Homogeneity of VacJ outer membrane lipoproteins among Pasteurella multocida strains and heterogeneity among members of Pasteurellaceae

Maintenance of bacterial outer membrane lipid asymmetry: insight into MlaA

The outer membrane (OM) of Gram-negative bacteria acts as an effective barrier to protect against toxic compounds. By nature, the OM is asymmetric with the highly packed lipopolysaccharide (LPS) at the outer leaflet and glycerophospholipids at the inner leaflet. OM asymmetry is maintained by the Mla system, in which is responsible for the retrograde transport of glycerophospholipids from the OM to the inner membrane. This system is comprised of six Mla proteins, including MlaA, an OM lipoprotein involved in the removal of glycerophospholipids that are mis-localized at the outer leaflet of the OM. Interestingly, MlaA was initially identified - and called VacJ - based on its role in the intracellular spreading of Shigella flexneri.Many open questions remain with respect to the Mla system and the mechanism involved in the translocation of mislocated glycerophospholipids at the outer leaflet of the OM, by MlaA. After summarizing the current knowledge on MlaA, we focus on the impact of mlaA deletion on OM lipid composition and biophysical properties of the OM. How changes in OM lipid composition and biophysical properties can impact the generation of membrane vesicles and membrane permeability is discussed. Finally, we explore whether and how MlaA might be a candidate for improving the activity of antibiotics and as a vaccine candidate.Efforts dedicated to understanding the relationship between the OM lipid composition and the mechanical strength of the bacterial envelope and, in turn, how such properties act against external stress, are needed for the design of new targets or drugs for Gram-negative infections.

Immunogenicity and protective efficacy of the recombinant Pasteurella multocida lipoproteins VacJ and PlpE, and outer membrane protein H from P. multocida A:1 in ducks

Duck cholera (duck hemorrhagic septicemia) is a highly contagious disease caused by Pasteurella multocida, and is one of the major bacterial diseases currently affecting the duck industry. Type A is the predominant pathogenic serotype. In this study, the genes encoding the lipoproteins VacJ, PlpE, and the outer membrane protein OmpH of P. multocida strain PMWSG-4 were cloned and expressed as proteins in E. coli. The recombinant VacJ (84.4 kDa), PlpE (94.8 kDa), and OmpH (96.7 kDa) proteins were purified, and subunit vaccines were formulated with a single water-in-oil adjuvant, while killed vaccines were prepared using a single oil-coated adjuvant. Antibody responses in ducks vaccinated with recombinant VacJ, PlpE, and OmpH proteins formulated with adjuvants were significantly antigenic (p<0.005). Protectivity of the vaccines was evaluated via the intraperitoneal challenge of ducks with 20 LD50 doses of P. multocida A: 1. The vaccine formulation consisting of rVacJ, rPlpE, rOmpH, and adjuvant provided 33.3%, 83.33%, and 83.33% protection, respectively, the vaccine formulation consisting of three recombinant proteins, rVacJ, rPlpE, rOmpH and adjuvant, was 100% protective, and the killed vaccine was 50% protective. In addition, it was shown through histopathological examination and tissue bacterial load detection that all vaccines could reduce tissue damage and bacterial colonization to varying (p<0.001). These findings indicated that recombinant PlpE or OmpH fusion proteins formulated with oil adjuvants have the potential to be used as vaccine candidates against duck cholera subunits.

Conservation of ptfA gene encoded Type IV fimbrial protein among circulating Pasteurella multocida serogroup A strains causing pneumonia in sheep

Respiratory infections are most common in small ruminants. Differentiation of homogenous bacterial strains originated from similar clinical forms (acute or chronic) of infections in a particular or diverse host origin and regions posses a greater challenge for rapid epidemiological studies. Pasteurella multocida, being a multi-host pathogen with wide range of infections among small ruminants especially sheep, is of greater economical concern among small and marginal farmers. In our study, we report ptfA gene sequence based analysis of circulating P. multocida strains recovered from clinically ailing sheep either with pneumonia or septicaemia belonging to different geographical regions of Karnataka. All the 29 P. multocida strains were characterized by conventional methods as well as molecular methods which indicated homogeneity as they belonged to serogroup A and possessed highly conserved ptfA gene by phylogenetic analysis. The study highlighted the conservation of ptfA gene/fimbrial protein among P. multocida strains from identical/diverse clinical conditions and could be employed in rapid epidemiological studies in routine surveillance of circulating pathogenic bacterial strains as well as pasteurellosis outbreak investigations among animals and birds.

