Phenotypic switching in mycoplasmas: phase variation of diverse surface lipoproteins

Phase variation of the multiple banded protein in Ureaplasma urealyticum and Ureaplasma parvum

Ureaplasma urealyticum and U. parvum are common commensals and, possibly, pathogens of the human urogenital tract. Like other Mycoplasmatales they possess variable surface proteins. The multiple banded (MB) protein shows a striking variability of its molecular weight. This is caused by changes of the number of C-terminal repeating units. In this study, selective pressure was imposed against cytadherence of U. urealyticum and U. parvum. Ureaplasmas were co-incubated with either erythrocytes or HeLa cells and the cell-bound fraction was removed. Additionally, U. urealyticum populations were transferred serially through broth containing specific polyclonal antibodies. Both approaches led to the emergence of escape variants in which no MB protein was detectable. PCR studies with several primers on different parts of the mba gene indicated major differences between wild-type strains and MB-negative escape variants. In experiments with clonal lineages, however, the loss of the MB protein was shown to be reversible. Therefore, it is proposed that the multiple banded proteins of U. urealyticum and U. parvum are subjected to a phase-switching mechanism as it has already been described for several other Mycoplasmatales.

Multiple promoter inversions generate surface antigenic variation in Mycoplasma penetrans

Mycoplasma penetrans is a newly identified species of the genus MYCOPLASMA: It was first isolated from a urine sample from a human immunodeficiency virus (HIV)-infected patient. M. penetrans changes its surface antigen profile with high frequency. The changes originate from ON<==>OFF phase variations of the P35 family of surface membrane lipoproteins. The P35 family lipoproteins are major antigens recognized by the human immune system during M. penetrans infection and are encoded by the mpl genes. Phase variations of P35 family lipoproteins occur at the transcriptional level of mpl genes; however, the precise genetic mechanisms are unknown. In this study, the molecular mechanisms of surface antigen profile change in M. penetrans were investigated. The focus was on the 46-kDa protein that is present in M. penetrans strain HF-2 but not in the type strain, GTU. The 46-kDa protein was the product of a previously reported mpl gene, pepIMP13, with an amino-terminal sequence identical to that of the P35 family lipoproteins. Nucleotide sequencing analysis of the pepIMP13 gene region revealed that the promoter-containing 135-bp DNA of this gene had the structure of an invertible element that functioned as a switch for gene expression. In addition, all of the mpl genes of M. penetrans HF-2 were identified using the whole-genome sequence data that has recently become available for this bacterium. There are at least 38 mpl genes in the M. penetrans HF-2 genome. Interestingly, most of these mpl genes possess invertible promoter-like sequences, similar to those of the pepIMP13 gene promoter. A model for the generation of surface antigenic variation by multiple promoter inversions is proposed.

The complete genomic sequence of Mycoplasma penetrans, an intracellular bacterial pathogen in humans

The complete genomic sequence of an intracellular bacterial pathogen, Mycoplasma penetrans HF-2 strain, was determined. The HF-2 genome consists of a 1 358 633 bp single circular chromosome containing 1038 predicted coding sequences (CDSs), one set of rRNA genes and 30 tRNA genes. Among the 1038 CDSs, 264 predicted proteins are common to the Mycoplasmataceae sequenced thus far and 463 are M.penetrans specific. The genome contains the two-component system but lacks the essential cellular gene, uridine kinase. The relatively large genome of M.penetrans HF-2 among mycoplasma species may be accounted for by both its rich core proteome and the presence of a number of paralog families corresponding to 25.4% of all CDSs. The largest paralog family is the p35 family, which encodes surface lipoproteins including the major antigen, P35. A total of 44 genes for p35 and p35 homologs were identified and 30 of them form one large cluster in the chromosome. The genetic tree of p35 paralogs suggests the occurrence of dynamic chromosomal rearrangement in paralog formation during evolution. Thus, M.penetrans HF-2 may have acquired diverse repertoires of antigenic variation-related genes to allow its persistent infection in humans.

Virulence of recent isolates of Mycoplasma synoviae in turkeys

Systemic Mycoplasma synoviae (MS) infection was induced experimentally in commercial turkeys with recent MS isolates (K4822D and K4774J) from turkey breeder flocks that exhibited no clinical signs typical of MS infection except for a low incidence of swollen footpads. The virulence of each strain was compared by evaluating gross and microscopic lesions, serologic responses, and MS isolation rates at 10 and 21 days postchallenge and by comparing these results with those obtained from a known virulent isolate (K1968), another previously characterized field isolate (K4463B), and unchallenged controls. All strains induced lesions typical of infectious synovitis but showed distinct differences in the extent of the gross and microscopic lesions and in the isolation rates from the tissues in turkeys. K1968 induced the most extensive lesions in hock and stifle joints and footpads, but strains K4822D, K4774J, and K4463B all induced synovitis and were similar in virulence for synovial tissues. Very mild respiratory lesions were induced by all of the strains studied. All strains yielded strong positive serologic responses. We concluded that these recent field isolates, although able to induce synovitis, are less virulent for turkeys than a known virulent strain. Nevertheless, under severe experimental challenge, these strains have the capability of causing lesions that may be incompatible with economical turkey production.

Phase variation among major surface antigens of Mycoplasma penetrans

The pathogenicity and prevalence of Mycoplasma penetrans, a Mycoplasma species recently isolated from humans, are still debated. A major P35 antigen, which is used as target epitope in serological assays, was shown to be a phase-variable lipid-associated membrane protein (LAMP). In this study, we performed a comparative analysis of the LAMP patterns from five M. penetrans clinical isolates and from the type strain. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis profiles and immunoblots with sera serially collected from an M. penetrans-infected patient indicated that these strains expressed different LAMP repertoires. Furthermore, the intraclonal variation in the expression of LAMPs (P34A, P34B, P35, and P38) was monitored by immunoblot analysis with three specific monoclonal antibodies (MAbs) developed in this study and MAb 7 to P35. The phase variation of these LAMPs occurs in an independent manner, with frequencies of variation ranging from 10(-2) to 10(-4) per cell per generation. Consistent with their amphipathic nature, the P34B and P38 antigens were found exposed at the cell surface. The DNA sequence encoding the P38 antigen was defined and found to be related to those of the P35 gene and other putative LAMP-encoding genes, suggesting that these variable antigens are encoded by a family of related genes. Finally, the serum samples from an M. penetrans-infected patient contained antibodies that reacted with a P36 antigen expressed in different M. penetrans strains but not in the isolate recovered from this patient. This result suggested that in vivo phase variation of P36 occurred, which would support a role for these LAMP variations in avoiding the host's immune vigilance.

Characterization and analysis of a stable serotype-associated membrane protein (P30) of Mycoplasma agalactiae

The gene for a 30-kDa immunodominant antigen, P30, of Mycoplasma agalactiae was cloned from type strain PG2 and expressed in Escherichia coli. P30 is encoded on a monocistronic operon determined by two -10 boxes and a possible -35 region constituting the potential promoter, and a transcription termination site. The gene for the 266-amino-acid protein is preceded by a polypurine-rich region designed as the consensus sequence for a ribosome-binding site. Analysis of the amino acid sequence of P30 revealed the presence of a recognition site for a prokaryotic signal peptidase II at amino acid (aa) 24, indicating that P30 is a transmembrane protein. Moreover, Triton X-114 phase partitioning of M. agalactiae PG2 total antigen revealed that P30 is strongly hydrophobic and hence a possible membrane component. Immunoblot analysis using the monospecific polyclonal anti-P30-His serum indicated that P30 is specific to M. agalactiae. Furthermore, PCR amplification with specific primers for p30 and Southern blot analysis revealed the presence of the gene in all M. agalactiae strains tested and its absence in the other mycoplasma species. Among 27 strains of M. agalactiae studied, 20 strains belonging to the common serotypes A to D, including PG2, expressed P30 or part of it as detected by the monospecific polyclonal anti-P30 antibodies. The other seven strains belonging to the rarely isolated serotypes E to H were negative for P30. The p30 gene was sequenced in 15 strains of M. agalactiae, 10 of which expressed P30 or at least part of it and 5 of which did not express P30. The negative strains carried mutations in both -10 boxes of the promoters. These mutations seem to be responsible for the lack of P30 expression in these strains. Analysis of sera from sheep that were experimentally infected with M. agalactiae revealed that P30 induced a strong and persistent immune response which was still very high two months after infection. In contrast, currently used enzyme-linked immunosorbent assay serology gave only low titers.

