Plant-adapted Escherichia coli show increased lettuce colonizing ability, resistance to oxidative stress and chemotactic response.

BACKGROUND: Escherichia coli is a widespread gut commensal and often a versatile pathogen of public health concern. E. coli are also frequently found in different environments and/or alternative secondary hosts, such as plant tissues. The lifestyle of E. coli in plants is poorly understood and has potential implications for food safety. METHODS/PRINCIPAL FINDINGS: This work shows that a human commensal strain of E. coli K12 readily colonizes lettuce seedlings and produces large microcolony-like cell aggregates in leaves, especially in young leaves, in proximity to the vascular tissue. Our observations strongly suggest that those cell aggregates arise from multiplication of single bacterial cells that reach those spots. We showed that E. coli isolated from colonized leaves progressively colonize lettuce seedlings to higher titers, suggesting a fast adaptation process. E. coli cells isolated from leaves presented a dramatic rise in tolerance to oxidative stress and became more chemotactic responsive towards lettuce leaf extracts. Mutant strains impaired in their chemotactic response were less efficient lettuce colonizers than the chemotactic isogenic strain. However, acclimation to oxidative stress and/or minimal medium alone failed to prime E. coli cells for enhanced lettuce colonization efficiency. CONCLUSION/SIGNIFICANCE: These findings help to understand the physiological adaptation during the alternative lifestyle of E. coli in/on plant tissues.

High lipid productivity of an Ankistrodesmus-Rhizobium artificial consortium.

Microalgae have great potential as alternative productive platforms for sustainable production of bioenergy, food, feed and other commodities. Process optimization to realize the claimed potential often comprises strains selection and improvement and also developing of more efficient cultivation, harvesting and downstream processing technology. In this work we show that inoculation with the bacterium Rhizobium strain 10II resulted in increments of up to 30% in chlorophyll, biomass and lipids accumulation of the oleaginous microalgae Ankistrodesmus sp. strain SP2-15. Inoculated cultures have reached a high lipid productivity of up to 112 mg L(-1) d(-1) after optimization. The resulting biomass presented significant levels of Ω3 fatty acids including stearidonic acid, suggesting potential as an alternative land-based source of essential fatty acids.

Association with an ammonium-excreting bacterium allows diazotrophic culture of oil-rich eukaryotic microalgae.

Concerns regarding the depletion of the world's reserves of oil and global climate change have promoted an intensification of research and development toward the production of biofuels and other alternative sources of energy during the last years. There is currently much interest in developing the technology for third-generation biofuels from microalgal biomass mainly because of its potential for high yields and reduced land use changes in comparison with biofuels derived from plant feedstocks. Regardless of the nature of the feedstock, the use of fertilizers, especially nitrogen, entails a potential economic and environmental drawback for the sustainability of biofuel production. In this work, we have studied the possibility of nitrogen biofertilization by diazotrophic bacteria applied to cultured microalgae as a promising feedstock for next-generation biofuels. We have obtained an Azotobacter vinelandii mutant strain that accumulates several times more ammonium in culture medium than wild-type cells. The ammonium excreted by the mutant cells is bioavailable to promote the growth of nondiazotrophic microalgae. Moreover, this synthetic symbiosis was able to produce an oil-rich microalgal biomass using both carbon and nitrogen from the air. This work provides a proof of concept that artificial symbiosis may be considered an alternative strategy for the low-N-intensive cultivation of microalgae for the sustainable production of next-generation biofuels and other bioproducts.