Sundarbans mangroves harbour antibiotic resistant bacteria

  • Researchers have documented the presence of antibiotic resistant bacteria (ARB) in the microbial milieu of the Sundarbans mangroves in a new study.
  • The study sheds light on the potential influence of human interventions in shaping the distribution of antibiotic resistant genes (ARG) and bacteria in the mangrove sediments.
  • More the heavy metals and polyaromatic hydrocarbons detected in the sediment, greater was the abundance of antibiotic resistance genes (ARGs).
  • Researchers emphasised on the importance of treating wastewater, conserving and regenerating mangroves. Mangrove microbial community is an important component of biodiversity that remains unexplored, they said.

Researchers have documented the presence of antibiotic resistant bacteria (ARB) in the microbial milieu of the Sundarbans mangroves in a new study that sheds light on the potential influence of human interventions in shaping the distribution of antibiotic resistant genes (ARG) and bacteria in the mangrove sediments.

The study speculates that dumping untreated industrial wastewater and agricultural run-off into rivers and streams may have triggered a section of bacteria in the world’s largest mangroves, the Sundarbans, to become immune to antibiotics.

The study led by Maitree Bhattacharyya from the University of Calcutta and Abhrajyoti Ghosh from Bose Institute, calls for looking at the environmental dimension of antimicrobial resistance on an “equal footing” with antibiotic overuse and exposure in humans and animals.

“We suspect that antibiotic resistance in bacteria in the mangrove soil may be linked to discharge of heavy metals and polyaromatic hydrocarbons (PAHs) in untreated industrial waste and to chemical fertilisers in farming activity related run-off (organochlorine and organophosphate) into rivers and streams that merge with the Sundarbans estuary,” Ghosh told Mongabay-India.

These pollutants enter the deltaic system and are deposited in the soil, said Ghosh.

The World Health Organisation has labelled antibiotic resistance as one of the biggest threats to global health, food security and development today.

The scientists of the Sundarbans study stressed on paying attention to previously overlooked environments, such as waste water system and rivers, sea and sediment laden with agricultural run-offs, that may harbour and spawn resistant bacteria.

“We need to look at the complex issue of antibiotic resistance in a holistic manner. Framing and strict enforcement of policies to ensure wastewater is treated before release into rivers and stream could hold key to reducing the prevalence of antibiotic resistance genes in the environment. Our observations also support the practice of organic farming to combat resistance,” Ghosh said.

In addition to emphasising the importance of treating wastewater, the study also underscores the importance of conserving and regeneration of mangroves.

“Mangroves act as a sink for heavy metals to a certain extent. So saving them could help us tackle antibiotic resistance,” Ghosh added.

Spanning 10,000 square km along the coast of India and Bangladesh, the Sundarbans represent the largest expanse of contiguous mangrove forests in the world.

This mosaic of waterways, mudflats and forested islands lies at the edge of the Bay of Bengal within the delta of the Ganges, Brahmaputra and Meghna rivers. The Indian side of Sundarbans has 102 islands.

The mangroves at Sundarbans. Photo by Anish Bhattacharyya.

In the Sundarbans mangroves, the microbiome is saddled with pressure from natural elements such as high salinity, critical inundation effects, anoxic environment, nutritional limitations and inter- as well as intra-species competitions.

But the added stress of man-made factors such as pollutants (with heavy metals) may have set off the microbiome to evolve survival strategies including picking up genes from their surroundings to shield themselves from the effects of these contaminants.

“When the bacteria that is dealing with stress, pick up genes from their surroundings to cope with the stress, certain genes conferring antibiotic resistance may also get in through a process called horizontal gene transfer. When these strains reproduce, they multiply and pass on the resistance,” Ghosh explained.

On examining soil samples across eight locations in the mangroves with varying levels of human interference, the researchers discovered that more the heavy metals and PAHs detected in the sediment, more was the abundance of antibiotic resistance genes (ARGs).

“This implies that heavy metals and PAHs in an environment facilitate dissemination of ARGs within the same habitat. Such a correlation allowed us to say that abundance and dissemination of ARGs are linked to pollution. We need to do further studies to flesh out the correlation,” Ghosh said.

Commonly used antibiotics in medical practice and in animal feed industry include beta-lactams (ampicillin), aminoglycoside (kanamycin), glycopeptide (vancomycin), tetracyclines, and macrolides (clarithromycin).

“We therefore sought to check bacterial resistance to these antibiotics. As clarithromycin is known to inhibit bacterial biofilms, we also studied the effect of this antibiotic on multi-drug resistant (ampicillin, kanamycin, tetracyclin and vancomycin) bacterial biofilm. To our surprise, we observed that bacterial biofilms are also resistant to clarithromycin,” Ghosh said.

A biofilm is an architectural colony of microorganisms, within a matrix of extracellular polymeric substance that they produce.

The team discovered the presence of 18 bacterial strains exhibiting resistance to ampicillin, kanamycin, tetracycline and vancomycin. Further, several of the strains showed resistance to multiple drugs (multi-drug resistance bacteria or MDRB).

“Besides, as expected, all the MDRB were found to be highly salt tolerant with considerable growth in salt concentrations ranging from 12 percent to 21 percent,” the study notes.

