SP0454, a putative threonine dehydratase, is required for pneumococcal virulence in mice

Dual RNA-Seq of Flavobacterium psychrophilum and Its Outer Membrane Vesicles Distinguishes Genes Associated with Susceptibility to Bacterial Cold-Water Disease in Rainbow Trout (Oncorhynchus mykiss)

Flavobacterium psychrophilum (Fp), the causative agent of Bacterial Cold-Water disease in salmonids, causes substantial losses in aquaculture. Bacterial outer membrane vesicles (OMVs) contain several virulence factors, enzymes, toxins, and nucleic acids and are expected to play an essential role in host–pathogen interactions. In this study, we used transcriptome sequencing, RNA-seq, to investigate the expression abundance of the protein-coding genes in the Fp OMVs versus the Fp whole cell. RNA-seq identified 2190 transcripts expressed in the whole cell and 2046 transcripts in OMVs. Of them, 168 transcripts were uniquely identified in OMVs, 312 transcripts were expressed only in the whole cell, and 1878 transcripts were shared in the two sets. Functional annotation analysis of the OMV-abundant transcripts showed an association with the bacterial translation machinery and histone-like DNA-binding proteins. RNA-Seq of the pathogen transcriptome on day 5 post-infection of Fp-resistant versus Fp-susceptible rainbow trout genetic lines revealed differential gene expression of OMV-enriched genes, suggesting a role for the OMVs in shaping the host–microbe interaction. Interestingly, a cell wall-associated hydrolase (CWH) gene was the most highly expressed gene in OMVs and among the top upregulated transcripts in susceptible fish. The CWH sequence was conserved in 51 different strains of Fp. The study provides insights into the potential role of OMVs in host–pathogen interactions and explores microbial genes essential for virulence and pathogenesis.

Heterologous prime-boost immunization with live SPY1 and DnaJ protein of Streptococcus pneumoniae induces strong Th1 and Th17 cellular immune responses in mice

Streptococcus pneumoniae is a leading cause of infectious diseases in children under 5-year-old. Vaccine has been used as an indispensable strategy to prevent S. pneumoniae infection for more than 30 years. Our previous studies confirmed that mucosal immunization with live attenuated strain SPY1 can protect mice against nasopharyngeal colonization of S. pneumoniae and lethal pneumococcal infection, and the protective effects are comparable with those induced by commercially available 23-valent polysaccharide vaccine. However, live attenuated vaccine SPY1 needs four inoculations to get satisfactory protective effect, which may increase the risk of virulence recovery. It is reported that heterologous primeboost approach is more effective than homologous primeboost approach. In the present study, to decrease the doses of live SPY1 and improve the safety of SPY1 vaccine, we immunized mice with SPY1 and DnaJ protein alternately. Our results showed that heterologous prime-boost immunization with SPY1 and DnaJ protein could significantly reduce the colonization of S. pneumoniae in the respiratory tract of mice, and induce stronger Th1 and Th17 cellular immune responses than SPY1 alone. These results indicate heterologous prime-boost immunization method not only elicits better protective effect than SPY1 alone, but also reduces the doses of live SPY1 and decreases the risk of SPY1 vaccine. This work is the first time to study the protective efficiency with two different forms of S. pneumoniae candidate vaccine, and provides a new strategy for the development of S. pneumoniae vaccine.

MRA_1571 is required for isoleucine biosynthesis and improves Mycobacterium tuberculosis H37Ra survival under stress

Threonine dehydratase is a pyridoxal 5-phosphate dependent enzyme required for isoleucine biosynthesis. Threonine dehydratase (IlvA) participates in conversion of threonine to 2-oxobutanoate and ammonia is released as a by-product. MRA_1571 is annotated to be coding for IlvA in Mycobacterium tuberculosis H37Ra (Mtb-Ra). We developed a recombinant (KD) Mtb-Ra strain by down-regulating IlvA. The growth studies on different carbon sources suggested reduced growth of KD compared to wild-type (WT), also, isoleucine concentration dependent KD growth restoration was observed. The expression profiling of IlvA suggested increased expression of IlvA during oxygen, acid and oxidative stress. In addition, KD showed reduced survival under pH, starvation, nitric oxide and peroxide stresses. KD was more susceptible to antimycobacterial agents such as streptomycin (STR), rifampicin (RIF) and levofloxacin (LVF), while, no such effect was noticeable when exposed to isoniazid. Also, an increase in expression of IlvA was observed when exposed to STR, RIF and LVF. The dye accumulation studies suggested increased permeability of KD to ethidium bromide and Nile Red as compared to WT. TLC and Mass studies confirmed altered lipid profile of KD. In summary down-regulation of IlvA affects Mtb growth, increases its susceptibility to stress and leads to altered cell wall lipid profile.

