The medical and agricultural importance of members of the genus Mycoplasma and related genera has led to the extensive cataloging of many of these organisms by culture, serology, and small subunit rRNA gene and whole genome sequencing.
A recent focus in the sub-discipline of molecular phylogenetics has both clarified and confused certain aspects of the organization of the class Mollicutes, and while a truce of sorts has been reached, the area is still somewhat of a moving target (Johansson and Pettersson, 2002).
The name mollicutes is derived from the Latin mollis (soft) and cutes (skin), and all of these bacteria do lack a cell wall and the genetic capability to synthesize peptidoglycan.
While the trivial name 'mycoplasmas' has commonly denoted all members of this class, this usage is somewhat imprecise and will not be used as such here.
Despite the lack of a cell wall, Mycoplasma and relatives have been classified in the phylum Firmicutes consisting of low G+C Gram-positive bacteria such as Clostridium, Lactobacillus, and Streptococcus based on 16S rRNA gene analysis.
The cultured members of Mollicutes are currently arranged into four orders: Acholeplasmatales, Anaeroplasmatales, Entomoplasmatales, and Mycoplasmatales.
The order Mycoplasmatales contains a single family, Mycoplasmataceae, which contains two genera: Mycoplasma and Ureaplasma. Historically, the description of a bacterium lacking a cell wall was sufficient to classify it to the genus Mycoplasma and as such it is the oldest and largest genus of the class with about half of the class' species (107 validly described) each usually limited to a specific host and with many hosts harboring more than one species, some pathogenic and some commensal.
In later studies, many of these species were found to be phylogenetically distributed among at least three separate orders. A limiting criterion for inclusion within the genus Mycoplasma is that the organism have a vertebrate host.
In fact, the type species, M. mycoides , along with other significant mycoplasma species like M. capricolum, is evolutionarily more closely related to the genus Spiroplasma in the order Entomoplasmatales than to the other members of the Mycoplasma genus. This and other discrepancies will likely remain unresolved because of the extreme confusion that change could engender among the medical and agricultural communities.
The remaining species in the genus Mycoplasma are divided into two non-taxonomic groups, hominis and pneumoniae, based on 16S rRNA gene sequences.
The hominis group contains the phylogenetic clusters of M. bovis, M. pulmonis, and M. hominis, among others.
The pneumoniae group contains the clusters of M. muris, M. fastidiosum, U. urealyticum, the currently unculturable haemotrophic mollicutes, informally referred to as haemoplasmas (recently transferred from the genera Haemobartonella and Eperythrozoon), and the M. pneumoniae cluster.
This cluster contains the species (and the usual or likely host) M. alvi (bovine), M. amphoriforme (human), M. gallisepticum (avian), M. genitalium (human), M. imitans (avian), M. pirum (uncertain/human), M. testudinis (tortoises), and M. pneumoniae (human). Most if not all of these species share some otherwise unique characteristics including an attachment organelle, homologs of the M. pneumoniae cytadherence-accessory proteins, and specialized modifications of the cell-division apparatus.
A detailed analysis of the 16S rRNA genes from the order Mollicutes by Maniloff has given rise to a view of the evolution of these bacteria that includes an estimate of the time-scale for the emergence of some groups or features (Maniloff, 2002). This analysis suggests that about 600 million years ago (MYA), late in the Proterozoic era, Mollicutes branched away from the low G+C Gram-positive ancestor of the streptococci, losing their cell wall.
At this time on Earth, molecular oxygen was present in the atmosphere at 1%, and the fossil record shows that multicellular marine animals had recently spread in the Cambrian explosion.
One hundred million years later the requirement for sterols in the cytoplasmic membrane evolved along with the change to the alternate genetic code.
Also, the ancestor of the genera Spiroplasma and Entomoplasma (primarily plant and insect pathogens) and Mycoplasma emerged at this time and would itself diverge into the Spiroplasma-Entomoplasma and Mycoplasma lineages approximately 100 million years after that.
