The vlhA loci of Mycoplasma synoviae are confined to a restricted region of the genome

Differential systemic antibody responses to Mycoplasma synoviae MSPA variants after vaccination and/ or field challenge

Mycoplasma synoviae has a major immunodominant surface protein VlhA (variable lipoprotein and haemagglutinin) which undergoes post-translational cleavage to form two separate proteins, MSPA and MSPB. Both these proteins are targets of the host immune response undergo high frequency antigenic variation and are thought to contribute to the pathogenesis of M. synoviae infection. In order to assess variation of MSPA in response to vaccination and/ or field challenge host systemic humoral responses after inoculation with M. synoviae strain 86079/7NS (7NS), vaccine strain MS-H and/or field strain 94011/v-18d (v-18d) were evaluated against eight variable regions of MSPA derived from M. synoviae type strain WVU-1853. At 21 days post-infection, chickens inoculated with 7NS had significantly higher antibody levels against MSPA B3 and B4 compared to those inoculated with the vaccine strain MS-H. Also, at 21 days post-challenge, chickens which had been inoculated with the MS-H vaccine strain prior to challenge had significantly lower antibody responses against MSPA C2 compared to those which had not been. This suggests that variants of MSPA are differentially expressed after vaccination and/ or challenge and antibodies against these variants may be strain dependent. The results of this study have implications for the differentiation of serological responses after M. synoviae vaccination and field challenge.

A sweeping view of avian mycoplasmas biology drawn from comparative genomic analyses

BACKGROUND

Avian mycoplasmas are small bacteria associated with several pathogenic conditions in many wild and poultry bird species. Extensive genomic data are available for many avian mycoplasmas, yet no comparative studies focusing on this group of mycoplasmas have been undertaken so far.

RESULTS

Here, based on the comparison of forty avian mycoplasma genomes belonging to ten different species, we provide insightful information on the phylogeny, pan/core genome, energetic metabolism, and virulence of these avian pathogens. Analyses disclosed considerable inter- and intra-species genomic variabilities, with genome sizes that can vary by twice as much. Phylogenetic analysis based on concatenated orthologous genes revealed that avian mycoplasmas fell into either Hominis or Pneumoniae groups within the Mollicutes and could split into various clusters. No host co-evolution of avian mycoplasmas can be inferred from the proposed phylogenetic scheme. With 3,237 different gene clusters, the avian mycoplasma group under study proved diverse enough to have an open pan genome. However, a set of 150 gene clusters was found to be shared between all avian mycoplasmas, which is likely encoding essential functions. Comparison of energy metabolism pathways showed that avian mycoplasmas rely on various sources of energy. Superposition between phylogenetic and energy metabolism groups revealed that the glycolytic mycoplasmas belong to two distinct phylogenetic groups (Hominis and Pneumoniae), while all the arginine-utilizing mycoplasmas belong only to Hominis group. This can stand for different evolutionary strategies followed by avian mycoplasmas and further emphasizes the diversity within this group. Virulence determinants survey showed that the involved gene arsenals vary significantly within and between species, and could even be found in species often reported apathogenic. Immunoglobulin-blocking proteins were detected in almost all avian mycoplasmas. Although these systems are not exclusive to this group, they seem to present some particular features making them unique among mycoplasmas.

CONCLUSION

This comparative genomic study uncovered the significant variable nature of avian mycoplasmas, furthering our knowledge on their biological attributes and evoking new hallmarks.

Comparative Genomics of Mycoplasma synoviae and New Targets for Molecular Diagnostics

