How heavy metals foster antibiotic resistance
Usually industry is regarded as the chief purveyor of heavy metal pollution. Roadways and automobiles constitute one of the largest sources of pollution by heavy metals. Various types of fuels, tire wear, batteries and brake emissions release significant amounts of heavy metals into the environment. Leaching from soil because of acid rain is another source of heavy metals. Occupational and household exposure may also lead to toxicity. Unregulated herbal medicines pose serious threat to public health as many of these traditional remedies contain dangerous levels of heavy metals.
The adverse effects of heavy metals, however, are not confined to direct toxicity on the living system. They can also blunt the beneficial effects of antibiotics. To understand this we need to know a bit about antibiotic resistance.
The discovery of a number of antibiotics, including the antitubercular drug streptomycin, during the 1940s and the 1950s led to a euphoria that all infectious diseases could be cured by antibiotics. But the harsh reality surfaced within a couple of years after the introduction of antibiotics in clinical practice. It became evident that pathogenic microorganisms can evolve and develop mechanisms that enable them to inactivate antibiotics.
Antibiotic resistance is not the characteristic of any particular organism. It may occur in nature both in antibiotic-sensitive and antibiotic-resistant forms. Resistant microbes transfer resistance-conferring genes to sensitive microbes by various mechanisms. Soil, water, humans, plants and animals play host to both the sensitive and resistant varieties. Sensitive microbes outnumber the resistant ones, and not all of them are harmful to us. But by indiscriminate use of antibiotics, adding antibiotics to the animal feed and throwing unused antibiotic formulations here and there, we are suppressing the friendly microbes.
Antibiotic-resistant microbes could, however, thrive even if we take care of such wanton practices. This is where heavy metals come in. In an environment polluted with heavy metals, organisms sensitive to heavy metals are killed and only those resistant to heavy metals survive. But then these survivors have plasmids that also carry antibiotic-resistance genes. In other words, the organisms are simultaneously resistant to various heavy metals and antibiotics.
A survey by the Centre for Cellular and Molecular Biology in Hyderabad and Leibniz Institute of Freshwater Ecology and Inland Fisheries in IGB-Neuglobsow, Germany, provides some insight into the links between antibiotic resistance and heavy metal tolerance. The study was conducted on 66 bacterial isolates, obtained from some lakes of northern Germany. The lakes are located in thinly populated areas, so the chances of human-induced antibiotic resistance is remote. But the bacteria displayed resistance to various therapeutically useful antibiotics. Forty-six per cent to 50 per cent of the microbes were resistant to trimethoprim and ampicillin, 24 per cent to erythromycin and 19 per cent to chloramphenicol. Resistance to chloramphenicol is surprising since the drug is no longer used in clinical practice in Western countries. Over 90 per cent of the isolates were tolerant to zinc chloride, over 80 per cent were tolerant to potassium chromate and over 60 per cent were tolerant to cadmium chloride. The isolates also showed resistance to combinations of antibiotics and heavy metals. Almost 15 per cent were resistant to five antibiotics and three heavy metals, one showed resistance to seven antibiotics and three heavy metals, and another was tolerant to six antibiotics and three heavy metals.
This shows tolerance of bacteria to various inhibitory substances does not require direct contact with the inhibitors. Heavy metals are just as culpable. The investigation underscores the urgent need for adopting stringent measures to contain the environmental pollution by heavy metals.
Madhab K Chattopadhyay works with the Centre for Cellular and Molecular Biology, Hyderabad. Hans-Peter Grossart works with Leibniz Institute of Freshwater Ecology and Inland Fisheries in IGB-Neuglobsow in Germany
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