Coupled phase-variable expression and epitope masking of selective surface lipoproteins increase surface phenotypic diversity in Mycoplasma hominis

A new mechanism expanding mycoplasmal surface diversity is described. Exposure of surface epitopes on a constitutively expressed membrane protein (P56) of Mycoplasma hominis was subject to high-frequency phase variation due to phase-variable expression of the P120 antigen and its selective masking of P56 epitopes. Phase-variable masking may confer previously unrealized adaptive capabilities on mycoplasmas.

VraA (BBI16) protein of Borrelia burgdorferi is a surface-exposed antigen with a repetitive motif that confers partial protection against experimental Lyme borreliosis

We have previously described the expression cloning of nine Borrelia burgdorferi antigens, using rabbit serum enriched for antibodies specific for infection-associated antigens, and determined that seven of these antigens were associated with infectious B. burgdorferi strain B31. One of these infection-associated antigens encoded a 451-amino-acid putative lipoprotein containing 21 consecutive and invariant 9-amino-acid repeat sequences near the amino terminus that we have designated VraA for virulent strain-associated repetitive antigen A. The vraA locus (designated BBI16 by The Institute for Genomic Research) maps to one of the 28-kb linear plasmids (designated lp28-4) that is not present in noninfectious strain B31 isolates. Subsequent PCR analysis of clonal isolates of B. burgdorferi B31 from infected mouse skin revealed a clone that lacked only lp28-4. Southern blot and Western blot analyses indicated that the lp28-4 and VraA proteins, respectively, were missing from this clone. We have also determined that VraA is a surface-exposed protein based on protease accessibility assays of intact whole cells. Furthermore, vraA expression is modestly derepressed when cells are grown at 37 degrees C relative to cells grown at 32 degrees C, suggesting that VraA is, in part, a temperature-inducible antigen. Homologues cross-reactive to B. burgdorferi B31 VraA, most with different molecular masses, were identified in several B. burgdorferi sensu lato isolates, including B. andersonii, suggesting that the immunogenic epitope(s) present in strain B31 VraA is conserved between Borrelia spp. In protection studies, only 8.3% of mice (1 of 12) immunized with full-length recombinant VraA fused to glutathione S-transferase (GST) were susceptible to infectious challenge with 10(2) B. burgdorferi strain B31, whereas naive mice or mice immunized with GST alone were infected 40% or 63 to 67% (depending on tissues assayed) of the time, respectively. As such, the partial protection elicited by VraA immunization provides an additional testable vaccine candidate to help protect against Lyme borreliosis.

Molecular characterization of the Mycoplasma gallisepticum pvpA gene which encodes a putative variable cytadhesin protein

A putative cytadhesin-related protein (PvpA) undergoing variation in its expression was identified in the avian pathogen Mycoplasma gallisepticum. The pvpA gene was cloned, expressed in Escherichia coli, and sequenced. It exhibits 54 and 52% homology with the P30 and P32 cytadhesin proteins of the human pathogens Mycoplasma pneumoniae and Mycoplasma genitalium, respectively. In addition, 50% homology was found with the MGC2 cytadhesin of M. gallisepticum and 49% homology was found with a stretch of 205 amino acids of the cytadherence accessory protein HMW3 of M. pneumoniae. The PvpA molecule possesses a proline-rich carboxy-terminal region (28%) containing two identical directly repeated sequences of 52 amino acids and a tetrapeptide motif (Pro-Arg-Pro-X) which is repeated 14 times. Genetic analysis of several clonal isolates representing different expression states of the PvpA product ruled out chromosomal rearrangement as the mechanism for PvpA phase variation. The molecular basis of PvpA variation was revealed in a short tract of repeated GAA codons, encoding five successive glutamate resides, located in the N-terminal region and subject to frequent mutation generating an in-frame UAA stop codon. Size variation of the PvpA protein was observed among M. gallisepticum strains, ranging from 48 to 55 kDa and caused by several types of deletions occurring at the PvpA C-terminal end and within the two directly repeated sequences. By immunoelectron microscopy, the PvpA protein was localized on the mycoplasma cell surface, in particular on the terminal tip structure. Collectively, these findings suggest that PvpA is a newly identified variable surface cytadhesin protein of M. gallisepticum.

Gene families encoding phase- and size-variable surface lipoproteins of Mycoplasma hyorhinis

A prototype family of seven genes encoding the variable surface lipoproteins (Vlps) of Mycoplasma hyorhinis is characterized in the pathogenic SK76 strain, using long-range PCR to amplify and analyze the single chromosomal region containing expressed genes vlpA to -G, each of which is subject to phase and size variation. Smaller families of vlp genes in subclones of SK76 or in another strain of M. hyorhinis, GDL, can be attributed to deletions of specific vlp genes from the prototype array described here. Two genes, vlpA and the newly revealed vlpG, contain repeat motifs in their 3' coding regions that differ from the short tandem repeats in other vlp genes yet retain structural features common to all vlp gene products. SK76 and GDL vlp gene families are similarly organized and show sequence similarity between corresponding individual vlp genes. In light of the extensive potential for diversity within the vlp gene system, such conservation provides a provisional basis to hypothesize that vlp genes may exist in specific arrays that endow selected functions while retaining common structural features required during phase-variable expression of this set of gene products.

Host-pathogen interactions in mycoplasma pathogenesis: virulence and survival strategies of minimalist prokaryotes

Despite their very small genomes mycoplasmas are successful pathogens of man and a wide range of animal hosts. Because of the lack of effective therapeutics and vaccines, mycoplasma diseases continue to be a significant problem for public health as well as livestock production with major socio-economic consequences worldwide. Recent outbreaks and epidemiological studies predict that the incidence of human and animal mycoplasma diseases might increase which indicates the urgent need to develop new approaches for prevention and therapy. Development of such reagents, however, requires a solid understanding of the molecular biology of mycoplasma infections. Knowledge in this field has considerably increased during the past decade since new techniques have been developed and adapted to mycoplasmas that allow these organisms to be studied at the molecular level. Research on the two human pathogens Mycoplasma pneumoniae and Mycoplasma genitalium of which the genome sequences have recently been completed as well as the substantial number of studies carried out on the AIDS-associated mycoplasmas, Mycoplasma penetrans and Mycoplasma fermentans, has led the way, but a number of animal mycoplasmas are becoming increasingly appreciated as models for the study of the molecular basis of mycoplasma diseases. This review summarizes and highlights some of the recent findings concerning the molecular interactions that occur between pathogenic mycoplasmas and their hosts, both the common strategies as well as some unique approaches evolved by particular mycoplasma pathogens, including adherence to and uptake into non-phagocytic host cells, as well as mechanisms of escaping the host immune system.

The vsp locus of Mycoplasma bovis: gene organization and structural features

Major lipoprotein antigens, known as variable membrane surface lipoproteins (Vsps), on the surface of the bovine pathogen Mycoplasma bovis were shown to spontaneously undergo noncoordinate phase variation between ON and OFF expression states. The high rate of Vsp phenotypic switching was also shown to be linked with DNA rearrangements that occur at high frequency in the M. bovis chromosome (I. Lysnyansky, R. Rosengarten, and D. Yogev, J. Bacteriol. 178:5395-5401, 1996). In the present study, 13 single-copy vsp genes organized in a chromosomal cluster were identified and characterized. All vsp genes encode highly conserved N-terminal domains for membrane insertion and lipoprotein processing but divergent mature Vsp proteins. About 80% of each vsp coding region is composed of reiterated coding sequences that create a periodic polypeptide structure. Eighteen distinct repetitive domains of different lengths and amino acid sequences are distributed within the products of the various vsp genes that are subject to size variation due to spontaneous insertions or deletions of these periodic units. Some of these repeats were found to be present in only one Vsp family member, whereas other repeats recurred at variable locations in several Vsps. Each vsp gene is also 5' linked to a highly homologous upstream region composed of two internal cassettes. The findings that rearrangement events are associated with Vsp phenotypic switching and that multiple regions of high sequence similarity are present upstream of the vsp genes and within the vsp coding regions suggest that modulation of the Vsp antigenic repertoire is determined by recombination processes that occur at a high frequency within the vsp locus of M. bovis.

