Relapsed Melioidosis Model in C57BL/6 Mice.

BACKGROUND: Burkholderia pseudomallei are the causative agents of melioidosis, a disease that has a high relapse rate in endemic areas. The mechanism of relapse is unclear OBJECTIVE: This study aimed to establish relapsed melioidosis in C57BL/6 mice by induction with B. pseudomallei. MATERIAL AND METHOD: Low doses of B. pseudomallei H777 and its biofilm defective mutant (M10) were intra-gastric fed to C57BL/6 mice. All the infected mice had suppressed immune status by intra-peritoneal injection of hydrocortisone at 2.5 mg per mouse at day 60 post-infection. Inflammatory response to the infection was investigated by histo-pathological studies and monitoring bacterial counts in the blood and organs. RESULTS: All the infected mice were found to have a high infiltration of mononuclear cells at day 60 post-infection. The results showed high bacterial counts in the blood in both strains post-suppressed immune status after two days. The biofilm mutant and wild type strains produced relapse in C57BL/6 mice but the latter was responsible for significantly more severe inflammation than the biofilm mutant. CONCLUSION: Low immune status may cause relapsed melioidosis in hosts with chronic inflammation.

High-temperature protein G is an essential virulence factor of Leptospira interrogans.

Leptospira interrogans is a global zoonotic pathogen and is the causative agent of leptospirosis, an endemic disease of humans and animals worldwide. There is limited understanding of leptospiral pathogenesis; therefore, further elucidation of the mechanisms involved would aid in vaccine development and the prevention of infection. HtpG (high-temperature protein G) is the bacterial homolog to the highly conserved molecular chaperone Hsp90 and is important in the stress responses of many bacteria. The specific role of HtpG, especially in bacterial pathogenesis, remains largely unknown. Through the use of an L. interrogans htpG transposon insertion mutant, this study demonstrates that L. interrogans HtpG is essential for virulence in the hamster model of acute leptospirosis. Complementation of the htpG mutant completely restored virulence. Surprisingly, the htpG mutant did not appear to show sensitivity to heat or oxidative stress, phenotypes common in htpG mutants in other bacterial species. Furthermore, the mutant did not show increased sensitivity to serum complement, reduced survival within macrophages, or altered protein or lipopolysaccharide expression. The underlying cause for attenuation thus remains unknown, but HtpG is a novel leptospiral virulence factor and one of only a very small number identified to date.

Leptospiral LruA is required for virulence and modulates an interaction with mammalian apolipoprotein AI.

Leptospirosis is a worldwide zoonosis caused by spirochetes of the genus Leptospira. While understanding of pathogenesis remains limited, the development of mutagenesis in Leptospira has provided a powerful tool for identifying novel virulence factors. LruA is a lipoprotein that has been implicated in leptospiral uveitis as a target of the immune response. In this study, two lruA mutants, M754 and M765, generated by transposon mutagenesis from Leptospira interrogans serovar Manilae, were characterized. In M754, the transposon inserted in the middle of lruA, resulting in no detectable expression of LruA. In M765, the transposon inserted toward the 3' end of the gene, resulting in expression of a truncated protein. LruA was demonstrated to be on the cell surface in M765 and the wild type (WT). M754, but not M765, was attenuated in a hamster model of acute infection. A search for differential binding to human serum proteins identified a serum protein of around 30 kDa bound to the wild type and the LruA deletion mutant (M754), but not to the LruA truncation mutant (M765). Two-dimensional separation of proteins from leptospiral cells incubated with guinea pig serum identified the 28-kDa apolipoprotein A-I (ApoA-I) as a major mammalian serum protein that binds Leptospira in vitro. Interestingly, M754 (with no detectable LruA) bound more ApoA-I than did the LruA-expressing strains Manilae wild type and M765. Our data thus identify LruA as a surface-exposed leptospiral virulence factor that contributes to leptospiral pathogenesis, possibly by modulating cellular interactions with serum protein ApoA-I.

Leptospiral outer membrane protein LipL41 is not essential for acute leptospirosis but requires a small chaperone protein, lep, for stable expression.

