Why horizontal gene exchange in Enterobacteria matters

Escherichia coli DNA fragments encoding enzymes, transcriptional regulators and virulence factors are fluxing through bacterial taxonomic walls by horizontal gene transfer. These elements often endow environmental and clinical strains of bacteria with new properties, including an enhanced virulence. Lateral genetic exchange, particularly of drug tolerance genes has been recognized for a long time; however, our understanding of this phenomenon is limited. Ontology and phylogeny of laterally transferred genetic elements are difficult to investigate, let alone the predictions of their insertion sites in hosts chromosomes.

An outbreak of the lethal Escherichia coli in Europe in 2011 highlighted the shortcoming of our knowledge on the basic principles of evolutionary trends of new pathogens. The outbreak first occurred in Germany in May 2011 where a rare enterohemorrhagic strain Escherichia coli O104:H4 caused haemolytic-uremic syndrome. The infection spread fast through many other European countries and sickened thousands of people. The level of lethality associated with the production of Shiga toxin by the strain and its resistance against many antibiotics was significant. A number of isolates from this outbreak have been sequenced and annotated. Based on the unique combination of genomic features these strains were suggested to represent a new pathotype Entero-Aggregative-Haemorrhagic E. coli (EAHEC).

 

Mainstreams of Horizontal Gene Exchange in Enterobacteria: Consideration of the Outbreak of Enterohemorrhagic E. coli O104:H4 in Germany in 2011. (2011) PLoS ONE 6(10): e25702. doi:10.1371/journal.pone.0025702
Escherichia coli O104:H4 caused a severe outbreak in Europe in 2011. The strain TY-2482 sequenced from this outbreak allowed the discovery of its closest relatives but failed to resolve ways in which it originated and evolved. On account of the previous statement, may we expect similar upcoming outbreaks to occur recurrently or spontaneously in the future? The inability to answer these questions shows limitations of the current comparative and evolutionary genomics methods. The study revealed oscillations of gene exchange in enterobacteria, which originated from marine γ-Proteobacteria. These mobile genetic elements have become recombination hotspots and effective ‘vehicles’ ensuring a wide distribution of successful combinations of fitness and virulence genes among enterobacteria. Two remarkable peculiarities of the strain TY-2482 and its relatives were observed: i) retaining the genetic primitiveness by these strains as they somehow avoided the main fluxes of horizontal gene transfer which effectively penetrated other enetrobacteria; ii) acquisition of antibiotic resistance genes in a plasmid genomic island of β-Proteobacteria origin which ontologically is unrelated to the predominant genomic islands of enterobacteria. Oscillations of horizontal gene exchange activity were reported which result from a counterbalance between the acquired resistance of bacteria towards existing mobile vectors and the generation of new vectors in the environmental microflora. We hypothesized that TY-2482 may originate from a genetically primitive lineage of E. coli that has evolved in confined geographical areas and brought by human migration or cattle trade onto an intersection of several independent streams of horizontal gene exchange. Development of a system for monitoring the new and most active gene exchange events was proposed.

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