This diversity coincided with the origin of land plants 500 MYA. It appears that the calculated rate of evolution for the Mycoplasma group increased several fold about 190 MYA, soon after the appearance of vertebrates, while the Spiroplasma-Entomoplasma ancestor continued to evolve at the previously shared slower rate until about 100 MYA, when angiosperms and their associated pollinating insects appeared.
Then the evolution rate of these bacteria appears to have also increased significantly. This is an attractive hypothesis, but while it tracks the emergence of several of the unusual characteristics of Mycoplasma and related organisms, it does not address the selective pressures driving their evolution, except perhaps the widespread close association of a parasite with a specific host.
The advantages of a reduced genome, cell wall-less structure, and alternate genetic code remain murky.
International Committee on Systematics of Prokaryotes; Subcommittee on the taxonomy of Mollicutes/2002
Several novel species in the class Mollicutes were described in the previous biennium. These included Mycoplasma agassizii, Mycoplasma alligatoris and Mycoplasma microti, which were isolated from the desert tortoise, the American alligator and the prairie vole, respectively.
Some organisms were also transferred into the genus Mycoplasma from other genera: Mycoplasma haemofelis, Mycoplasma haemomuris, Mycoplasma suis and Mycoplasma haemocanis were transferred from the genus Haemobartonella and Mycoplasma wenyonii from the genus Eperythrozoon.
Three novel `Candidatus Mycoplasma' species were described. These were `Candidatus Mycoplasma haemodidelphidis' from the opossum, `Candidatus Mycoplasma haemolamae' from the alpaca and `Candidatus Mycoplasma heamominutum' from the cat.
Ureaplasma parvum was split off from Ureaplasma urealyticum as a separate novel species and there was a novel `Candidatus Phytoplasma' species, `Candidatus Phytoplasma brasiliense'.
Minute 4. Spiroplasma taxonomy. R. F. Whitcomb summarized recent work on the phylogeny and taxonomy of spiroplasmas and other mollicutes that included serology, 16S rRNA gene sequencing, phylogeny and the acquisition of 16 candidate novel species from tabanids (horseflies) collected in Costa Rica and Australia.
Phylogenetic trees constructed by using maximum-parsimony (the preferred method), maximum-likelihood or distance-matrix-based methods gave trees with similar topologies. Serology had been fundamental in the establishment of the existing spiroplasma species and groups.
More than 5000 serological cross-tests were carried out and none disagreed with the phylogenetic trees.
In terms of host association, the spiroplasmas from horseflies fell into two groups, which were earlier judged as short or long in morphology. Members of the former group grow to 1011 colony-forming units (c.f.u.) in culture, are easily filterable, utilize arginine, and the G+C content of their DNA is high (29-31 mol%).
Although the second group of horsefly spiroplasmas is technically not monophyletic, its members occupy a group of closely related clades of the apis group. The G+C content of the DNA of members of this clade set is 26 mol%, the members' cells are very long, grow to 1010 c.f.u. in culture and glucose, but not arginine, is utilized.
These two groups appear as clades in the overall tree. The research group has compiled the polyphasic classification using all the available information on G+C content, genome size, serology, sterol requirement, glucose utilization etc. This approach permits an organism to be classified in the absence of a 16S rRNA gene sequence.
Serological methods for identification of spiroplasmas were regarded as highly appropriate, especially when used in conjunction with 16S rRNA gene sequence analysis. These tests are less cumbersome than the DNA-DNA reassociation method. Furthermore, serology operates at the same level of taxonomic resolution as DNA-DNA reassociation.
Availability of antisera, although not currently a problem, could become one in the future. At this point, antisera to all existing species, groups, and candidate species and/or groups are available.
Minute 5. Phytoplasma taxonomy. G. Firrao reported on phylogeny and taxonomy of phytoplasmas. Classification of phytoplasmas will in the near future be based on polyphasic taxonomy. In a paper on the aster yellows cluster of phytoplasmas, classification was based on sequence comparisons of the 16S rRNA genes and the tuf genes [Marcone et al., Int J Syst Evol Microbiol 50 (2000), 1703-1713].
