Characterization of a multigene family undergoing high-frequency DNA rearrangements and coding for abundant variable surface proteins in Mycoplasma agalactiae

Mycoplasma agalactiae Vaccines: Current Status, Hurdles, and Opportunities Due to Advances in Pathogenicity Studies

Contagious agalactia (CA) is a serious multietiological disease whose classic etiological agent is Mycoplasma agalactiae and which causes high morbidity and mortality rates in infected herds. CA is classified as a notifiable disease by the World Organization for Animal Health due to its significant worldwide economic impact on livestock, primarily involving goat and sheep farms. The emergence of atypical symptoms and strains of M. agalactiae in wildlife ungulates reestablishes its highly plastic genome and is also of great epidemiological significance. Antimicrobial therapy is the main form of control, although several factors, such as intrinsic antibiotic resistance and the selection of resistant strains, must be considered. Available vaccines are few and mostly inefficient. The virulence and pathogenicity mechanisms of M. agalactiae mainly rely on surface molecules that have direct contact with the host. Because of this, they are essential for the development of vaccines. This review highlights the currently available vaccines and their limitations and the development of new vaccine possibilities, especially considering the challenge of antigenic variation and dynamic genome in this microorganism.

Mycoplasma ovipneumoniae: A Most Variable Pathogen

Mycoplasma ovipneumoniae, a well-established respiratory pathogen of sheep and goats, has gained increased importance recently because of its detection in wild ruminants including members of the Cervidae family. Despite its frequent isolation from apparently healthy animals, it is responsible for outbreaks of severe respiratory disease which are often linked to infections with multiple heterologous strains. Furthermore, M. ovipneumoniae is characterized by an unusually wide host range, a high degree of phenotypic, biochemical, and genomic heterogeneity, and variable and limited growth in mycoplasma media. A number of mechanisms have been proposed for its pathogenicity, including the production of hydrogen peroxide, reactive oxygen species production, and toxins. It shows wide metabolic activity in vitro, being able to utilize substrates such as glucose, pyruvate, and isopropanol; these patterns can be used to differentiate strains. Treatment of infections in the field is complicated by large variations in the susceptibility of strains to antimicrobials, with many showing high minimum inhibitory concentrations. The lack of commercially available vaccines is probably due to the high cost of developing vaccines for diseases in small ruminants not presently seen as high priority. Multiple strains found in affected sheep and goats may also hamper the development of effective vaccines. This review summarizes the current knowledge and identifies gaps in research on M. ovipneumoniae, including its epidemiology in sheep and goats, pathology and clinical presentation, infection in wild ruminants, virulence factors, metabolism, comparative genomics, genotypic variability, phenotypic variability, evolutionary mechanisms, isolation and culture, detection and identification, antimicrobial susceptibility, variations in antimicrobial susceptibility profiles, vaccines, and control.

Eating the Enemy: Mycoplasma Strategies to Evade Neutrophil Extracellular Traps (NETs) Promoting Bacterial Nucleotides Uptake and Inflammatory Damage

Neutrophils are effector cells involved in the innate immune response against infection; they kill infectious agents in the intracellular compartment (phagocytosis) or in the extracellular milieu (degranulation). Moreover, neutrophils release neutrophil extracellular traps (NETs), complex structures composed of a scaffold of decondensed DNA associated with histones and antimicrobial compounds; NETs entrap infectious agents, preventing their spread and promoting their clearance. NET formation is triggered by microbial compounds, but many microorganisms have evolved several strategies for NET evasion. In addition, the dysregulated production of NETs is associated with chronic inflammatory diseases. Mycoplasmas are reduced genome bacteria, able to induce chronic infections with recurrent inflammatory symptoms. Mycoplasmas' parasitic lifestyle relies on metabolite uptake from the host. Mycoplasmas induce NET release, but their surface or secreted nucleases digest the NETs' DNA scaffold, allowing them to escape from entrapment and providing essential nucleotide precursors, thus promoting the infection. The presence of Mycoplasma species has been associated with chronic inflammatory disorders, such as systemic lupus erythematosus, rheumatoid arthritis, inflammatory bowel disease, Crohn's disease, and cancer. The persistence of mycoplasma infection and prolonged NET release may contribute to the onset of chronic inflammatory diseases and needs further investigation and insights.

Serum Resistance of Mycoplasma agalactiae Strains and Mutants Bearing Different Lipoprotein Profiles

In order to spread systemically, resistance against complement and other factors present in serum is an important trait in pathogenic bacteria. The variable proteins of Mycoplasma agalactiae (Vpmas) have been shown to affect differential adhesion, invasion and immune evasion, and undergo high-frequency phase-variation in expression. However, nothing is known about their involvement in M. agalactiae’s serum susceptibility. To evaluate this, the PG2 strain, the GM139 strain and the six Vpma phase-locked mutants (PLMs, PLMU to PLMZ) were tested for their ability to survive in the presence of non-sensitized and sensitized sheep serum, as well as guinea pig complement. Additionally, the reactivity of the sensitized sheep serum was analysed on the strains via western blotting. Overall data demonstrate PG2 strain to be more susceptible to sheep serum compared to the GM139 strain bearing a different Vpma profile. Significant differences were also observed between the different PLMs, with PLMU and PLMX showing the highest serum susceptibility in serum, while the other PLMs expressing longer Vpma proteins were more resistant. The results are in good correlation with previous studies where shorter lipoprotein variants contributed to a higher susceptibility to complement. Since none of the tested strains and PLMs were susceptible to non-sensitized sheep serum, antibodies seem to play an important role in serum killing.

Predominant Single Stable VpmaV Expression in Strain GM139 and Major Differences with Mycoplasma agalactiae Type Strain PG2

Simple Summary

MALDI-ToF MS analysis demonstrates major differences between Mycoplasma agalactiae type strain PG2 and strain GM139. Evaluation of the Vpma (variable proteins of M. agalactiae) phenotypic profiles of these two strains reveals that unlike PG2, GM139 expresses a single Vpma, namely VpmaV, and without any visible phase variation. Although the presence of only one Vpma seems to be sufficient for the pathogenesis of the GM139 strain, the absence of phase variation can compromise immune system evasion and spread to other anatomical sites in the host.

Abstract

Although mycoplasmas have a reduced genome and no cell wall, they have important mechanisms for the antigenic variation in surface lipoproteins that modulate their interactions with the host. Mycoplasma agalactiae, the main etiological agent of contagious agalactia, has a multigene family involved in the high-frequency phase variation in surface lipoproteins called variable proteins of M. agalactiae (Vpmas). The Vpma lipoproteins are involved in the immune evasion, colonization, dissemination, and persistence of M. agalactiae in the host. In this paper, we evaluate the Vpma phenotypic profiles of two different strains of M. agalactiae, namely, GM139 and the type strain PG2, to assess possible correlations between Vpma phase variability and the geographic localization, animal origin, and pathogenicity of these two strains. Using monospecific Vpma antibodies against individual Vpmas in immunoblots, we demonstrate that, unlike PG2, which expresses six Vpma proteins with high-frequency phase variation, colonies of GM139 predominantly express VpmaV and do not exhibit any sectoring phenotype for any Vpma. Since VpmaV is one of the most important Vpmas for cell adhesion and invasion, its predominant sole expression in GM139 without high-frequency variation may be the basis of the differential pathogenicity of GM139 and PG2. Additionally, MALDI-ToF MS analysis also demonstrates significant differences between these two strains and their relatedness with other M. agalactiae strains.