Among other isolates, members of genus Oceanobacillus, Bacillus, Marinobacter, Thalassocella, Nitratireductor, Halobacillus have been identified as MDRB in the present study.

In general, most of the multi-drug resistant bacteria (MDRB) were resilient to kanamycin (used to treat MDR Tuberculosis), while vancomycin was found to be the most effective in killing bacteria at higher concentrations.

According to Joakim Larsson of Centre for Antibiotic Resistance Research at the University of Gothenburg, Sweden, one needs much more to conclude there is a link than the pure correlation between potentially selective agents, such as established dose-response relationships.

“Importantly, there are often many confounding factors that can lead you astray, such as the more anthropogenic impact overall  – the more human input, the more likely it is to find both a very large range of pollutants as well as faecal bacteria (which are often more resistant!). That does not mean causality between any of the pollutants and the resistant bacteria, just that both come from humans,” Larsson told Mongabay India.

Antibiotic resistance infographic. Photo by CDC/Wikimedia Commons.

Emergence of global observations

The study builds upon global observations on the potential link between environmental pollutants and antibiotic resistance. For example, in 2017, experts from India and Sweden had flagged concerns over uncontrolled dumping of partly treated/untreated urban waste into rivers − leading to the spread of antibiotic resistance. They studied wastewater discharge in the river Mutha in Pune, Maharashtra.

According to the report “Antimicrobial Resistance: Investigating the Environmental Dimension”, the discharge of untreated sewage is likely to be an important driver of increasing antibiotic resistance in the environment, however it is a very challenging problem to solve.

A United Nations Environment Programme (UNEP) report has said there is “clear evidence” that the release into the environment of antimicrobial compounds in effluents from households, hospitals and pharmaceutical facilities, and in agricultural run-off, combined with direct contact between natural bacterial communities and discharged resistant bacteria, is driving bacterial evolution and the emergence of more resistant strains.

The eight sampling stations of this study by Bhattacharyya, Ghosh et al. spanned from the northernmost island of these mangrove forests (Godkhali) to the penultimate island Dhulibhashani, that is closer to the sea and remains virtually untouched by human influence.

Godkhali being a small-scale port and the largest marketplace in the neighbouring islands has a massive human footprint: oil spillage, industrial, agricultural and hospital wastes and household drainage contaminations. Its mangroves occur in patches.

On the other hand, the pristine status of Dhulibhashani may be attributed to a series of factors such as the distance of the island from the nearest inhabited area as well as the periodic cleansing of sediments by both riverine and oceanic tides.

“Stations at Kalash, Lothian, Sushnir char, Dhanchi and Dhulibhashani, that were located in the southern section of the Sundarban estuary, are considerably less influenced by human activities as well as anthropogenic factors. These places had low level of ARGs in comparison to islands such as Godkhali that had more human footprint,” Ghosh said.

Map of the study stations in the Sundarbans. Photo by researchers.

Unexplored biodiversity of mangroves: microbial community

Mangroves cover up to 152,000 square km globally. However, their range is decreasing due to pollution, urbanisation and other human activities.

“There is an increasing urgency to understand the structural and functional architecture that underlies the mangrove ecosystem and the microbial community is an important component that remains unexplored,” explained Ranjith Kumavath of the department of genomic science, Central University of Kerala.

In recent studies, Kumavath and collaborators have shown that the mangrove ecosystem of Kerala (India) harbours a rich reservoir of microbial communities distinct from other mangroves around the globe.

The research team also found high prevalence of antibiotic resistance genes as well as heavy metal resistant genes in Kerala mangrove microbiome as compared to Brazilian and Saudi Arabian mangrove microbial ecosystems and land and deep marine environments.

“Increasing antibiotic resistance is a global health issue. However, the good news is that, these antibiotic resistant genes found in these mangroves sediments rather than resisting drugs, are involved in basic physiological functions to survive in the environment where they compete with a number of other microbes,” Kumavath said in a press release.

The negative aspect of this discovery is that such antibiotic resistance genes can have serious consequences if they “move” from non-harmful microbes to human pathogens through horizontal gene transfer.

“On the other hand, the heavy metal resistance genes of these microbial ecosystems can be wisely used in developing transgenic organisms towards bioremediation. Further study is currently being designed to evaluate the dissemination of such antibiotic resistant genes to pathogens,” Kumavath added.

The mangrove ecosystem of Kerala harbours a rich reservoir of microbial communities. Photo by Ranjith Kumavath.



Bhattacharyya, A., Haldar, A., Bhattacharyya, M., & Ghosh, A. (2018). Anthropogenic influence shapes the distribution of antibiotic resistant bacteria (ARB) in the sediment of Sundarban estuary in India. Science of The Total Environment. DOI10.1016/j.scitotenv.2018.08.038.

Imchen, M., Kumavath, R., Barh, D., Azevedo, V., Ghosh, P., Viana, M., & Wattam, A. R. (2018). Author Correction: Searching for signatures across microbial communities: Metagenomic analysis of soil samples from mangrove and other ecosystems. Scientific Reports, 8(1), 165.

Imchen, M., Kumavath, R., Barh, D., Vaz, A., Góes-Neto, A., Tiwari, S., … & Azevedo, V. (2018). Comparative mangrove metagenome reveals global prevalence of heavy metals and antibiotic resistome across different ecosystems. Scientific Reports, 8(1), 11187.

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