The distinctive properties of Brucella outer membrane have been considered to be critical for Brucella sp. virulence. Among the outer membrane molecules possibly related to these properties, Omp10 and Omp19 are immunoreactive outer membrane lipoproteins. Moreover, these proteins of Brucella could constitute a new family of outer membrane proteins specifically encountered in the family Rhizobiaceae. We evaluated the impact of omp10 or omp19 deletion on Brucella abortus outer membrane properties and virulence in mice. The omp10 mutant was dramatically attenuated for survival in mice and was defective for growth in minimal medium but was not impaired in intracellular growth in vitro, nor does it display clear modification of the outer membrane properties. Significantly fewer brucellae were recovered from the spleens of mice infected with the omp19 mutant than from those of mice infected with the parent strain at 4 and 8 weeks postinfection. The omp19 mutant exhibited an increase in sensitivity to the polycation polymyxin B and to sodium deoxycholate. These results indicate that inactivation of the omp19 gene alters the outer membrane properties of B. abortus.Brucellae are gram-negative bacteria that cause human disease and significant worldwide economic losses due to infection of livestock. These bacteria are able to multiply within professional and nonprofessional phagocytes, but the exact mechanisms whereby Brucella spp. intracellularly parasitize the host are still to be defined (4,5,7,15,30,36,37).The Brucella outer membrane has been proposed to be involved in virulence (i.e., resistance to bactericidal cationic peptides and polycations, permeability to hydrophobic agents, resistance to divalent cation chelators, and poor activation of bactericidal mechanisms by lipopolysaccharide [LPS]) (for a review, see reference 35). Correlation between these properties and specific surface molecules can be studied by genetically engineering mutations in the genes and determining the resultant phenotype.The analysis of genetically defined rough mutants of Brucella melitensis and Brucella abortus confirmed the involvement of lipopolysaccharide O side chain in Brucella in vivo survival (1,24,25,34,45,46). The recently identified BvrR-BvrS twocomponent regulatory system is involved in the control of outer membrane properties such as resistance to bactericidal polycations and is highly relevant for the virulence of B. abortus (40).The molecular characterization of several Brucella outer membrane proteins (Omps) has been reported over the past years. The genes omp25, omp31, and omp2b (which encode the major 25-, 31-, and 36-kDa Omps, respectively) and the genes pal, omp10, omp19, and omp1 (which encode the 16-, 10-, 19-, and 89-kDa minor Omps, respectively) have been cloned and sequenced (10,