Gull droppings at beaches in the Porto region of Portugal are riddled with bacteria that are resistant to the ‘last-resort’ antibiotic colistin, undermining efforts in the livestock sector to reduce colistin-resistance, according to a paper published in Environmental Microbiology, an Applied Microbiology International publication.


Researchers at the University of Porto have warned that urgent environmental containment strategies for managing food waste and sewage as well as the increasing gull populations are needed to control the spread of mcr genes - genes that make bacteria resistant to colistin.

“Colistin is a last-resort antibiotic vital for treating patients with severe infections, but is still highly used in animal farming globally,” said Professor Patrícia Antunes, assistant professor at the Faculty of Nutrition and Food Sciences, University of Porto in Porto, Portugal.

“Our group have been studying the expansion of colistin resistance and mcr-carrying bacteria at farm-to-fork level and the impact of the EU/national ongoing actions involving colistin restrictions among food-animal production to limit the the spread of mobilised colistin resistance (mcr) genes.”

Mcr are a family of genes that confer colistin resistance and can also be horizontally transferred between bacteria due to their common location in mobile genetic elements such as plasmids. In recent years mcr global expansion has been strongly linked to food-producing animals where extensive use of colistin occurs.

Role of gulls

However, the new research suggests that the development of large gull colonies in the big coastal cities of the Mediterranean is also playing an important role in the spread of antimicrobial resistant bacteria.

“By representing the environment-human interface, our study aimed to elucidate if gulls might be important mcr-spreaders and a further risk factor undermining strategies for reduction of colistin-resistance and MCR-producers successfully achieved through ongoing EU colistin restrictions in the food-animal sector,” Professor Antunes said.

“We firstly unveil a high rate of gull pooled faecal samples (36%) carrying E coli with mcr-1 gene over seven collection months. We found a high diversity of global pathogenic multidrug resistant Escherichia coli clones, including the zoonotic relevant B2-ST131-H22, ST10 or ST162. 

“In most cases the mcr-1 gene was located in widespread plasmids and diverse genetic contexts. Whole genome sequencing revealed an enrichment of these E coli strains on diverse antibiotic resistance, virulence, and metal tolerance genes. 

“Our results underscore gulls as important spreaders of high priority bacteria and genes (mcr-carrying bacteria) between wildlife, the environment, humans and potentially food-producing animals, potentially undermining ongoing strategies at the livestock sector to reduce colistin resistance.”

Last resort

Colistin is a last-resort antibiotic reserved for treating severe infections, but to safeguard its effectiveness for human health, all sectors that could contribute to the spread of mcr genes between environments must be involved, Professor Antunes said.

“The increasing population of gulls in coastal cities together with their mobility and intruder behaviour enhance the opportunities for short and long-distance mcr environmental dissemination through gulls’ faecal contamination of water, public areas and livestock/agriculture,” she said. 

“The possibilities of mcr transmission are aggravated due to the proximity and interactions between gulls and human urban areas, which constitute a public health concern.

“Thus, data from our study support the need for One Health strategies underlining that community and environmental sectors should act together with food-producing animal and healthcare sectors in order to reduce colistin resistance spread towards health and sustainability in line with the European Green Deal.”

Public health concern

The results suggest a real public health concern, especially in view of intruder behaviour of gulls across the globe, along with their easy access to food-animal wastes, food-producing animals/aquacultures, wastewater treatment plants, and sewage effluents.

“Urgent environmental containment strategies for managing food wastes or sewages and the increasing gull populations are needed to control the dissemination of mcr genes associated with successful clones,” Professor Antunes said. 

“We also need further studies at the environment-livestock-human interface linked to the ongoing international strategies to combat antimicrobial resistance (AMR). 

“Thus, our group have ongoing studies monitoring colistin resistance/mcr spread in different contexts to depict other factors that could mitigate ’One Health’ actions with implications for food safety and the environment.”

‘High diversity of pathogenic Escherichia coli clones carrying mcr-1 among gulls underlines the need for strategies at the environment-livestock-human interface’ appears in Environmental Microbiology.

Patrícia Antunes is Assistant Professor at Faculty of Nutrition and Food Sciences, University of Porto, Porto, Portugal.

Researcher at UCIBIO - Applied Molecular Biosciences Unit, REQUIMTE, Laboratory of Microbiology, Department of Biological Sciences, Faculty of Pharmacy, University of Porto, Porto, Portugal / Associate Laboratory i4HB - Institute for Health and Bioeconomy, Faculty of Pharmacy, University of Porto, Porto, Portugal.