The characterization of Mycoplasma synoviae EF-Tu protein and proteins involved in hemadherence and their N-terminal amino acid sequences

An abundant cytoplasmic 43-kDa protein from Mycoplasma synoviae, a major pathogen from poultry, was identified as elongation factor Tu. The N-terminal amino acid sequence (AKLDFDRSKEHVNVGTIGHV) has 90% identity with the sequence of the Mycoplasma hominis elongation factor Tu protein. Monoclonal antibodies reacting with the M. synoviae elongation factor Tu protein also reacted with 43-kDa proteins from the avian Mycoplasma species Mycoplasma gallinarum, Mycoplasma gallinaceum, Mycoplasma pullorum, Mycoplasma cloacale, Mycoplasma iners and Mycoplasma meleagridis, but not with the proteins from Mycoplasma gallisepticum, Mycoplasma imitans or Mycoplasma iowae. In addition, two groups of phase variable integral membrane proteins, pMSA and pMSB, associated with hemadherence and pathogenicity of M. synoviae strains AAY-4 and ULB925 were identified. The cleavage of a larger hemagglutinating protein encoded by a gene homologous to the vlhA gene of M. synoviae generates pMSB1 and pMSA1 proteins defined by mAb 125 and by hemagglutination inhibiting mAb 3E10, respectively. The N-terminal amino acid sequences of pMSA proteins (SENKLI ... and SENETQ ...) probably indicate the cleavage site of the M. synoviae strain ULB 925 hemagglutinin.

Molecular biology and pathogenicity of mycoplasmas

The recent sequencing of the entire genomes of Mycoplasma genitalium and M. pneumoniae has attracted considerable attention to the molecular biology of mycoplasmas, the smallest self-replicating organisms. It appears that we are now much closer to the goal of defining, in molecular terms, the entire machinery of a self-replicating cell. Comparative genomics based on comparison of the genomic makeup of mycoplasmal genomes with those of other bacteria, has opened new ways of looking at the evolutionary history of the mycoplasmas. There is now solid genetic support for the hypothesis that mycoplasmas have evolved as a branch of gram-positive bacteria by a process of reductive evolution. During this process, the mycoplasmas lost considerable portions of their ancestors' chromosomes but retained the genes essential for life. Thus, the mycoplasmal genomes carry a high percentage of conserved genes, greatly facilitating gene annotation. The significant genome compaction that occurred in mycoplasmas was made possible by adopting a parasitic mode of life. The supply of nutrients from their hosts apparently enabled mycoplasmas to lose, during evolution, the genes for many assimilative processes. During their evolution and adaptation to a parasitic mode of life, the mycoplasmas have developed various genetic systems providing a highly plastic set of variable surface proteins to evade the host immune system. The uniqueness of the mycoplasmal systems is manifested by the presence of highly mutable modules combined with an ability to expand the antigenic repertoire by generating structural alternatives, all compressed into limited genomic sequences. In the absence of a cell wall and a periplasmic space, the majority of surface variable antigens in mycoplasmas are lipoproteins. Apart from providing specific antimycoplasmal defense, the host immune system is also involved in the development of pathogenic lesions and exacerbation of mycoplasma induced diseases. Mycoplasmas are able to stimulate as well as suppress lymphocytes in a nonspecific, polyclonal manner, both in vitro and in vivo. As well as to affecting various subsets of lymphocytes, mycoplasmas and mycoplasma-derived cell components modulate the activities of monocytes/macrophages and NK cells and trigger the production of a wide variety of up-regulating and down-regulating cytokines and chemokines. Mycoplasma-mediated secretion of proinflammatory cytokines, such as tumor necrosis factor alpha, interleukin-1 (IL-1), and IL-6, by macrophages and of up-regulating cytokines by mitogenically stimulated lymphocytes plays a major role in mycoplasma-induced immune system modulation and inflammatory responses.

Expression of the pMGA genes of Mycoplasma gallisepticum is controlled by variation in the GAA trinucleotide repeat lengths within the 5' noncoding regions

We analyzed the segment of DNA which contains the expressed pMGA gene from one strain of Mycoplasma gallisepticum in normal (strain S6) cells and in cells in which pMGA1.1 gene expression had ceased as a consequence of in vitro culture in the presence of pMGA1. 1-specific antibodies. Sequence analysis of isolates lacking pMGA1.1 expression revealed that this gene, which is typically expressed, exhibited sequence changes within a region 5' to its promoter. Specifically, pMGA1.1(+) cells contained a (GAA)12 motif upstream of the promoter, whereas in pMGA1.1(-) cells the corresponding region contained a (GAA)10 motif; when such cells were grown in medium no longer containing pMGA-specific antibodies, pMGA1.1 was reexpressed and the 5' (GAA)12 motif was restored. Two other genes, pMGA1.9 and pMGA1.2, were also shown to acquire a (GAA)12 motif in clones which expressed these genes. The results imply the evolution by the pMGA genes of M. gallisepticum of a novel transcriptional requirement which facilitates rapid and reversible switches in the pMGA expression pattern.

Truncation as a novel form of variation of the p50 gene in Mycoplasma hominis

A characteristic feature of the mycoplasmas is the presence of variable surface proteins which may play an important role in the adaptation of the cell-wall-less organisms to their host environments. In addition, this antigen variation may be an important pathogenic property of the organism. The ubiquity of the gene encoding P50, an adhesin of Mycoplasma hominis FBG, and its transcription were analysed in different isolates of M. hominis. The p50 gene was present in all isolates tested. Based on Southern blot analysis and sequencing of the gene, the isolates could be classified into one of three distinct groups. Within two groups specific truncations of the p50 gene occurred. The reduction of the gene size was confirmed in Northern blot analysis of representative isolates from each group, with a decrease in transcript length from 1.6 kb in group G-1 down to 0.76 kb in group G-3. In addition to truncation, a coincidental duplication of some gene segments was detected. This work has provided evidence for the genetic basis of a further variation in the M. hominis P50 adhesin.

Cell culture contamination: sources, consequences, prevention, and elimination

The subject of the chapter is cell culture contamination. Contamination may enter the cell culture system as a physical, chemical, and/or biological component of the environment. The potential sources and consequences of cell culture contamination are unique to the cell culture system and the contaminant. A basic understanding of cell culture contamination is necessary to appreciate the need to develop and practice standardized cell culture procedures. General sources, consequences, and preventative measures are discussed for physical and chemical contamination based on current technology. Mycoplasmal contamination is the focus of the discussion on biological contamination and its impact on cell cultures. The introduction of other biological contaminants should be controlled by the institution of cell culture management procedures needed to minimize the incidence of mycoplasmal contamination. The need to eliminate the routine use of antibiotics in cell culture systems and institute routine testing to detect contamination is emphasized. More rapid detection of contamination should reduce the incidence of cross-contamination and minimize the consequences of any contamination event.

Expression of two members of the pMGA gene family of Mycoplasma gallisepticum oscillates and is influenced by pMGA-specific antibodies

Certain monoclonal antibodies and polyclonal antisera directed to pMGA, the major protein of Mycoplasma gallisepticum, were tested for the ability to influence the surface phenotype of the cell population which resulted from their inclusion in growth medium. The polyclonal antiserum and one monoclonal antibody (MAb 66) resulted in an alteration of surface phenotype; specifically, populations of cells grown either on plates or in broth cultures which contained these reagents ceased the expression of pMGA and instead expressed an antigenically unrelated new polypeptide (p82). Upon the removal of antibody, the progeny of these cells regained pMGA expression and produced antigenically sectored colonies. The basis of this switch between pMGA+ and pMGA- states was shown to be transcriptional. The p82 polypeptide, the expression of which resulted from growth of cells in antibodies, was another member of the pMGA gene family and was located just downstream from the pMGA gene normally expressed by the M. gallisepticum cells used. Collectively the results of this work suggest that this organism has evolved an unusual means of altering the antigenic composition of its surface in response to antibodies or to other environmental cues.