Genomic Islands in Mycoplasmas

Bacteria of the Mycoplasma genus are characterized by the lack of a cell-wall, the use of UGA as tryptophan codon instead of a universal stop, and their simplified metabolic pathways. Most of these features are due to the small-size and limited-content of their genomes (580–1840 Kbp; 482–2050 CDS). Yet, the Mycoplasma genus encompasses over 200 species living in close contact with a wide range of animal hosts and man. These include pathogens, pathobionts, or commensals that have retained the full capacity to synthesize DNA, RNA, and all proteins required to sustain a parasitic life-style, with most being able to grow under laboratory conditions without host cells. Over the last 10 years, comparative genome analyses of multiple species and strains unveiled some of the dynamics of mycoplasma genomes. This review summarizes our current knowledge of genomic islands (GIs) found in mycoplasmas, with a focus on pathogenicity islands, integrative and conjugative elements (ICEs), and prophages. Here, we discuss how GIs contribute to the dynamics of mycoplasma genomes and how they participate in the evolution of these minimal organisms.

Mycoplasmas under experimental antimicrobial selection: The unpredicted contribution of horizontal chromosomal transfer

Horizontal Gene Transfer was long thought to be marginal in Mycoplasma a large group of wall-less bacteria often portrayed as minimal cells because of their reduced genomes (ca. 0.5 to 2.0 Mb) and their limited metabolic pathways. This view was recently challenged by the discovery of conjugative exchanges of large chromosomal fragments that equally affected all parts of the chromosome via an unconventional mechanism, so that the whole mycoplasma genome is potentially mobile. By combining next generation sequencing to classical mating and evolutionary experiments, the current study further explored the contribution and impact of this phenomenon on mycoplasma evolution and adaptation using the fluoroquinolone enrofloxacin (Enro), for selective pressure and the ruminant pathogen Mycoplasma agalactiae, as a model organism. For this purpose, we generated isogenic lineages that displayed different combination of spontaneous mutations in Enro target genes (gyrA, gyrB, parC and parE) in association to gradual level of resistance to Enro. We then tested whether these mutations can be acquired by a susceptible population via conjugative chromosomal transfer knowing that, in our model organism, the 4 target genes are scattered in three distinct and distant loci. Our data show that under antibiotic selective pressure, the time scale of the mutational pathway leading to high-level of Enro resistance can be readily compressed into a single conjugative step, in which several EnroR alleles were transferred from resistant to susceptible mycoplasma cells. In addition to acting as an accelerator for antimicrobial dissemination, mycoplasma chromosomal transfer reshuffled genomes beyond expectations and created a mosaic of resistant sub-populations with unpredicted and unrelated features. Our findings provide insights into the process that may drive evolution and adaptability of several pathogenic Mycoplasma spp. via an unconventional conjugative mechanism.

Variable Membrane Protein A of Flavescence Dorée Phytoplasma Binds the Midgut Perimicrovillar Membrane of Euscelidius variegatus and Promotes Adhesion to Its Epithelial Cells

Phytoplasmas are uncultivated plant pathogens and cell wall-less bacteria and are transmitted from plant to plant by hemipteran insects. The phytoplasma's circulative propagative cycle in insects requires the crossing of the midgut and salivary glands, and primary adhesion to cells is an initial step toward the invasion process. The flavescence dorée (FD) phytoplasma possesses a set of variable membrane proteins (Vmps) exposed on its surface, and this pathogen is suspected to interact with insect cells. The results showed that VmpA is expressed by the flavescence dorée phytoplasma present in the midgut and salivary glands. Phytoplasmas cannot be cultivated at present, and no mutant can be produced to investigate the putative role of Vmps in the adhesion of phytoplasma to insect cells. To overcome this difficulty, we engineered the Spiroplasma citri mutant G/6, which lacks the ScARP adhesins, for VmpA expression and used VmpA-coated fluorescent beads to determine if VmpA acts as an adhesin in ex vivo adhesion assays and in vivo ingestion assays. VmpA specifically interacted with Euscelidius variegatus insect cells in culture and promoted the retention of VmpA-coated beads to the midgut of E. variegatus In this latest case, VmpA-coated fluorescent beads were localized and embedded in the perimicrovillar membrane of the insect midgut. Thus, VmpA functions as an adhesin that could be essential in the colonization of the insect by the FD phytoplasmas.IMPORTANCE Phytoplasmas infect a wide variety of plants, ranging from wild plants to cultivated species, and are transmitted by different leafhoppers, planthoppers, and psyllids. The specificity of the phytoplasma-insect vector interaction has a major impact on the phytoplasma plant host range. As entry into insect cells is an obligate process for phytoplasma transmission, the bacterial adhesion to insect cells is a key step. Thus, studying surface-exposed proteins of phytoplasma will help to identify the adhesins implicated in the specific recognition of insect vectors. In this study, it is shown that the membrane protein VmpA of the flavescence dorée (FD) phytoplasma acts as an adhesin that is able to interact with cells of Euscelidius variegatus, the experimental vector of the FD phytoplasma.

Novel role of Vpmas as major adhesins of Mycoplasma agalactiae mediating differential cell adhesion and invasion of Vpma expression variants

Mycoplasma agalactiae exhibits antigenic variation by switching the expression of multiple surface lipoproteins called Vpmas. Although implicated to have a significant influence on the pathogenicity, their exact role in pathogen-host interactions has not been investigated so far. Initial attachment to host cells is regarded as one of the most important steps for colonization but this pathogen lacks the typical mycoplasma attachment organelle. The aim of this study was to determine the role of Vpmas in adhesion of M. agalactiae to host cells. 'Phase-Locked' Mutants (PLMs) steadily expressing single well-characterized Vpma lipoproteins served as ideal tools to evaluate the role of each of the six Vpmas in cytadhesion, which was otherwise not possible due to the high-frequency switching of Vpmas in the wildtype strain PG2. Using in vitro adhesion assays with HeLa and sheep mammary epithelial (MECs) and stromal (MSCs) cells, we could demonstrate differences in the adhesion capabilities of each of the six PLMs compared to the wildtype strain. The PLMV mutant expressing VpmaV exhibited the highest adhesion rate, whereas PLMU, which expresses VpmaU showed the lowest adhesion values explaining the reduced in vivo fitness of PLMU in sheep during experimental intramammary and conjunctival infections. Furthermore, adhesion inhibition assays using Vpma-specific polyclonal antisera were performed to confirm the role of Vpmas in M. agalactiae cytadhesion. This led to a significant decrease (p<0.05) in the adhesion percentage of each PLM. Immunofluorescence staining of TX-114 phase proteins extracted from each PLM showed binding of the respective Vpma to HeLa cells and MECs proving the direct role of Vpmas in cytadhesion. Furthermore, as adhesion is a prerequisite for cell invasion, the ability of the six PLMs to invade HeLa cells was also evaluated using the gentamicin protection assay. The results showed a strong correlation between the adhesion rates and invasion frequencies of the individual PLMs. This is the first report that describes a novel function of Vpma proteins in cell adhesion and invasion. Besides the variability of these proteins causing surface antigenic variation, the newly identified phenotypes are likely to play critical roles in the pathogenicity potential of this ruminant pathogen.

