A non-invasive intratracheal inoculation method for the study of pulmonary melioidosis

Pulmonary melioidosis, a disease manifestation caused by the bacterium Burkholderia pseudomallei, has been studied using aerosols or intranasal (IN) inoculation in small animal models. Both have inherent disadvantages which may not accurately model primary pulmonary melioidosis in humans. Intratracheal inoculation (IT) by direct visualization of the tracheal opening offers an alternative technique for infection that overcomes the disadvantages of aerosol and IN challenge. In this study, we describe a method which requires relatively inexpensive equipment, little training, and is compliant with the operational constraints of a BSL3 laboratory. Results obtained using trypan blue demonstrated that an inoculum can be accurately delivered into the lungs of mice within a biosafety cabinet (BSC). Whole body imaging and histopathology confirmed that mice inoculated intratracheally with B. pseudomallei develop the primary focus of infection in the lungs, and not the nasal passages which can lead to invasion of the central nervous system and potential neurologic complications. Further, based on colony counts and bioluminescent imaging, dissemination to secondary organs occurred as expected. Taken together, this intratracheal method of inoculation fulfills four goals: (1) to accurately deliver B. pseudomallei into the lungs of the animal model, (2) to avoid potentially confounding complications due to primary infections at sites other than the lung, (3) to maintain normal organ dissemination, and (4) to be BSL3 compliant.

Nitrosative damage to free and zinc-bound cysteine thiols underlies nitric oxide toxicity in wild-type Borrelia burgdorferi

Borrelia burgdorferi encounters potentially harmful reactive nitrogen species (RNS) throughout its infective cycle. In this study, diethylamine NONOate (DEA/NO) was used to characterize the lethal effects of RNS on B. burgdorferi. RNS produce a variety of DNA lesions in a broad spectrum of microbial pathogens; however, levels of the DNA deamination product, deoxyinosine, and the numbers of apurinic/apyrimidinic (AP) sites were identical in DNA isolated from untreated and DEA/NO-treated B. burgdorferi cells. Strains with mutations in the nucleotide excision repair (NER) pathway genes uvrC or uvrB treated with DEA/NO had significantly higher spontaneous mutation frequencies, increased numbers of AP sites in DNA and reduced survival compared with wild-type controls. Polyunsaturated fatty acids in B. burgdorferi cell membranes, which are susceptible to peroxidation by reactive oxygen species (ROS), were not sensitive to RNS-mediated lipid peroxidation. However, treatment of B. burgdorferi cells with DEA/NO resulted in nitrosative damage to several proteins, including the zinc-dependent glycolytic enzyme fructose-1,6-bisphosphate aldolase (BB0445), the Borrelia oxidative stress regulator (BosR) and neutrophil-activating protein (NapA). Collectively, these data suggested that nitrosative damage to proteins harbouring free or zinc-bound cysteine thiols, rather than DNA or membrane lipids underlies RNS toxicity in wild-type B. burgdorferi.

Borrelia burgdorferi membranes are the primary targets of reactive oxygen species

Spirochetes living in an oxygen-rich environment or when challenged by host immune cells are exposed to reactive oxygen species (ROS). These species can harm/destroy cysteinyl residues, iron-sulphur clusters, DNA and polyunsaturated lipids, leading to inhibition of growth or cell death. Because Borrelia burgdorferi contains no intracellular iron, DNA is most likely not a major target for ROS via Fenton reaction. In support of this, growth of B. burgdorferi in the presence of 5 mM H(2)O(2) had no effect on the DNA mutation rate (spontaneous coumermycin A1 resistance), and cells treated with 10 mM t-butyl hydroperoxide or 10 mM H(2)O(2) show no increase in DNA damage. Unlike most bacteria, B. burgdorferi incorporates ROS-susceptible polyunsaturated fatty acids from the environment into their membranes. Analysis of lipoxidase-treated B. burgdorferi cells by Electron Microscopy showed significant irregularities indicative of membrane damage. Fatty acid analysis of cells treated with lipoxidase indicated that host-derived linoleic acid had been dramatically reduced (50-fold) in these cells, with a corresponding increase in the levels of malondialdehyde by-product (fourfold). These data suggest that B. burgdorferi membrane lipids are targets for attack by ROS encountered in the various stages of the infective cycle.

