Gene discovery through genomic sequencing of Brucella abortus.

Brucella abortus is the etiological agent of brucellosis, a disease that affects bovines and human. We generated DNA random sequences from the genome of B. abortus strain 2308 in order to characterize molecular targets that might be useful for developing immunological or chemotherapeutic strategies against this pathogen. The partial sequencing of 1,899 clones allowed the identification of 1,199 genomic sequence surveys (GSSs) with high homology (BLAST expect value < 10(-5)) to sequences deposited in the GenBank databases. Among them, 925 represent putative novel genes for the Brucella genus. Out of 925 nonredundant GSSs, 470 were classified in 15 categories based on cellular function. Seven hundred GSSs showed no significant database matches and remain available for further studies in order to identify their function. A high number of GSSs with homology to Agrobacterium tumefaciens and Rhizobium meliloti proteins were observed, thus confirming their close phylogenetic relationship. Among them, several GSSs showed high similarity with genes related to nodule nitrogen fixation, synthesis of nod factors, nodulation protein symbiotic plasmid, and nodule bacteroid differentiation. We have also identified several B. abortus homologs of virulence and pathogenesis genes from other pathogens, including a homolog to both the Shda gene from Salmonella enterica serovar Typhimurium and the AidA-1 gene from Escherichia coli. Other GSSs displayed significant homologies to genes encoding components of the type III and type IV secretion machineries, suggesting that Brucella might also have an active type III secretion machinery.

Protection against infection in mice vaccinated with a Brucella abortus mutant.

This study determines whether a genetically engineered mutant of Brucella abortus, strain M-1, possesses differences in protective properties compared to the parental strain, vaccine S19. M-1 is a mutant unable to express BP26, a periplasmic protein with potential use in diagnosis. Mice vaccinated with S19 developed antibodies against BP26, while those vaccinated with M-1 did not. However, mice vaccinated with S19 or M-1 were similarly protected against challenge with pathogenic strain 2308, suggesting that the lack of BP26 does not affect the induction of the protective immune response exerted by S19. These and previous results showing that bacterial invasion and growth or replication in mouse spleens were indistinguishable between strains M-1 and S19 could indicate that the mutant is an attenuated strain which maintains the same protective properties as S19.

Cloning of Brucella abortus gene and characterization of expressed 26-kilodalton periplasmic protein: potential use for diagnosis.

Brucella spp. are the causative agents of brucellosis in many different hosts, including humans. Most of the serological methods of diagnosis are based on the detection of antilipopolysaccharide antibodies, which makes the differentiation of vaccinated animals from infected animals difficult. By using molecular biology techniques, a gene that encodes a 26-kDa protein (BP26) was isolated from a Brucella abortus S19 genome lambda gt11 library. This protein is in the periplasm of B. abortus and in transformed Escherichia coli. It is exported to the periplasm via a preprotein of 29 kDa with a signal sequence of 28 amino acids. The nucleotide and amino acid sequences of this gene and protein did not show any similarity with those of previously sequenced genes. The use of this protein in Western blotting allowed the differentiation between vaccinated bovines from infected bovines and the detection of infected rams: on the other hand, sera from human patients with active brucellosis were positive, while sera from human patients with chronic brucellosis or without clinical signs were nonreactive. BP26 might be of value as an antigen for serological diagnosis of brucellosis in different mammals.