According to World Bank data, the amount of solid waste generated by the world in a year is about 2 billion tonnes. It is estimated that at least 33 percent of this is not managed in an environmentally safe manner [1]. Among the toxic pollutants are agricultural pesticides, pharmaceuticals and byproducts from petrochemical industries.
Pesticides
Ever since the industrialization of agriculture, pesticide use has become widespread. Pesticides are chemical substances or mixtures of substances that protect crops and plants from pests, weeds and diseases. Humans and other living organisms are exposed to pesticides through contact with the skin, inhalation and oral ingestion. Several research studies show that these chemicals are bio-accumulated in the body. Bio-accumulation refers to the uptake of substances from the environment by an organism and its accumulation or retention in the body over time. Adverse health effects such as diseases related to the skin, gastrointestinal tract, nervous system, respiratory, endocrine and reproductive systems and cancer have been linked with exposure to pesticides. High levels of exposure can even result in death. Although the concentration of pesticides found in food such as fruits, vegetables and cereals is within the legislative “safe limits”, the combined exposure to two or more different chemical substances may have synergistic effects that could pose serious health risks [2].
Pharmaceuticals
Similar to pesticides, pharmaceutical residues from the medical sector are also present in the environment. Several analytical studies have shown the widespread occurrence of these chemical compounds in soil and water samples. Puckowski et al. did a rigorous review of analytical studies on pharmaceuticals in the environment, such as antibiotics, anti-hypertensives, antidepressants, analgesics, anti-inflammatory drugs and steroids[3]
Pharmaceuticals are inherently bioactive compounds. Though they are present in the environment at low concentrations, their continuous input into the environment could lead to prolonged exposure, thereby causing adverse effects on non-target organisms. Furthermore, the rate at which these pharmaceuticals are introduced into the environment exceeds their degradation rate, making them a persistent toxic contaminant in the environment [3].
Just like pesticides, these pharmaceuticals also tend to bio-accumulate in the body of organisms. While available research studies refer only to selected compounds, in reality, organisms are exposed to hundreds of pharmaceutical compounds simultaneously. The health risks due to such synergistic effects have not been investigated.
Toxin exposure and neurodegenerative diseases
Widespread exposure to toxic chemicals and their bio-accumulation have been confirmed as a cause of cognitive dysfunction and dementia[4]. The Center for Disease Control and Prevention, USA, has found that most people have accumulated an assortment of toxic chemicals in their body, such as heavy metals, pesticides and flame retardants. It is well-established that many of these chemicals are neurotoxins; they have a neurodegenerative as well as neurodevelopmental impact as they lead to pathological mechanisms such as disruption of neurotransmitter regulation[4].
The most common type of dementia, Alzheimer’s disease, is now growing at an epidemic pace, with a huge burden of healthcare costs that could devastate the world’s economies, health care systems and families [4].
Bioremediation
Fortunately, toxicant exposure is reversible and it is possible to detoxify the environment of these chemicals. This method is called bioremediation, defined as the use of naturally occurring microbes such as bacteria and fungi or genetically engineered micro-organisms to detoxify man-made pollutants. Earthworms, for instance, have the capacity to detoxify several pesticides due to their intestinal microflora.
Principle of Bioremediation
(Image courtesy: www.earthreminders.com)
One of the recent advances in bioremediation technology is the use of cow dung microbial consortia for detoxification of pollutants. A microbial consortium refers to two or more bacterial groups living in a symbiotic relationship (a close, mutually beneficial biological relationship between two different organisms).
Microbial consortium: Two or more bacterial groups living in a close, mutually beneficial relationship.
(Image Courtesy: https://growersnetwork.org/cultivation/what-is-a-consortium-of-bacteria/)
Cow dung for bioremediation
In the Indian tradition, cow dung is considered a highly purifying substance. It is used to plaster the walls and floor of traditional homes, owing to its activity against disease causing pathogens and several other properties beneficial to human health. Even today, it is a common sight to observe women sprinkle cow dung mixed with water over the portico of the house early in the morning. In the historical epic, the Mahabharata, the great sage Vasishta describes the glories of cows and explains how the cows practised austere penances for a hundred thousand years with the goal of attaining to a position of great eminence and becoming the rescuers of the worlds.
“By bathing in water mixed with our dung, people shall become sanctified. The deities and men shall use our dung for the purpose of purifying all creatures mobile and immobile.”
Such is the boon granted by Lord Brahma, the creator of the universe, to the cows [5]
Today. scientific evidence shows that the microbial communities present in cow dung possess remarkable potential for bioremediation of pesticides, pharmaceuticals and other pollutants. Let us look at the findings of a few studies.
Geetha et al. investigated the bioremediation of commonly used pesticides such as chlorpyrifos, fenvalerate, cypermethrin, butoxyethyl ester at different concentrations using cow dung microbial consortia.The process of bioremediation of the pesticide mixed soil was studied till the parent compound was converted into less harmful compounds, which eventually become a part of the microbial food chain and get integrated with humic substances (organic matter) in soil [6].
