Leptospirosis is one of the most common zoonotic diseases in the world, resulting in high morbidity and mortality in humans and affecting global livestock production. Most infections are caused by either Leptospira borgpetersenii or Leptospira interrogans, bacteria that vary in their distribution in nature and rely on different modes of transmission. We report the complete genomic sequences of two strains of L. borgpetersenii serovar Hardjo that have distinct phenotypes and virulence. These two strains have nearly identical genetic content, with subtle frameshift and point mutations being a common form of genetic variation. Starkly limited regions of synteny are shared between the large chromosomes of L. borgpetersenii and L. interrogans, probably the result of frequent recombination events between insertion sequences. The L. borgpetersenii genome is Ϸ700 kb smaller and has a lower coding density than L. interrogans, indicating it is decaying through a process of insertion sequence-mediated genome reduction. Loss of gene function is not random but is centered on impairment of environmental sensing and metabolite transport and utilization. These features distinguish L. borgpetersenii from L. interrogans, a species with minimal genetic decay and that survives extended passage in aquatic environments encountering a mammalian host. We conclude that L. borgpetersenii is evolving toward dependence on a strict host-to-host transmission cycle.genome sequence ͉ leptospirosis ͉ spirochete

Brucellosis is a worldwide disease of humans and livestock that is caused by a number of very closely related classical Brucella species in the alpha-2 subdivision of the Proteobacteria. We report the complete genome sequence of Brucella abortus field isolate 9-941 and compare it to those of Brucella suis 1330 and Brucella melitensis 16 M. The genomes of these Brucella species are strikingly similar, with nearly identical genetic content and gene organization. However, a number of insertion-deletion events and several polymorphic regions encoding putative outer membrane proteins were identified among the genomes. Several fragments previously identified as unique to either B. suis or B. melitensis were present in the B. abortus genome. Even though several fragments were shared between only B. abortus and B. suis, B. abortus shared more fragments and had fewer nucleotide polymorphisms with B. melitensis than B. suis. The complete genomic sequence of B. abortus provides an important resource for further investigations into determinants of the pathogenicity and virulence phenotypes of these bacteria.Brucellosis is a bacterial disease of animals that can be transmitted to humans. The primary impact of brucellosis stems from losses due to reproductive failure in food animals and the loss of human productivity. Since brucellosis threatens the food supply and causes undulant fever, a long, debilitating disease in humans, Brucella species are recognized as potential agricultural, civilian, and military bioterrorism agents. Brucellosis in food animals is controlled by vaccination. Human brucellosis is treatable with antibiotics, though the course of antibiotic treatment must be prolonged due to the intracellular nature of Brucella.Analysis of 16S rRNA sequences places Brucella spp. as members of the alpha-2 Proteobacteria (31). The genus Brucella has six recognized species, all of which exhibit distinct host preferences (25,26). The high degree of similarity among the brucellae (1, 3, 13, 33) lends support to the proposal that the classical species of Brucella are actually strains of Brucella melitensis (40). However, this view conflicts with the hypothesized evolutionary isolation of these classical species due to their intracellular existence and host preference (29). Common host-pathogen associations among the classical Brucella species are as follows: B.