By this approach, the number of groups was reduced, although it may not be applicable to all phytoplasmas. A short manuscript on the genus `Candidatus Phytoplasma' was in preparation in which the essential properties of this Candidatus genus are described. A list of species will probably also be included. The proposed classification will be based on results already published and an alignment of all 16S rRNA gene sequences currently available for phytoplasmas (about 150) will be provided. About 30 `Candidatus Phytoplasma' species will be described, based on a cut-off level of 97.5 % sequence similarity of the 16S rRNA genes.
The difficulties associated with defining a type for a Candidatus genus were discussed and another concern was that authors who determined the 16S rRNA gene sequences of a particular `Candidatus Phytoplasma' species might not necessarily be involved in naming the species, thus denying them some credit for their work. It was suggested that a series of short papers defining the various novel species should be published in association with the paper on the genus `Candidatus Phytoplasma'.
A problem among the phytoplasmas is the existence of isolates that are very similar, and which would normally be classified as members of the same species, but which occur in different hosts and cause very different diseases. For plant pathologists, it would be useful to have such pathogenic strains designated taxonomically. However, the solution is not to contravene the accepted species concept in bacteria [Wayne et al., Int J Syst Bacteriol 37 (1987), 463-464]. The problem can be resolved by using pathovar designations. This system has been used successfully with other plant-pathogenic prokaryotes.
Minute 6. Ureaplasma taxonomy. J. A. Robertson, in a written statement, pointed out the limitations in our understanding of ureaplasma pathogenicity. She emphasized that differences in molecular structure and function, even small differences between strains, may be useful in studying evolution and phylogeny but may not be of any value for predicting the biological or pathogenic properties.
Minute 7. Final report on 16S rRNA gene sequencing of the genus Mycoplasma. K.-E. Johansson reported that the 16S rRNA gene sequences from all members of the genus Mycoplasma were available from GenBank and a list of the most recently sequenced species could be found in the book Molecular Biology and Pathogenicity of Mycoplasmas, edited by S. Razin and R. Herrmann (Kluwer, New York, 2002). Sequencing of the 16S rRNA genes of the remaining species of the genus Acholeplasma should be completed by the July 2004 meeting of the subcommittee.
Minute 8. Problems in differentiation of Mycoplasma agalactiae and Mycoplasma bovis. Discussion took place on the similarity between Mycoplasma agalactiae and Mycoplasma bovis and the associated problems in differentiation of these species. To illustrate this, R. F. Rosenbusch described a disease with up to 10 % mortality in American bison, which had a mycoplasmal aetiology. 16S rRNA gene sequencing suggested that the mycoplasma could be Mycoplasma agalactiae, which is exotic to the USA; however, RFLP and serology suggested that it was Mycoplasma bovis, and sequencing of the uvrC gene finally confirmed this. It should be noted that the intraspecific variation in the 16S rRNA genes is high within Mycoplasma agalactiae and Mycoplasma bovis and that there are strains that are difficult to identify by 16S rRNA gene sequencing alone.
Minute 9. Problems arising from Candidatus proposals. D. G. Pitcher described problems arising during the previous biennium as a result of the transfer of members of the genera Eperythrozoon and Haemobartonella to the genus Mycoplasma and `Candidatus Mycoplasma' species status. These organisms apparently have a close phylogenetic relationship to mollicutes. The 16S rRNA genes of a few members of Eperythrozoon and Haemobartonella have been sequenced, and the results indicate that these organisms form a distinct clade within the pneumoniae group. A letter of objection had been received by the Editor of the International Journal of Systematic and Evolutionary Microbiology indicating that the genus Eperythrozoon should have priority over the genus Mycoplasma because it was published a year earlier.
Minute 10. A common name for Mycoplasma mycoides subsp. capri and Mycoplasma mycoides subsp. mycoides LC. At the 2000 subcommittee meeting in Fukuoka, there appeared to be no arguments against considering these two organisms as a single species, with the possible name Mycoplasma capri, although there was no clear agreement as to whether there should be a single species or two subspecies. E. Vilei, on behalf of J. Frey, reported that a common name for Mycoplasma mycoides subsp. capri and Mycoplasma mycoides subsp. mycoides LC had also been discussed at former COST meetings. (COST is an intergovernmental framework for European Co-operation in the field of Scientific and Technical Research.)