Zinc is the metal cofactor of Borrelia burgdorferi peptide deformylase

Peptide deformylase (PDF, E.C. 3.5.1.88) catalyzes the removal of N-terminal formyl groups from nascent ribosome-synthesized polypeptides. PDF contains a catalytically essential divalent metal ion, which is tetrahedrally coordinated by three protein ligands (His, His, and Cys) and a water molecule. Previous studies revealed that the metal cofactor is a Fe2+ ion in Escherichia coli and many other bacterial PDFs. In this work, we found that PDFs from two iron-deficient bacteria, Borrelia burgdorferi and Lactobacillus plantarum, are stable and highly active under aerobic conditions. The native B. burgdorferi PDF (BbPDF) was purified 1200-fold and metal analysis revealed that it contains approximately 1.1 Zn2+ ion/polypeptide but no iron. Our studies suggest that PDF utilizes different metal ions in different organisms. These data have important implications in designing PDF inhibitors and should help address some of the unresolved issues regarding PDF structure and catalytic function.

Insights into the complex regulation of rpoS in Borrelia burgdorferi

Co-ordinated regulation of gene expression is required for the transmission and survival of Borrelia burgdorferi in different hosts. The sigma factor RpoS (sigma(S)), as regulated by RpoN (sigma(54)), has been shown to regulate key virulence factors (e.g. OspC) required for these processes. As important, multiple signals (e.g. temperature, pH, cell density, oxygen) have been shown to increase the expression of sigma(S)-dependent genes; however, little is known about the signal transduction mechanisms that modulate the expression of rpoS. In this report we show that: (i) rpoS has a sigma(54)-dependent promoter that requires Rrp2 to activate transcription; (ii) Rrp2Delta123, a constitutively active form of Rrp2, activated sigma(54)-dependent transcription of rpoS/P-lacZ reporter constructs in Escherichia coli; (iii) quantitative reverse transcription polymerase chain reaction (QRT-PCR) experiments with reporter cat constructs in B. burgdorferi indicated that Rrp2 activated transcription of rpoS in an enhancer-independent fashion; and finally, (iv) rpoN is required for cell density- and temperature-dependent expression of rpoS in B. burgdorferi, but histidine kinase Hk2, encoded by the gene immediately upstream of rrp2, is not essential. Based on these findings, a model for regulation of rpoS has been proposed which provides mechanisms for multiple signalling pathways to modulate the expression of the sigma(S) regulon in B. burgdorferi.

Borrelia burgdorferi bb0728 encodes a coenzyme A disulphide reductase whose function suggests a role in intracellular redox and the oxidative stress response

The cellular responses of Borrelia burgdorferiTo reactive oxygen species (ROS) encountered during the different stages of its infective cycle are poorly understood. Few enzymes responsible for protecting proteins, DNA/RNA and lipids from damage by ROS have been identified and characterized. Data presented here suggest that bb0728 encodes an enzyme involved in this process. Biochemical analyses on purified recombinant BB0728 indicated that it functioned as a coenzyme A disulphide reductase (CoADR) (specific activity approximately 26 units per mg of protein). This enzyme was specific for coenzyme A (CoA) disulphide, required NADH and had no significant activity against other disulphides, such as oxidized glutathione or thioredoxin. The high intracellular concentration of reduced CoA (CoASH) in B. burgdorferi cells ( approximately 1 mM) and absence of glutathione suggest that CoA is the major low-molecular-weight thiol in this spirochete. Interestingly, CoASH was able to reduce H(2)O(2) and be regenerated by CoADR suggesting one role for the system may be to protect B. burgdorferi from ROS. Further, mobility-shift assays and transcriptional fusion data indicated that bb0728 was positively regulated by the Borrelia oxidative stress response regulator, BosR. Taken together, these data suggest a role for BB0728 in intracellular redox and the oxidative stress response in B. burgdorferi.