The comparative metabolism of the mollicutes (Mycoplasmas): the utility for taxonomic classification and the relationship of putative gene annotation and phylogeny to enzymatic function in the smallest free-living cells

Mollicutes or mycoplasmas are a class of wall-less bacteria descended from low G + C% Gram-positive bacteria. Some are exceedingly small, about 0.2 micron in diameter, and are examples of the smallest free-living cells known. Their genomes are equally small; the smallest in Mycoplasma genitalium is sequenced and is 0.58 mb with 475 ORFs, compared with 4.639 mb and 4288 ORFs for Escherichia coli. Because of their size and apparently limited metabolic potential, Mollicutes are models for describing the minimal metabolism necessary to sustain independent life. Mollicutes have no cytochromes or the TCA cycle except for malate dehydrogenase activity. Some uniquely require cholesterol for growth, some require urea and some are anaerobic. They fix CO2 in anaplerotic or replenishing reactions. Some require pyrophosphate not ATP as an energy source for reactions, including the rate-limiting step of glycolysis: 6-phosphofructokinase. They scavenge for nucleic acid precursors and apparently do not synthesize pyrimidines or purines de novo. Some genera uniquely lack dUTPase activity and some species also lack uracil-DNA glycosylase. The absence of the latter two reactions that limit the incorporation of uracil or remove it from DNA may be related to the marked mutability of the Mollicutes and their tachytelic or rapid evolution. Approximately 150 cytoplasmic activities have been identified in these organisms, 225 to 250 are presumed to be present. About 100 of the core reactions are graphically linked in a metabolic map, including glycolysis, pentose phosphate pathway, arginine dihydrolase pathway, transamination, and purine, pyrimidine, and lipid metabolism. Reaction sequences or loci of particular importance are also described: phosphofructokinases, NADH oxidase, thioredoxin complex, deoxyribose-5-phosphate aldolase, and lactate, malate, and glutamate dehydrogenases. Enzymatic activities of the Mollicutes are grouped according to metabolic similarities that are taxonomically discriminating. The arrangements attempt to follow phylogenetic relationships. The relationships of putative gene assignments and enzymatic function in My. genitalium, My. pneumoniae, and My. capricolum subsp. capricolum are specially analyzed. The data are arranged in four tables. One associates gene annotations with congruent reports of the enzymatic activity in these same Mollicutes, and hence confirms the annotations. Another associates putative annotations with reports of the enzyme activity but from different Mollicutes. A third identifies the discrepancies represented by those enzymatic activities found in Mollicutes with sequenced genomes but without any similarly annotated ORF. This suggests that the gene sequence is significantly different from those already deposited in the databanks and putatively annotated with the same function. Another comparison lists those enzymatic activities that are both undetected in Mollicutes and not associated with any ORF. Evidence is presented supporting the theory that there are relatively small gene sequences that code for functional centers of multiple enzymatic activity. This property is seemingly advantageous for an organism with a small genome and perhaps under some coding restraint. The data suggest that a concept of "remnant" or "useless genes" or "useless enzymes" should be considered when examining the relationship of gene annotation and enzymatic function. It also suggests that genes in addition to representing what cells are doing or what they may do, may also identify what they once might have done and may never do again.

Immunoblot examination of humoral response of chickens infected with Mycoplasma gallisepticum at various ages

Mycoplasma gallisepticum- and Mycoplasma synoviae-free chickens were infected with 0.2 ml broth culture of M. gallisepticum strain 1226 intra air sac at 3, 14, 18, 28, 42, 49 and 65 days of age. Blood samples were taken 0-5 weeks before infection and 1-6 weeks after infection (depending on age of infection). The antibody response was examined by Western blot. As a control of infection, serum plate agglutination test (SPA), pathological lesions, and presence of Mycoplasma in air sacs were used. Antibodies to p64-67 kDa appeared in all groups of birds on the first week post-infection. Antibodies to p56 were detected from the second week post-challenge if infection was performed at 3 or 14 days of age, while on first week if challenge was done at 18, 28, 42, 49 or 65 days of age. Antibodies to p200, p120, p98, p80, p75, p72, p60, p50, p45, p40, p35, p33, p31, p28, p26, p24 and p22 were also detected.

Localized frameshift mutation generates selective, high-frequency phase variation of a surface lipoprotein encoded by a mycoplasma ABC transporter operon

The wall-less mycoplasmas have revealed unusual microbial strategies for adaptive variation of antigenic membrane proteins exposed during their surface colonization of host cells. In particular, high-frequency mutations affecting the expression of selected surface lipoproteins have been increasingly documented for this group of organisms. A novel manifestation of mutational phase variation is shown here to occur in Mycoplasma fermentans, a chronic human infectious agent and possible AIDS-associated pathogen. A putative ABC type transport operon encoding four gene products is identified. The 3' distal gene encoding P78, a known surface-exposed antigen and the proposed substrate-binding lipoprotein of the transporter, is subject to localized hypermutation in a short homopolymeric tract of adenine residues located in the N-terminal coding region of the mature product. High-frequency, reversible insertion/deletion frameshift mutations lead to selective phase variation in P78 expression, whereas the putative nucleotide-binding protein, P63, encoded by the most 5' gene of the operon, is continually expressed. Mutation-based phase variation in specific surface-exposed microbial transporter components may provide an adaptive advantage for immune evasion, while continued expression of other elements of the same transporter may preserve essential metabolic functions and confer alternative substrate specificity. These features could be critical in mycoplasmas, where limitations in both transcriptional regulators and transport systems may prevail. This study also documents that P63 contains an uncharacteristic hydrophobic sequence between predicted nucleotide binding motifs and displays an amphiphilic character in detergent fractionation. Both features are consistent with an evolutionary adaptation favoring integral association of this putative energy-transducing component with the single mycoplasma membrane.

Adherence of Mycoplasma gallisepticum involves variable surface membrane proteins

Adherence of Mycoplasma gallisepticum to erythrocytes was examined by colony immunoblotting, detergent phase fractionation, trypsin treatment, comparison of protein profiles, and comparison of erythrocyte-bound mycoplasma protein fractions of hemadsorption-positive and -negative mutants. The binding of M. gallisepticum to chicken or human erythrocytes was found to be mediated via surface-exposed membrane proteins undergoing high-frequency phase variation.

Elongated versions of Vlp surface lipoproteins protect Mycoplasma hyorhinis escape variants from growth-inhibiting host antibodies

Variation in Vlp surface proteins of Mycoplasma hyorhinis was evaluated in terms of its role in determining susceptibility of organisms to growth inhibition by host antibodies (Abs). High-frequency switching of Vlp surface lipoproteins has been studied in isogenic lineages of M. hyorhinis SK76. In these lineages, the products of three genes, vlpA, vlpB, and vlpC, are subject to phase and size variation in vitro, which occur through distinct mutator elements that independently govern the expression of each vlp gene (promoter mutations) or the size of the vlp gene product (by intragenic expansion or contraction of a 3' region containing tandem repeats). Isogenic clonal variants of M. hyorhinis SK76 expressing distinct profiles of Vlp products were assessed for their susceptibility to complement-independent growth inhibition by serum Abs of swine experimentally infected with the arthritigenic SK76 strain. Invariably, variants expressing longer versions of VlpA, VlpB, or VlpC (each expressed individually) were completely resistant to host immune serum Abs, whereas variants expressing shorter allelic versions of each Vlp were susceptible. The target of growth-inhibiting Abs was not the Vlp products, since removal of anti-Vlp Abs had no effect on the inhibitory activity of the host immune serum on susceptible variants. Escape variant populations derived by propagating susceptible variants in an immune (versus control) host serum revealed a strong selection for the long-Vlp phenotype, irrespective of the identity of the Vlp expressed. Apparent mutational pathways of acquiring the protective phenotype included mutational switches to express long vlp genes that had been transcriptionally silent or switches to elongate expressed vlp genes. These results suggest that a major function of the Vlp system is to shield the wall-less mycoplasma surface from host Abs capable of binding vital (and as-yet-unidentified) surface antigens of this organism.

