It's generally understood how genetic information is spread: from an organism to its offspring. For this reason, the traditional representation of evolution and the relationship between organisms is portrayed as a "tree," with an increasing number of branches emanating from the origin. But it turns out that this well-understood structure doesn't apply to microbes.
Bacteria don't just transmit genetic information to their offspring, they also exchange genes with other bacteria -- even bacteria from different species. This "horizontal gene transfer" happens with great regularity, and is responsible for a nearly 10% of the "gene transfer events" in bacterial evolution. As a result, the relationship between different microbial species is closer to a network than a tree, with otherwise distant types connected through gene swapping. Now researchers at the European Bioinformatics Institute (EBI) have determined what that bacterial network looks like -- and it turns out that the bacterial horizontal gene transfer network (PDF) has a lot in common with the world wide web.
This network appears to behave in a 'scale-free' manner. This term was first coined by physicist Albert-Laszlo Barabasi and his colleagues at the University of Notre Dame, Indiana, in the US. In 1998, they mapped the connectedness of the World Wide Web and found, to their surprise, that the web did not have an even distribution of connectivity (so-called 'random connectivity') but instead, a very few network nodes (called 'hubs') were far more connected than other nodes.One property of scale-free networks is their 'small-world' nature: travelling from one node to any other is very fast. Other well-known examples of small-world networks include social networks and air-travel connections.
These characteristics allow the hubs to serve as bacterial 'gene banks', providing a medium to acquire and redistribute genes in microbial communities.
So what does this mean? The biggest implication concerns evolved resistance to antibiotics. Because of the horizontal gene transfer, a resistance that evolved naturally in one species can be transferred by chance, fully formed, to another bacterial species.
According to Christos Ouzounis, this pattern has important implications for the understanding of horizontal gene transfer because, in small-world networks, the shortest path between any two network nodes is relatively small. In other words, 'a gene can rapidly be disseminated from organism to organism through very few horizontal gene transfer events', explains the scientist. [...]'It's entirely possible that apparently harmless organisms are quietly spreading antibiotic resistance under our feet,' concludes Christos Ouzounis.
Unstated in the article, but implied by this analysis, is that thinking of bacterial relationships as a scale-free network could lead to new approaches to the control of antibiotic resistance, and possibly even to new types of anti-bacterial treatments; it also means that novel antibiotics may have greater-than-anticipated effects on seemingly unrelated bacteria (most of which are quite helpful and important to ecological processes).
In addition -- and this is informed conjecture on my part -- this discovery could lead to new insights into how information networks can, in turn, evolve resistances to external attacks (such as spam, viruses and "denial of service" episodes).
