Disinfectants: Are they all the same?

Johannesburg, 29 July 2020: The spread of the common cold, influenza and many other infectious diseases may be prevented by using the correct disinfectants on surfaces as well as in hand sanitation on a regular basis.  Many respiratory diseases affecting humans and animals globally, such as avian influenza virus (AIV), tuberculosis as well as the recent outbreak of the coronavirus, more specifically the COVID-19, are caused by agents including bacteria, fungi as well as viruses. These organisms and especially viruses are secreted in large amounts by the infected human or animal which leads to contamination of the environment in which they live or move around in.  Disinfection, as well as proper cleaning, plays a crucial part in the control and elimination of these pathogens.

A disinfectant is defined as a chemical substance which has the ability to destroy vegetative forms of harmful micro-organisms, such as bacteria, viruses, moulds, mildews and fungi, on inanimate objects as well as provide effective protection when used as hand sanitation devices.  The active ingredient within a disinfectant formula is able to eliminate pathogens by disrupting and or damaging the active cells and or protein structure and or nucleic acid components depending on the entity. Disinfectants act in two different ways, either as growth inhibitors (i.e. bacteriostasis, fungistasis) or in a lethal way (bactericidal, fungicidal or virucidal). Disinfectants are required to be effective and safe regardless of any other compounds present during use.  It must be stable in both diluted and concentrated forms.

The effectivity of a disinfectant is compared to the phenol standard according to the corresponding system called the “Phenolic coefficient”.  A new or commercial disinfectant is to be tested by comparing it with phenol on a standard micro-organism such as Salmonella typhi or Staphylococcus aureus.  Disinfectants are graded according to their effectivity against the microbes compared to phenol, either with more effectivity than phenol thus having a coefficient of more than 1 and those having less effectivity than phenol having a coefficient of less than 1.  Another measure to determine disinfectant effectiveness is the United States Environmental Protection Agency (EPA) classification scale as either high, intermediate or low levels of disinfection.  High-level disinfection refers to the disinfectant killing all organisms, except high levels of bacterial spores. The intermediate level disinfectants are classified by their ability to kill mycobacteria, most viruses as well as bacteria. Finally, low-level disinfectants are classified as agents which kill some viruses and bacteria. Another alternative assessment measurement of disinfectants is based on the Minimum Inhibitory Concentrations (MIC) of the disinfectant product against selected micro-organisms. These tests are performed using a microbroth dilution test.

A good disinfectant is characterised according to the following factors:

  1. Broad-spectrum germicidal, sporicidal, and fungicidal effectivity
  2. Slight to moderate efficiency in the presence of organic matter
  3. Slight residual activity
  4. Must be both active as well as stable
  5. Safe for humans and animals, moderate toxicity
  6. Environmentally safe
  7. Leaves no damage or odour
  8. Non-corrosive
  9. Affordable
  10. Straightforward usage.

Disinfectant formulas contain an active ingredient which is capable of killing pathogens by either causing disruption of structure (e.g. RNA) or damage to their cells.  Most disinfectants contain substances which aid the active ingredient which is carefully chosen to assist the purpose of the disinfectant (synergistic effect). There are many broad categories of disinfectants which are commonly used for the purpose of sterilisation within the commercial and industrial industries.  Below are the most commonly used disinfectant categories and examples of each.

 

Alcohols

Alcohols such as ethanol and isopropanol, between 60–90%, are considered to be good general disinfectants which provide disinfection by precipitation of proteins within pathogens as well as denaturing lipids. However, the usefulness of alcohol decreases as the concentration used is below 50%. It is important to understand that higher concentrations of alcohol within a disinfectant do not necessarily generate more desirable effects against bacteria, viruses, and fungi.

These are fast-acting disinfectants with rapid evaporation time which leave no residue.  Alcohols may have a strong smell and have the ability to harden certain rubbers and plastics.  Alcohols are active against bacteria, fungi and tuberculosis (TB) causing organisms.  However, alcohols have more limited activity against viruses and have no activity against spores.  Solutions with an isopropyl alcohol concentration above 91% have the ability to kill bacteria but may require longer contact time for disinfection and thus allow spores to lie dormant without being killed. Alcohols are also highly flammable.

