When sewage plants were developed in late 19th century, neither antibiotics were discovered nor antibiotic resistance was known about. In consequence, wastewater treatment plants were not developed to specifically reduce antibiotic residues or resistant bacteria but by their very design, they might actually rather promote their development. Why is that?
Sewage plants process water of different origins. Nowadays that waters might carry residues of antibiotics. They originate from pharmaceutical production sites, hospitals, droppings of treated humans or animals and direct deposition of antibiotics down the drain. Those antibiotics interact with different communities of bacteria which are present in the wastewater. The aeration basin at the end of the plant provides the best conditions for microorganisms to thrive.
Antibiotic resistance development in sewage plants
The presence of antibiotics facilitates the development of antibiotic resistant bacteria strains. Bacteria sensitive to antibiotics die off or are not able to proliferate, resistant strains survive and thrive in the favourable conditions found in sewage plant clarifiers. The development of resistances depends on the concentration of antibiotics in the wastewater and their degradation rate. For Penicillin, the degradation rate is very high: 99,8 % in 100 days. For other antibiotics like Fluoroquinolones, only 50 % are degraded during this period.
Despite the treatment in sewage plants the water re-entering the environment contains resistant bacteria. In a swiss study, the analyses of water collected downstream of a sewage plant showed higher bacterial load than analyses of water upstream. The accumulation of resistant bacteria in the water is also shown in a study conducted in UK. There, a team of the University of Exeter Medical School analysed faecal samples of 273 people, with half of the participants being surfers. The samples were tested for Cefotaxime resistant bacteria. The differences have been significant: 9 % of the surfers but only 3 % of the non-surfers carried E. Coli resistant to this clinically important antibiotic. Other studies show the elimination of antimicrobials in sewage treatment plants: In a Chinese study an almost complete elimination of fluoroquinolones from the effluent could be shown. Possible explanations were that the antibiotics have been biodegraded or they accumulated elsewhere. Further analyses showed a high accumulation of the antibiotics in the sewage sludge.
Re-entry of AMR via crop production
The spread of AMR via crop production is not well understood. A very high percentage of antibiotics used are applicated in animal production. As these substances rarely are metabolized completely, high amounts of them end up in slurry and litter. When slurry or sewage sludge are used as fertilizers, there is a certain risk that resistant bacteria are brought back into the environment and into waters. Therefore, in some countries the disposal of sewage sludge into the fields is already forbidden.
The only way the development of antimicrobial resistance can be slowed down is through the reduction in the use of antimicrobials. For animal producers there are alternatives available. Decreasing the antibiotic use in agriculture reduces the total use of antibiotics and therefore the development of antibiotic resistance.