Leptospirosis is a worldwide zoonosis caused by pathogenic Leptospira spp., but knowledge of leptospiral pathogenesis remains limited. However, the development of mutagenesis systems has allowed the investigation of putative virulence factors and their involvement in leptospirosis. LipL41 is the third most abundant lipoprotein found in the outer membranes of pathogenic leptospires and has been considered a putative virulence factor. LipL41 is encoded on the large chromosome 28 bp upstream of a small open reading frame encoding a hypothetical protein of unknown function. This gene was named lep, for LipL41 expression partner. In this study, lipL41 was found to be cotranscribed with lep. Two transposon mutants were characterized: a lipL41 mutant and a lep mutant. In the lep mutant, LipL41 protein levels were reduced by approximately 90%. Lep was shown through cross-linking and coexpression experiments to bind to LipL41. Lep is proposed to be a molecular chaperone essential for the stable expression of LipL41. The roles of LipL41 and Lep in the pathogenesis of Leptospira interrogans were investigated; surprisingly, neither of these two unique proteins was essential for acute leptospirosis.

Leptospira interrogans catalase is required for resistance to H2O2 and for virulence.

Pathogenic Leptospira spp. are likely to encounter higher concentrations of reactive oxygen species induced by the host innate immune response. In this study, we characterized Leptospira interrogans catalase (KatE), the only annotated catalase found within pathogenic Leptospira species, by assessing its role in resistance to H(2)O(2)-induced oxidative stress and during infection in hamsters. Pathogenic L. interrogans bacteria had a 50-fold-higher survival rate under H(2)O(2)-induced oxidative stress than did saprophytic L. biflexa bacteria, and this was predominantly catalase dependent. We also characterized KatE, the only annotated catalase found within pathogenic Leptospira species. Catalase assays performed with recombinant KatE confirmed specific catalase activity, while protein fractionation experiments localized KatE to the bacterial periplasmic space. The insertional inactivation of katE in pathogenic Leptospira bacteria drastically diminished leptospiral viability in the presence of extracellular H(2)O(2) and reduced virulence in an acute-infection model. Combined, these results suggest that L. interrogans KatE confers in vivo resistance to reactive oxygen species induced by the host innate immune response.

Cross-protective immunity against leptospirosis elicited by a live, attenuated lipopolysaccharide mutant.

BACKGROUND: Leptospira species cause leptospirosis, a zoonotic disease found worldwide. Current vaccines against leptospirosis provide protection only against closely related serovars. METHODS: We evaluated an attenuated transposon mutant of Leptospira interrogans serovar Manilae (M1352, defective in lipopolysaccharide biosynthesis) as a live vaccine against leptospirosis. Hamsters received a single dose of vaccine and were challenged with the homologous serovar (Manilae) and a serologically unrelated heterologous serovar (Pomona). Comparisons were made with killed vaccines. Potential cross-protective antigens against leptospirosis were investigated. RESULTS: Live M1352 vaccine induced superior protection in hamsters against homologous challenge. The live vaccine also stimulated cross-protection against heterologous challenge, with 100% survival (live M1352) versus 40% survival (killed vaccine). Hamsters receiving either vaccine responded to the dominant membrane proteins LipL32 and LipL41. Hamsters receiving the live vaccine additionally recognized LA3961/OmpL36 (unknown function), Loa22 (OmpA family protein, recognized virulence factor), LA2372 (general secretory protein G), and LA1939 (hypothetical protein). Manilae LigA was recognized by M1352 vaccinates, whereas LipL36 was detected in Pomona. CONCLUSION: This study demonstrated that a live, attenuated vaccine can stimulate cross-protective immunity to L. interrogans and has identified antigens that potentially confer cross-protection against leptospirosis.

Identification of genes essential for leptospirosis.

The development of methods for the construction of defined mutants of pathogenic Leptospira has been a breakthrough in the study of leptospiral virulence. These methods have allowed the identification of genes essential for infection in animal models. This chapter describes methods for random transposon mutagenesis of pathogenic leptospires, identification of transposon insertion sites using direct sequencing from genomic DNA and a nested PCR utilizing degenerate oligonucleotides, and methods for testing mutant attenuation in the hamster model of infection.

Characterization of BPSS1521 (bprD), a regulator of Burkholderia pseudomallei virulence gene expression in the mouse model.