Identification of novel species-specific antigens of Mycoplasma hyopneumoniae by preparative SDS-PAGE ELISA profiling

Mycoplasma hyopneumoniae, M. hyorhinis and M. flocculare are commonly isolated from the respiratory tract of pigs and are phylogenetically related. The identification and characterization of antigens specific for M. hyopneumoniae is crucial for the development of serological reagents and for understanding the mechanisms of pathogenicity of this pathogen. Protein and antigen profiles of six strains of M. hyopneumoniae, four strains of M. hyorhinis and a type strain of M. flocculare were compared using SDS-PAGE and immunoblotting. Five strains of M. hyopneumoniae originally isolated from diverse geographical regions produced similar protein and antigen profiles. One strain, C1735/2, produced a unique protein profile and was poorly immunoreactive, suggesting that some strains of M. hyopneumoniae may possess a structurally modified repertoire of antigens. Major M. hyopneumoniae antigens with molecular masses of approximately 36, 43, 48, 52, 76, 78, 80, 82, 94, 106, 114 and 200 kDa were identified by immunoblotting using hyperimmune pig sera raised against both high and low passage strains of M. hyopneumoniae. Porcine hyperimmune sera raised against the GDL type strain of M. hyorhinis reacted strongly with all M. hyorhinis strains although the profiles displayed considerable variation. Major antigens of molecular mass 42, 49, 52, 78, 80 and 82 kDa were identified in type strains GDL and BTS-7 and field strain 2; however, field strain 1 produced a unique profile. A preparative SDS-PAGE profiling (PPP) technique was developed which enabled quantification of the immunoreactivity of denatured antigens with porcine serum by ELISA. PPP facilitated the rapid identification of species-specific and cross-reactive antigens among the three mycoplasma species. PPP studies revealed several strongly immunoreactive M. hyopneumoniae-specific antigens of 43, 76, 94, 114 and 200 kDa as well as antigens of molecular mass between 52 and 62 kDa which were not apparent in immunoblotting studies. Rabbit monospecific anti-42 kDa serum reacted specifically with a 43 kDa antigen in whole cell lysates of geographically diverse strains of M. hyopneumoniae and failed to cross-react with M. flocculare or M. hyorhinis whole cell lysates. This study has identified a number of M. hyopneumoniae-specific antigens which warrant further investigation to determine their potential as diagnostic reagents and the role they play, if any, in pathogenicity.

Variable expression and geographic distribution of Mycoplasma agalactiae surface epitopes demonstrated with monoclonal antibodies

Species-specific monoclonal antibodies (MAbs) were developed against Mycoplasma agalactiae reference strain PG2 and French isolate P89 to study the in vitro expression of surface epitopes and to probe the antigenic profiles of 245 field isolates originating from 10 different countries. Colony immunostaining with MAbs on clonal lineage showed that 4 out of 9 species-specific epitopes exhibited a high rate of variation, demonstrating that M. agalactiae possesses a capacity for phenotypic diversification of its surface antigenicity. The emphasis was on dot immunobinding screening of the field isolates with MAbs recognizing permanently expressed epitopes. Eight different profiles could be defined. Great differences in epitope conservation were demonstrated with some area-specific strains completely lacking certain specific determinants. These results indicate that the antigenic variability of M. agalactiae relies not only upon surface switching mechanisms but also upon true epitope differences, partially related to the geographic origin of the isolates.

Molecular biology of mycoplasmas

Although mycoplasmas lack cell walls, they are in many respects similar to the gram-positive bacteria with which they share a common ancestor. The molecular biology of mycoplasmas is intriguing because the chromosome is uniquely small (< 600 kb in some species) and extremely A-T rich (as high as 75 mol% in some species). Perhaps to accommodate DNA with a lower G + C content, most mycoplasmas do not have the "universal" genetic code. In these species, TGA is not a stop codon; instead it encodes tryptophan at a frequency 10 times greater than TGG, the usual codon for this amino acid. Because of the presence of TGA codons, the translation of mycoplasmal proteins terminates prematurely when cloned genes are expressed in other eubacteria, such as Escherichia coli. Many mycoplasmas possess strikingly dynamic chromosomes in which high-frequency changes result from errors in DNA repair or replication and from highly active recombination systems. Often, high-frequency changes in the mycoplasmal chromosome are associated with antigenic and phase variation, which regulate the production of factors critical to disease pathogenesis.

Phenotypic switching of variable surface lipoproteins in Mycoplasma bovis involves high-frequency chromosomal rearrangements

Mycoplasma bovis, an important pathogen of cattle, was recently shown to possess a family of phase- and size-variable membrane surface lipoprotein antigens (Vsps). These proteins spontaneously undergo noncoordinate phase variation between ON and OFF expression states, generating surface antigenic variation. In the present study, we show that the spontaneously high rate of Vsp phenotypic switching involves DNA rearrangements that occur at high frequency in the M. bovis chromosome. A 1.5-kb HindIII genomic fragment carrying the vspA gene from M. bovis PG45 was cloned and sequenced. The deduced VspA amino acid sequence revealed that 80% of the VspA molecule is composed of reiterated intragenic coding sequences, creating a periodic polypeptide structure. Four distinct internal regions of repetitive sequences in the form of in-tandem blocks extending from the N-terminal to the C-terminal portion of the Vsp product were identified. Southern blot analysis of phenotypically switched isogenic lineages representing ON or OFF phase states of Vsp products suggested that changes in the Vsp expression profile were associated with detectable changes at the DNA level. By using a synthetic oligonucleotide representing a sequence complementary to the repetitive vspA gene region as a probe, we could identify the vspA-bearing restriction fragment undergoing high-frequency reversible rearrangements during oscillating phase transition of vspA. The 1.5-kb HindIII fragment carrying the vspA gene (on state) rearranged and produced a 2.3-kb HindIII fragment (OFF state) and vice versa. Two newly discovered vsp genes (vspE and vspF) were localized on two HindIII fragments flanking the vsp gene upstream and downstream. Southern blot hybridization with vspE- and vspF-specific oligonucleotides as probes against genomic DNA of VspA phase variants showed that the organization and size of the fragments adjacent to the vspA gene remained unchanged during VspA ON-OFF switching. The mechanisms regulating the vsp genes are yet unknown; our findings suggest that a recombinative mechanism possibly involving DNA inversions, DNA insertion, or mobile genetic elements may play a role in generating the observed high-frequency DNA rearrangements.

Molecular basis of size and antigenic variation of a Mycoplasma hominis adhesin encoded by divergent vaa genes

The molecular basis for the size and antigenic diversity of the variable adherence-associated (Vaa) antigen, a major surface protein and a putative adhesin (of Mycoplasma hominis, is described. Size-variant alleles of the single-copy vaa gene encode abundant surface lipoproteins containing one to four nearly identical, tandem repetitive units of 121 amino acids in the central region of the mature Vaa product. Gain or loss of central repeats in vaa genes gives rise to distinct size-variant Vaa antigens in clonal populations of this organism. The N-terminal and repeat regions of Vaa contain highly conserved sequences, while the C-terminal region, implicated as the adherence-mediating module, is highly variable and divergent among different strains of this pathogen. Sequence variation in this region may underlie the strain-dependent binding of some monoclonal antibodies to Vaa products. The Vaa antigen is expressed in vivo during chronic, active arthritis associated with M. hominis infection and is highly immunogenic in the human host. Size variation and C-terminal antigenic divergence of Vaa could affect the adherence of M. hominis and evasion of antibody-mediated immunity, thereby contributing to the organism's adaptive capability in the human host. Variation in vaa genes reveals a distinct pattern of mutations generating mycoplasma surface variation.