Mechanisms for bioremediation: The cow dung was characterized for physico-chemical and microbial parameters. Two key factors were found to be the reason for cow dung’s potential in bioremediation:
The cow dung slurry contained a robust, large, diverse population of microorganisms such as bacteria, fungi and actinomycetes, which were effective in the biodegradation of pesticide amended soil.
The high concentration of nutrients in cow dung such as organic carbon, nitrogen, phosphorus, sulphate, calcium chloride, sodium, potassium and magnesium provide micro- and macro- nutrition for the microbial community to grow and proliferate [7].
In other words, the large microbial population and higher nutrient availability was found to have a significant influence on the bioremediation of pesticides.
Adaptation responses of bacteria: Research on microbial mechanisms of bioremediation of toxic compounds reveals that successful decontamination requires bacterial strains that have the appropriate enzymes and the ability to degrade particular toxins. This ability to degrade toxins primarily depends on the adaptation mechanisms that allow bacteria to eliminate toxic compounds in their environment and survive. Many adaptation responses have been observed in bacteria that counteract the effects of toxic pollutants in the environment. Examples include saturation-rigidification of the cell membrane, increased content of specific fatty acids, production of stress proteins and so on. Since the cell membrane is the first point of contact between the toxin and the bacterial cell, most adaptive mechanisms are to do with the maintenance of the cell membrane fluidity. As a result of these adaptive processes, the toxic compounds in their environment are eliminated or their negative effects minimized [8].
Randhawa et al did a review of research papers on bioremediation of biomedical wastes, heavy metals, arsenic, petroleum, benzene and phenol with cow dung [9]. Some of the key findings are discussed below.
The management of biomedical wastes poses a challenge because they pollute the soil, water and air, if incinerated (as incinerators produce toxic gases like dioxins). In one research study, it was found that a certain species of fungus known as Periconiella present in cow dung, was an excellent degrader of biomedical waste such as contaminated bandages and cotton.
Another study found that Pseudomonas plecoglossicida, isolated from cow dung, was effective in the bioremediation of insecticides like cypermethrin (a fast-acting neurotoxin for insects). Similarly, Pseudomonas aeruginosa could be used in the bioremediation of chlorpyrifos, another pesticide that is a neurotoxin.
Such studies highlight the potential for these bacterial isolates from cow dung to be applied in the bioremediation of pesticide contaminated soil and water. The microbes in cow dung also have the ability to reduce arsenic concentration in water.
Petroleum refineries and petrochemical industries release complex organic wastes such as benzene, phenol, toluene, naphthalene, paraffin, etc. Pseudomonas and Bacillus species of bacteria present in cow dung were identified as petroleum utilizing bacteria that could degrade petroleum. Pseudomonas putida was found to be a potential benzene and phenol degrader.
In summary, bioremediation using naturally occurring microbes in cow dung is a powerful, low-cost approach to detoxify the environment of persistent and bioaccumulative chemical residues. It is a resource that is abundantly available in rural areas of India. Further research and technology development in this area could go a long way in reclaiming our environment and our health for a sustainable future.
REFERENCES
[1] Trends in Solid Waste Management n.d. https://datatopics.worldbank.org/what-a-waste/trends_in_solid_waste_management.html (accessed March 28, 2021).
[2] Nicolopoulou-Stamati P, Maipas S, Kotampasi C, Stamatis P, Hens L. Chemical Pesticides and Human Health: The Urgent Need for a New Concept in Agriculture. Front Public Heal 2016;4:148. https://doi.org/10.3389/fpubh.2016.00148.
[3] Puckowski A, Mioduszewska K, Łukaszewicz P, Borecka M, Caban M, Maszkowska J, et al. Bioaccumulation and analytics of pharmaceutical residues in the environment: A review. J Pharm Biomed Anal 2016;127:232–55. https://doi.org/10.1016/j.jpba.2016.02.049.
[4] Genuis SJ, Kelln KL. Toxicant exposure and bioaccumulation: a common and potentially reversible cause of cognitive dysfunction and dementia. Behav Neurol 2015;2015:620143. https://doi.org/10.1155/2015/620143.
[5] The Mahabharata, Book 13: Anusasana Parva: Section LXXIX n.d. https://www.sacred-texts.com/hin/m13/m13b044.htm .
[6] Geetha M, Fulekar M. Bioremediation of pesticides in surface soil treatment unit using microbial consortia. African J Environ Sci Technol 2008;2:36–45.
[7] Fulekar MH. Bioremediation technology: recent advances 2012.
[8] Donyinah SK. Persistent Organic Pollutants. BoD–Books on Demand; 2019.
[9] Randhawa GK, Kullar JS. Bioremediation of pharmaceuticals, pesticides, and petrochemicals with gomeya/cow dung. ISRN Pharmacol 2011;2011:362459. https://doi.org/10.5402/2011/362459.