Comparative genetic diversity analysis based on virulence and repetitive genes profiling of circulating Pasteurella multocida isolates from animal hosts

Virulence associated and/or housekeeping/repetitive genes either in single or multiple copies are being extensively targeted for bacterial pathogen detection and differentiation in epidemiological studies. In the present study, isolation of Pasteurella multocida from different animals and their genetic profiling based on the capsular types, virulence and repetitive elements (ERIC/REP) were carried out. A total 345 clinical samples from apparently healthy and diseased (pneumonic, septicaemia) animals (sheep, goat, pig, cattle, buffalo and rabbits) from different geographical regions of Karnataka, Uttar Pradesh, Mizoram and Assam states of India were screened. A total of 32% of the samples were found positive, of which 41 P. multocida isolates recovered. Virulence profiling of isolates indicated that omp87, ompA, ptfA, sodA, sodC, nanB, fur and exbB were present in 100% of isolates. Whereas, prevalence of other genes were; nanH (90%), ompH (71%), pfhA (63%), plpB (80%), hsf-1 (12%), hsf-2 (37%), pmHAS (78%), toxA (73%), hgbA (37%), hgbB (81%), tbpA (78%) and fimA (98%), among isolates. There was no influence of host or place on prevalence of virulence genes when assessed by fitting a Hierarchial Bayesian ordinal regression model. There was correlation (positive and negative) between broad groups of virulence genes. Both repetitive gene profiles (ERIC and REP) generated multiple amplicons (~200 to ~4000 bp). Cluster analysis with ERIC profiles revealed 5 clusters and 3 non- typable isolates with higher discriminatory power (D = 0.7991) than the REP-PCR profiles (D = 00.734) which revealed 4 clusters and 6 non- typable isolates. The results showed that a considerable level of genetic diversity exists among circulating P. multocida isolates despite belonging to the same geographical origin. The genetic diversity or clustering based on either virulence or repetitive elements among isolates could be largely driven by multiple factors acting together which lead to manifestations of particular disease symptoms.

Variability in in vitro biofilm production and antimicrobial sensitivity pattern among Pasteurella multocida strains

Biofilm production, hitherto an uncharacterized feature among circulating Pasteurella multocida strains, was studied along with the antibiotic susceptibility pattern. On the basis of biofilm formation ability, all the strains were categorized into four groups under six different culture conditions: strong biofilm-forming (22%), moderate (19%), weak (51%), and non-adherent (7%). Strains from serogroups A and B formed significant biofilms in at least one culture condition whereas strains from serogroup D were unable to form biofilms. All strains were found to be susceptible to tetracycline. In addition, the correlation between diverse factors (host, capsule type, clinical condition and the tadD gene) as well as antimicrobial susceptibility in biofilm production were analyzed by Joint distribution models, and showed that enrofloxacin and azithromycin resistant strains were positively correlated with strong biofilm production.

Immunogenicity of recombinant Omp16 protein of Pasteurella multocida B:2 in mouse model

Bacterial peptidoglycan-associated lipoproteins (PAL) are potential targets for the development of diagnostics/subunit vaccines for infectious diseases. Most commonly prevalent Omp16 lipoprotein is absolutely conserved among Pasteurella multocida strains, which are involved in multiple infectious diseases of livestock worldwide. In the present study, we cloned omp16 gene encoding for mature Omp16 of P. multocida B:2 strain P52 and overexpressed as a fusion protein in Escherichia coli. Mice immunized with purified recombinant non-lipidated Omp16 fusion protein (~32 kDa) resulted in elicitation of significant antigen specific serum antibody titres (total IgG and subtypes). A more pronounced increase in Th2 response (IgG1) was noticed. The study indicated the potential possibilities to use lipidated recombinant Omp16 protein in developing a composite subunit vaccine along with suitable adjuvant for haemorrhagic septicaemia/ pasteurellosis in livestock.

Deletion of the vacJ gene affects the biology and virulence in Haemophilus parasuis serovar 5

