Bacteria become resistant to antibiotics through many mechanisms causing mutations by selecting or acquiring genes for resistance, through evolution, attributed mainly to horizontal gene transfer (HGT). Mis-use of these antibiotics and their release from wastewater treatment plants (WWTP) have contributed greatly to the emergence and spread of resistant bacteria, including bacteria that cause human and animal infections. Aquatic environments, especially, provide ideal conditions in the water column and sediment for the horizontal exchange of mobile genetic elements (MGEs).
Resistance in nature has an ancient origin in which organisms use as a defence mechanism to protect against harmful substances of others, encoded by a group of genes known as the resistome. Studies have collectively shown that the environment harbours a large majority of antibiotic-resistant genes (ARGs) but only in the last 10 years evidence has been demonstrating mobilisation of resistance into human pathogens.
Aquatic ecosystems are used as a way of disposal and therefore now play a significant role in ARG transfer, ecology and evolution. WWTPs provide an ideal environment for HGT as they contain high densities of bacteria, high oxygen levels, high nutrient concentrations and are in constant contact with antibiotics and resistant bacteria. A study conducted found that high concentrations of ARGs were found in biofilms downstream of a WWTP suggesting that they encourage the spread of resistance in the environment.
Biofilms are a natural dense community of microbes which could act as an ideal site for HGT of ARGs. Bacteria also aggregate in the sediments which facilitates the movement of genetic elements especially where antibiotics are used, in aquaculture and from other anthropogenic sources. Tetracycline resistant genes were found 100 times higher in sediments than in the water column, and these concentrations depended on the degree of anthropogenic stresses in that area.
Many other anthropogenic factors contribute to the spread of ARGs including sewage, agricultural runoff, heavy metal pollution and temperature. A natural phenomena that provides a surface for the movement of MGEs is the biofilms formed on chitin. Chitin is a very common element used in structural components on many crustaceans and molluscs and therefore these sites are abundant in which HGT can occur.
ARG's are very abundant in the environment but to prevent further mutations of human and animal pathogens, use and disposal of antibiotics into the environment needs to be monitored and limited as it could pose a serious threat to global health.
Marti, E., Variatza, E. and Balcazar, J.L. (2013) The role of aquatic ecosystems as reservoirs of antibiotic resistance. Trends in Microbiology. XX:1-6