Harmless bacteria can promote pathogenesis

Is your enemy's friend also your enemy? Most research into bacterial infections has focused on the relationship between the pathogen and the host plant or animal, but little is known about the influence of seemingly harmless bacteria residing alongside the pathogen in the host. Are they innocent bystanders or do they aid the pathogenic bacteria?

Biologists have known for over a decade that bacteria communicate with other members of their species using a signalling molecule particular to that species. When the concentration of this molecule passes a certain threshold, pathogenic bacteria will start to attack their host.

However, Vittorio Venturi, a plant bacteriologist at the International Centre for Genetic Engineering and Biotechnology in Trieste, Italy, wanted to investigate whether any additional media exist between the pathogen and other, seemingly harmless, bacteria that reside in the host.

Venturi and his colleagues, including researchers at the Hassan II Institute of Agriculture and Veterinary Medicine, Morocco, examined the interaction between the pathogen Pseudomonas savastanoi (Psv) — which causes olive knot disease that infects a variety of plants and disrupts agriculture in the Middle East and Southern Europe — and two other resident bacterial species often found in olive knot, Pantoea agglomerans and Erwinia toletana.

The team incapacitated the pathogen, so that it could not release its signalling molecule, an N-acyl-homoserine lactone. In theory, this should have stopped the spread of disease, as Psv should have no way to communicate with other members of its species. However, they found that the two resident bacteria had evolved a signalling molecule with the same structure as the pathogen, allowing the resident to communicate in the pathogen's stead, triggering the attack.

The researchers suggest that collaboration between bacteria of different species can enhance the pathogenicity of olive knot disease.

"These findings tell us that plant diseases can be polymicrobial and that we may need to target both pathogens and resident bacteria," says Venturi.

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