Mycoplasma synoviae is an important pathogen of poultry, causing significant economic losses in this industry. Analysis of the unique genes and shared genes among different M. synoviae strains and among related species is helpful for studying the molecular pathogenesis of M. synoviae and provides valuable molecular diagnostic targets to facilitate the identification of M. synoviae species. We selected a total of 46 strains, including six M. synoviae strains, from 25 major animal (including avian) Mycoplasma species/subspecies that had complete genome sequences and annotation information published in GenBank, and used them for comparative genomic analysis. After analysis, 16 common genes were found in the 46 strains. Thirteen single-copy core genes and the 16s rRNA genes were used for genetic evolutionary analysis. M. synoviae was found to have a distant evolutionary relationship not only with other arthritis-causing mycoplasmas, but also with another major avian pathogen, Mycoplasma gallisepticum, that shares the major virulence factor vlhA with M. synoviae. Subsequently, six unique coding genes were identified as shared among these M. synoviae strains that are absent in other species with published genome sequences. Two of the genes were found to be located in the genetically stable regions of the genomes of M. synoviae and were determined to be present in all M. synoviae isolated strains (n = 20) and M. synoviae-positive clinical samples (n = 48) preserved in our laboratory. These two genes were used as molecular diagnostic targets for which SYBR green quantitative PCR detection methods were designed. The two quantitative PCR methods exhibited good reproducibility and high specificity when tested on positive plasmid controls and genomic DNA extracted from different M. synoviae strains, other major avian pathogenic bacteria/mycoplasmas, and low pathogenic Mycoplasma species. The detection limit for the two genes was 10 copies or less per reaction. The clinical sensitivity and specificity of the quantitative PCR methods were both 100% based on testing chicken hock joint samples with positive or negative M. synoviae infection. This research provides a foundation for the study of species-specific differences and molecular diagnosis of M. synoviae.

Mini-review: Strategies for Variation and Evolution of Bacterial Antigens

Across the eubacteria, antigenic variation has emerged as a strategy to evade host immunity. However, phenotypic variation in some of these antigens also allows the bacteria to exploit variable host niches as well. The specific mechanisms are not shared-derived characters although there is considerable convergent evolution and numerous commonalities reflecting considerations of natural selection and biochemical restraints. Unlike in viruses, mechanisms of antigenic variation in most bacteria involve larger DNA movement such as gene conversion or DNA rearrangement, although some antigens vary due to point mutations or modified transcriptional regulation. The convergent evolution that promotes antigenic variation integrates various evolutionary forces: these include mutations underlying variant production; drift which could remove alleles especially early in infection or during life history phases in arthropod vectors (when the bacterial population size goes through a bottleneck); selection not only for any particular variant but also for the mechanism for the production of variants (i.e., selection for mutability); and overcoming negative selection against variant production. This review highlights the complexities of drivers of antigenic variation, in particular extending evaluation beyond the commonly cited theory of immune evasion. A deeper understanding of the diversity of purpose and mechanisms of antigenic variation in bacteria will contribute to greater insight into bacterial pathogenesis, ecology and coevolution with hosts.

Characterization of the antigenic and functional domains of a Mycoplasma synoviae variant vlhA gene

The Mycoplasma synoviae haemagglutinin gene, vlhA, encodes two major immunodominant and surface-exposed membrane proteins, MSPB and MSPA. Both products are antigenically variable but only MSPA mediates binding to erythrocytes. Previously we have shown that M. synoviae type strain WVU 1853 could express a variant vlhA gene, referred to as MS2/28.1, with a considerably shorter and divergent MSPA region. A finding that prompted detailed characterization of its antigenic and functional properties. Here we mutagenized each of the six opal codons of the variant MS2/28.1 vlhA member into tryptophan, thus allowing its expression in Escherichia coli as well as its cleavage products, MSPB and MSPA. In addition, we expressed 5 contiguous regions of MS2/28.1 extending from the last part of MSPB to the C-terminus of MSPA. Colony immunostaining with region-specific antisera mapped antigenic variation to the N-terminal half of MS2/28.1 MSPA. No haemagglutinating activity was observed for MSPB, but consistent haemadsorption inhibition was mapped to the region extending from amino acid 325 to 344. Inhibition of both haemagglutination and haemadsorption activities were obtained with sera directed against the C-terminal region of MSPA, with the highest titers (1/320 and 1/160, respectively) being recorded for its last 60 residues. Importantly, antibodies to this region also yielded the highest metabolic inhibition titer of 1/1280. Overall, aside from mapping the functional domains of a M. synoviae highly divergent haemagglutinin gene, this study shows that the C-terminal half of its MSPA region induced the highest titers of antibodies inhibiting haemagglutination, haemadsorption, and metabolism.