A conservative amino acid change alters the function of BosR, the redox regulator of Borrelia burgdorferi

Borrelia burgdorferi, the aetiologic agent of Lyme disease, modulates gene expression in response to changes imposed by its arthropod vector and mammalian hosts. As reactive oxygen species (ROS) are known to vary in these environments, we asked how B. burgdorferi responds to oxidative stress. The B. burgdorferi genome encodes a PerR homologue (recently designated BosR) that represses the oxidative stress response in other bacteria, suggesting a similar function in B. burgdorferi. When we tested the sensitivity of B. burgdorferi to ROS, one clonal non-infectious B. burgdorferi isolate exhibited hypersensitivity to t-butyl hydroperoxide when compared with infectious B. burgdorferi and other non-infectious isolates. Sequence analysis indicated that the hypersensitive non-infectious isolates bosR allele contained a single nucleotide substitution, converting an arginine to a lysine (bosRR39K). Mutants in bosRR39K exhibited an increase in resistance to oxidative stressors when compared with the parental non-infectious strain, suggesting that BosRR39K functioned as a repressor. Complementation with bosRR39K and bosR resulted in differential sensitivity to t-butyl hydroperoxide, indicating that these alleles are functionally distinct. In contrast to BosR, BosRR39K did not activate transcription of a napA promoter-lacZ reporter in Escherichia coli nor bind the napA promoter/operator domain. However, we found that both BosR and BosRR39K bound to the putative promoter/operator region of superoxide dismutase (sodA). In addition, we determined that cells lacking BosRR39K synthesized fourfold greater levels of the decorin binding adhesin DbpA suggesting that BosRR39K regulates genes unrelated to oxidative stress. Based on these data, we propose that the single amino acid substitution, R39K, dramatically alters the activity of BosR by altering its ability to bind DNA at target regulatory sequences.

Dissolved oxygen levels alter gene expression and antigen profiles in Borrelia burgdorferi

The Lyme disease spirochete, Borrelia burgdorferi, encounters many environmental signals as it cycles between the arthropod vector and mammalian hosts, including temperature, pH, and other host factors. To test the possibility that dissolved oxygen modulates gene expression in B. burgdorferi, spirochetes were exposed to differential levels of dissolved oxygen, and distinct alterations were observed at both the transcriptional and translational levels. Specifically NapA, a Dps/Dpr homologue involved in the oxidative stress response in other bacteria, was reduced when B. burgdorferi was grown under oxygen-limiting conditions. In contrast, several immunoreactive proteins were altered when tested with infection-derived sera from different hosts. Specifically, OspC, DbpA, and VlsE were synthesized at greater levels when cells were grown in limiting oxygen, whereas VraA was reduced. The levels of oxygen in the medium did not affect OspA production. Real-time reverse transcription-PCR analysis of RNA isolated from infectious isolates of strains B31 and cN40 indicated that the expression of ospC, dbpA, and vlsE increased while napA expression decreased under dissolved-oxygen-limiting conditions, whereas flaB was not affected. The reverse transcription-PCR results corroborated the immunoblot analyses and indicated that the increase in OspC, DbpA, and VlsE was due to regulation at the transcriptional level of the genes encoding these antigens. These results indicate that dissolved oxygen modulates gene expression in B. burgdorferi and imply that the redox environment may be an additional regulatory cue that spirochetes exploit to adapt to the disparate niches that they occupy in nature.

Borrelia oxidative stress response regulator, BosR: a distinctive Zn-dependent transcriptional activator

The ability of a pathogen to cause infection depends on successful colonization of the host, which, in turn, requires adaptation to various challenges presented by that host. For example, host immune cells use a variety of mechanisms to control infection by bacterial pathogens, including the production of bactericidal reactive oxygen species. Prokaryotic and eukaryotic cells have developed ways of protecting themselves against this oxidative damage; for instance, Borrelia burgdorferi alters the expression of oxidative-stress-related proteins, such as a Dps/Dpr homolog NapA (BB0690), in response to increasing levels of oxygen and reactive oxygen species. These stress-related genes appear to be regulated by a putative metal-dependent DNA-binding protein (BB0647) that has 50.7% similarity to the peroxide-specific stress response repressor of Bacillus subtilis, PerR. We overexpressed and purified this protein from Escherichia coli and designated it Borrelia oxidative stress regulator, BosR. BosR bound to a 50-nt region 180 bp upstream of the napA transcriptional start site and required DTT and Zn2+ for optimal binding. Unlike the Bacillus subtilis PerR repressor, BosR did not require Fe2+ and Mn2+ for binding, and oxidizing agents, such as t-butyl peroxide, enhanced, not eliminated, BosR binding to the napA promoter region. Surprisingly, transcriptional fusion analysis indicated that BosR exerted a positive regulatory effect on napA that is inducible with t-butyl peroxide. On the basis of these data, we propose that, despite the similarity to PerR, BosR functions primarily as a transcriptional activator, not a repressor of oxidative stress response, in B. burgdorferi.