The usefulness of environmental decontamination has perhaps been best demonstrated during outbreaks of Clostridium difficile infection.

In a before and after intervention study (5) 293 patients were studied in 3 units (a bone marrow transplantation unit, a neurosurgical ITU and general medical unit). Hypochlorite replaced quaternary ammonium compounds for patients with C difficile infection. C difficile rates in the bone marrow transplantation reduced from 8.6 to 3.3 per 1000 patient days. There was no effect in the other 2 units although this observation may have been due to the small number of patients in the study.

While the requirement for environmental cleaning relates more to public perception than to real risk, contamination can be substantial in outbreaks (especially those caused by vancomycin resistant enterococci and C difficile. It is also clear that decontamination can help halt outbreaks of C difficile infection and perhaps also MRSA and resistant Gram negative rods.

Disinfectants; an important component of Infection Control

Although most environmental areas need no more than cleaning, certain areas benefit from disinfection. These areas include trolleys for clean instruments and surfaces which have been contaminated with infective or potentially infective material. One of the most important examples of the latter is surfaces which have been contaminated with blood or body fluids. These areas need decontaminating with a disinfectant active against blood borne viruses and chlorine releasing agents are the most commonly chosen agents for this purpose. Unfortunately chlorine releasing agents are corrosive to metal and bleach carpets and fabrics. Metal surfaces can be effectively disinfected with 70% alcohol but a suitable agent for cleaning carpets is not available. Steam cleaning is the most appropriate process for this sort of material.

Existing Classes of Environmental and Instrument Disinfectants

Alcohols

Alcohol, when used as a disinfectant is usually in the form of ethyl or isopropyl alcohol. Both these chemicals are rapidly bactericidal against vegetative organisms as well as being tuberculocidal, viricidal and fungicidal but have no activity against bacterial spores. They are most active when used at a concentration of 60-90%. The activity of alcohol is probably due to its ability to denature proteins.

Alcohols are useful for disinfecting instruments such as thermometers, stethoscopes and laryngoscopes. They are also suitable for disinfecting horizontal surfaces and, because they evaporate rapidly, tend to leave equipment dry. The main disadvantage of alcohols is their flammability and thus they need to be used with care and stored appropriately. Another important disadvantage is incompatibility with some lens cements which makes alcohol inappropriate for disinfecting many endoscopes

Aldehydes including glutaraldehyde

Because of their toxicity, aldehydes are used less commonly in the UK than previously. However, they are still used in health care settings in many countries, predominantly as formaldehyde or glutaraldehyde. Their activity is due to alkylation of sulfhydryl and other groups which causes alterations to proteins and nucleic acids. Formaldehyde has excellent microbicidal activity, including cidal activity against vegetative bacteria, mycobacteria, viruses, fungi and bacterial spores. It is a toxic agent and consequently is controlled under COSHH regulations. There are also limits for the acceptable environmental levels of formaldehyde. It is no longer used except for fumigation of high risk areas such as category 3 rooms.

Glutaraldehyde is widely used in health care settings worldwide as an endoscope disinfectant. Most preparations are used at 2% and need activating to the alkaline form as this renders the agent more active but less stable. It is bactericidal and virucidal but only slowly sporicidal requiring 3 hours to produce a greater than 5 log10 reduction of bacterial spores. It is also mycobactericidal producing a 5 log10 reduction of viable M tuberculosis in 20 minutes.21 Atypical mycobacteria are less susceptible; M avium-intracellulare requires 1 hour exposure for the same level of kill. The activated form has a limited shelf life (14 to 28 days) because of polymerisation of the glutaraldehyde molecule.

Chlorine releasing agents

Commonly used chlorine releasing agents include hypochlorites and sodium dichloroisicyanourate (NaDCC). They are rapidly bactericidal, virucidal and tuberculocidal. Their mechanism of action is mediated by release of hypochlorous acid which interacts with key metabolic processes via protein denaturition. Chlorine releasing agents are corrosive and thus cannot be used on many metallic instruments. They demonstrate poor activity at low concentrations under dirty conditions because they are relatively easily inactivated by organic matter. It is important, therefore, that they are used on clean surfaces washer disinfectors may need to be modified to make them compatible with Nu Cidex. An integrated washer disinfector using peracetic acid (Steris) is also available.

Chlorine Dioxide

Chlorine dioxide is a relatively recently introduced instrument disinfectant with good broad spectrum activity. Its mode of action has not been thoroughly investigated but it is likely that it acts by production of free chlorine. It is rapidly bactericidal achieving a 5 log10 kill of Pseudomonas aeruginosa and other vegetative organisms in one minute although its activity may be reduced under dirty conditions. It is fungicidal, sporicidal and tuberculocidal achieving a greater than 5 log10 reduction in viable count for M tuberculosis, M. avium intracellulare and glutaraldehyde resistant atypical mycobacteria

Hydrogen peroxide

Hydrogen peroxide acts by the production of free hydroxyl radicals which denature cell walls and essential bacterial enzymes resulting in bactericidal activity. The fact that it is unstable and breaks down into water and oxygen makes it environmentally friendly but also means that it is easily inactivated. It has been used in concentrations varying from 3% for routine disinfection to 25% for high level disinfection. It has been shown to be bactericidal (including activity against spores), virucidal, tuberculocidal and fungicidal. Hydrogen peroxide is an irritant chemical and has been implicated in corneal damage following disinfection of tonometer heads and colitis following use in gastrointestinal endoscopy. It is therefore not widely used.

