Antibiotics resistant (ABR) bacteria reach humans via the food chain through multiple pathways. First is through consumption of both animal products and vegetables which are exposed to antibiotics. A second route is the possible presence of resistance genes in bacteria that are intentionally added during the processing of food (starter cultures, probiotics, bioconserving microorganisms and bacteriophages). A last way is through cross-contamination with antimicrobial resistant bacteria during food processing.
Excessive and unsupervised use of antibiotics in poultry, dairy, aquaculture and food crops have not only resulted in resistance gain and transfer among gut bacteria but has also resulted in treatment failure in veterinary and aquaculture animals. For example, pork and poultry meat can both be sources of transfer of antimicrobial resistant Salmonella Typhimurium strains to humans. A recent study estimated the probability of exposure to 1,000 colony forming units of cephalosporin resistant Escherichia coli (CREC) through consumption of a meal containing chicken meat as low as 1.5 per cent.
Similarly, excessive pollution of rivers and farming lands with domestic-industrial sewage and sludge have also resulted in introduction of antibiotic residues and resistant bacteria into farming lands. In India, use of partially treated and untreated wastewater for irrigation is a common practice, especially in drought ridden places. Application of sewage sludge from wastewater treatment plants, animal waste as manure and soil amendment is also a common practice. Wastewater and sludge are rich sources of antibiotics and resistant bacteria.
Antibiotics have also been directly applied to crops as a means to improve production by preventing diseases. Crops exposed to antibiotics and resistant bacteria in one or the other way can bioaccumulate antibiotics and promote resistant bacteria. Consumption of fresh or uncooked vegetables exposed to antibiotic residues and resistant bacteria could pose health hazards in humans and animals.
Food processing can also be another unintended route of transmission of resistance. Preservation techniques such as heat and cold treatment inactivate or kill bacteria. Stress conditions such as cold stress, heat stress, acid stress, freeze injury among others may trigger several mechanisms in bacterial cells, e.g., stress adaptation, cellular repair, application of response mechanisms and enhanced virulence . Antimicrobial resistance genes that are present in partly inactivated, stressed cells may be transferred to commensals and pathogens, both in the foodstuff and after ingestion in the digestive system of humans. This may be achieved either by conjugation, when resistance is located on mobilisable elements, or by transformation and transduction, however to a lower degree.
Use of antibiotics in animals is often unsupervised. Antibiotic residues and resistant bacteria have been widely reported in animal products such as meat, egg, milk and fish. Consumption of raw and semi-cooked animal products could possibly increase the exposure to drug resistant bacteria in humans eventually leading to drug resistant infections and failure of treatment. According to a study conducted by Center for Science and Environment in 2014 of the 70 chicken samples collected from Delhi and NCR regions 40 per cent of the samples contained at least one antibiotic. The concentration of six antibiotics viz; oxytetracycline, chlortetracycline, doxycycline, enrofloxacin, ciprofloxacin and neomycin were in the range of 3.37-131.75 μg/kg. According to the Food Safety and Standards Authority of India (FSSAI) antibiotic tolerance limit for 76 antibiotics in milk, meat and meat products, poultry and fish are 0.01 mg/kg and maximum residue limit is 1 μg/kg.
Salmonella and Campylobacter are the most common causes of bacterial foodborne diseases in industrialised countries and an increasing prevalence of antimicrobial drug resistance has been recognised. Studies have shown that infections with antimicrobial resistant Salmonella and Campylobacter can result in higher mortality as compared to infections with susceptible strains. Therefore, special attention has to be given to reduce the prevalence of these pathogens on food products and to reduce the presence of antimicrobial resistance genes in these strains. Also, the antimicrobial resistance of zoonotic pathogens, including those that confer a risk by direct contact with living animals throughout the food chain as seen for e.g., livestock-associated methicillin resistant Staphylococcus aureus (LA-MRSA), have to be reduced. The situation has been recently worsened with the emergence of antibiotic-resistant bacteria having significant pandemic potential, such as carbapenem-resistant Enterobacteriaceae (CRE) (harboring a VIM-1 carbapenemase resistant to the beta-lactam antibiotics family plus additional co-resistance) and colistin-resistant E. coli.
Lack of strict and stringent monitoring of food quality coupled with unawareness among the large population increases the risk of exposure to antibiotic residue and resistant bacteria. Food safety can be gradually improved if more attention is paid to quality of the animal and vegetable produce for domestic use rather than only for export. Spreading awareness among farmers, use of organic produce, stringent monitoring of antibiotic application for veterinary and crop cultivation can break the antibiotic resistance spread via the food chain. With advancement in analytical technologies such as LC MS/MS and RT-PCR, regular testing of animal and vegetable produce for the presence of antibiotic residues and resistant bacteria before the product reaches the market can certainly improve the produce quality and ensure food safety.
The Food and Agriculture Organisation of the United Nations (FAO), the World Organisation for Animal Health (OIE), and the World Health Organisation (WHO) endorsed the One Health approach, affirming that healthy animals contribute to healthy people and environments. In keeping with this, supranational programmes and systems for monitoring antimicrobial resistance in animals and foodborne (namely originating from food or food products) pathogens, namely, the Global Foodborne Infections Network, the WHO’s Advisory Group on Integrated Surveillance of Antimicrobial Resistance and the Codex Alimentarius Commission, have been established. The WHO’s Global Action Plan and FAO’s Action Plan on Antimicrobial Resistance were recently published to address this worldwide threat.
A number of low- and middle-income countries (LMICs) have also initiated efforts to contain antibiotic resistance (ABR), albeit with a focus on human health but they have a long way to go. India introduced and has begun to implement national guidelines for antibiotic use in 2013 and National Action Plan on AMR in 2018. China launched a National Antibiotic Restraining Policy to reduce antibiotic consumption in the country, Thailand implemented a national strategy for emerging diseases, including ABR, and a policy for rational drug use in 2011, as well as an ABR Containment Programme for 2012–2016. South Africa developed and implemented a National Framework of Antimicrobial Resistance for 2014–2024. Given the expansion of the human population, globalisation of trade in animals and food products, international travels and host movements, AMR can easily spread globally via the food chain. If preventive and containment measures are not applied locally, nationally, regionally and internationally, the limited interventions in one country or continent, for instance, in the developing world, can compromise the efficacy and endanger the policies of containment of AMR implemented in other parts of the world.
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Authors’ details- Prof. Indumathi Nambi, Environmental and Water Resources Engineering, Department of Civil Engineering, IIT Madras, & Sasikala R., Research Scholar, Environment and Water Resources, Department of Civil Engineering, IIT Madras