Infection prevention With Table 1, the optimal characteristics
have been shaded. Based on the outcomes, the choice for an effective disinfectant for healthcare with a wide spectrum of activity, rapid contact time22
and meeting safety criteria
often rests between hydrogen peroxide blends, amphoterics or a QAC. Amphoteric compounds, such as those consisting of long-chain alkyl amino acids, are characterised by the presence of both anionic and cationic groups within the same molecule and combine the anionic detergent properties and cationic disinfection properties.23
There are
a number of amphoteric compounds used as disinfectants and their antimicrobial activity depends on the molecule used. Hydrogen peroxide-based disinfectants are sometimes blended with other chemicals, such as peracetic acid; alternatively, additives like silver ions are combined with peroxide. Hydrogen peroxide based disinfectants have poor penetrative capacity when soil is present on a surface (which places an emphasis upon effective cleaning). Amphoterics and QACs are less likely to be inactivated by high levels of organic materials, with amphoteric compounds slightly less effected. However, this difference can be rendered marginal through effective cleaning prior to disinfection21
or by using a
repeat application or leaving the QAC in contact with the surface for a longer period. The addition of cleaning chemicals to a QAC formulation also counters the effects of residual organic materials. The spectrum of activity and mode of action against vegetative organisms is similar. Each agent is effective against a range of different material types, including steel, aluminium, and plastic (demonstrating shorter contact times on stainless steel compared with other materials).24 In terms of sporicidal properties, the QAC and hydrogen peroxide disinfectants are often sporicidal (provided they have been evaluated against a recognised standard). Mycobacteria need to be considered as a
separate category since these organisms are more resistant to disinfection and they fall between vegetative bacteria and endospores in terms of their resistance to chemical disinfection due to their hydrophobic waxy cell wall. Here peroxide blends and QACs are superior to the amphoterics. Each chemical is non-corrosive (unlike
more aggressive chemicals such as chlorine- based compounds) and are not abrasive to the materials commonly used as surfaces within the healthcare setting. Where QACs have an advantage is with lower residue formation (a property shared with peroxide blends although peroxides have a more pungent
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odour, and many require personnel to wear respiratory equipment). This reduces the risk of incompatibility with detergents and with other disinfectants (especially if disinfectants are rotated). Furthermore, QACs and peroxides have a lower concentration exponent compared with amphoterics. Where the concentration exponent is higher, then the effect of a change in concentration is much more significant than for those with a low concentration exponent. This can be illustrated in the following examples: l For a QAC with a concentration exponent of 1, a two-fold dilution will reduce the activity by a factor of 2 to the power of 1(21
) (it will take
twice as long for the same level of activity), a threefold dilution will reduce it by a factor of 3.
l For an amphoteric compound with a concentration exponent of 9, a two-fold dilution will reduce the activity by 2 to the power 9 (29
reduce it by a factor of 39
referred to a ‘residual power,’ and it is an ability to continue the disinfectant activity for a prolonged period, a characteristic that QACs share with phenolics. With prolonged activity, it is generally thought that autolysis is taking place26
(however, care needs to be
taken with this factor since dirt build up over time will create a protective film around any microorganisms contaminating the surface, creating a residual barrier; hence, regular application of a disinfectant is important). Therefore, on balance, selecting a QAC class
of disinfectant is the optimal choice for the healthcare facility.
) or 512, a three-fold dilution will or 19,683.
Amphoteric disinfectants increase in activity and kill rate when subjected to higher temperatures (optimally at 37o
C); this effect is
not as pronounced with QACs although there is a measurable increase in activity against the temperature quotient (heating peroxides creates a vapour and this presents safety concerns in uncontained areas). This property makes amphoterics useful when
operating cleaning / disinfection cycles that involve heat. QACs show enhanced microbial kill in proportion to the contact time applied (that is the longer the QAC is in contact with surface the greater the reduction in the microbial population). This effect is less apparent with amphoterics once the target contact time has been reached.25
This phenomenon is sometimes
Environmental health and safety concerns As with many types of industrial chemicals, care must be taken when disposing of used QAC solutions since adverse ecotoxicological hazard effects can occur, especially presenting chronic toxicity to susceptible aquatic organisms (uncontrolled wastewater discharges can lead to the presence of QACs in downstream aquatic ecosystems).27 QACs are persistent because of they do not volatilise significantly from soil or water.28
In
addition, QACs must be used in ventilated areas29 and personnel must wear personal protective clothing when managing QACs to avoid the development of dermal and respiratory ill- effects.30
Conclusion QACs are disinfectants of longstanding and with a long history of use in pharmaceuticals and healthcare. QACs have therefore contributed to maintaining appropriate contamination control standards through reducing bioburden microbial levels overall and reducing the probability of
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