Three publications had indicated the similarity of these mycoplasmas: comparison of 16S rRNA gene sequences [Pettersson et al., J Bacteriol 178 (1996), 4131-4142], comparison of a gene coding for a putative membrane protein [Thiaucourt et al., Vet Microbiol 72 (2000), 251-268] and comparison of the gene encoding lipoprotein LppA [Monnerat et al., Clin Diagn Lab Immunol 6 (1999), 224-230]. The differences found by 16S rRNA gene sequencing, and by protein analysis, suggest that the organisms represent different serovars, but not different subspecies. Therefore, to avoid confusing the pathogenic Mycoplasma mycoides subsp. mycoides SC with the LC subspecies, LC strains might be grouped with Mycoplasma mycoides subsp. capri and the `LC' terminology abandoned. The name Mycoplasma mycoides subsp. mycoides could then be reserved for SC strains. Considerable discussion ensued as to whether enough strains within each taxon had been examined to justify these taxonomic changes and whether DNA-DNA reassociation values could be established on more strains. The potential value of using a pathovar system of classification was again noted.
K. Sachse, in studying the 23S rRNA gene region of these organisms, had found that the SC and LC strains and Mycoplasma mycoides subsp. capri clustered very closely together. However, he suggested that further evidence from different loci should be explored for these taxa. K. S. Wise cautioned that mobile elements and gene order might be of great importance when interpreting relationships at this level. He wondered if there were suitable housekeeping genes that might become a `standard set' for determining phylogenetic distances in addition to the 16S rRNA gene. R. F. Whitcomb suggested the 23S-16S intergenic spacer region (ISR) as a possible locus since data on this region are growing and it appears to function taxonomically. D. G. Pitcher agreed, indicating that the ISRs of Mycoplasma spp. that he had examined were very conserved within a species but very different from one another.
At the conclusion of these discussions it was clear that some subcommittee members were in favour of a revised nomenclature while others preferred a more cautious approach pending further studies. The Chairman, J. M. Bradbury, suggested that those directly involved in the problem should pool their data, prepare a summary and then take informal advice from members of the Judicial Commission. K.-E. Johansson (Secretary) pointed out that the subcommittee itself cannot suggest a new name but can give a recommendation; the interested researchers should collaborate to draw up a proposal for publication.
Minute 11. Species or subspecies status for bovine group 7. On behalf of J. Frey, E. Vilei introduced the complex situation of mycoplasmas in bovine group 7. The relationship between this group and Mycoplasma capricolum subsp. capripneumoniae, Mycoplasma capricolum subsp. capricolum and Mycoplasma mycoides subsp. mycoides SC had not yet been clearly resolved. It would be helpful to give species designation to the bovine group 7 mycoplasmas because there are numerous reports describing their isolation from arthritic joints and mastitic milk. The Swiss group was inclined to propose the name Mycoplasma mycoides subsp. boviarthritidis. R. Rosenbusch pointed out that Australian strains of bovine group 7 are more pathogenic than those seen in the USA and a method of distinguishing between them is needed for regulatory purposes.
K. S. Wise again warned of the possible pitfalls of comparing pieces of mycoplasma genomes because they represent large mixtures of mosaics of genes and because they appear to be more `scrambled' in mollicutes than in many other groups of bacteria. Once complete genome sequences are known it might be necessary to re-examine the whole problem since the genomes of mollicutes may be unstable. He urged caution when starting to use new regions of the genome.
The Chairman reminded the meeting that there was still in existence an ad hoc group on the taxonomy of the Mycoplasma mycoides cluster. Current members included K.-E. Johansson, S. Razin and J. G. Tully. It was suggested that K. Sachse, J. Frey and S. Djordjevic join these members. K.-E. Johansson expressed the wish to step down as Chair of this group and suggested J. Frey as his successor.