Proteolytic Post-Translational Processing of Adhesins in a Pathogenic Bacterium

Mollicutes, including mycoplasmas and spiroplasmas, have been considered as good representatives of the « minimal cell » concept: these wall-less bacteria are small in size and possess a minimal genome and restricted metabolic capacities. However, the recent discovery of the presence of post-translational modifications unknown so far, such as the targeted processing of membrane proteins of mycoplasma pathogens for human and swine, revealed a part of the hidden complexity of these microorganisms. In this study, we show that in the phytopathogen, insect-vectored Spiroplasma citri GII-3 adhesion-related protein (ScARP) adhesins are post-translationally processed through an ATP-dependent targeted cleavage. The cleavage efficiency could be enhanced in vitro when decreasing the extracellular pH or upon the addition of polyclonal antibodies directed against ScARP repeated units, suggesting that modification of the surface charge and/or ScARP conformational changes could initiate the cleavage. The two major sites for primary cleavage are localized within predicted disordered regions and do not fit any previously reported cleavage motif; in addition, the inhibition profile and the metal ion requirements indicate that this post-translational modification involves at least one non-conventional protease. Such a proteolytic process may play a role in S. citri colonization of cells of the host insect. Furthermore, our work indicates that post-translational cleavage of adhesins represents a common feature to mollicutes colonizing distinct hosts and that processing of surface antigens could represent a way to make the most out of a minimal genome.

Comparison of phenotypic and genotypic profiles among caprine and ovine Mycoplasma ovipneumoniae strains

Mycoplasma ovipneumoniae (Movp) is considered to be one of the most important mycoplasmas causing respiratory disease in small ruminants. Most epidemiologic and characterisation studies have been conducted on strains collected from sheep. Information on the presence and characteristics of Movp in healthy and pneumonic goats is limited. Phenotypic or genotypic differences between sheep and goat isolates have never been studied. The objective of our study was to characterise and compare the similarities and differences between caprine and ovine Movp strains isolated from affected and asymptomatic animals in order to elucidate phenotypic and genotypic variability. Four different techniques were used on a set of 23 Movp isolates. These included SDS-PAGE, Western blotting, random amplified polymorphic DNA and the heat shock protein 70 gene sequence-based method. A high degree of phenotypic and genotypic heterogeneity among Movp strains was demonstrated in this study. Our results demonstrated differences between goat and sheep strains, revealing not only a link between strains and host ruminant species, but by geographical origin as well. However, the finding of immunodominant antigens of molecular masses 36, 38, 40 and 70 kDa (±3 kDa) in Movp isolates from sheep and goats foretells their potential use in the development of serological diagnostic tests and vaccines.

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.

Disruption of the pdhB pyruvate dehydrogenase [corrected] gene affects colony morphology, in vitro growth and cell invasiveness of Mycoplasma agalactiae

The utilization of available substrates, the metabolic potential and the growth rates of bacteria can play significant roles in their pathogenicity. This study concentrates on Mycoplasma agalactiae, which causes significant economic losses through its contribution to contagious agalactia in small ruminants by as yet unknown mechanisms. This lack of knowledge is primarily due to its fastidious growth requirements and the scarcity of genetic tools available for its manipulation and analysis. Transposon mutagenesis of M. agalactiae type strain PG2 resulted in several disruptions throughout the genome. A mutant defective in growth in vitro was found to have a transposon insertion in the pdhB gene, which encodes a component of the pyruvate dehydrogenase complex. This growth difference was quite significant during the actively dividing logarithmic phase but a gradual recovery was observed as the cells approached stationary phase. The mutant also exhibited a different and smaller colony morphology compared to the wild type strain PG2. For complementation, pdhAB was cloned downstream of a strong vpma promoter and upstream of a lacZ reporter gene in a newly constructed complementation vector. When transformed with this vector the pdhB mutant recovered its normal growth and colony morphology. Interestingly, the pdhB mutant also had significantly reduced invasiveness in HeLa cells, as revealed by double immunofluorescence staining. This deficiency was recovered in the complemented strain, which had invasiveness comparable to that of PG2. Taken together, these data indicate that pyruvate dehydrogenase might be an important player in infection with and colonization by M. agalactiae.

Development of a specific immunomagnetic capture-PCR for rapid detection of viable Mycoplasma agalactiae in sheep milk samples

AIMS

To develop an immunomagnetic capture (IMC) to detect viable Mycoplasma agalactiae in routine ovine milk samples.

METHODS AND RESULTS

Polyclonal antibodies against two M. agalactiae membrane surface proteins (P80 and P55) were covalently conjugated to magnetic beads (MBs) to form MB-Ab80 and MB-Ab55. Mycoplasma agalactiae cells were captured by a specific antigen-antibody reaction and magnetic separation. Immunomagnetic capture (IMC) was used to isolate and concentrate M. agalactiae in serial decimal dilutions and in artificially contaminated milk to facilitate subsequent detection by PCR. A 375-bp fragment of M. agalactiae was amplified using a pair of M. agalactiae-specific primers in PCR. The limit of detection of IMC-PCR method ranged from 10 to 10(2)  CCU ml(-1) when mycoplasmas were resuspended in PBS and from 10(2) to 10(3)  CCU ml(-1) when mycoplasmas were resuspended in uncontaminated ovine milk. This study also describes the application of IMC-PCR method to test for M. agalactiae in 516 milk samples collected from sheep with suspected contagious agalactia. Its performance was evaluated relative to culture.

CONCLUSIONS

This report has demonstrated for the first time, the effective use of rapid and reliable IMC combined with PCR assay for the detection of viable M. agalactiae.

SIGNIFICANCE AND IMPACT OF THE STUDY

The method IMC-PCR provides an alternative to conventional microbiological detection, method and it could be applied to quick detection of M. agalactiae in routine sheep milk samples.

Molecular characterisation of the Mycoplasma cynos haemagglutinin HapA

Mycoplasma (M.) cynos is a proven pathogen of dogs causing respiratory infections including pneumonia. We examined 19 M. cynos strains isolated from different organs of dogs in Austria, Denmark and Israel. All strains agglutinated mammalian and chicken erythrocytes. Using erythrocytes of chickens or dogs as specific ligands we isolated an approximately 65 kDa protein from cell-free supernatants of 3 M. cynos strains, which showed an apparent capacity for haemagglutination. The N-terminal sequence of a 25 kDa fragment of this protein was identified as NNEMTPKVTVEAKSMELLLSVEK. The identical amino acid sequence is encoded by the gene MCYN_0308 in the genome of M. cynos C142. This gene belongs to a family of some 20 genes which encode putative lipoproteins with proline-rich regions (PRR) in the first third of their molecules. We termed the 65 kDa haemagglutinin HapA and sequenced hapA gene homologues of 16 M. cynos strains. Analyses of hapA gene homologues revealed similar but not identical sequences, some having insertions and/or deletions in the PRR. We produced a recombinant HapA protein (rHapA) and also mouse monoclonal antibodies (mAbs) recognizing HapA. However, enzyme immunoassays using native M. cynos colonies and mAbs 5G2 or 3B7 showed variable expression of HapA in all M. cynos strains. This was further confirmed by Western blot analyses which showed different HapA quantities and also size-variation of HapA among strains. Analyses of cDNA of the expressed hapA genes showed that besides the hapA gene cultures of M. cynos (strains 105, 2002, 2297) can also express other forms of hap genes. In addition, in cloned cultures of strain 2297 altered HapA epitopes for mAbs 5G2 and 3B7 with distinct hapA gene mutations that resulted in altered HapA amino acid sequence were found. Most of the dogs examined had serum antibodies to rHapA. In conclusion, we characterized the M. cynos haemagglutinin HapA protein and encoding gene hapA, a factor involved in cytadherence to host cells and therefore important for M. cynos infection, and showed that expression of HapA is varied in M. cynos by two distinct mechanisms; differential gene expression and nucleic acid substitution within hapA homologues.