The Gram-negative saprophytic bacterium Burkholderia pseudomallei is the causative agent of melioidosis, a severe infectious disease of both humans and animals. Severity of the disease is thought to be dependent on both the health status of the host, including diabetes mellitus and kidney disease, and bacterial-derived factors. To identify the bacterial factors important during an acute infection, gene expression profiles in the spleen, lung, and liver of BALB/c (Th2 prototype) and C57BL/6 mice (Th1 prototype) were determined using DNA microarrays. This analysis identified BPSS1521 (bprD), a predicted transcriptional regulator located in the type III secretion system (T3SS-3) operon, to be up regulated, specifically in C57BL/6 mice. BALB/c mice infected with a bprD mutant showed a shorter time to death and increased inflammation, as determined by histopathological analysis and enumeration of bacteria in the spleen. Elevated numbers of multinucleated giant cells (MNGCs), which is the hallmark of melioidosis, were detected in both the wild-type and the bprD mutants; a similar elevation occurs in melioidosis patients. One striking observation was the increased expression of BPSS1520 (bprC), located downstream of bprD, in the bprD mutant. BprC is a regulator of T6SS-1 that is required for the virulence of B. pseudomallei in murine infection models. Deletion of bprD led to the overexpression of bprC and a decreased time to death. bprD expression was elevated in C57BL/6--as compared to BALB/c--mice, suggesting a role for BprD in the natural resistance of C57BL/6 mice to B. pseudomallei. Ultimately, this analysis using mice with different immune backgrounds may enhance our understanding of the outcomes of infection in a variety of models.

Use of a high-throughput screen to identify Leptospira mutants unable to colonize the carrier host or cause disease in the acute model of infection.

The molecular basis for leptospirosis infection and colonization remains poorly understood, with no efficient methods available for screening libraries of mutants for attenuation. We analysed the attenuation of leptospiral transposon mutants in vivo using a high-throughput method by infecting animals with pooled sets of transposon mutants. A total of 95 mutants was analysed by this method in the hamster model of acute infection, and one mutant was identified as attenuated (M1233, lb058 mutant). All virulence factors identified in Leptospira to date have been characterized in the acute model of infection, neglecting the carrier host. To address this, a BALB/c mouse colonization model was established. The lb058 mutant and two mutants defective in LPS synthesis were colonization deficient in the mouse model. By applying the high-throughput screening method, a further five colonization-deficient mutants were identified for the mouse model; these included two mutants in genes encoding proteins with a predicted role in iron uptake (LB191/HbpA and LB194). Two attenuated mutants had transposon insertions in either la0589 or la2786 (encoding proteins of unknown function). The final attenuated mutant had an unexpected deletion of genes la0969-la0975 at the point of transposon insertion. This is the first description of defined, colonization-deficient mutants in a carrier host for Leptospira. These mutants were either not attenuated or only weakly attenuated in the hamster model of acute leptospirosis, thus illustrating that different factors that may be required in the carrier and acute models of leptospiral infection. High-throughput screening can reduce the number of animals used in virulence studies and increase the capacity to screen mutants for attenuation, thereby enhancing the likelihood of detecting unique virulence factors. A comparison of virulence factors required in the carrier and acute models of infection will help to unravel colonization and dissemination mechanisms of leptospirosis.

Genomic islands as a marker to differentiate between clinical and environmental Burkholderia pseudomallei.

Burkholderia pseudomallei, as a saprophytic bacterium that can cause a severe sepsis disease named melioidosis, has preserved several extra genes in its genome for survival. The sequenced genome of the organism showed high diversity contributed mainly from genomic islands (GIs). Comparative genome hybridization (CGH) of 3 clinical and 2 environmental isolates, using whole genome microarrays based on B. pseudomallei K96243 genes, revealed a difference in the presence of genomic islands between clinical and environmental isolates. The largest GI, GI8, of B. pseudomallei was observed as a 2 sub-GI named GIs8.1 and 8.2 with distinguishable %GC content and unequal presence in the genome. GIs8.1, 8.2 and 15 were found to be more common in clinical isolates. A new GI, GI16c, was detected on chromosome 2. Presences of GIs8.1, 8.2, 15 and 16c were evaluated in 70 environmental and 64 clinical isolates using PCR assays. A combination of GIs8.1 and 16c (positivity of either GI) was detected in 70% of clinical isolates and 11.4% of environmental isolates (P<0.001). Using BALB/c mice model, no significant difference of time to mortality was observed between K96243 isolate and three isolates without GIs under evaluation (P>0.05). Some virulence genes located in the absent GIs and the difference of GIs seems to contribute less to bacterial virulence. The PCR detection of 2 GIs could be used as a cost effective and rapid tool to detect potentially virulent isolates that were contaminated in soil.