Advances in Recovering Plasmids from Wastewater: A State of the Science

Plasmids are small rings of double stranded DNA that are found in all three domains of life: the Bacteria, the Archaea, and the Eukarya. Plasmids encode for proteins that provide their host organisms multiple abilities, such as the ability to transfer genetic information, degrade xenobiotic compounds, resist antibiotics, and outcompete other cells. Plasmids are similar in size to viruses, mostly in the submicron size range; however, plasmid size is normally described by the number of DNA base pairs as opposed to the physical size. Although plasmids are found in all three domains of life, plasmids are most commonly associated with bacteria. Because wastewater harbors bacteria and WRRFs harness bacteria for biological treatment processes, wastewater contains a variety of plasmids and plasmid hosts. Sources of plasmids in wastewater include bacteria excreted from humans and bacteria present in the environment. Researchers have found high levels of bacteria with antibiotic resistant plasmids in the fecal matter of antibiotic-treated children and adults. Bacterial hosts transport plasmids throughout wastewater treatment processes, and as bacteria die and rupture the plasmids are released into the wastewater. Although research suggests that naked plasmids degrade in wastewater, WRRF effluents and biosolids have been shown to contain plasmids and may contribute to the spread of antibiotic resistance. Plasmids and their associated genetic information represent a potential recoverable resource from wastewater. These small rings of intracellular DNA not only encode the genes that facilitate their own transfer and propagation but also harbor functional genes related to a variety of processes. The biotechnology industry has harnessed the information on plasmids to improve multiple industries, such as agriculture, chemical production, paper, textiles, health care, environment, and biotechnology. Plasmids that function to produce antibiotics, degrade xenobiotic compounds, and serve as biocatalysts have been isolated from wastewater. At the same time, the purposes and functions of significant proportions of plasmid DNA recovered from wastewater samples remain undetermined and represent an unknown value to society. Nevertheless, research suggests that wastewater contains genetic information that is of value to society. Plasmids are unlike traditional recoverable commodities, because they are not sold by weight. The information encoded on plasmids and the intellectual property protections provide the value. The actual plasmid has little to no value. Therefore, traditional methods for valuing resource recovery are not transferable to plasmids, as bulk plasmids with no information on the encoded functions or intellectual property protection have little to no value. Hence, a methodology for valuing plasmids must involve sequencing and/or culture-based determination of the gene functions available thereon. Once determined, plasmid products suitable for market could take one or more of several forms, consisting of the plasmid itself, its intellectual property rights, the products of its functional genes, or any combination of the above. Plasmid based products, such as enzymes, are used in multiple industries. Using market estimates from multiple biotechnology sectors, the market for plasmids and plasmid based products and intellectual property could be expected to grow between 5% and 10% combined annual growth rate, and the biotechnology sector has a current value of greater than $270 billion. Yet plasmid recovery, if evaluated using an expected value economic approach, likely represents a net loss for WRRFs at this time due to the substantial risks and capital costs involved compared to the potential returns. As a result, plasmid recovery and the recovery of genetic materials from municipal wastewater is a long-term objective, and a pathway for developing the product recovery potential of plasmids is provided over a 20-year time horizon.