Contagious agalactia due to Mycoplasma spp. in small dairy ruminants: epidemiology and prospects for diagnosis and control

Contagious agalactia (CA) is a serious disease of small dairy ruminants that has a substantial economic impact on the goat and sheep milk industries. The main aetiological agent of the disease is Mycoplasma agalactiae, although other species, such as Mycoplasma mycoides subsp. capri, Mycoplasma capricolum subsp. capricolum and Mycoplasma putrefaciens, are pathogenic in goats. There are two clinical-epidemiological states of CA in sheep and goats; herds and flocks may exhibit outbreaks of CA or may be chronically infected, the latter with a high incidence of subclinical mastitis and only occasional clinical cases. The complex epidemiology of CA is related to the genetic characteristics and mechanisms of molecular variation of the Mycoplasma spp. involved, along with presence of CA-mycoplasmas in wild ruminant species. In goats, the situation is particularly complex and asymptomatic carriers have been detected in chronically infected herds. The coexistence of other non-pathogenic mycoplasmas in the herd further complicates the diagnosis of CA and the design of efficient strategies to control the disease. Routes of infection, such as the venereal route, may be involved in the establishment of chronic infection in herds. Current challenges include the need for improved diagnostic methods for detection of chronic and subclinical infections and for the design of more efficient vaccines.

Role of the GapA and CrmA cytadhesins of Mycoplasma gallisepticum in promoting virulence and host colonization

Mycoplasma gallisepticum is an important avian pathogen that commonly induces chronic respiratory disease in chicken. To better understand the mycoplasma factors involved in host colonization, chickens were infected via aerosol with two hemadsorption-negative (HA(-)) mutants, mHAD3 and RCL2, that were derived from a low passage of the pathogenic strain R (Rlow) and are both deficient in the two major cytadhesins GapA and CrmA. After 9 days of infection, chickens were monitored for air sac lesions and for the presence of mycoplasmas in various organs. The data showed that mHAD3, in which the crmA gene has been disrupted, did not promote efficient colonization or significant air sac lesions. In contrast, the spontaneous HA(-) RCL2 mutant, which contains a point mutation in the gapA structural gene, successfully colonized the respiratory tract and displayed an attenuated virulence compared to that of Rlow. It has previously been shown in vitro that the point mutation of RCL2 spontaneously reverts with a high frequency, resulting in on-and-off switching of the HA phenotype. Detailed analyses further revealed that such an event is not responsible for the observed in vivo outcome, since 98.4% of the mycoplasma populations recovered from RCL2-infected chickens still display the mutation and the associated phenotype. Unlike Rlow, however, RCL2 was unable to colonize inner organs. These findings demonstrate the major role played by the GapA and CrmA proteins in M. gallisepticum host colonization and virulence.

Mycoplasma agalactiae MAG_5040 is a Mg2+-dependent, sugar-nonspecific SNase recognised by the host humoral response during natural infection

In this study the enzymatic activity of Mycoplasma agalactiae MAG_5040, a magnesium-dependent nuclease homologue to the staphylococcal SNase was characterized and its antigenicity during natural infections was established. A UGA corrected version of MAG_5040, lacking the region encoding the signal peptide, was expressed in Escherichia coli as a GST fusion protein. Recombinant GST-MAG_5040 exhibits nuclease activity similar to typical sugar-nonspecific endo- and exonucleases, with DNA as the preferred substrate and optimal activity in the presence of 20 mM MgCl2 at temperatures ranging from 37 to 45°C. According to in silico analyses, the position of the gene encoding MAG_5040 is consistently located upstream an ABC transporter, in most sequenced mycoplasmas belonging to the Mycoplasma hominis group. In M. agalactiae, MAG_5040 is transcribed in a polycistronic RNA together with the ABC transporter components and with MAG_5030, which is predicted to be a sugar solute binding protein by 3D modeling and homology search. In a natural model of sheep and goats infection, anti-MAG_5040 antibodies were detected up to 9 months post infection. Taking into account its enzymatic activity, MAG_5040 could play a key role in Mycoplasma agalactiae survival into the host, contributing to host pathogenicity. The identification of MAG_5040 opens new perspectives for the development of suitable tools for the control of contagious agalactia in small ruminants.

Role of Vpma phase variation in Mycoplasma agalactiae pathogenesis

Compared with other bacterial pathogens, the molecular mechanisms of mycoplasma pathogenicity are largely unknown. Several studies in the past have shown that pathogenic mycoplasmas are equipped with sophisticated genetic systems that allow them to undergo high-frequency surface antigenic variations. Although never clearly proven, these variable mycoplasma surface components are often implicated in host immune evasion and adaptation. Vpma surface lipoproteins of the ruminant pathogen Mycoplasma agalactiae are encoded on a genomic pathogenicity island-like locus and are considered as one of the well-characterized model systems of mycoplasma surface antigenic variation. The present study assesses the role of these phase-variable Vpmas in the molecular pathogenesis of M. agalactiae by testing the wild-type strain PG2 in comparison with the xer1-disrupted Vpma 'phase-locked' mutants in sheep infection models. The data clearly illustrate that although Xer1 recombinase is not a virulence factor of M. agalactiae and Vpma phase variation is not necessary for establishing an infection, it might critically influence the survival and persistence of the pathogen under natural field conditions, mainly due to a better capacity for dissemination and evoking systemic responses. This is the first study where mycoplasma 'phase-locked' mutants are tested in vivo to elucidate the role of phase variation during infection.

Comparative assessment of two commonly used commercial ELISA tests for the serological diagnosis of contagious agalactia of small ruminants caused by Mycoplasma agalactiae

Background

Contagious agalactia (CA) of sheep and goats caused by Mycoplasma agalactiae is a widely occurring economically important disease that is difficult to control. The ELISA is commonly used for the serological detection of CA but it has some limitations and the performance of the available tests have not been properly evaluated.

Two commercial ELISA kits are widely used, one involving a fusion protein as target antigen and the other a total antigen. The objectives were to compare these tests by evaluating:

i. Their diagnostic sensitivity and specificity, the relevance of the recommended cut-off points, the correlation between the two tests, and, the correlation between serology data and the milk shedding of M. agalatiae;

ii. The influence of extrinsic factors such as the targeted animal species, geographical origin of the samples, intra-specific variability of M. agalactiae and concurrent mycoplasma infections.

A sample of 5900 animals from 211 farms with continuous CA monitoring for 20 years and no prior vaccination history was used. The infection status was known from prior bacteriological, epidemiological and serological monitoring with a complementary immunoblotting test.

Results

The average diagnostic sensitivity was 56% [51.8–59.8] for the fusion protein ELISA and 84% [81.3–87.2] for the total antigen ELISA, with noteworthy flock-related variations. The average diagnostic specificity for the fusion protein ELISA was 100% [99.9–100], and for the total antigen ELISA differed significantly between goats and sheep: 99.3% [97.4–99.9] and 95.7% [93.8–97.2] respectively.

Experimental inoculations with different M. agalactiae strains revealed that the ELISA kits poorly detected the antibody response to certain strains. Furthermore, test performances varied according to the host species or geographical origin of the samples.

Finally, the correlation between milk shedding of M. agalactiae and the presence of detectable antibodies in the blood was poor.

Conclusions

These serological tests are not interchangeable. The choice of a test will depend on the objectives (early detection of infection or disease control program), on the prevalence of infection and the control protocol used. Given the variety of factors that may influence performance, a preliminary assessment of the test in a given situation is recommended prior to widespread use.

