GM Crop Pest Control Bacteria Jump To Humans

From insects to man: identification of virulence factors involved in the adaptation of the emerging pathogen Photorhabdus asymbiotica to human hosts

Abstract number: 1733_565

Forquin M., Esque J., Pimenta A.

Photorhabdus are entomopathogenic bacteria currently used for the biological control of crop pests. Recently, an increasing number of Photorhabdus strains have been isolated from human clinical specimens, associated with locally invasive soft tissue infections and disseminated bacteraemia.

Objectives: In view of their growing use in biological control, which increases the human potential rate of exposure to these pathogens, we undertook a comparative study between insect and human pathogenic strains of Photorhabdus, in an attempt to understand the genetic mechanisms involved in their recently acquired capacity to infect humans.

Methods: Three Photorhabdus strains, two human (P. asymbiotica SN and ATCC43949) and one insect pathogen (Photorhabdus sp Q617), were tested for their ability to bind to two major components of the human extracellular matrix (ECM), fibronectin (Fn) and vitronectin (Vn), to infect or invade human epithelial cells in culture, and to interact with the cellular cytoskeleton. Adherence, infection and invasion experiments were performed in 96-well microtiter plates coated either with Fn, Vn or with monolayers of human epithelial HaCaT cells. Bacterial invasion was quantified by the gentamicin survival assay. In inhibition assays, anti-Fn, anti-Vn antibodies, or genistein were added to epithelial cell monolayers. For immunofluorescence, HaCaT monolayers on glass coverslips were infected and labelled with phalloidine-TRITC and DAPI.

Results: Only P. asymbiotica human pathogenic isolates ATCC43949 and SN were able to adhere to human fibronectin and/or vitronectin, and to infect/invade HaCaT cells. This was inhibited in the presence of antibodies against human Fn or Vn. Only strain ATCC43949 induced cytotoxic effects after 6h co-culture with human cells. Cytoskeleton involvement in the P. asymbiotica infective process was indicated by blockage of infection and invasion by genistein, and by immunofluorescence analysis of infected cells.

Conclusion: The acquisition of virulence factors enabling P. asymbiotica to interact with human ECM and to human cells is a key factor in its evolution towards the colonisation of human hosts. ECM proteins are involved in the primary events during P. asymbiotica colonisation of human epithelial tissues, and the host cytoskeleton is solicited during the P. asymbiotica-epithelial cells interaction. Possible mechanisms used by P. asymbiotica to colonise human tissues are discussed.
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