DISTRIBUTION OF ANTIBIOTIC RESISTANCE GENES IN THE ENVIRONMENT:
THE ROLE OF MINERAL FACILITATED HORIZONTAL GENE TRANSFER
Combining recent research across disciplines, I see evidence that minerals hold a high and unrecognized potential for enhancing the distribution of the ARg in the environment. Adsorption of ARg to minerals significantly increases the ARg’s lifetime and facilitates their distribution by sedimentary transport processes. In addition, minerals also serve as a) sites for horizontal gene transfer (HGT), b) platforms for microbial growth and, hence 3) act as hot spots for propagation of adsorbed ARg to other microbes. However, some minerals and ARg are bound more strongly than others and various bacteria have different affinities toward various minerals. Those variations in affinity are poorly quantified but vital for predicting the distribution of ARg in the environment.
Bacterial colony formation.
Image by Lisselotte Jauffred (collaborator from NBI)
The spread of antibiotic resistance genes (ARg) is a worldwide health risk1 and is no longer only a clinical issue. Vast reservoirs of ARg are found in natural environments2–4 such as soils, sediments and oceans. The emergence and release of ARg to the environment is in particular caused by extended use of antibiotics in farming, e.g. where the genes dissipate from the manure.5 Once in the environment, the ARg are surprisingly rapidly propagated. It is well known that the ARg are distributed to neighbour bacteria through processes of both cell sharing or through horizontal gene transfer (HGT) where one species acquirer resistance from another.6,7 Most HGT responsible for the spread of ARg are assumed to be through direct microbe-microbe contact. However, I find that the outcome of non-contact transfer is grossly underestimated. In the HGT mechanism called “Transformation”, free ARg in suspension or adsorbed to a mineral can be picked up and incorporated into non-related organisms. Considering that free DNA only can survive for a few weeks in sea- and freshwater environments,8–10 any HGT from free DNA can rightly be assumed to be local, but if the DNA gets adsorbed to a mineral, it can survive for several hundred thousands of years.11–14 If this also holds for ARg, then minerals offer a potent mechanism for distributing ARg through our environments my means of sedimentary processes.
ENGINEERED ELECTRON TRANSFER
Tuning mineral reactivity
Can we tune the subsurface mineralogy and reactivity by engineering microbes?
Iron oxides in our subsurface can greatly aid in e.g. contaminant removal from ground water reservoirs or soils. It is hard, though, to control the determing mineral paramaters such as phase and reactivity of an already formed sediment. Dissimilatory microbes, however, readily can transform iron oxides by delivering electrons produced as part of their metabolismcan.
The rate of this reduction process is key for the final mineral phase. Another player is the bacterial associated extracellular matrices (EM) which offer preferential binding sites for nucleation and waslily can adsorb free ions. The variation in the EM and electron production rate between bacteria make it hard to predict the end-product from microbial induced transformations of existing nanoparticulate iron oxide.
Shewanella oneidensis is a naturally occurring biofilm producing metal-reducing bacterium. It transports electrons via the Mtr pathway which is a series of transmembrane intermolecular electron transfer events to extracellular solid minerals. In contrast the popular model microbe Escherichia coli lacks the Mtr pathway and is hence not able to reduce solid metals. We are using an engineered E.coli that expres the Mtr pathway to try and optimize controls on transformation by electron transport.