Proteomics and pathway analyses of the milk fat globule in sheep naturally infected by Mycoplasma agalactiae provide indications of the in vivo response of the mammary epithelium to bacterial infection

Milk fat globules (MFGs) are vesicles released in milk as fat droplets surrounded by the endoplasmic reticulum and apical cell membranes. During formation and apocrine secretion by lactocytes, various amounts of cytoplasmic crescents remain trapped within the released vesicle, making MFGs a natural sampling mechanism of the lactating cell contents. With the aim of investigating the events occurring in the mammary epithelium during bacterial infection, the MFG proteome was characterized by two-dimensional difference gel electrophoresis (2-D DIGE), SDS-PAGE followed by shotgun liquid chromatography-tandem mass spectrometry (GeLC-MS/MS), label-free quantification by the normalized spectral abundance factor (NSAF) approach, Western blotting, and pathway analysis, using sheep naturally infected by Mycoplasma agalactiae. A number of protein classes were found to increase in MFGs upon infection, including proteins involved in inflammation and host defense, cortical cytoskeleton proteins, heat shock proteins, and proteins related to oxidative stress. Conversely, a strikingly lower abundance was observed for proteins devoted to MFG metabolism and secretion. To our knowledge, this is the first report describing proteomic changes occurring in MFGs during sheep infectious mastitis. The results presented here offer new insights into the in vivo response of mammary epithelial cells to bacterial infection and open the way to the discovery of protein biomarkers for monitoring clinical and subclinical mastitis.

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.

Ureaplasma antigenic variation beyond MBA phase variation: DNA inversions generating chimeric structures and switching in expression of the MBA N-terminal paralogue UU172

Phase variation of the major ureaplasma surface membrane protein, the multiple-banded antigen (MBA), with its counterpart, the UU376 protein, was recently discussed as a result of DNA inversion occurring at specific inverted repeats. Two similar inverted repeats to the ones within the mba locus were found in the genome of Ureaplasma parvum serovar 3; one within the MBA N-terminal paralogue UU172 and another in the adjacent intergenic spacer region. In this report, we demonstrate on both genomic and protein level that DNA inversion at these inverted repeats leads to alternating expression between UU172 and the neighbouring conserved hypothetical ORF UU171. Sequence analysis of this phase-variable ‘UU172 element’ from both U. parvum and U. urealyticum strains revealed that it is highly conserved among both species and that it also includes the orthologue of UU144. A third inverted repeat region in UU144 is proposed to serve as an additional potential inversion site from which chimeric genes can evolve. Our results indicate that site-specific recombination events in the genome of U. parvum serovar 3 are dynamic and frequent, leading to a broad spectrum of antigenic variation by which the organism may evade host immune responses.

Complete genome sequence of Mycoplasma bovis type strain PG45 (ATCC 25523)

This complete and fully assembled genome sequence of Mycoplasma bovis type strain PG45 is the first available for this species and offers a framework for comparison with additional pathogenic isolates. The single circular chromosome of 1,003,404 bp reveals multiple gene sets and mechanisms involved in variable expression of surface antigens and the incursion of numerous and assorted mobile elements, despite its reduced size.

The liposoluble proteome of Mycoplasma agalactiae: an insight into the minimal protein complement of a bacterial membrane

BACKGROUND

Mycoplasmas are the simplest bacteria capable of autonomous replication. Their evolution proceeded from gram-positive bacteria, with the loss of many biosynthetic pathways and of the cell wall. In this work, the liposoluble protein complement of Mycoplasma agalactiae, a minimal bacterial pathogen causing mastitis, polyarthritis, keratoconjunctivitis, and abortion in small ruminants, was subjected to systematic characterization in order to gain insights into its membrane proteome composition.

RESULTS

The selective enrichment for M. agalactiae PG2T liposoluble proteins was accomplished by means of Triton X-114 fractionation. Liposoluble proteins were subjected to 2-D PAGE-MS, leading to the identification of 40 unique proteins and to the generation of a reference 2D map of the M. agalactiae liposoluble proteome. Liposoluble proteins from the type strain PG2 and two field isolates were then compared by means of 2D DIGE, revealing reproducible differences in protein expression among isolates. An in-depth analysis was then performed by GeLC-MS/MS in order to achieve a higher coverage of the liposoluble proteome. Using this approach, a total of 194 unique proteins were identified, corresponding to 26% of all M. agalactiae PG2T genes. A gene ontology analysis and classification for localization and function was also carried out on all protein identifications. Interestingly, the 11.5% of expressed membrane proteins derived from putative horizontal gene transfer events.

CONCLUSIONS

This study led to the in-depth systematic characterization of the M. agalactiae liposoluble protein component, providing useful insights into its membrane organization.

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.

Xer1-mediated site-specific DNA inversions and excisions in Mycoplasma agalactiae

Surface antigen variation in Mycoplasma agalactiae, the etiologic agent of contagious agalactia in sheep and goats, is governed by site-specific recombination within the vpma multigene locus encoding the Vpma family of variable surface lipoproteins. This high-frequency Vpma phase switching was previously shown to be mediated by a Xer1 recombinase encoded adjacent to the vpma locus. In this study, it was demonstrated in Escherichia coli that the Xer1 recombinase is responsible for catalyzing vpma gene inversions between recombination sites (RS) located in the 5'-untranslated region (UTR) in all six vpma genes, causing cleavage and strand exchange within a 21-bp conserved region that serves as a recognition sequence. It was further shown that the outcome of the site-specific recombination event depends on the orientation of the two vpma RS, as direct or inverted repeats. While recombination between inverted vpma RS led to inversions, recombination between direct repeat vpma RS led to excisions. Using a newly developed excision assay based on the lacZ reporter system, we were able to successfully demonstrate under native conditions that such Xer1-mediated excisions can indeed also occur in the M. agalactiae type strain PG2, whereas they were not observed in the control xer1-disrupted VpmaY phase-locked mutant (PLMY), which lacks Xer1 recombinase. Unless there are specific regulatory mechanisms preventing such excisions, this might be the cost that the pathogen has to render at the population level for maintaining this high-frequency phase variation machinery.

Comparative genomic and proteomic analyses of two Mycoplasma agalactiae strains: clues to the macro- and micro-events that are shaping mycoplasma diversity

BACKGROUND

While the genomic era is accumulating a tremendous amount of data, the question of how genomics can describe a bacterial species remains to be fully addressed. The recent sequencing of the genome of the Mycoplasma agalactiae type strain has challenged our general view on mycoplasmas by suggesting that these simple bacteria are able to exchange significant amount of genetic material via horizontal gene transfer. Yet, events that are shaping mycoplasma genomes and that are underlining diversity within this species have to be fully evaluated. For this purpose, we compared two strains that are representative of the genetic spectrum encountered in this species: the type strain PG2 which genome is already available and a field strain, 5632, which was fully sequenced and annotated in this study.

RESULTS

The two genomes differ by ca. 130 kbp with that of 5632 being the largest (1006 kbp). The make up of this additional genetic material mainly corresponds (i) to mobile genetic elements and (ii) to expanded repertoire of gene families that encode putative surface proteins and display features of highly-variable systems. More specifically, three entire copies of a previously described integrative conjugative element are found in 5632 that accounts for ca. 80 kbp. Other mobile genetic elements, found in 5632 but not in PG2, are the more classical insertion sequences which are related to those found in two other ruminant pathogens, M. bovis and M. mycoides subsp. mycoides SC. In 5632, repertoires of gene families encoding surface proteins are larger due to gene duplication. Comparative proteomic analyses of the two strains indicate that the additional coding capacity of 5632 affects the overall architecture of the surface and suggests the occurrence of new phase variable systems based on single nucleotide polymorphisms.

CONCLUSION

Overall, comparative analyses of two M. agalactiae strains revealed a very dynamic genome which structure has been shaped by gene flow among ruminant mycoplasmas and expansion-reduction of gene repertoires encoding surface proteins, the expression of which is driven by localized genetic micro-events.