Spiralin polymorphism in strains of Spiroplasma citri is not due to differences in posttranslational palmitoylation

Spiralin is defined as the major membrane protein of the helical mollicute Spiroplasma citri. According to the S. citri strain used, spiralin shows polymorphism in its electrophoretic mobility. The spiralin gene sequences of eight S. citri strains were determined by direct sequencing of the PCR-amplified genes. All spiralins were found to be 241 amino acids long, except for the spiralin of strain Palmyre, which is 242 amino acids long. The molecular masses calculated from these sequences did not explain the differences observed in the electrophoretic mobilities. In all of the spiralins examined, the first 24 N-terminal amino acids were conserved, including a cysteine at position 24, and had the features of typical signal peptides of procaryotic lipoproteins. When S. citri strains were grown in the presence of [3H]palmitic acid, at least 10 proteins, including spiralin, became labeled. In the presence of globomycin, a lipoprotein signal peptidase inhibitor in eubacteria, apparently unprocessed spiralin could be detected. Formic acid hydrolysis of the [3H]palmitic acid-labeled spiralins of four representative S. citri strains yielded two peptide fragments for each spiralin, as expected from the gene sequence. On fragment was [3H]palmitic acid labeled, and it had almost the same electrophoretic mobility irrespective of the spiralins used. Samples of the unlabeled peptide fragments from the four representative strains had slightly different electrophoretic mobilities (delta Da approximately equal to 800 Da); however, these were much smaller than those of the whole spiralins before formic acid hydrolysis (delta Da approximately equal to 8,000 Da). These results suggest that spiralin polymorphism in S. citri is not due to differences in posttranslational modification by palmitic acid and is certainly a structural property of the whole protein or could result from an unidentified posttranslational modification of spiralin.

Variant colony surface antigenic phenotypes within mycoplasma strain populations: implications for species identification and strain standardization

Immunobinding assays with mycoplasma colonies on agar plates (immunofluorescence and immunoperoxidase techniques) or with imprints of colonies transferred to solid supports (colony immunoblotting) are widely used as standard diagnostic tests for serological species identification of mycoplasma isolates. However, in light of the high rate of variability of surface antigens in many mycoplasmas, diagnostic data obtained with these techniques require a more critical evaluation. In this report, we demonstrate with some examples that mycoplasma surface variability based on alterations in expression, in size, and in surface presentation of integral and peripheral membrane proteins may lead to misinterpretation of colony immunostaining reactions obtained by using specific monoclonal antibodies as well as conventional diagnostic hyperimmune sera. To more easily identify phenotypically mixed isolates or samples which contain more than one species, we have introduced some minor modifications of the colony immunoblot technique which provide sharp signals of positive as well as negative reactions and enable identification of cryptic epitopes. It is further demonstrated that because of the variability in colony surface antigenic phenotype, mycoplasma strains, including well-established reference and other prototype strains which are used under the same designation in many laboratories, can differ markedly in their antigen profiles and their potentially virulence-related surface properties, since they are usually purified by filter cloning and often propagated by subcultivation of randomly selected agar-grown subpopulations. We conclude from this study that because of this surface variability, the establishment of criteria for standardization of mycoplasma strains and diagnostic antisera is urgently required in order to obtain reproducible results in different laboratories.

Increased structural and combinatorial diversity in an extended family of genes encoding Vlp surface proteins of Mycoplasma hyorhinis

Variable lipoproteins (Vlp) constitute the major coat protein of Mycoplasma hyorhinis. They are products of multiple, divergent, single-copy genes organized in a chromosomal cluster. Three genes, vlpA, vlpB, and vlpC, have been previously identified in clonal isolates of M. hyorhinis SK76. Each is linked to a characteristic promoter region containing a homopolymeric tract of adenine residues [poly(A) tract], subject to hypermutation, that transcriptionally controls phase variation of vlp genes and leads to combinatorial surface mosaics of distinct Vlp products. The size of the natural vlp gene repertoire is unknown but may critically determine the degree of structural and combinatorial diversity available in this species. In this study, the vlp repertoire of M. hyorhinis GDL-1 was characterized and shown to contain three additional genes, vlpD, vlpE, and vlpF, clustered with other known vlp genes in the order 5'-vlpD-vlpE-vlpF-IS-vlpA-IS-vlpB-vlpC+ ++-3', where IS represents copies of the IS1221 element of M. hyorhinis. The 5' boundary of this expanded family was identical to that of the more limited family 5'-vlpA-IS-vlpB-vlpC-3' previously described in a clonal isolate of strain SK76. A recombinant construct containing vlpD, vlpE, and vlpF expressed antigenically distinguishable products corresponding to each gene. These genes encode characteristic C-terminal repetitive regions that are subject to size variation by insertion or deletion of intragenic repeats but maintain an extended, charged structure. Each vlp gene also contained characteristic alternative open reading frames, which provide a potential reservoir of coding sequence for Vlp diversity, possibly recruited through insertion and/or deletion mutations. These findings demonstrate a vastly expanded potential for structural diversity and combinatorial display of surface mosaics on this organism and suggest that modulation of the vlp repertoire, possibly in conjunction with mobile elements, may determine the capacity for surface variation in natural populations and laboratory strains of this mycoplasma species.

Comparison of Mycoplasma arthritidis strains by enzyme-linked immunosorbent assay, immunoblotting, and DNA restriction analysis

Twenty Mycoplasma arthritidis strains or isolates were compared by a combination of enzyme-linked immunosorbent assay by an antiserum adsorption technique, Western immunoblotting, and restriction analysis of chromosomal DNA. Antigenic markers that defined strains related to strains 158p10p9, PG6, and H606 were identified. In addition, restriction analysis allowed all 20 strains to be divided into six groups. Results of restriction analysis corresponded generally with antigenic similarities, although the former did not allow grouping with as fine a precision as the latter. However, intrastrain antigenic variability, which is common among many Mycoplasma species, including M. arthritidis, introduced a complicating factor into our attempts at antigenic analysis. While serologic and antigenic analyses remain useful, we recommend that they be used with caution and in combination with other techniques for identifying and characterizing new isolates and newly acquired strains. Combinations of these techniques have proven to be useful in our laboratory for quality control and for uncovering interesting relationships among strains subjected to animal passage and their less virulent antecedents and among strains originally classified as the same but obtained from different sources and maintained, sometimes for decades, in different laboratories.

In vivo variation of Mycoplasma gallisepticum antigen expression in experimentally infected chickens

The antigen expression profiles of Mycoplasma gallisepticum isolates obtained from tracheal swabs of chickens after aerosol-inoculation with M. gallisepticum strain R or clonal variant R/E were examined in western immunoblots. A reference anti-M. gallisepticum chicken antiserum and antisera from individual infected chickens as well as monoclonal antibodies (mAbs) specific for surface proteins were used to monitor in vivo antigenic variation. mAbs 1E5 and 12D8, recognizing PvpA and p67a, recently shown to undergo high-frequency in vitro phase variation, were used for consecutive staining of colony and western immunoblots in order to distinguish between the resultant phenotypes with respect to the corresponding epitopes. Marked differences in the expression of major immunogenic proteins, including p67a, were observed between the two inocula as well as among reisolates recovered at different times of infection. Comparative western immunoblot analysis of the rapidly changing chicken serum antibody response and reisolates recovered during the course of an experimental infection with M. gallisepticum R or clonal variant R/E suggest that immune modulation may have a key role in generating surface diversity. In addition, comparison of colony immunoblots of strain R inoculum and of reisolated colonies from tracheas of birds 8 days post infection indicated an in vivo selection of the PvpA+p67a- phenotype. This study established that surface antigens of M. gallisepticum are subjected in vivo to rapid alteration in their expression. This variability may function as a crucial adaptive mechanism, enabling the organism to escape from the host immune defense and to adapt to the changing host environment at different stages of a natural infection.

