistance to
biocides.