Critical role of dispensable genes in Mycoplasma agalactiae interaction with mammalian cells

Mycoplasmas are minimal bacteria whose genomes barely exceed the smallest amount of information required to sustain autonomous life. Despite this apparent simplicity, several mycoplasmas are successful pathogens of humans and animals, in which they establish intimate interactions with epithelial cells at mucosal surfaces. To identify biological functions mediating mycoplasma interactions with mammalian cells, we produced a library of transposon knockout mutants in the ruminant pathogen Mycoplasma agalactiae and used this library to identify mutants displaying a growth-deficient pheonotype in cell culture. M. agalactiae mutants displaying a 3-fold reduction in CFU titers to nearly complete extinction in coculture with HeLa cells were identified. Mapping of transposon insertion sites revealed 18 genomic regions putatively involved in the interaction of M. agalactiae with HeLa cells. Several of these regions encode proteins with features of membrane lipoproteins and/or were involved in horizontal gene transfer with phylogenetically distant pathogenic mycoplasmas of ruminants. Two mutants with the most extreme phenotype carry a transposon in a genomic region designated the NIF locus which encodes homologues of SufS and SufU, two proteins presumably involved in [Fe-S] cluster biosynthesis in Gram-positive bacteria. Complementation studies confirmed the conditional essentiality of the NIF locus, which was found to be critical for proliferation in the presence of HeLa cells and several other mammalian cell lines but dispensable for axenic growth. While our results raised questions regarding essential functions in mycoplasmas, they also provide a means for studying the role of mycoplasmas as minimal pathogens.

Common strategies for antigenic variation by bacterial, fungal and protozoan pathogens

The complex relationships between infectious organisms and their hosts often reflect the continuing struggle of the pathogen to proliferate and spread to new hosts, and the need of the infected individual to control and potentially eradicate the infecting population. This has led, in the case of mammals and the pathogens that infect them, to an 'arms race', in which the highly adapted mammalian immune system has evolved to control the proliferation of infectious organisms and the pathogens have developed correspondingly complex genetic systems to evade this immune response. We review how bacterial, protozoan and fungal pathogens from distant evolutionary lineages have evolved surprisingly similar mechanisms of antigenic variation to avoid eradication by the host immune system and can therefore maintain persistent infections and ensure their transmission to new hosts.

Striking diversity of vmp1, a variable gene encoding a putative membrane protein of the stolbur phytoplasma

Studies of phytoplasma-insect vector interactions and epidemiological surveys of plant yellows associated with the stolbur phytoplasma (StolP) require the identification of relevant candidate genes and typing markers. A recent StolP genome survey identified a partial coding sequence, SR01H10, having no homologue in the "Candidatus Phytoplasma asteris" genome but sharing low similarity with a variable surface protein of animal mycoplasmas. The complete coding sequence and its genetic environment have been fully characterized by chromosome walking. The vmp1 gene encodes a protein of 557 amino acids predicted to possess a putative signal peptide and a potential C-terminal transmembrane domain. The mature 57.8-kDa VMP1 protein is likely to be anchored in the phytoplasma membrane with a large N-terminal hydrophilic part exposed to the phytoplasma cell surface. Southern blotting experiments detected multiple sequences homologous to vmp1 in the genomes of nine StolP isolates. vmp1 is variable in size, and eight different vmp1 RsaI restriction fragment length polymorphism types could be distinguished among 12 StolP isolates. Comparison of vmp1 sequences revealed that insertions in largest forms of the gene encode an additional copy of a repeated domain of 81 amino acids, while variations in 11-bp repeats led to gene disruption in two StolP isolates. vmp1 appeared to be much more variable than three housekeeping genes involved in protein translation, maturation, and secretion and may therefore be involved in phytoplasma-host interactions.

Occurrence, plasticity, and evolution of the vpma gene family, a genetic system devoted to high-frequency surface variation in Mycoplasma agalactiae

Mycoplasma agalactiae, an important pathogen of small ruminants, exhibits a very versatile surface architecture by switching multiple, related lipoproteins (Vpmas) on and off. In the type strain, PG2, Vpma phase variation is generated by a cluster of six vpma genes that undergo frequent DNA rearrangements via site-specific recombination. To further comprehend the degree of diversity that can be generated at the M. agalactiae surface, the vpma gene repertoire of a field strain, 5632, was analyzed and shown to contain an extended repertoire of 23 vpma genes distributed between two loci located 250 kbp apart. Loci I and II include 16 and 7 vpma genes, respectively, with all vpma genes of locus II being duplicated at locus I. Several Vpmas displayed a chimeric structure suggestive of homologous recombination, and a global proteomic analysis further indicated that at least 13 of the 16 Vpmas can be expressed by the 5632 strain. Because a single promoter is present in each vpma locus, concomitant Vpma expression can occur in a strain with duplicated loci. Consequently, the number of possible surface combinations is much higher for strain 5632 than for the type strain. Finally, our data suggested that insertion sequences are likely to be involved in 5632 vpma locus duplication at a remote chromosomal position. The role of such mobile genetic elements in chromosomal shuffling of genes encoding major surface components may have important evolutionary and epidemiological consequences for pathogens, such as mycoplasmas, that have a reduced genome and no cell wall.

Alternate phase variation in expression of two major surface membrane proteins (MBA and UU376) of Ureaplasma parvum serovar 3

Ureaplasma urealyticum and Ureaplasma parvum are commensals and pathogens of the human urogenital tract and of newborn infants. There are four distinct U. parvum serovars and 10 distinct U. urealyticum serovars. Both species possess a distinct immunodominant variable surface protein, the multiple banded antigen (MBA), which shows size variability among isolates as a result of changes in the number of C-terminal repeating units. Adjacent to the MBA gene (UU375) lies UU376, which was annotated as 'Ureaplasma-specific conserved hypothetical gene'. In four different strains of U. parvum serovar 3, we demonstrated expression of UU376 by Western blot analysis and phase variation between UU376, here designated Upvmp376 (Ureaplasma phase-variable membrane protein 376), and MBA after application of selective pressure with hyperimmune antisera directed against either protein. By Southern blot analysis, we found that the switch between MBA and Upvmp376 expression is associated with a DNA inversion event in which the nonrepetitive region of the MBA gene and its putative promoter region are opposed to either the repetitive region of MBA or UU376. We propose that in U. parvum serovar 3, and presumably in all U. parvum and U. urealyticum, an inversion event at specific sites effects an alternate ON/OFF switching of the genes UU375 and UU376.

Genetic immunization with the immunodominant antigen P48 of Mycoplasma agalactiae stimulates a mixed adaptive immune response in BALBc mice

A DNA vaccine against contagious agalactia was developed for the first time, encoding the P48 of Mycoplasma agalactiae. Specific immune responses elicited in BALB/c mice were evaluated. Both total IgG and IgG1 were detected in mice vaccinated with pVAX1/P48. Proliferation of mononuclear cells of the spleen, levels of gamma interferon, interleukin-12, and interleukin-2 mRNAs were enhanced in immunized animals. Results indicate that pVAX1/P48 vaccination induced both T(h)1 and T(h)2 immune responses. Nucleic acid immunization could be a new strategy against M. agalactiae infections and may be potentially used to develop vaccines for other Mycoplasma diseases.

VNTR analysis reveals unexpected genetic diversity within Mycoplasma agalactiae, the main causative agent of contagious agalactia

BACKGROUND

Mycoplasma agalactiae is the main cause of contagious agalactia, a serious disease of sheep and goats, which has major clinical and economic impacts. Previous studies of M. agalactiae have shown it to be unusually homogeneous and there are currently no available epidemiological techniques which enable a high degree of strain differentiation.