Peracetic acid

Peracetic acid is a strong oxidizing agent and has rapid bactericidal activity against a range of vegetative organisms and spores. It is also mycobactericidal (achieving a 5 log reduction of M tuberculosis in less than 5 minutes), virucidal and fungicidal. A formulation of 0.35% (Nu Cidex)has been marketed as a possible replacement for glutaraldehyde in endoscopy and has appropriately rapid activity for this use. Unfortunately peracetic acid is corrosive to brass and copper components and therefore washer disinfectors may need to be modified to make them compatible with Nu Cidex. An integrated washer disinfector using peracetic acid (Steris) is also available.

Phenolics

Carbolic acid (phenol) was used by Lister as an antiseptic and its antibacterial activities are well known. Halogenated derivatives of carbolic acid are known as phenolics and tend to have improved antibacterial properties compared with the parent compound. Their mode of action is by interference with essential bacterial enzyme systems. Phenolic disinfectants are bactericidal, fungicidal and tuberculocidal, although different compounds vary in their activity. Phenolic disinfectants are not sporicidal and have little or no virucidal activity. Activity against blood borne viruses is important for disinfectants (see "Activity" above) and therefore the use of phenolics has largely ceased.

The main concern about phenolic disinfectants is toxicity. They tend to be irritant and therfore are not recommended as instrument disinfectants. They also are unpleasant to work with and alternatives such as chlorine releasing agents are generally preferable as routine environmental disinfectants. There are reports that phenolics can cause hyperbilirubinaemia in infants and their use in special care baby units and nurseries should be discouraged.

Quaternary ammonium compounds (QACs)

The first commercially available QAC was benzalkonium chloride. It gained poularity due to its good antibacterial activity. However it lost favour due to the ease with which it is inactivated by organic material and hard water, coupled with its poor mycobactericidal and virucidal activity. It is believed that their activity is due to inactivation of cell metabolic pathways and denaturition of proteins. More recently developed QACs have better tolerance of hard water and are bactericidal and virucidal against lipophilic viruses. They are generally not tuberculocidal or sporicidal and have poor activity against hydrophilic viruses.

Superoxidised water

The concept of electrolysing saline to create a disinfectant is attractive as the basic materials are cheap and the end product is not damaging to the environment. A commercial adaptation of this technique is marketed as Sterilox. The mode of action is probably relates to the hypochlorous acid content as well as production of superoxide and other free radicals. Unfortunately the equipment required to produce this product with an acceptable degree of reliability is expensive as Several parameters must be carefully monitored. These include pH and redox potential as well as the current driving the electrolysis reaction. There is some concern that superoxidised water is damaging to materials and at least oneflexible endoscope manufacturer has warned that the guarantee on its endoscopes may be invalidated if superoxidised water is used to disinfect them. However, new coatings have been developed whichappear to protect endoscopes from this damaging effect.

The profile of an ideal candidate

The ideal disinfectant would be completely safe and non-toxic to users (healthcare workers) andpatients, would be immediately active against all vegetative bacteria, spores and viruses, would be pleasant to use and would be inexpensive. Needless to say, such a product does not exist.

While it is of utmost importance that a disinfectant is active against relevant pathogens, it is almost as important that it is acceptable to the users. Skin irritancy, respiratory irritancy and unpleasant odour can all make the agent less likely to be used and if the agent is not used its activity becomes irrelevant. For similar reasons the chemical components of disinfectant agents need to be safe and must comply with Control of Substances Hazardous to Health (COSHH) regulations.

However the most important property of a disinfectant is that it actually works; in other words it must have adequately rapid activity against the most important pathogens in its proposed area of use. The required spectrum of activity must be given careful consideration. On the one hand the chosen disinfectant may have activity against pathogens that are not important (for example some disinfectants have sporicidal activity which is not required in most clinical areas). Such unnecessary enhanced activity may have implications on toxicity and cost. On the other hand, a disinfectant used for blood spills must have rapid virucidal activity and an agent which does not inactivate viruses would be inappropriate and may even be unsafe in this situation.

Finally, it is important that the agent is cost effective. Some disinfectants (such as chlorine releasing agents) are very inexpensive but are corrosive to many metals. This may render these agents inappropriate for some uses. Alternatively agents used for disinfecting flexible bronchoscopes need rapid activity against M tuberculosis and it is well worth extra expense to ensure this activity and thereby protect patients.

REFERENCES

Boyce JM et al 1994. Outbreak of multidrug resistant Enterococcus faecium with transferable vanB class vancomycin resistance. J Clin Microbiol. 32:1148-1153

Crossley K, Landesman B & Zaske D. 1979. An outbreak of infections caused by strains of Staphylococcus aureus resistant to methicillin and aminoglycosides. II. Epidemiologic studies. J Infect Dis. 139:280-7

French GL et al. 2004. Tackling contamination of the hospital environment by methicillin resistant Staphylococcus aureus (MRSA): a comparison between conventional terminal cleaning and hydrogen peroxide vapour decontamination. J Hosp Infect. 2004 57: 31-37.

Bates CJ, Pearse R. 2005. Use of hydrogen peroxide vapour for environmental control during a Serratia outbreak in a neonatal intensive care unit. J Hosp Infect 2005. 61: 364-366.

Mayfield JL et al.2000. Environmental control to reduce transmission of Clostridium difficile. Clin Infect Dis 31: 995-1000.