Minute 12. Problems involved in classification schemes based on a single genomic locus or region. K. Sachse re-emphasized that the taxonomy of the mollicutes is in a transitional period before whole genome sequences are available. In the meantime, he suggested that workers should base new classifications not only on 16S rRNA but also on the sequences of more genomic loci. His team had sequenced 44 species of mycoplasma in the 23S rRNA gene signature region (the entire gene is almost 3 kb) and found a rather characteristic central domain of about 500 nucleotides, which has the potential to differentiate between species of Mycoplasma and might also prove useful for other mollicutes. He suggested that future classifications and definitions might be based on 16S rRNA gene sequences, the ISR and this 23S rRNA gene signature region, and possibly also the elongation factor tu. By using four sets of sequences, problems such as those of Mycoplasma bovis and Mycoplasma agalactiae (Minute 8 above) might be avoided.
B. C. Kirkpatrick suggested that the ISR, in phytoplasma work, is good for identification purposes but not for constructing a phylogeny. Other workers have had some success in generating trees from these data, so the utility of this region appears to differ in its potential utility among taxa. The possibility of using amplified fragment length polymorphism for examining diversity was raised but K.-E. Johansson pointed out that, while the technique is useful for examining strains or closely related species, it is not useful for work concerning distinct species.
Minute 13. Higher classification of mollicutes. R. F. Whitcomb described the polyphyletic nature of the genus Mycoplasma, which consists essentially of two monophyletic clusters of organisms (although the hominis and pneumoniae groups are technically paraphyletic). As a possible taxonomic resolution, he introduced the concept of subgenus status for the Mycoplasma mycoides phylogenetic group and for the group containing the remaining members of the genus Mycoplasma. He discussed the pros and cons of an active versus passive approach to taxonomic resolution of this problem. Support for the proposal was mixed and, in a written statement, J. G. Tully suggested that opinions should be sought from experts in veterinary medicine and taxonomy on the implications of such name changes, especially as there may be far-reaching consequences for some changes regarding international quarantine regulations etc.
R. F. Whitcomb raised another problem in higher classification (genus) with regard to testing for sterol requirement. The genera Spiroplasma, and especially Mesoplasma and Entomoplasma, are polyphyletic with regard to this requirement and work performed [Rose et al., Int J Syst Bacteriol 43 (1993), 527-532] after the 1993 revision of Mollicutes indicated that the ability to grow in Tween 80 and, in fact, the sterol test itself, were not particularly helpful in classifying these organisms. He pointed out that the results from the division of mesoplasmas and entomoplasmas using this test do not correlate with any other phenotypic property. In a written statement J. G. Tully agreed that, based on 16S rRNA gene sequencing, members of the Entomoplasmataceae probably represent a single cluster. However, there were more than 12 mesoplasma strains that should be sequenced before changes could be proposed regarding sterol and Tween 80 tests. The subcommittee would make every effort to encourage completion of the 16S rRNA gene sequencing of this group. There was some concern that any changes regarding recommendations for these tests would not be in the new edition of Bergey's Manual. However it was agreed that a statement could be added to indicate that changes are expected and would appear in the relevant literature.
Minute 14. Revision of minimum standards. It was agreed that, now that the 16S rRNA gene sequences were available for most Mollicutes, including all members of the genus Mycoplasma, it would be appropriate to revise the minimum standards for the description of novel species. Much revision could be agreed by email correspondence. The subcommittee considered that determination of the 16S rRNA gene sequence should be mandatory, although K. S. Wise pointed out that large insertion elements could contribute up to 15 % of the genome size and, because their presence or absence might not be consistent within a species, results could be misleading. It was felt that the determination of genome size, while potentially useful in some cases, should not be mandated. It was agreed that genome size could be useful in ruling out the presence of an L-phase bacterial variant. This was also true of G+C determination.
Minute 15. The proposal to modify recommendation 30b of the Bacteriological Code. A proposal had been made [Christensen et al., Int J Syst Evol Microbiol 51 (2001), 2221-2225] that descriptions of novel species of bacteria should be based on as many strains as possible, with a minimum of five. The subcommittee agreed that this condition would be impossible to fulfil for many fastidious mollicutes. For example, D. Taylor-Robinson pointed out in a written statement that Mycoplasma genitalium, a human pathogen of emerging importance, would never have been described under those circumstances since there are so few isolates. It was agreed that the proposal could inhibit the advancement of the field even though there was merit in examining as many strains as practical under the prevailing circumstances.
