Bacteria associated with the nematode Bursaphelenchus xylophilus, a pathogen of trees and the causal agent of
pine wilt disease (PWD) may play a role in the disease. In order to evaluate their role (positive or negative to the
tree), strains isolated from the track of nematodes from infected Pinus pinaster trees were screened, in vitro, for their
nematicidal potential. The bacterial products, from strains more active in killing nematodes, were screened in order to
identify and characterize the nematicidal agent. Forty-seven strains were tested and, of these, 21 strains showed
capacity to produce extracellular products with nematicidal activity. All Burkholderia strains were non-toxic. In
contrast, all Serratia strains except one exhibited high toxicity. Nematodes incubated with Serratia strains showed, by
SEM observation, deposits of bacteria on the nematode cuticle. The most nematicidal strain, Serratia sp. A88copa13,
produced proteases in the supernatant. The use of selective inhibitors revealed that a serine protease with 70 kDa
was majorly responsible for the toxicity of the supernatant. This extracellular serine protease is different
phylogenetically, in size and biochemically from previously described proteases. Nematicidal assays revealed
differences in nematicidal activity of the proteases to different species of Bursaphelenchus, suggesting its usefulness
in a primary screen of the nematodes. This study offers the basis for further investigation of PWD and brings new
insights on the role bacteria play in the defense of pine trees against B. xylophilus. Understanding all the factors
involved is important in order to develop strategies to control B. xylophilus dispersion.This research was partially supported by Direcção Regional de Florestas, Fundo Florestal Permanente and Autoridade Florestal Nacional,
through a national project ‘O nemátode-da-madeira-do-pinheiro (NMP), Bursaphelenchus xylophilus’ and by FEDER funds through the Programa
Operacional Factores de Competitividade – COMPETE and by national funds through the Fundação para a Ciência e Tecnologia (FCT), Portugal, under
the project PTDC/AGR-CFL/115373/2009. D.N.P. was supported by FCT, Portugal, graduate fellowship SFRH/BD/61311/2009. G.P. was supported by
FCT, Portugal, fellowship PTDC/AGR-CFL/115373/2009