Alkalis

Calcium hydroxide, sodium carbonate and calcium oxide are only a few of the disinfectants which form part of the alkali group of disinfecting agents.  Alkalis destroy pathogens by altering the pH through hydroxyl ions and have the ability to saponify fats.  Alkalis have a slower acting disinfecting time which is affected by pH and perform optimally at higher temperatures.  These agents are corrosive to metals and cause severe skin burns as well as mucous membrane irritation.  Alkalis are bactericidal, virucidal, fungicidal as well as sporicidal.  Alkalis show limited action against TB bacilli.

Aldehydes

This class of disinfectants includes agents such as formaldehyde, glutaraldehyde and ortho-phthal-aldehyde.  An example of a good disinfectant is glutaraldehyde, which is classified as a saturated dialdehyde disinfectant, sterilant and active ingredient of some hygiene products.  It has gained wide acceptance as a high-level disinfectant as well as a chemical sterilant.  It works by denaturing the proteins of pathogens and has the ability to alkylate nucleic acids (DNA and RNA).  The biocidal activity of glutaraldehyde results from its ability to alkylate sulfhydryl, hydroxyl, carboxyl, and amino groups of micro-organisms, which alters RNA, DNA, and protein synthesis.  It is a slow-acting disinfectant which is affected by both pH and temperature.  It is a colourless liquid with a pungent smell and is classified as non-corrosive.  Glutaraldehyde is proven to be bactericidal, virucidal, fungicidal, highly tuberculocidal as well as sporicidal. A 2% solution of glutaraldehyde exhibits good activity against vegetative bacteria, spores as well as viruses.  Glutaraldehyde is 10-fold more effective than formaldehyde and is less toxic. It is non-staining and relatively non-corrosive and can be used as a sterilant on plastics, rubber, lenses, stainless steel as well as most items which cannot be autoclaved.

Adapted from McDonnell, G. & Russell, A.D. 1999

When tested, a 40% as well as a 70% glutaraldehyde solution is able to kill a minimum of 99,9% of Escherichia coli micro-organisms as well as the PHI 174 bacteriophage (single-stranded DNA (ssDNA) virus that infects Escherichia coli) within a 15 minute exposure period. Newer patents include glutaraldehyde surfactant combinations allowing for better surface penetration of organisms and translating into a more effective product.

Oxidising Agents: (Halogens: Chlorine & Iodine; Peroxygen Compounds)

Halogen disinfectants containing chlorine such as sodium hypochlorite (bleach), calcium hypochlorite and chlorine dioxide act by denaturing pathogen proteins.  These are fast-acting agents which are affected by pH and require frequent application and are inactivated by UV radiation.  These agents have the ability to corrode metals, rubbers as well as fabrics.  Chlorine halogens cause mucous membrane irritation and may release a toxic gas if mixed with strong acids or ammonia.  Chlorine disinfectants are active against bacteria, viruses, fungi, TB bacilli as well as bacterial spores.

Halogen disinfectants containing iodine such as povidone-iodine destroy pathogens by denaturing microbial proteins.  These agents are stable in storage but are, however, affected by pH and require frequent application.  Iodine is corrosive and stains clothing as well as treated surfaces.  Disinfectants containing iodine are active against bacteria, viruses, fungi, TB bacilli and have limited activity against spores.

Peroxygen compounds such as hydrogen peroxide, pancreatic acid and potassium peroxymonosulphate all form part of the peroxygen disinfectants which eliminate pathogens by denaturation of proteins as well as lipids.  These are fast-acting disinfectants which may damage some metals such as lead, copper, brass and zinc.  These agents are also available in a powdered form which may cause mucous membrane irritation but have low toxicity at lower levels and are considered generally environmentally friendly.  Peroxygen disinfectants are all active against bacteria and viruses as well as spores.  These agents pose limited effectivity against fungi as well as TB bacilli.