Variation of vlhA gene in Mycoplasma synoviae clones isolated from chickens

Mycoplasma synoviae synthesizes haemagglutinin VlhA, which cleaves into the N-terminal part, a lipoprotein MSPB, and a C-terminal part MSPA. Previous studies have shown that the 3'-end of the expressed vlhA gene can recombine with vlhA pseudogenes in a process called gene conversion, but there have been no data about diversification of the expressed vlhA gene in M. synoviae populations replicating in chickens. Following intratracheal inoculation with the M. synoviae strain ULB 02/T6, which showed only minor vlhA gene variation prior to inoculation, we investigated temporal changes in MSPB epitopes defined by monoclonal antibodies (mAbs) 3B4 and 50, as well as diversification of the vlhA gene sequence in M. synoviae populations recovered from chicken tracheas. In cultures isolated 8 and 18 days post inoculation (p.i.), most colonies showed variation of MSPB epitopes for mAbs 3B4 and 50. They also changed 3'-end vlhA gene sequences. Further diversity of the vlhA gene occurred in cultures isolated 8 weeks and 5 months p.i. The vlhA gene sequences from isolated cultures shared only 65 to 80% sequence identity with vlhA gene of the inoculated ULB 02/T6 culture. Notably, in most of those cultures their vlhA gene sequences contained stop codons potentially causing premature terminations of translation. Interestingly, in one culture isolated 8 weeks p.i. (clone T6-8W/IT2A) the 3'-vlhA gene sequence was identical in the last 1140 bases to that of the first vlhA pseudogene positioned the most far (upstream) of the expressed vlhA gene. This is the first demonstration of temporal diversity of the vlhA gene in M. synoviae populations isolated from chicken tracheas.

Analysis of the 16S to 23S rRNA intergenic spacer region of Mycoplasma synoviae field strains

Mycoplasma synoviae has been associated with economic loss in the chicken and turkey industries. The molecular characterization of M. synoviae at strain level allows the analysis of relationships between strains that may be valuable in epidemiological investigations. In the present study, the intergenic spacer region (ISR) between the 16S and 23S rRNA genes was examined to see whether useful information about strains could be derived. M. synoviae has two copies of this region, which may not be exactly the same (intercistronic heterogeneity). Sequencing of the ISRs of 21 M. synoviae isolates and the type strain revealed that 19 of them had such heterogeneity so DNA cloning was performed where necessary. All sequences were analysed and aligned; the percentage similarity of the DNA was calculated and a dendrogram was constructed. The length of the ISRs varied between 305 and 309 base pairs. Apart from having extra A/Ts in poly-A or poly-T regions and the presence of a few polymorphisms, the sequences of the M. synoviae strains were similar. Based on phylogenetic analysis, the strains were assigned to 10 groups-taking into account that within each group the DNA similarity was 100%, while the lowest similarity between groups was 95.8%. The results were compared with those obtained with the vlhA gene, resulting in very similar M. synoviae groups. Although the ISR could be a good target for strain typing, as has been shown by others for Mycoplasma gallisepticum, the method may be too cumbersome for routine use with M. synoviae because of complications with intercistronic heterogeneity. However, if the ISR sequence information was to be combined with other mutation detection techniques it could increase the discriminatory power.

The central role of lipoproteins in the pathogenesis of mycoplasmoses

Mycoplasmas are a diverse group of pathogens responsible for disease in a wide range of animal species. In recent years there have been considerable advances in knowledge of the proteins and structures involved in adherence in some mycoplasmas, but understanding of the biochemical functions and roles in virulence of another central feature of mycoplasmas, their lipoproteins, continues to develop. The aim of this review is to examine current knowledge of the roles of lipoproteins in the pathogenicity and the evolution of virulence in those mycoplasmas causing disease in domestic animals. Those lipoproteins that have been characterised have roles in adherence, in transport of nutrients into the mycoplasma cell, and in enzymatic interactions with the host. Furthermore they appear to play a prominent role in both inducing the host immune response to infection and in facilitating evasion of this response, particularly through the generation of dramatic levels of antigenic variation on the cell surface. Recent genomic comparisons of several pathogenic mycoplasmas have identified a further level of interaction between lipoproteins and pathogenicity. In several pathogens large scale horizontal gene transfer between distantly related mycoplasma species has resulted in the acquisition of a large number of genes, including those encoding lipoproteins thought to play a role in virulence, by one mycoplasma from another inhabiting the same host species. The interactions between these horizontally transferred genes, their new mycoplasma host and the animal that it infects may be an important contributing factor in the pathogenesis of some mycoplasmoses.

