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.
CALCITE GROWTH AND INHIBITION
Despite being one of the most common minerals understanding the growth and nucleation of calcite is still a standing frontier.
We measured growth rates on calcite spirals to explain the differences in growth rate that are observed on the structurally distinct sites on calcite steps at nonstoichiometric conditions and used process based growth models to interpret the data. We found that none of the existing models were able to explain the experimentally observed behaviour. We subsequently developed a microkinetic model using density functional theory that accurately reproduces calcite growth over a very wide range of published experimental data, solution composition, saturation index, pH, impurities and additives. In addition we demonstrate that polynuclear complexes play a central role in mineral growth at high supersaturation and that a classical complexation model is sufficient to reproduce measured rates.
We used the constant composition method to characterize the inhibitor index by a range of concentrations of Mg, SO4 and MgSO4. The results are applicable to scaling problematics and can be modeled using the microkinetic model.
GROWTH SPIRALS-
-role of saturation index,
Ca:CO3 ratio and surface structure
PROCESS BASED GROWTH MODELS
-fitting our experimental data poorly
MICROKINETIC MODEL
-satisfactory fit to our experimental data, literature values as well as for additives.
INORGANICALLY INDUCED INHIBITION
-growth inhibition of Mg, SO4 and MgSO4.