Phenols

Disinfectants including ortho-phenylphenol and orthobenzylpara-chlorophenol disrupt cell wall synthesis as well as denature pathogens’ proteins.  These agents may leave a residual film on treated surfaces and can damage rubber and plastic but are non-corrosive.  Phenols are stable in storage.  Phenols cause irritation to the skin as well as to eyes.  Phenols are considered toxic to animals especially to cats and pigs.  Phenols are bactericidal, fungicidal, virucidal, as well as tuberculocidal but have no effectivity against spores.

Quaternary Ammonium Compounds

Benzylkonium chloride and alkyldimethyl ammonium chloride provide disinfection by denaturing pathogen proteins and binding phospholipids of the cell membranes.  These agents are stable in storage and provide best activity at neutral or alkaline pH.  Quaternary Ammonium Compounds are effective at high temperatures.  Higher concentrations are corrosive to metals and cause irritation to skin, eyes and the respiratory tract.  Quaternary Ammonium Compounds are effective against bacterial, selectively enveloped viruses, fungi and spores.  These agents show no activity against tuberculocidal pathogens.

Combination products

In many cases, the combination of two or more different disinfectants poses beneficial in the sterilization of surfaces and or when used to sterilise medical equipment as well as in skin sanitation products. However, disinfectants cannot merely be mixed or used in combination without the proper knowledge of each mechanism of action as different disinfectants may either enhance the action of the other or cause reactions which in turn delay or stop disinfection completely.

When looking at combinations of disinfectants, the use of quaternary ammonia and glutaraldehyde as a disinfectant against enveloped and non-enveloped viruses provides a good example of where disinfection is enhanced when a combination of disinfecting agents is used.

Soap versus disinfectant

The recent COVID-19 pandemic has highlighted the importance of handwashing as well as the use of sanitisers in the prevention of infecting others. As we move into the colder weather, skin is likely to become prone to dryness and the skin barrier is then essentially compromised. The aim is to promote skin health while also maintaining the maximum possible degree of infection prevention.

Cleaning with water and the use of soap is referred to as the physical removal of dirt and grease and in process some of the pathogens which may cause disease.  When cleaning with the use of soap one simply moves pathogens from one surface to another.  Disinfection allows for the elimination of pathogens from these surfaces as well as rendering these pathogens incapable of reproduction.

Many viruses, including the novel coronavirus, are compiled of self-assembled nanoparticles which exhibits the most vulnerable structure as the outer bilayer. Soap has the ability to dissolve this lipid membrane which in turn breaks the virus apart, leaving it inactive. Soaps are alkaline surfactants, able to pick up particles such as dirt, bacteria as well as viruses from the skin to be rinsed off with water. The duration of time spent hand washing has proven to be the most effectively correlated with removing viruses from the skin’s surface. A recent study done on over two thousand participants proved that those spending only 5-10 seconds washing hands with soap were at increased risk of more frequent influenza-like illness, compared with those who washed their hands for 15 seconds or longer.

 

Conclusion

Alcohol is the most commonly used disinfectant since the 1800s. Most alcohol-based hand disinfectants contain isopropanol, ethanol, N-propanol or a combination of these products. As previously noted, alcohol solutions containing higher concentrations than 95% of alcohol are less effective as these contain very little water, which is necessary to denature proteins easily. Alcohol strips the skin of moisture, thus many newer formulations of hand sanitisers contain a humectant in the form of glycerine and/or aloe vera which aids in replacing water content to the skin surface. Both hand washing and sanitation are equally important when looking to prevent the risk of infection transmission as well as to protect oneself from contracting many viruses and or bacteria.  However, one needs to also ensure that the skin’s integrity is maintained to ensure no hiding spaces are created for microorganisms and that the skin’s barrier remains intact. The latter can be achieved with frequent use of moisturisers as part of the cleaning and sanitising “ritual”.

Then lastly, it remains important to ensure that products used are quality controlled and approved by regulatory agencies such as the SABS and registered and tested as effective disinfectants able to substantiate the claims on the label.

 

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