RESULTS

We have developed variable number tandem repeat (VNTR) analysis using the sequenced genome of the M. agalactiae type strain PG2. The PG2 genome was found to be replete with tandem repeat sequences and 4 were chosen for further analysis. VNTR 5 was located within the hypothetical protein MAG6170 a predicted lipoprotein. VNTR 14 was intergenic between the hypothetical protein MAG3350 and the hypothetical protein MAG3340. VNTR 17 was intergenic between the hypothetical protein MAG4060 and the hypothetical protein MAG4070 and VNTR 19 spanned the 5' end of the pseudogene for a lipoprotein MAG4310 and the 3' end of the hypothetical lipoprotein MAG4320. We have investigated the genetic diversity of 88 M. agalactiae isolates of wide geographic origin using VNTR analysis and compared it with pulsed field gel electrophoresis (PFGE) and random amplified polymorphic DNA (RAPD) analysis. Simpson's index of diversity was calculated to be 0.324 for PFGE and 0.574 for VNTR analysis. VNTR analysis revealed unexpected diversity within M. agalactiae with 9 different VNTR types discovered. Some correlation was found between geographical origin and the VNTR type of the isolates.

CONCLUSION

VNTR analysis represents a useful, rapid first-line test for use in molecular epidemiological analysis of M. agalactiae for outbreak tracing and control.

Phase-locked mutants of Mycoplasma agalactiae: defining the molecular switch of high-frequency Vpma antigenic variation

Mycoplasma agalactiae, an important pathogen of small ruminants, exhibits antigenic diversity by switching the expression of multiple surface lipoproteins called Vpmas (Variable proteins of M. agalactiae). Although phase variation has been shown to play important roles in many host–pathogen interactions, the biological significance and the mechanism of Vpma oscillations remain largely unclear. Here, we demonstrate that all six Vpma proteins are expressed in the type strain PG2 and all undergo phase variation at an unusually high frequency. Furthermore, targeted gene disruption of the xer1 gene encoding a putative site-specific recombinase adjacent to the vpma locus was accomplished via homologous recombination using a replicon-based vector. Inactivation of xer1 abolished further Vpma switching and the ‘phase-locked’ mutants (PLMs) continued to steadily express only a single Vpma product. Complementation of the wild-type xer1 gene in PLMs restored Vpma phase variation thereby proving that Xer1 is essential for vpma inversions. The study is not only instrumental in enhancing our ability to understand the role of Vpmas in M. agalactiae infections but also provides useful molecular approaches to study potential disease factors in other ‘difficult-to-manipulate’ mycoplasmas.

Being pathogenic, plastic, and sexual while living with a nearly minimal bacterial genome

Mycoplasmas are commonly described as the simplest self-replicating organisms, whose evolution was mainly characterized by genome downsizing with a proposed evolutionary scenario similar to that of obligate intracellular bacteria such as insect endosymbionts. Thus far, analysis of mycoplasma genomes indicates a low level of horizontal gene transfer (HGT) implying that DNA acquisition is strongly limited in these minimal bacteria. In this study, the genome of the ruminant pathogen Mycoplasma agalactiae was sequenced. Comparative genomic data and phylogenetic tree reconstruction revealed that ∼18% of its small genome (877,438 bp) has undergone HGT with the phylogenetically distinct mycoides cluster, which is composed of significant ruminant pathogens. HGT involves genes often found as clusters, several of which encode lipoproteins that usually play an important role in mycoplasma–host interaction. A decayed form of a conjugative element also described in a member of the mycoides cluster was found in the M. agalactiae genome, suggesting that HGT may have occurred by mobilizing a related genetic element. The possibility of HGT events among other mycoplasmas was evaluated with the available sequenced genomes. Our data indicate marginal levels of HGT among Mycoplasma species except for those described above and, to a lesser extent, for those observed in between the two bird pathogens, M. gallisepticum and M. synoviae. This first description of large-scale HGT among mycoplasmas sharing the same ecological niche challenges the generally accepted evolutionary scenario in which gene loss is the main driving force of mycoplasma evolution. The latter clearly differs from that of other bacteria with small genomes, particularly obligate intracellular bacteria that are isolated within host cells. Consequently, mycoplasmas are not only able to subvert complex hosts but presumably have retained sexual competence, a trait that may prevent them from genome stasis and contribute to adaptation to new hosts.

Mycoplasmas are cell wall–lacking prokaryotes that evolved from ancestors common to Gram-positive bacteria by way of massive losses of genetic material. With their minimal genome, mycoplasmas are considered to be the simplest free-living organisms, yet several species are successful pathogens of man and animal. In this study, we challenged the commonly accepted view in which mycoplasma evolution is driven only by genome down-sizing. Indeed, we showed that a significant amount of genes underwent horizontal transfer among different mycoplasma species that share the same ruminant hosts. In these species, the occurrence of a genetic element that can promote DNA transfer via cell-to-cell contact suggests that some mycoplasmas may have retained or acquired sexual competence. Transferred genes were found to encode proteins that are likely to be associated with mycoplasma–host interactions. Sharing genetic resources via horizontal gene transfer may provide mycoplasmas with a means for adapting to new niches or to new hosts and for avoiding irreversible genome erosion.

Development of a sensitive and specific enzyme-linked immunosorbent assay based on recombinant antigens for rapid detection of antibodies against Mycoplasma agalactiae in sheep

We developed a new recombinant enzyme-linked immunosorbent assay (rELISA) for serodiagnosis of contagious agalactia (CA), a disease caused by Mycoplasma agalactiae in sheep and goats. The assay is based on two M. agalactiae surface proteins, namely, P80 and P55. Identification of these immunodominant and common antigens was accomplished by examining the antibody response elicited in sheep during experimental infection and comparing it to the protein expression profiles of 75 M. agalactiae field strains. Our rELISA was tested with 343 sera, collected from sheep with a laboratory-confirmed diagnosis of CA (n = 223) and from healthy animals (n = 120). All sera had previously been tested by Western blotting (WB) for reactivity against M. agalactiae. In addition, our rELISA was compared with a commercial routine ELISA based on inactivated antigens (CHEKiT). Among the 223 samples that were WB positive for M. agalactiae, 209 (93.7%) tested positive for rP80-P55 with our ELISA, whereas only 164 (73.8%) tested positive with the CHEKiT ELISA. Among the 120 samples tested that were WB negative for M. agalactiae, 96.7% were confirmed as negative with our rELISA, while only 75.8% were confirmed as negative with the CHEKiT ELISA. A comparison of the results with receiver operating characteristic curves indicated that the differences observed between our rELISA and the CHEKiT ELISA are statistically significant. The use of recombinant peptides instead of inactivated antigens could significantly improve the discrimination of positive and negative animals, bringing significant advantages in controlling the import/export of live animals and helping in eradication of this economically detrimental disease.