Haemophilus parasuis is an important pathogen causing severe infections in pigs. However, the specific bacterial factors that participate in pathogenic process are poorly understood. VacJ protein is a recently discovered outer membrane lipoprotein that relates to virulence in several pathogens. To characterize the function of the vacJ gene in H. parasuis virulent strain HS49, a vacJ gene-deletion mutant ΔvacJ and its complemented strain were constructed. Our findings supported that VacJ is essential for maintenance of cellular integrity and stress tolerance of H. parasuis, by the demonstrations that the ΔvacJ mutant showed morphological change, increased NPN fluorescence and, and decreased resistance to SDS-EDTA, osmotic and oxidation pressure. The increased susceptibility to several antibiotics in the ΔvacJ mutant further suggested that the stability of the outer membrane was impaired as a result of the mutation in the vacJ gene. Compared to the wild-type strain, the ΔvacJ mutant strain caused a decreased survival ratio from the serum and complement killing, and exhibited a significant decrease ability to adhere to and invade PK-15 cell. In addition, the ΔvacJ mutant showed reduced biofilm formation compared to the wild-type strain. Furthermore, the ΔvacJ was attenuated in a murine (Balb/C) model of infection and its LD₅₀ value was approximately fifteen-fold higher than that of the wild-type or complementation strain. The data obtained in this study indicate that vacJ plays an essential role in maintaining outer membrane integrity, stress tolerance, biofilm formation, serum resistance, and adherence to and invasion of host cells related to H. parasuis and further suggest a putative role of VacJ lipoprotein in virulence regulation.

Outer membrane lipoprotein VacJ is required for the membrane integrity, serum resistance and biofilm formation of Actinobacillus pleuropneumoniae

The outer membrane proteins of Actinobacillus pleuropneumoniae are mediators of infection, acting as targets for the host's defense system. The outer membrane lipoprotein VacJ is involved in serum resistance and intercellular spreading in several pathogenic bacteria. To investigate the role of VacJ in the pathogenicity of Actinobacillus pleuropneumoniae, the vacJ gene-deletion mutant MD12 ΔvacJ was constructed. The increased susceptibility to KCl, SDS plus EDTA, and several antibiotics in the MD12ΔvacJ mutant suggested that the stability of the outer membrane was impaired as a result of the mutation in the vacJ gene. The increased NPN fluorescence and significant cellular morphological variation in the MD12ΔvacJ mutant further demonstrated the crucial role of the VacJ lipoprotein in maintaining the outer membrane integrity of A. pleuropneumoniae. In addition, the MD12ΔvacJ mutant exhibited decreased survival from the serum and complement killing compared to the wild-type strain. Interestingly, the MD12ΔvacJ mutant showed reduced biofilm formation compared to the wild-type strain. To our knowledge, this is the first description of the VacJ lipoprotein contributing to bacterial biofilm formation. The data presented in this study illustrate the important role of the VacJ lipoprotein in the maintenance of cellular integrity, serum resistance, and biofilm formation in A. pleuropneumoniae.

Outer membrane proteome analysis of Indian strain of Pasteurella multocida serotype B:2 by MALDI-TOF/MS analysis

Identification of outer membrane proteins (OMPs) is important to understand the bacteria structure and function, host-pathogen interaction, development of novel vaccine candidates, and diagnostic antigens. But till now the key antigens of P. multocida B:2 isolate causing haemorrhagic septicaemia (HS) in animals are not clearly defined. In this study, P52 strain of P. multocida serotype B:2 was grown in vitro under iron-rich and iron-limited condition. The OMPs were extracted by sarkosyl method followed by SDS-PAGE and the proteins were identified by MALDI-TOF/MS analysis. In total, 22 proteins were identified, of which 7 were observed exclusively under iron-limited condition. Most of the high molecular weight proteins (TbpA, HgbA, HgbB, HasR, IroA, and HemR) identified in this study were involved in iron acquisition. Some hypothetical proteins (HP-KCU-10206, HP and AAUPMB 08244, HP AAUPMB 21592, HP AAUPMB 19766, AAUPMB 11295) were observed for the first time in this study which could be unique to serotype B:2. Further functional in vivo study of the proteins identified are required to explore the utility of these proteins in developing diagnostics and vaccine against HS.

Recombinant transferrin binding protein A (rTbpA) fragments of Pasteurella multocida serogroup B:2 provide variable protection following homologous challenge in mouse model

Transferrin binding protein A (TbpA), an iron acquisition surface protein that also acts as virulence factor, is widely distributed among strains of Pasteurella multocida. In the present study, a total of seven clones of TbpA fragments (39D to F777; 39D to Q697; 188V to F777; 188V to Q697; 39D to P377; 188V to P377 and 39D to F187) belonging to P. multocida B:2 were constructed, over-expressed and purified as recombinant fusion proteins from Escherichia coli using affinity chromatography. Immunization of mice with rTbpA fragments resulted in a significant (p < 0.05) rise in antigen specific serum total IgG and subtypes (IgG1 and IgG2a) tires. All immunized mice challenged with 8 LD50 of P. multocida B:2 resulted in a variable protective efficacy up to 50%. The study indicated the potential possibilities to incorporate full length TbpA in subunit vaccine formulation composed of synergistic subunit antigens against haemorrhagic septicaemia (HS) in cattle and buffalo.