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.
Much of the latest research on CaCO3 polymorphs precipitated with and without additives discusses the processes of formation of the precipitates. Two prevalent growth processes have been proposed in this connection: classical and non-classical crystal growth. The classical theory is dependent on supersaturation and describes an ion-by-ion or single-molecule attachment to a critical crystal nucleus that only grows when the threshold value for the free-energy is larger than the critical radius and owth (Everett, 1988; Söhnel; 1992). The newer non-classical theory describes a nanoparticle mediated crystallization of primary nanoparticles. The aggregation of the nanoparticles can take several routes. In a pure system the nanoparticles are thought to evolve into an iso-oriented crystal through oriented attachment (Colfen and Mann, 2003). If polymers or other additives are involved, the process of nanoparticle aggregation is argued to occur via a mesoscale assembly into a single crystalline mesocrystal (Wohlrab et al., 2005). Aggregation of nanoparticles can also result in polycrystalline crystals if the aggregation is nonoriented (Kulak et al., 2007). The formation processes of crystals are difficult to address, however the non-classical theory is often claimed based on line broadening in a X-ray diffraction pattern or a polymorphic crystal. As a consequence, vaterite crystals with similar morphologies have been assigned to various formation processes.
In my studies I have not found any evidence in favor of the non-classical pathway and all my resulting growth rates and morphologies can be explained through the classical nucleation pathway. Both in pure systems and when additives are present. The below work represents some of my early work in the CaCO3 system.
Specifically I find that the aragonite morphology can be explained by a twinning processes and not as a result of nano aggregation. Both the branching aragonite structure and vaterite can be explained by classical spherulitic growth process.
Aragonite twinning
Spherulitic growth