Distinctive repertoire of contingency genes conferring mutation- based phase variation and combinatorial expression of surface lipoproteins in Mycoplasma capricolum subsp. capricolum of the Mycoplasma mycoides phylogenetic cluster

The generation of surface variation among many divergent species of Mollicutes (mycoplasmas) occurs through stochastic expression patterns of diverse lipoprotein genes. The size and wide distribution of such variable gene sets in minimal (approximately 0.6- to 1.4-Mb) mycoplasmal genomes suggest their key role in the adaptation and survival of these wall-less monoderms. Diversity through variable genes is less clearly established among phylogenetically similar mycoplasmas, such as the Mycoplasma mycoides cluster of ruminant pathogens, which vary widely in host range and pathobiology. Using (i) genome sequences from two members of this clade, Mycoplasma capricolum subsp. capricolum and M. mycoides subsp. mycoides small colony biotype (SC), (ii) antibodies to specific peptide determinants of predicted M. capricolum subsp. capricolum gene products, and (iii) analysis of the membrane-associated proteome of M. capricolum subsp. capricolum, a novel set of six genes (vmcA to vmcF) expressing distinct Vmc (variable M. capricolum subsp. capricolum) lipoproteins is demonstrated. These occur at two separate loci in the M. capricolum subsp. capricolum genome, which shares striking overall similarity and gene synteny with the M. mycoides subsp. mycoides SC genome. Collectively, Vmc expression is noncoordinate and combinatorial, subject to a single-unit insertion/deletion in a 5' flanking dinucleotide repeat that governs expression of each vmc gene. All vmc genes share modular regions affecting expression and membrane translocation. In contrast, vmcA to vmcD genes at one locus express surface proteins with highly structured size-variable repeating domains, whereas vmcE to vmcF genes express products with short repeats devoid of predicted structure. These genes confer a distinctive, dynamic surface architecture that may represent adaptive differences within this important group of pathogens as well as exploitable diagnostic targets.

Characterisation of protein and antigen variability among Mycoplasma mycoides subsp. mycoides (LC) and Mycoplasma agalactiae field strains by SDS-PAGE and immunoblotting

Mycoplasma mycoides subsp. mycoides (LC) (Mmm LC) and Mycoplasma agalactiae are the most important mycoplasma species involved in the contagious agalactia syndrome. A total of 25 field strains from Spain and the two type strains were analysed by SDS-PAGE and immunoblotting. Two polyclonal antisera (PAbs) raised against a pool of strains of each mycoplasma species were used. The results revealed a high degree of protein variability among the field strains. The type strain of Mmm LC appeared to be representative of the field strains of this species, whereas this was not the case with the M. agalactiae type strain. Whereas M. agalactiae is known to possess a gene family regulating surface antigen diversity, there is a need to study the mechanisms used byMmm LC to generate antigenic variability in more detail.

Comparison of four diagnostic tests for the identification of serum antibodies in small ruminants infected withMycoplasma agalactiae

AIM

To determine the diagnostic capability of a newly developed Western blot (WB) assay for the detection of serum antibodies against Mycoplasma agalactiae compared with conventional serological tests, and to identify the best test for routine diagnostic use.

METHODS

The serological test methods used were: two commercial indirect enzyme-linked immunosorbent assays (ELISA), viz ELISA-1, using a bacterial antigen preparation, and ELISA-2, using a recombinant protein (lipoprotein p48) antigen; the complement fixation test (CFT); and a newly developed WB assay, the latter both using a bacterial antigen preparation. Thirty sera from goats infected with M. agalactiae and 97 sera from non-infected sheep were tested using all four methods.

RESULTS

Staining patterns in the WB were quite variable. An immuno-dominant band of 41 kDa was detected in 63% of sera from infected animals. The same band also appeared, although mostly very weakly, in 10% of sera from non-infected animals. When suspicious or very weak reactors were omitted, the diagnostic sensitivity (DSE) and diagnostic specificity (DSP), respectively, for the four assays were: WB=56.7%, 97.9%; ELISA-1=76.7%, 99.0%; ELISA-2=56.7%, 100%; and CFT=40.0%, 94.8%.

CONCLUSIONS

ELISA-1 performed best in this comparison. While the WB can be used, it did not have a technical advantage over the ELISA. The CFT should be discouraged as the primary screening method for contagious agalactia and should be replaced by ELISA-1. Results from this study confirm that serological test methods for contagious agalactia are useful for the detection of infected flocks but will not detect every individual infected animal.

Construction of the first shuttle vectors for gene cloning and homologous recombination in Mycoplasma agalactiae

Mycoplasma agalactiae is a worldwide ruminant pathogen that causes significant economic losses by inflicting contagious agalactia in sheep and goats. The development of efficient control strategies requires a better understanding of the mycoplasma factors that promote successful infection. However, lack of genetic tools has been a major impediment in studying the pathogenic mechanisms of M. agalactiae. This study describes the identification and cloning of the M. agalactiae origin of replication (oriC) in order to construct the first shuttle vectors for targeted gene disruption, gene complementation and expression studies. Additionally, this report provides the first evidence of the occurrence of homologous recombination and the functionality of heterologous tetM determinant in this pathogen.

Rapid differential diagnosis of Mycoplasma agalactiae and Mycoplasma bovis based on a multiplex-PCR and a PCR-RFLP

The membrane-protein 81 gene (mb-mp81) of Mycoplasma bovis was cloned, sequenced and compared to membrane-protein 81 gene (ma-mp81) of Mycoplasma agalactiae. After alignment of both sequences, specific primers pairs were designed from variable or unchanging nucleotide segments. In this study, we describe the development and optimization of a multiplex-PCR (MPCR) for the rapid detection of M. agalactiae and M. bovis strains. In addition, a simple and rapid PCR-restriction fragment length polymorphism (RFLP) assay, using the restriction enzymes AluI, DraI, RsaI and XbaI, is described to distinguish between both species. The results suggest that MPCR and PCR-RFLP assays could be used as an alternative method in routine diagnosis for rapid and specific simultaneous detection of M. agalactiae and M. bovis.

Avoidance of the host immune system through phase variation in Mycoplasma pulmonis

Phase-variable lipoproteins are commonly found in Mycoplasma species. Mycoplasma pulmonis contains a family of extensively studied phase- and size-variable lipoproteins encoded by the vsa locus. The Vsa surface proteins vary at a high frequency, the in vivo significance of which has yet to be determined. We investigated the role of Vsa phase variation in respect to tissue tropism and avoidance of the immune system in the mouse host. Mycoplasmas were cultured 3, 14, and 21 days postinoculation from the nose, lung, trachea, liver, and spleen of experimentally infected C57BL/6 (wild-type), C57BL/6-RAG-1-/- (RAG-/-), and C57BL/6-inducible nitric oxide synthase (iNOS)-/- (iNOS-/-) mice. In wild-type and iNOS-/- mice, a large number of Vsa variants were seen by 21 days postinoculation. In contrast, little Vsa variation occurred in all tissues of RAG-/- mice. Analysis of isolates from 14 days postinoculation revealed less variation of the Vsa proteins in iNOS-/- mice than in the wild type. Western blot analysis of isolates from each strain of mouse demonstrated that Vsa phase variation occurred independently of size variation, indicating no obvious selection pressure for size variants. Additionally, these experiments provided no evidence that mycoplasmas producing particular Vsa proteins adhered only to specific tissues. The data strongly indicate that Vsa phase variation is a mechanism for avoidance of the immune system with no obvious contribution to tissue tropism.

First steps towards the genetic manipulation of Mycoplasma agalactiae and Mycoplasma bovis using the transposon Tn4001mod

Mycoplasma agalactiae and M. bovis rank amongst the most serious pathogenic mycoplasmas infecting small ruminants and cattle, respectively. Despite considerable advances made in Mycoplasma molecular genetics in the past decade, there is still a complete lack of genetic tools to assess the pathogenic mechanisms of these two species. Studies were undertaken to develop a genetic system for the analysis of potential virulence factors of these pathogens. Transposon Tn4001mod was successfully introduced into various chromosomal sites of M. agalactiae and M. bovis with an optimal frequency of 10(-6) per viable colony-forming unit (CFU). This is the first report that demonstrates the amenability of these agents to transformation and to genetic manipulation. Furthermore, Tn4001 is implicated as the first potential genetic tool available for these ruminant pathogens.