Mucosal immunization with the live attenuated vaccine SPY1 induces humoral and Th2-Th17-regulatory T cell cellular immunity and protects against pneumococcal infection

Mucosal immunization with attenuated vaccine can protect against pneumococcal invasion infection, but the mechanism was unknown. Our study found that mucosal delivery with the live attenuated SPY1 vaccine strain can confer T cell- and B cell-dependent protection against pneumococcal colonization and invasive infection; yet it is still unclear which cell subsets contribute to the protection, and their roles in pneumococcal colonization and invasion remain elusive. Adoptive transfer of anti-SPY1 antibody conferred protection to naive μMT mice, and immune T cells were indispensable to protection examined in nude mice. A critical role of interleukin 17A (IL-17A) in colonization was demonstrated in mice lacking IL-17A, and a vaccine-specific Th2 immune subset was necessary for systemic protection. Of note, we found that SPY1 could stimulate an immunoregulatory response and that SPY1-elicited regulatory T cells participated in protection against colonization and lethal infection. The data presented here aid our understanding of how live attenuated strains are able to function as effective vaccines and may contribute to a more comprehensive evaluation of live vaccines and other mucosal vaccines.

A novel protein, RafX, is important for common cell wall polysaccharide biosynthesis in Streptococcus pneumoniae: implications for bacterial virulence

Teichoic acid (TA), together with peptidoglycan (PG), represents a highly complex glycopolymer that ensures cell wall integrity and has several crucial physiological activities. Through an insertion-deletion mutation strategy, we show that ΔrafX mutants are impaired in cell wall covalently attached TA (WTA)-PG biosynthesis, as evidenced by their abnormal banding patterns and reduced amounts of WTA in comparison with wild-type strains. Site-directed mutagenesis revealed an essential role for external loop 4 and some highly conserved amino acid residues in the function of RafX protein. The rafX gene was highly conserved in closely related streptococcal species, suggesting an important physiological function in the lifestyle of streptococci. Moreover, a strain D39 ΔrafX mutant was impaired in bacterial growth, autolysis, bacterial division, and morphology. We observed that a strain R6 ΔrafX mutant was reduced in adhesion relative to the wild-type R6 strain, which was supported by an inhibition assay and a reduced amount of CbpA protein on the ΔrafX mutant bacterial cell surface, as shown by flow cytometric analysis. Finally, ΔrafX mutants were significantly attenuated in virulence in a murine sepsis model. Together, these findings suggest that RafX contributes to the biosynthesis of WTA, which is essential for full pneumococcal virulence.

Serotype-independent protection against pneumococcal infections elicited by intranasal immunization with ethanol-killed pneumococcal strain, SPY1

The 23-valent polysaccharide vaccine and the 7-valent pneumococcal conjugate vaccine are licensed vaccines that protect against pneumococcal infections worldwide. However, the incidence of pneumococcal diseases remains high in low-income countries. Whole-cell vaccines with high safety and strong immunogenicity may be a favorable choice. We previously obtained a capsule-deficient Streptococcus pneumoniae mutant named SPY1 derived from strain D39. As an attenuated live pneumococcal vaccine, intranasal immunization with SPY1 elicits broad serotype-independent protection against pneumococcal infection. In this study, for safety consideration, we inactivated SPY1 with 70% ethanol and intranasally immunized BALB/c mice with killed SPY1 plus cholera toxin adjuvant for four times. Results showed that intranasal immunization with inactivated SPY1 induced strong humoral and cellular immune responses. Intranasal immunization with inactivated SPY1 plus cholera toxin adjuvant elicited effective serotype-independent protection against the colonization of pneumococcal strains 19F and 4 as well as lethal infection of pneumococcal serotypes 2, 3, 14, and 6B. The protection rates provided by inactivated SPY1 against lethal pneumococcal infection were comparable to those of currently used polysaccharide vaccines. In addition, vaccine-specific B-cell and T-cell immune responses mediated the protection elicited by SPY1. In conclusion, the 70% ethanol-inactivated pneumococcal whole-cell vaccine SPY1 is a potentially safe and less complex vaccine strategy that offers broad protection against S. pneumoniae.