Revised Mycoplasma synoviae vlhA PCRs

Mycoplasma synoviae (MS) is an important pathogen of chickens and turkeys. In recent years sequence analysis of the partial MS variable lipoprotein and hemagglutinin A (vlhA) gene PCR product has been utilized routinely for MS strain genotyping. Several PCR assays have been proposed for the amplification of the conserved upstream region of the MS vlhA gene; however, in several clinical instances the published assays failed to generate vlhA PCR products from confirmed MS-positive cases. These occurrences hindered our capability to genotype those cases. In silico analysis of the published MS vlhA PCRs raised concerns, which were addressed by the design of revised MS vlhA PCRs. The published and revised assays were tested for their relative sensitivity and specificity with laboratory and clinical MS-positive samples. One of the revised MS vlhA PCRs (revised Hong) was demonstrated to be more sensitive and specific, and amplified all clinical samples analyzed in this study.

Phase and antigenic variation in mycoplasmas

With their reduced genome bound by a single membrane, bacteria of the Mycoplasma species represent some of the simplest autonomous life forms. Yet, these minute prokaryotes are able to establish persistent infection in a wide range of hosts, even in the presence of a specific immune response. Clues to their success in host adaptation and survival reside, in part, in a number of gene families that are affected by frequent, stochastic genotypic changes. These genetic events alter the expression, the size and the antigenic structure of abundant surface proteins, thereby creating highly versatile and dynamic surfaces within a clonal population. This phenomenon provides these wall-less pathogens with a means to escape the host immune response and to modulate surface accessibility by masking and unmasking stably expressed components that are essential in host interaction and survival.

Characterization of a variant vlhA gene of Mycoplasma synoviae, strain WVU 1853, with a highly divergent haemagglutinin region

Background

In Mycoplasma synoviae, type strain WVU 1853, a single member of the haemaglutinin vlhA gene family has been previously shown to be expressed. Variants of vlhA are expressed from the same unique vlhA promoter by recruiting pseudogene sequences via site-specific recombination events, thus generating antigenic variability. Using a bacterial stock of M. synoviae WVU 1853 that had been colony purified thrice and maintained in our laboratory at low passage level, we previously identified a vlhA gene-related partial coding sequence, referred to as MS2/28.1. The E. coli-expressed product of this partial coding sequence was found to be immunodominant, suggesting that it might be expressed.

Results

Reverse transcription-PCR amplification (RT-PCR), using a sense primer located at the 5'-end region of the expected vlhA transcript and a reverse primer located at the 3' end of MS2/28.1 coding sequence, yielded a consistent amplification product showing that MS2/28.1 was indeed transcribed. Nucleotide sequence analysis of the RT-PCR product identified an 1815-nucleotide full-length open reading frame (ORF), immediately preceded by a nucleotide sequence identical to that previously reported for expressed vlhA genes. PCR amplifications using genomic DNA isolated from single colonies further confirmed that the full-length ORF of MS2/28.1 was located downstream of the unique vlhA promoter sequence. The deduced 604-amino acid (aa) sequence showed a perfect sequence identity to the previously reported vlhA expressed genes along the first 224 residues, then highly diverged with only 37.6% aa identity. Despite the fact that this M. synoviae clone expressed a highly divergent and considerably shorter C-terminal haemagglutinin product, it was found to be expressed at the surface of the bacterium and was able to haemagglutinate chicken erythrocytes. Importantly, the E. coli-expressed C-terminal highly divergent 60 residues of MS2/28.1 proved haemagglutination competent.

Conclusions

In contrast to the previously characterized vlhA expressedvariants, MS2/28.1 displayed a highly divergent sequence, while still able to haemagglutinate erythrocytes. Overall, the data provide an indication as to which extent the M. synoviae vlhA gene could vary its antigenic repertoire.

