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Dr Richard Joseph, Advisor, Schevaran Innovation Centre, gives Clean India Journal an overview of the nature of germicides in relation to their application purpose and place of need. It may be useful to become aware of the grey areas in the endeavor for initiating attempts to refine the procedures for ideal end results.
The nation is seized of the most peremptorily needed ‘Clean India Mission’. Cleaning as we would know is to get rid of dirt, dust and anything that mars a place. In this, a targeted approach is to remove, avoid or destroy harmful germs that can cause disease in people, animals and plants, and the procedure is referred to as hygiene maintenance. The chemical agents employed in hygiene maintenance are known as germicides.
In the cleaning process, the removal of dust and dirt is easily discerned, but not the germs. Indeed, there was a time when germs were not of any consideration, because their existence itself was unknown. In particular, diseases were attributed to varied curious things, spirits and gods included. There were however, some enlightened speculation on ‘invisible living creatures’ as the causative agents. The germs, as we know today are too small for the human eye to see. They were seen and officially reported for the first time in the 17th century by a Dutch businessman, Antony van Leeuwenhoek who is credited as the inventor of microscope. It was eventually a big revelation that these germs or microorganisms do have a profound influence on the larger living beings including humans in a love-hate relationship. Yes, though some of them are deadly disease causative agents, many were found to be not merely harmless, but essential for our healthful existence.
Targeting the disease-causing germs is a major task in the medical field, where hygiene maintenance is of primary importance. Hygiene maintenance and infection control are vitally important in many other places where people gather for work, entertainment, recreation or just in their homes where they may spend much time. Modern methods of cleaning in any place combines hygiene maintenance which is largely accomplished by use of carefully chosen germicidal chemical agents.
Attributes of germicides:
Germicide (-cide, Latin. caedere = to kill) denotes any agent that will act against the all microorganisms in any form. The availability of a fairly good number of germicidal chemicals with varied attributes provides the opportunity to choose the right ones for the job and place. Among the germicides, the term Disinfectant is used when the agent kills the growing microbial cells (vegetative form), but not the spores of the diseasecausing microorganism.
Disinfectants are mostly relevant for application on inanimate objects. Sterilant is an agent that will kill all types of microorganism (whether disease causing or not) including spores. Antiseptic has function similar to disinfectants, but it usually refers to chemical agent applied topically on the body. Some chemical agents retard or check microbial growth, but not necessarily kill them.
The first ever chemical agent employed was phenol extracted from coal tar over a century ago. This formed the bench-mark for comparison of efficiency of several others that were subsequently found useful in this regard. Phenol happens to be a harsh and corrosive chemical. Currently, with the finding of new chemical agents, final selection of chemicals is made if they conform to the “green” criteria, viz. they are non-toxic, non-irritating, non-corrosive, ecofriendly and cost effective.
Microorganisms vary greatly in their susceptibility to germicides. Recognizing this, microbiologists have delineated the germicides into the following categories:
1. Low level disinfectants: Examples in this category are the quaternary ammonium salts and phenolics. Their mode of action is by disruption of the plasma membrane of the microorganisms. The disinfectants act against Staph. aureus, Salmonella typhi, Pseudomonas auroginosa. (representative examples of pathogenic bacteria). They are also active against most of the enveloped viruses which include the Human immunodeficiency virus (HIV), Hepatitis B-, C- viruses
2. Intermediary level disinfectants: Exemplified by alcohols (ethanol, isopropanol), hypochlorites, iodine. Their mode of action is by denaturation of proteins, oxidation of thiol groups and inhibition of DNA synthesis. These disinfectants are active against all those microbe groups mentioned under aforementioned low-level disinfectants. And, in addition, they are active against some fungi which include Candida sp., Cryptococcus, Aspergillus sp. Dermatophytes etc.
3. High level disinfectants: Examples in this category are hydrogen peroxide and peracetic acid. They act by generating free radicals (+OH) thereby damaging DNA and also by oxidation of thiol groups. These disinfectants are active against all those microbes mentioned in categories 1 and 2 above. In addition, they are also active against Mycobacteria and non-enveloped viruses.
4. Chemical sterilants: Examples in this category are glutaraldehyde, orthophthaldehyde, formaldehyde and ethylene oxide. This group of chemicals can cause total destruction of microorganisms including their spores. Some important points to note here are:
i) The alcohol based disinfectants can kill vegetative bacteria and enveloped viruses (e.g. Influenza viruses) almost instantly whereas the efficacy of the rest of the disinfectants would depend on time–concentration factors
ii) The formidable pathogenic bacteria (e.g. Mycobacterium spp.) can be inactivated by the disinfectants mentioned under the ‘High level disinfectant’ category iii) Cryptococcus, a basidiomycete yeast found in 1975 as a virulent pathogen capable of systemic infection can be inactivated by the intermediary level disinfectants
While most disinfectants mentioned are highly efficient, their usage can be limited by the side effects, such as adverse impact on environment and possible hazardous outcomes to the non-professional handlers. Hence a new set of disinfectants named as ‘Green Germicides’ have been brought up. Included in this group are the quaternary ammonium salts, silver compounds, hydrogen peroxide, triclosan, chlorhexidine, isothizolinones, para hydroxyl benzoate and potassium metabisulfite.
Future prospects and challenges: Currently a great deal of interest has been evoked to explore preparations from botanical sources to control and manage harmful microorganisms. Their success would depend upon sustained efficacy and cost effectiveness
There has been much concern about the development of drug resistance in pathogenic microorganisms in recent times. The focus has so far been on therapeutically relevant drugs. There was however, scant information sought or available on the effect of the chemicals employed in hygiene maintenance and their sustained applicability vis a vis the impact on environment. An important question would be about the environmental microorganisms’ ability to undergo adaptive changes to the point of failure of hygiene maintenance efforts. As such it is known that there is some intrinsic resistance present in microbial spores, their coat being impervious to chemicals. The non-sporulating Mycobacteria have hydrophobic cell wall that prevents many chemicals, if not all, from entering the cell. In this respect the Gram-negative bacteria are also shown to exhibit a relatively higher permeability barrier than the Grampositive ones.
There is an interesting physiological adaptation of the microorganisms detected, by which they form a kind of wall-adhering membrane like structure termed as ‘biofilm’. In this form, the microorganism exhibits resistance to germicidal agents. This apparently is an area meant for exploration to get new information and developing effective application procedures.