Susceptibility to biocides

Microorganisms are known to exhibit a wide range of tolerance to biocidal chemicals, with bacteria showing the highest resistance.With invertebrates, fouling starts with the larval settlement, and this stage is believed to be the most susceptible to biocidal chemicals. 

Carrier tests with dried cells on surfaces are not suitable for the evaluation of biocide efficacy towards biofilms. In general, microbial cells dried on carriers are less susceptible to biocides compared to planktonic cells however, established biofilms grown on surfaces usually display enhanced resistance compared to organisms simply dried on carriers (e.g., Samrakandi et al., 1994 Ntsama.Essomba et al., 1997). A possible reason may be physiological changes which are associated with biofilm formation, and can result in enhanced resistance to biocides. These processes are not involved in carrier tests. 

Before considering the mechanisms by which microorganisms exhibit resistance, it is well documented that different microorganisms have different responses or susceptibility to biocides. This is commonly known as the MIC or minimum inhibitory concentration that will affect the microbe. For example, hexachlorophene has a MIC of only 0.05 ug/ml with S. aureus, 12.5 ug/ml with E. coli, but a value of 250 ug/ml with P. aeruginosa conversely, the antimicrobial agent bronopol (2-bromo-2-nitro-1,3-propanediol) has a much narrower MIC range for these three bacteria (31.5 ug/ml for E. coli and P. aeruginosa and 62.5 ug/ml for S. aureus) [16]. 

Chlorine Tolerance Most of the best RO membranes are attacked by oxidants, and they are particularly susceptible to chlorine. A particularly sensitive locus for attack is the amidic hydrogen. Cellu-losic membranes are generally less sensitive, and pass the chlorine into the permeate giving downstream biocidal activity, veiy useful for under-the-sink RO. These factors are largely responsible for CA s survival in RO membranes. Chlorine, whatever its vices, has the virtue of being a known, effective, residual bactericide and a good inhibitor of... 

Low power ultrasound offers the possibility of enhancing the effects of chlorine. The results of a study of the combined effect of low power ultrasound and chlorination on the bacterial population of raw stream water are shown (Tab. 4.2). Neither chlorination alone nor sonication alone was able to completely destroy the bacteria present. When sonication is combined with chlorination however the biocidal action is significantly improved [10]. The effect can be ascribed partly to the break-up and dispersion of bacterial clumps and floes which render the individual bacteria more susceptible to chemical attack. In addition cavitation induced damage to bacterial cell walls will allow easier penetration of the biocide. 

Although the musk fragrances and biocides often are released to the aquatic environment, many of them may be transported to other media, including the atmosphere. As discussed above, the processes in the atmosphere are all chemical degradation processes and the ability for a chemical to reach the atmosphere will depend on the Henry s law constant (Hc). In addition, some of these products may be susceptible to chemical hydrolysis. Table 16.5 lists the musk fragrances and biocide compounds along with the Henry s law constant estimated by EPISuite... 

In nature, S. aureus may occur as mucoid strains, in which a slime layer surrounds the cells. Such mucoid strains are less susceptible than nonmucoid strains to chloroxylenol (27), cetrimide (28) or chlorhexidine (13) although there is little difference in response with phenols or chlorinated phenols [111]. Removal of slime by washing renders cells as phenotypically sensitive to biocides as non-mucoid cells [111]. The protective nature of slime could be achieved by its acting as either (i) a physical barrier to biocide penetration, or (ii) a loose layer that interacts with, or absorbs, the biocide. 

The response of a developing spore to a biocide depends upon its stage of development [30, 128-132]. Resistance increases during sporulation and may be an early, intermediate or late event [128-132], Resistance to chlor-hexidine takes place at an intermediate stage and appears to be associated with the development of the cortex [133]. In contrast, decreasing susceptibility to glutaraldehyde is a very late event [131, 132] and is linked to the biosynthesis of the spore coats. 

An association between antibiotic resistance and chlorhexidine and QAC resistance in Providencia stuartii and Proteus has been observed, but no evidence of a plasmid link obtained [25, 73, 287,288]. Chlorhexidine hypersensitivity has been noted in ciprofloxacin-resistant variants of Ps. aeruginosa [289] and vancomycin- and gentamicin-resistant strains of E. faecium retained sensitivity to the /usbiguanide [289, 290] and to other biocides [270-272], Anderson et al. [272] studied the inactivation kinetics of VRE and vancomycin-sensitive enterococci (VSE) exposed to environmental disinfectants at concentrations well below (extended dilutions) the recommended use-dilutions and found no differences in susceptibility of VRE and VSE. This type of approach is much more relevant than the widespread usage of MICs to measure responses to biocides. 

Biocides are used widely in industry. There are at least three main classes of industrial chemical biocides. The first class includes the oxidizing and bleaching agents, such as chlorine dioxide, hydrogen peroxide, and sodium hypochlorite. The oxidizing action may directly kill bacteria or fungi or weaken the cell walls so that they are more susceptible to other classes of biocides (see below). Sodium... 

Members of the genus Proteus are unusually resistant to high concentrations of chlorhexidine 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 reduced binding of, and hence increased resistance to, cationic biocides. Decreased susceptibility to EDTA may result from the reduced divalent cation content of the Proteus outer membrane. 

The broad-spectrum biocidal action induced by phototoxic allelochemicals has been demonstrated toward a range of organisms including bacteria, fungi, nematodes, insects as well as non-photo-toxin-containing plants. Table III summarizes the organisms susceptible to photosensltlzatlon by alpha-terthienyl, a phototoxin characteristic of many Asteraceae species (4, 10-11). The toxicity of this thiophenic polyine toward plant pathogens (e.g.,... 

Cationic surfactants fall into several categories depending on the nature of their cationic polar heads. Some of them have functional groups susceptible to protonation e.g. amines) and thus display cationic properties particularly in acidic media, while others, such as quaternary ammonium salts, exhibit a permanent positive charge. In household products, cationic surfactants are primarily applied in fabric softeners and hair preparations. Other applications of cationic surfactants include disinfectants and biocides, emulsifiers, wetting agents and processing additives. By volume, the most important cationic surfactants in household products are the alkyl ester ammonium salts that... 

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