INF ECTION P R EVENTION
agent to a surface or to hands is to reduce the population of potentially pathogenic microorganisms. More formally, when measured using defined norms for disinfectant efficacy, the chemical agent should reduce a known population of microorganisms by a pre-set logarithmic value and within a target period of time (the contact time). An ideal disinfectant will have a high inactivating capacity for a wide range of viruses, such as HIV, SARS-CoV-29 hepatitis10
and
, as well as being effective against a broad spectrum of pathogenic bacteria, including mycobacteria. In addition, disinfectants must be safe for staff to use; and be suitable for frequent application, in terms of not aggressively damaging surfaces. Disinfectants that are available in a range of formats, such as trigger sprays and pre- saturated wipes, provide greater flexibility for healthcare professionals. Some parts of the infection control protocol require the use of ‘high-level’ sporicidal agents (such as transfer disinfection into pharmacy compounding units and to remediate an outbreak of specific pathogens capable of forming endospores, such as Clostridium difficle). However, given the toxic and corrosive nature of sporicidal agents, wards and other parts of the hospital require regular application of a broad spectrum biocide to maintain contamination control. The focus of this article is on the application of quaternary ammonium compounds to help to deliver infection control. Biocidal QACs can be readily applied to hands (as an antiseptic at lower concentrations,11 such as <1%, formulated with appropriate moisturisers – for example, less aggressive QACs like cocamidopropyl betaine) and to surfaces (at higher concentrations, such as within the 3-5% range).
Global standards International data supports the use of cleaning and disinfection to deliver a reduction in the rate of healthcare-associated infections throughout the hospital setting. For this reason, disinfection measures
are captured in most national infection control standards, including guidelines drawn up for US and UK hospitals.12,13 To meet the required decontamination standards required for skin antisepsis and hard surface disinfection, QACs are one of the most common disinfectants used worldwide and these are deployed for a variety of clinical purposes. QACs ‘for use on skin’ are listed in the WHO hand hygiene guidance; recommended applications include preoperative disinfection of unbroken skin and general ward hand hygiene.14 References to the use of QACs for hand disinfection are also made in the CDC guidance.15
At higher concentrations, QACs
are used for the disinfection of surfaces, as described in separate CDC guidance documents.16, 17
Standards exist for ensuring
that staff are appropriately trained and that protocols are available.18
In addition, regular
auditing of ward and personnel hygiene by infection control specialists is recommended, together with signage as to the cleaned and disinfected status of the ward.
QAC chemistry and method of kill As indicated above, QACs are commonly used, broad-spectrum disinfectants. Whereas
many biocides are formulated as single and well-defined chemical substances, QACs are composed of a diverse, eclectic collection of substances that share a common chemical design: a molecular structure containing a positively charged nitrogen atom covalently bonded to four carbon atoms (hence ‘quaternary’). QACs are amphoteric surfactants (surface active agents), composed of positively charged polyatomic ions.19
An example of a QAC is
didecyldimethylammonium chloride (formed of alkyl-quaternary ammonium salts with typical alkyl chain lengths of C8, C10 and C12), a chemical that demonstrates strong microcidal activity.20
Other characteristics
are that QACs tend to be free from odour (or having a faint ethanolic odour) and colour; they are stable; exhibit low toxicity; and are non-corrosive in dilute form.21
QACs are
generally very stable, mostly unaffected by pH levels, and, once applied, they remain effective on a surface for a prolonged time.22 With microbial kill, the permanent positive charge makes QACs bind readily to the negatively charged surface of most microorganisms. The subsequent antimicrobial action of QACs against bacteria involves perturbation of cytoplasm and the phospholipid bilayers that form the bacterial cell membrane. On entering the bacterial cell, the QAC chemical forms mixed micelle aggregates with hydrophobic membrane components; these function to solubilize membranes, cause proteins and nucleic acids to degrade inside the cell, and trigger cell lysis and death.23
Of these
mechanisms, the antimicrobial activity of quaternary ammonium compounds is derived from their longest alkyl chain which acts as the predominant mode of action for the disruption of the cell membrane.24 Against fungi, the antifungal activities of QACs include impediment of hyphae formation through the reversal of cell surface
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