Note: The proposition by Christensen et al. was discussed by the International Committee on Systematics of Prokaryotes (ICSP) in July 2002, who decided that it would remind systematists of best practice but that there should be no hindrance to the description of single-strain taxa [Saddler, Int J Syst Evol Microbiol 55 (2005), 533-537].
Minute 16. The status of the Mycoplasma Culture Collection at the University of Florida. M. K. Davidson, the Director of the collection, reported approximately 85 antisera and 170 seed cultures had been supplied to workers in many different countries. The greatest numbers of requests had been for M. pneumoniae cultures by pharmaceutical companies. Charges were as recommended by the IOM: $100 per vial to non-IOM members and $50 to IOM members. Production of some antiserum could be funded from this income.
Discussion followed on the appropriateness of using type strain cultures from this collection vis � vis cultures from the American Type Culture Collection (ATCC). In the event of conflicting data, it would become necessary to use ATCC material, to be sure the discrepant result was not the result of a mix-up. It was agreed that the University of Florida collection should seek association with the World Federation of Culture Collections. K. B. Waites suggested that a list of available cultures and antiserum be posted on the IOM website.
Minute 17. Liaison with IRPCM. K. B. Waites, on behalf of the IRPCM, asked for up-to-date information concerning mollicute taxonomy and a list of all the currently recognized species to be supplied for the IOM website. (Note: this has now been accomplished through the auspices of the ICSP and has a link from the IOM website.)
Minute 18. Future meeting schedule. The next meeting is in Athens, GA, USA in July 2004. K. B. Waites indicated that there was increasing time pressure on IOM Congresses and the ancillary meetings, which include the subcommittee meeting. This could pose considerable problems for the subcommittee since it meets only once each biennium, and the agenda contains important and complex issues that require full discussion and debate. The Chairman agreed to ask the Congress organizers to allocate more time to the meeting and to avoid running it in parallel with scientific sessions. It was noted that local conference organizers have so far provided a meeting room from their budget.
Minute 19. Election of officers and membership changes. The Chairman and Secretary were re-elected to another term of office. A. Blanchard was appointed as a new member.
Minute 20. Current membership. The current membership of the subcommittee is as follows: J. M. Bradbury (Chairman; UK), K.-E. Johansson (Secretary; Sweden), A. Blanchard (France), J. M. Bov� (France), G. Christiansen (Denmark), G. Firrao (Italy), J. Frey (Switzerland), R. Harasawa (Japan), B. C. Kirkpatrick (USA), H. C. Neimark (USA), J. D. Pollack (USA), S. Razin (Israel), J. A. Robertson (Canada), R. F. Rosenbusch (USA), K. Sachse (Germany), J. G. Tully (USA) and R. F. Whitcomb (USA). Advisory member is D. Taylor-Robinson (UK). Ex-officio members are K. B. Waites (IOM IRPCM Chair; USA), M. K. Davidson and J. K. Davis (USA) and D. G. Pitcher (UK).
Minute 21. Adjournment. The meeting was adjourned at 12 : 35 on 7 July 2002.
Session 2 - Open meeting
Minute 22. Call to order. The open meeting of the subcommittee was called to order at 13 : 30 on 12 July 2002 by the Chairman, J. M. Bradbury.
Minute 23. Record of attendance. The members present were J. M. Bov�, J. M. Bradbury (Chairman), G. Christiansen, G. Firrao, K.-E. Johansson (Secretary), B. C. Kirkpatrick, J. D. Pollack, S. Razin, R. F. Rosenbusch. K. Sachse and R. F. Whitcomb. Ex-officio members present were D. G. Pitcher of the Public Health Laboratory Service (UK), M. K. Davidson of the University of Florida (USA) and K. B. Waites, Chair of the Board of the International Organization of Mycoplasmology (IOM) International Research Programme on Comparative Mycoplasmology (IRPCM).
