Gram-negative bacteria biocide resistance

Bacterial resistance to biocides (Table 13.2) is usually considered as being of two types (a) intrinsic (innate, natural), a natural property of an organism, or (b) acquired, either by chromosomal mutation or by the acquisition of plasmids or transposons. Intrinsic resistance to biocides is usually demonstrated by Gram-negative bacteria, mycobacteria and bacterial spores whereas acquired resistance can result by mutation or, more frequently, by the acquisition of genetic elements, e.g. plasmid- (or transposon-) mediated resistance to mercury compounds. Intrinsic resistance may also be exemplified by physiological (phenotypic) adaptation, a classical example of which is biofilm production. 

Members of the genus Proteus are unusually resistant to high concentrations of ehlorhexidine and other cationic biocides and are more resistant to EDTA than most other types of Gram-negative bacteria. A less acidic type of LPS may be responsible... 

For example, Candida albicans and (especially) Aspergillus niger are much more resistant to a variety of biocides than Gram-positive and Gram-negative bacteria. 

Plasmid R124 alters the cell surface of E. coli cells such that they show enhanced resistance to the QAC, cetrimide, and other agents [299]. Generally, plasmids do not promote resistance in Gram-negative bacteria to biocidal agents [300], although hospital isolates may be more resistant to biocides than laboratory strains [301]. It is to be wondered whether an Mdr system is associated with this resistance. 

Bacterial resistance to antibiotics and biocides is essentially of two types, intrinsic and acquired. Whilst the latter is of greater significance clinically with antibiotics, specific examples of intrinsic resistance to both antibiotics, e.g. mycobacteria, and biocides (e.g. mycobacteria, Gram-negative bacteria, spores) are also of importance. 

Different types of microbes show varying responses to biocides. This is demonstrated clearly in Table 18.2. Additionally, it must be noted that Grampositive bacteria such as staphylococci and streptococci are generally more sensitive to biocides than are Gram-negative bacteria. Enterococci are frequently antibiotic-resistant, but are not necessarily... 

As already pointed out, staphylococci and streptococci are generally more sensitive to biocides than Gram-negative bacteria examples are provided in Table 18.4. On the other hand, mycobacteria and especially bacterial spores are much more resistant. A major reason for this variation in response is associated with the chemical composition and structure of the outer cell layers such that there is restricted uptake of a biocide. In consequence of this cellular impermeability, a reduced concentration of the antimicrobial compound is available at the target site(s) so that the cell may escape severe injury. Another, less frequently observed, mechanism is the presence of constitutive, biocidedegrading enzymes. 

---