Identification and characterization of a Mycoplasma hyopneumoniae adhesin

An adhesin of Mycoplasma hyopneumoniae was identified and characterized in this study. A monoclonal antibody (MAb), F2G5, and its F(ab')2 fragments inhibited the adherence of M. hyopneumoniae to porcine tracheal cilia, the natural targets to which the mycoplasma binds during infection. MAb F2G5 detected multiple bands, but predominantly recognized a 97-kDa (P97) protein of M. hyopneumoniae on immunoblots. Affinity chromatography, conducted with immobilized MAb F2G5, mainly purified P97. The purified proteins were able to bind to cilia and blocked the adherence of intact M. hyopneumoniae cells to cilia. Immunolabeling of mycoplasmas with MAb F2G5 under electron microscopy demonstrated that the proteins recognized by MAb F2G5 were located at the surface of the mycoplasma, predominantly on a surface fuzzy layer. These results indicate that P97 functions as an adhesin of M. hyopneumoniae. The N-terminal amino acid sequence of P97 did not have significant homology with any known bacterial or mycoplasmal adhesins, suggesting that P97 is a novel protein. The predominant proteins detected by MAb F2G5 in different strains varied in size, indicating that the antigen bearing the epitope for MAb F2G5 undergo intraspecies size variation. Antigenic variation of adhesins may be a pathogenic mechanism utilized by M. hyopneumoniae to evade the porcine immune system.

Small repeating units within the Ureaplasma urealyticum MB antigen gene encode serovar specificity and are associated with antigen size variation

Ureaplasma urealyticum is a common commensal of the female lower urogenital tract, yet it has been shown to be an important cause of chorioamnion infection, respiratory and central nervous system disease, and death in premature infants. It has been suggested that only certain serovars are capable of producing invasive disease. However, we previously showed that many serotypes are invasive and that perhaps antigen variability and host factors are more important determinants of ureaplasma infections than are different serotypes per se. The molecular characterization in this report describes a mechanism available to ureaplasmas for producing antigen variation. That antigen, designated MB and previously identified on U. urealyticum, contains serovar-specific and cross-reactive epitopes, is produced both in vitro and in vivo, is a predominant antigen recognized during ureaplasma infections of humans, undergoes a high rate of size variation in vitro, and is size variable on invasive ureaplasma isolates. In the present study, we cloned and sequenced the gene of the MB antigen from serovar 3, the serovar most commonly isolated from humans. The 3' two-thirds of the gene was shown to contain identical 18-nucleotide tandem repeats. PCR analysis and direct sequencing of two variants indicated that alterations within this repeat region are responsible for the size variation of the MB antigen. Intact recombinant serovar 3 MB antigen and truncated products, expressed by coupled in vitro transcription and translation of the cloned gene, were immunoprecipitated by both a serovar-specific monoclonal antibody and the serum of a U. urealyticum-infected patient, and these results identified the repeat region of the MB antigen as serovar defining. Resolution of the precise amino acids responsible for specific epitopes and characterization of similar genes in the other serovars should yield reagents useful in elucidating the role of antigen size variants in disease production and the role of specific antibody in protection from ureaplasma disease.

Analysis of the variability in expression of Mycoplasma gallisepticum surface antigens

The in vitro expression of surface epitopes for different strains of Mycoplasma gallisepticum (MG) was studied with a panel of monoclonal antibodies (mAbs) using indirect colony immunostaining and Western blot (WB) analyses. Immunostaining of colonies with mAbs showed that five epitopes had different degrees of variable expression, while one epitope was permanently expressed in vitro. Colonies that failed to express the studied epitopes had the potential of phenotypically switching the expression of these epitopes in vitro. Variable and permanently expressed epitopes were associated with more than one protein and not all mAb-defined proteins were responsible for the immunostaining of intact MG colonies. The ability of MG to variably express their surface epitopes maybe the mechanism utilized by the microorganism to avoid the host immune response.

A surface epitope undergoing high-frequency phase variation is shared by Mycoplasma gallisepticum and Mycoplasma bovis

We have recently reported that three distinct size- and phase-variable surface lipoproteins (Vsps) of the bovine pathogen Mycoplasma bovis possess a common epitope recognized by monoclonal antibody 1E5. In the present study, we show that this epitope is also present on a size-variant protein (PvpA) of the avian pathogen Mycoplasma gallisepticum. Application of monoclonal antibody 1E5 in Western immunoblot analysis of Triton X-114 phase-fractionated proteins and in colony immunoblots, as well as in trypsin and carboxypeptidase digestion experiments, has demonstrated that (i) PvpA is an integral membrane protein with a free C terminus, (ii) the shared epitope is surface exposed, and (iii) PvpA is subjected to high-frequency phase variation in expression. By using serum antibodies from M. gallisepticum-infected chickens, we were able to demonstrate the immunogenic nature of PvpA and identify three additional highly immunogenic Triton X-114 phase proteins (p67, p72, and p75) also undergoing high-frequency phase variation spontaneously and independently. Metabolic labeling experiments with [14C]palmitate and [14C]oleate revealed that PvpA, in contrast to p67, p72, and p75, is not lipid modified. Southern blot hybridization with restriction fragments carrying the pvpA gene of M. gallisepticum or the vspA gene of M. bovis against digested genomic DNA of the two Mycoplasma species indicated the absence of genetic relatedness between the pvpA and vspA genes. The apparent complexity of the antigenic variation phenomenon in M. gallisepticum is discussed.

Antigen heterogeneity among isolates of Mycoplasma bovis is generated by high-frequency variation of diverse membrane surface proteins

The protein and antigen profiles of 11 isolates of Mycoplasma bovis were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblot analysis of whole organisms. The isolates examined included the type strain PG45 and 10 other filter-cloned strains or purified isolates both from animals without clinical signs and from clinical cases of bovine mastitis, arthritis, or pneumonia. While the overall protein patterns visualized by silver staining were very similar, marked differences in the antigen banding profiles were detected by rabbit antiserum prepared against whole organisms from one of the strains analyzed. This antigenic heterogeneity was shown to be independent of the geographical origin, the type of clinical disease, and the site of isolation and was also observed among serial isolates from a single animal. Antigen profiles were further monitored throughout sequentially subcloned populations of the PG45 strain. This clonal analysis revealed a high-frequency variation in the expression levels of several prominent antigens. All of these variable antigens were defined by detergent-phase fractionation with Triton X-114 as amphiphilic integral membrane proteins. A subset of different-sized membrane proteins was identified by a monoclonal antibody raised against a PG45 subclone expressing a 63- and a 46-kDa variant antigen within that set. The selective susceptibility of these proteins to trypsin treatment of intact organisms and their ability to bind the monoclonal antibody in colony immunoblots demonstrated that they were exposed on the cell surface. In addition, their preferential recognition by serum antibodies from individual cattle with naturally induced M. bovis mastitis or arthritis confirmed that they were major immunogens of this organism. These studies establish that the apparent antigenic heterogeneity among M. bovis isolates reported here does not represent stable phenotypic strain differences generated from accumulated mutational events but reflects distinct expression patterns of diverse, highly variable membrane surface proteins.

A family of phase- and size-variant membrane surface lipoprotein antigens (Vsps) of Mycoplasma bovis

A set of strain- and size-variant highly immunogenic membrane surface protein antigens of Mycoplasma bovis, which has been identified by a monoclonal antibody, is shown in this report to make up a family of antigenically and structurally related lipid-modified proteins, designated Vsps (variable surface proteins). By systematic analysis of several isogenic clonal lineages of the type strain PG45, three members of this family have been identified, VspA, VspB, and VspC, each of which was shown to undergo independent high-frequency changes in size as well as noncoordinate phase variation between ON and OFF expression states. The monoclonal antibody-defined epitope common to VspA, VspB, and VspC was accessible on the cell surface in most, but not all, of the clonal populations analyzed and was present on a C-terminal limit tryptic fragment of each Vsp variant that was released from the membrane surface. VspA and VspC were distinguished from VspB by their selective detection with colloidal gold and by their distinctive reaction with a polyclonal antibody against M. bovis D490. VspA, VspB, and VspC were further distinguishable from one another by their characteristic patterns of degradation at carboxypeptidase Y pause sites. While these Vsp-specific structural fingerprints with an irregular periodic spacing were constant for similarly sized variants of a defined Vsp product, they showed distinct differences among variants differing in size. This variability included gain or loss of individual bands within distinct subsets of bands, as well as shifts of the entire banding patterns up- or downwards, indicating that insertions or deletions underlying Vsp size variation can occur at various locations either within the C-terminal domain or within other regions of these proteins. This was similarly confirmed by comparative epitope mapping analysis of tryptic cleavage products generated from different Vsp size variants. The Vsp family of M. bovis described in this study represents a newly discovered system of surface antigenic variation in mycoplasmas displaying features which closely resemble but are also different from the characteristics reported for the Vlp (variable lipoprotein) system of M. hyorhinis. The isogenic lineages established here provide key populations for subsequent analysis of corresponding genes to further elucidate Vsp structure and variation, which may have important relevance for a better understanding of the pathogenicity of this agent.