Minute 24. J. M. Bradbury welcomed approximately 75 attendees to the meeting and gave a brief explanation of the function of the subcommittee, its relationship to the ICSP and of the arrangement that allows the subcommittee to meet alongside the biennial IOM congress. She acknowledged the valuable contributions of the late Roger Miles to the work of the subcommittee and also those of F. Laigret, who had recently tendered his resignation. The Chairman congratulated J. G. Tully on the award of the Bergey medal. This prestigious award is in recognition of his lifelong contributions to the field of systematic bacteriology.
Novel species published during the biennium included Mycoplasma agassizii, Mycoplasma alligatoris and Mycoplasma microti. Several organisms were transferred into the genus Mycoplasma from other genera. These were Mycoplasma haemofelis, Mycoplasma haemomuris, Mycoplasma suis and Mycoplasma haemocanis from the genus Haemobartonella and Mycoplasma wenyonii from the genus Eperythrozoon. Three novel `Candidatus Mycoplasma' species were also described: `Candidatus Mycoplasma haemodidelphidis' from opossum, `Candidatus Mycoplasma haemolamae' from alpaca and `Candidatus Mycoplasma haemominutum' from cats. Ureaplasma parvum was split off from Ureaplasma urealyticum as a separate novel species and one novel `Candidatus Phytoplasma' species was described. This was `Candidatus Phytoplasma brasiliense'.
Minute 25. Spiroplasma taxonomy. R. F. Whitcomb summarized the results of a large phylogenetic study by Gasparich and co-workers that had been undertaken with Spiroplasma, noting that they fell into distinct clades similar to those of Weisburg et al. [J Bacteriol 171 (1989), 6455-6467]. He emphasized the valuable part played by serology, which appears to have a resolving power similar to that of DNA-DNA reassociation to differentiate between species, but he drew attention to the difficulties involved in producing or obtaining antisera.
Minute 26. Phytoplasma taxonomy. G. Firrao reported that a manuscript was in preparation that would describe the properties of the genus `Candidatus Phytoplasma' that should prove valuable to workers in this field.
Minute 27. Mycoplasma taxonomy. K.-E. Johansson reported that the 16S rRNA gene sequences from all members of the genus Mycoplasma were now available from GenBank. The Chairman congratulated him on this achievement, which would be of great benefit to the field. J. G. Tully's contribution in assembling antisera to the vast majority of the species of Mollicutes was also recognized. The transfer of several members of the genus Haemobartonella and the genus Eperythrozoon to the genus Mycoplasma was described, together with the associated problems. K.-E. Johansson then summarized the difficulties regarding the possible amalgamation of Mycoplasma mycoides subsp. capri and Mycoplasma mycoides subsp. mycoides LC into a single taxon and indicated that some of the subcommittee favoured obtaining more evidence before deciding about further division into subspecies. The problems associated with classification of bovine group 7 were also pointed out.
Minute 28. Classification schemes based on a single genomic locus or region. K. Sachse emphasized that workers should base new classifications not only on 16S rRNA gene sequences but also on the sequences of more genomic loci. He mentioned the work of his team in sequencing part of the 23S rRNA gene signature region and its potential to differentiate between species of Mycoplasma. There was general consensus that future classifications should be based on more than just 16S rRNA gene sequences and that both the ISR and this 23S rRNA gene signature region could prove to be of value.
Minute 29. Higher classification of mollicutes. R. F. Whitcomb presented his ideas for proposing a subgenus status for the two polyphyletic groups within the genus Mycoplasma and the Chairman cautioned about the possible implications for veterinary medicine and quarantine regulations.
Minute 30. Revision of minimum standards. R. F. Whitcomb stated that members of the subcommittee were preparing new minimum standards for the description of novel species of Mollicutes. There were several issues to be resolved but the general aim would be to reduce the burden of serological testing by restricting the tests to the clade identified in 16S rRNA gene analyses; this approach had been recommended by K.-E. Johansson and his colleagues.
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