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Inactivation of the Pst System Reduces the Virulence of an Avian Pathogenic Escherichia coli O78 Strain

Groupe de Recherche sur les Maladies Infectieuses du Porc (GREMIP), Université de Montréal, Faculté de Médecine Vétérinaire, C.P. 5000, Saint-Hyacinthe, Québec, Canada J2S 7C6,¹ INRS-Institut Armand-Frappier, Université du Québec, 531 boul. des Prairies, Laval, Québec, Canada H7V-1B7,² Département de Microbiologie, Université de Montréal, C.P. 6128 succursale Centre-ville, Montréal, Canada H3C 3J7,³ Department of Biology, Washington University, St. Louis, Missouri 63130⁴

Received 18 November 2004/ Returned for modification 10 January 2005/ Accepted 28 February 2005

Escherichia coli O78 strains are frequently associated with extraintestinal diseases, such as airsacculitis and septicemia, in poultry, livestock, and humans. To understand the influence of the pst operon in the virulence of E. coli, we introduced mutations into the pst genes of the avian pathogenic E. coli (APEC) O78:K80 strain 7122 by allelic exchange. The mutation of pst genes led to the constitutive expression of the Pho regulon. Furthermore, the virulence of APEC strain 7122 in a chicken infection model was attenuated by inactivation of the Pst system. The pst mutant caused significantly fewer extraintestinal lesions in infected chickens, and bacterial numbers isolated from different tissues after infection were significantly lower for the mutant than for the wild-type strain. Moreover, resistance to the bactericidal effects of rabbit serum and acid shock was impaired in the pst mutant, in contrast to the wild-type strain. In addition, the MIC of polymyxin was twofold lower for the mutant than for the wild-type strain. Although the pst mutant demonstrated an increased susceptibility to rabbit serum, this strain was not killed by chicken serum, suggesting the presence of differences in host innate immune defenses and complement-mediated killing. In APEC O78 strain 7122, a functional Pst system is required for full virulence and resistance to acid shock and polymyxin. Our results suggest that the mutation of pst genes induces a deregulation of phosphate sensing and changes in the cell surface composition that lead to decreased virulence, indicating the importance of the Pst system for the virulence of pathogenic E. coli strains from different hosts.

Editor: A. D. O’Brien

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