Processing and surface presentation of the Mycoplasma hyorhinis variant lipoprotein VlpC

The variant surface lipoprotein VlpC of Mycoplasma hyorhinis was shown to be processed by cleavage of a characteristic prokaryotic prolipoprotein signal peptide. In addition, a vlpC::phoA fusion protein expressed and translocated in Escherichia coli was recognized by surface-binding monoclonal antibodies, which identified the characteristic region II of Vlps, containing divergent external sequences proximal to the membrane, as an exposed portion of these surface proteins subject to immune recognition and selection.

Membrane-associated nuclease activities in mycoplasmas

Membrane-associated nucleases of various mycoplasmal species were investigated by using two nuclease assays. A lambda DNA assay was developed to measure nuclease activity associated with whole-cell suspensions, activity released from intact cells, and activity associated with detergent-disrupted cells. In most species, nuclease activities were entirely membrane associated, and disruption by a detergent had a stimulatory effect on these activities. All mycoplasmal species contained nuclease activity, but Mycoplasma capricolum was unusual because its activity was dependent upon magnesium and was inhibited by calcium. We developed a sodium dodecyl sulfate-polyacrylamide gel electrophoresis system that produced reproducible nuclease patterns, and this system was used to determine the apparent molecular weights of the nuclease proteins. An examination of 20 mycoplasmal species failed to identify common bands in their nuclease patterns. An examination of 11 Mycoplasma pulmonis strains, however, indicated that nuclease patterns on polyacrylamide gels may provide a means for categorizing strains within a species. Our results suggest that nucleases are important constituents of mycoplasmal membranes and may be involved in the acquisition of host nucleic acids required for growth.

Differential protein expression and surface presentation generate high-frequency antigenic variation in Mycoplasma fermentans

Mycoplasma fermentans, a wall-less prokaryote, is currently under investigation as a potential human pathogen. Recently, several surface lipoproteins have been shown to vary in expression between M. fermentans strains. Using specific antibodies to these lipoproteins, we investigated the extent and nature of antigenic variation within this species. Immunoscreening of type strain PG18 agar-grown colonies revealed marked heterogeneity in expression of distinct surface lipoproteins. Subsequent isolation and propagation of clonal isolates established isogenic lineages which displayed high-frequency (10(-2) to 10(-5) per generation) antigenic phase variation. [35S]cysteine-labeled protein profiles and Western immunoblots of phase-variant clones showed that several distinct integral membrane proteins undergo noncoordinate variation in expression. In addition to differential expression of epitope-bearing lipoproteins, differential accessibility of epitopes to antibodies was also documented as a mechanism generating surface phenotypic variation. Examination of one strain-variant antigen showed high-frequency phase variation to underlie previously observed antigenic differences between strains of this species. Thus, M. fermentans has a complex system capable of creating rapid changes in surface mosaics. This may profoundly affect mycoplasma-host interactions and may limit the methods by which populations of M. fermentans may be studied in vivo.

A family of strain-variant surface lipoproteins of Mycoplasma fermentans

The wall-less procaryote Mycoplasma fermentans is currently being examined as an agent potentially associated with human disease, including infectious processes affecting immunocompromised individuals. To delineate and understand the interactions of M. fermentans with its host, specific membrane surface components were characterized as markers for detecting the organism and for assessing heterogeneity in antigenic surface architecture within this mycoplasma species. Detergent phase fractionation of metabolically labeled organisms of type strain PG18 identified a family of prominent integral membrane proteins; several of these labeled with 35S-cysteine and 3H-palmitate, which are characteristics of procaryotic lipoproteins. Specific monoclonal and polyclonal antibodies raised to strain PG18 components further distinguished seven of these membrane proteins, which were localized on the organism's surface by monitoring their selective susceptibility during trypsin treatment of intact cells. With these antibodies, Western immunoblot profiles of surface membrane antigens expressed on strain PG18 were compared with those expressed on the recently identified Incognitus strain of M. fermentans, as well as with several other human and animal mycoplasma species. While the antibodies were specific for M. fermentans, marked differences were observed between the strains in the size of one surface lipoprotein and in the apparent levels of several antigens expressed in the cultured populations analyzed. Some monoclonal antibodies to strain PG18 and a previously described monoclonal antibody to strain Incognitus showed apparent selectivity for the strain used for immunization. Monoclonal antibodies developed here recognize stable epitopes defining a family of surface lipoproteins and provide critical tools to determine the basis of surface variation in this mycoplasma species and to assess the location and antigenic phenotypes of organisms in the human host.

Mechanisms of attachment of Mycoplasma arthritidis to host cells in vitro

Although other investigators have reported that Mycoplasma arthritidis failed to attach to several types of mammalian cells in vitro, we showed that it attached well to rat synovial fibroblasts, lung cells, and skin cells but not to kidney cells, suggesting that receptor sites are unequally expressed or distributed among different rat tissues. M. arthritidis also attached poorly to canine kidney and to baby hamster kidney cells, although it did attach to human fetal lung and fetal amnion cells. Four M. arthritidis strains, two virulent and two avirulent, all attached equally well to rat lung cells. Attachment was inhibited by trypsinization, suggesting that the major adhesins are protein. Fab' fractions of rabbit antisera against four M. arthritidis strains partially inhibited adherence of both homologous and heterologous strains, although not to the same extent, indicating some degree of antigenic heterogeneity among their adhesins. A filter-cloned variant of M. arthritidis 158p10p9, designated LC1, attached poorly compared with the parent strain. Missing from this variant were two proteins migrating within the 81- to 90-kDa range by polyacrylamide gel electrophoresis; in their place was a 24-kDa antigen that may be a truncated version of one of these proteins. A monoclonal antibody that partially inhibited attachment recognized all these peptides by Western immunoblotting. An additional attachment-inhibiting monoclonal antibody recognized a 71-kDa antigen present in both low-adherence and fully adherent populations. The low-adherence variant LC1 induced slightly but significantly less arthritis in Lewis rats than did a fully adherent clone.

NaCl-tolerance of Campylobacter isolates from birds and Campylobacter type strains and variation of their serological behaviour

Growth on media containing 1.5% NaCl is one of the criteria for phenotypical differentiation of Campylobacter laridis from other thermophilic Campylobacter spp. Campylobacter isolates from birds and Campylobacter type strains could be adapted to growth at 3% NaCl within 19 to 72 subsequent passages on nutrient agar with increasing salt contents. The acquisition of salt-tolerance was stable after ten passages on media without salt and did not induce changes in other phenotypical characteristics. The results of slide agglutination demonstrate changes in the antigenic pattern of the Campylobacter strains after growth in salt. Heat-labile and heat-stable antigens of the salt-tolerant variants of Campylobacter type strains differed from those of the parent strains.

Adaptive surface variation in mycoplasmas

Mycoplasmas excel as infectious agents, despite their very small genomes. In one mycoplasma species, adaptive flexibility is enhanced by an elegant genetic system that diversifies the membrane surface through a set of variable lipoproteins (Vlps). A family of vlp genes supplies divergent coding sequences and undergoes high-frequency mutations, thus creating large repertoires of surface mosaics and structural variants.

Antigenic variation in Mycoplasma hyorhinis: increased repertoire of variable lipoproteins expanding surface diversity and structural complexity

VlpE is characterized as a new member in a family of variable surface lipoproteins (Vlps) of Mycoplasma hyorhinis. VlpE shows phenotypic variation in expression and size within isogenic lineages of some strains but is absent from lineages of other strains that express only three previously known Vlps. Expression of four Vlps in some cells further indicates the presence and usage of an expanded reservoir of Vlp coding sequences, which greatly increases the capacity for surface diversification.