Genetic variation in sialidase and linkage to N-acetylneuraminate catabolism in Mycoplasma synoviae

We explored the genetic basis for intraspecific variation in mycoplasmal sialidase activity that correlates with virulence, and its potentially advantageous linkage to nutrient catabolism. Polymorphism in N-acetylneuraminate scavenging and degradation genes (sialidase, N-acetylneuraminate lyase, N-acetylmannosamine kinase, N-acetylmannosamine-6-phosphate epimerase, N-acetylglucosamine-6-phosphate deacetylase, and glucosamine-6-phosphate deaminase) was evident among eight strains of the avian pathogen Mycoplasma synoviae. Most differences were single nucleotide polymorphisms, ranging from 0.34+/-0.04 substitutions per 100 bp for N-acetylmannosamine kinase to 0.65+/-0.03 for the single-copy sialidase gene nanI. Missense mutations were twice as common as silent mutations in nanI; 26% resulted in amino acids dissimilar to consensus; and there was a 12-base deletion near the nanI promoter in strain WVU1853(T), supporting a complex genetic basis for differences in sialidase activity. Two strains had identical frameshifts in the N-acetylneuraminate lyase gene nanA, resulting in nonsense mutations, and both had downstream deletions in nanA. Such genetic lesions uncouple extracellular liberation of sialic acid from generation of fructose-6-phosphate and pyruvate via intracellular N-acetylneuraminate degradation. Retention of nanI by such strains, but not others in the M. synoviae phylogenetic cluster, is evidence that sialidase has an important non-nutritional role in the ecology of M. synoviae and certain other mycoplasmas.

Organization of theMycoplasma synoviaeWVU 1853TvlhAgene locus

Mycoplasma synoviae cause respiratory disease and synovitis in poultry. It has two major membrane antigens of approximately 45 to 50 kDa, MSPA and MSPB. Both MSPA and MSPB ar encoded by a single gene, vlhA (variable lipoprotein and haemagglutinin), possibly with a post-translational cleavage generating the two proteins. The vlhA gene belongs to a large multigene family, but only one vlhA gene is expressed in any single cell; the other vlhA genes/fragments are transcriptionally silent. In order to characterize the vlhA gene locus, DNA fragments were cloned from three different M. synoviae WVU 1853(T) genomic DNA libraries and their nucleotide sequences were fully or partially determined. Analysis of the resultant nucleotide sequences identified a transcriptional termination signal for the expressed vlhA gene, determined the genes located downstream of the expressed vlhA gene and revealed that vlhA pseudogenes were arranged as tandem repeats upstream of the expressed vlhA gene. In order to determine whether vlhA was expressed as a monocistronic or polycistronic message, RNA from two M. synoviae clones expressing truncated or full-length versions of the vlhA gene product were purified and examined by northern blotting. Messages corresponding to the full length of the vlhA gene (approximately 2.4 kb) were detected in both clones, suggesting that truncation of the vlhA gene product was probably post-transcriptional. These studies have revealed the organization of the M. synoviae vlhA gene locus and established that the vlhA gene transcript is monocistronic.

Swine and poultry pathogens: the complete genome sequences of two strains of Mycoplasma hyopneumoniae and a strain of Mycoplasma synoviae

This work reports the results of analyses of three complete mycoplasma genomes, a pathogenic (7448) and a nonpathogenic (J) strain of the swine pathogen Mycoplasma hyopneumoniae and a strain of the avian pathogen Mycoplasma synoviae; the genome sizes of the three strains were 920,079 bp, 897,405 bp, and 799,476 bp, respectively. These genomes were compared with other sequenced mycoplasma genomes reported in the literature to examine several aspects of mycoplasma evolution. Strain-specific regions, including integrative and conjugal elements, and genome rearrangements and alterations in adhesin sequences were observed in the M. hyopneumoniae strains, and all of these were potentially related to pathogenicity. Genomic comparisons revealed that reduction in genome size implied loss of redundant metabolic pathways, with maintenance of alternative routes in different species. Horizontal gene transfer was consistently observed between M. synoviae and Mycoplasma gallisepticum. Our analyses indicated a likely transfer event of hemagglutinin-coding DNA sequences from M. gallisepticum to M. synoviae.