This is from a U.K. Institute of Food Research press release:

To find out more, the IFR team took the first comprehensive look at the differences between the populations of E. coli growing on crop plants and populations in the mammalian gut. Funded by the Biotechnology and Biological Sciences Research Council, they took over 100 isolates from leafy parts of vegetables growing in fields in England. Analysis of these showed that even within the same field the E. coli population is diverse and complex. They then compared these isolates with a standard reference collection of E. coli taken from mammals, including humans, from different continents. Profiling the two groups found a number of significant differences depending on the source of isolation. Compared to the habitat inside the gut, a leaf surface is a hostile environment for gut bacteria. The temperature fluctuates away from the constant 37 °C inside our bodies, and there is a greater risk of drying out.

The researchers found that E. coli populations derived from plants tended to form biofilms more readily. Biofilms are complex structures formed by populations of bacteria coming together to make a thin film over a surface. They are held together by a protective extracellular matrix of proteins and sugars, and the researchers saw that there was also an increase in the production of components of this matrix in E. coli derived from the fields. These strains also used sucrose and other plant-derived sugars more than the E. coli populations derived from mammalian sources.

Biofilms might help to prevent E. coli drying out outside of its host and being able to take advantage of plant sugars could also aid their survival outside the main host, although overall the plant strains showed lower growth on the usual carbon sources E. coli uses.

An analysis showed that these differences are associated with previously defined phylogentic groups of E. coli showing that different environmental conditions have a selective effect in the evolution of different groups. While some have become more generalised, adapting to life outside the mammalian gut, others have remained specialised for life in this environment, avoiding the associated growth penalty. “While it was known that different environments harboured different E. coli populations, we now have an idea on how and why this happens,” said Sacha Lucchini. “Knowledge of the mechanisms involved in plant colonisation by E. coli provides targets for developing strategies aimed at preventing potentially dangerous E. coli strains from colonising vegetables, thus keeping them off our plates.”

Reference: Phylogenetic distribution of traits associated with plant colonization in Escherichia coli, Environmental Microbiology doi:10.1111/j.1462-2920.2012.02852.x