Background on Mycoplasma taxonomy, physiology and pathologyThe taxonomic connection of mycoplasmas to various other microbes has actually beencontroversial(Razin, 1992).Prior to the 1930"s, mycoplasmas wereconsidered to it is in viruses due to the fact that they were so tiny that they passedthrough filters that blocked passage of ordinary bacteria. Later they werethought to be symbionts growing with the Streptobacillus bacteria, and also thenthey to be proposed come be plain bacteria that had actually lost their cell wall (Lform bacteria). By the 1960"s, both base composition and hybridizationanalysis of the genomic DNA verified that mycoplasmas to be not associated tostable L creates of ordinarily walled bacteria. However, currentexplanations for the advancement of mycoplasmas argue for "degenerative"evolution from walled bacteria; for this reason the induction the L forms may be apresent-day recapitulation that one action in mycoplasma evolution. Mycoplasmas are the smallest and also simplest self-replicating organisms.Their genome sizes variety from about 540 to 1300 kb, v a G+C content23-41 mol%. Back the little size, both the the mycoplasma and itsgenome, initially suggested that they to be the many primitive extantorganisms, nucleic mountain hybridization and sequencing studies currently indicatethat lock are obtained from the gram-positive branch that walled eubacteria.Their evolution from this walled bacteria requires a substantial reductionin genome size, including loss the the functions required because that synthesis andmaintenance the a bacterial cabinet wall. A complete of 92 types were recognized within the genus Mycoplasma in 1992, andseveral related genera, families and also orders space now known within theclass Mollicutes. The surname "mycoplasma" was acquired from the Greek wordsmykes, for fungus, and also plasma, because that something formed or molded. The classMollicutes is the only course in the division Tenericutes (wall-lessbacteria), which is in ~ an equivalent taxonomic level to the gram-positiveeubacteria, the gram-negative eubacteria, and the archaebacteria(comprising four departments of the kingdom Procaryotae in 1992). The termmycoplasma is widely provided to refer to any type of member that the class Mollicutes.Phylogenetic data ar the mycoplasmas together a monophyletic swarm withinthe gram-positive bacteria, specifically the family Bacillaceae, whichinclude the genera Clostridium, Lactobacillus, Bacillus,and Streptococcus(Maniloff, 1983;Maniloff, 1992).If undoubtedly mycoplasmasand the Lactobacillus team share a common ancestor, then it is most likely thatmycoplasma genomes advanced by "attrition" or"degenerative evolution" froman organism with a genome of about 2,200 to 2,500 kb (current sizes ofgenomes in the Lactobacillus group)(Maniloff, 1992).Thus theindependent taxonomic condition of mycoplasmas remains at ideal controversial.Mycoplasmas room evolving much more rapidly than are eubacteria(Maniloff, 1992).The smallest mycoplasmal genomes, of around 600 kb, may be the minimalgenetic intricacy for a living organism, since this is the minimal sizeobtained on at least three independent present of mycoplasmas(Maniloff, 1992). "Mycoplasmas typically exhibit a quite strict host and tissue specificity,probably reflecting their nutritionally exacting nature and also obligateparasite setting of life."(Razin, 1992).M. Pneumoniae was long regarded together a microorganism strictly limited to thetracheal epithelium the humans, however it has actually been recovered from joints ofimmunocompromised patients and also other tissues even in immunocompetentpatients. M. Genitalium was first isolated indigenous the urethral discharge oftwo guys with non-gonococcal urethritis (NGU) and also was proactively studied as apossible pathogen. However, subsequent initiatives to isolate an ext M.genitalium strains from the genital tract have not prospered (atleast up till 1992). M. Genitalium has actually been uncovered in several throatisolates in a mixture with M. Pneumoniae. "The 2 organisms share genomicsequences and also epitopes." M. Genitalium is a "fastidious"bacterium, whichprevents its main cultivation. (So possibly it is no surprising thatinvestigators have not to be able to culture it from vul or from throatvery easily). Application of PCR does show M. Genitalium in about 10 of150 genital tract specimens native 100 patient (with NGU?). Previous characterization of mycoplasmal genomes measured the size and also G+Ccontent because that M. Pneumoniae of about 840 kb and 41 mol% (the highest knownvalue for mycoplasmas) and for M. Genitalium of around 590 kb and also 32.5 mol%(Herrmann, 1992).There are countless repetitive DNA sequences however thesedo no lead to comprehensive genome instability. This contrasts v the"observation that an unusually high number of spontaneously emerging mutantscan it is in isolated." Mycoplasmal recurring DNA sequences include both multiple duplicates ofprotein-coding regions, such together that for portions of the P1 adhesin the M.pneumoniae and M. Genitalium, and insertion sequences such as RS-1,which belongs come the IS3 class of insertion assignment initiallycharacterized in E. Coli(McIntosh et al., 1992). Mycoplasma pneumoniae is the leading cause of pneumonia in enlarge childrenand young adults(Krause and also Taylor-Robinson, 1992).M. Pneumoniaecauses tracheobronchitis and also primary atypical pneumonia. The chronic,flu-like nature the the mycoplasma-caused disease, contrasting with theabrupt, rigorous onset of many bacterial pneumonias, has actually led to itsnickname "walking pneumonia". M. Pneumoniae has been isolated native theoropharynx and lower respiratory tract tract the infected humans. It clearly is apathogen, in contrast to several other mycoplasmal species that show up tobe part of the regular flora. "M. Pneumonia have the right to catabolize glucose ormannose but not make use of arginine together a carbon and energy source. That iscapable that reduction reactions under both anaerobic or aerobic conditions,and both hydrogen peroxide and the superoxide anion are by-products ofglucose metabolism. It lacks many common enzyme equipment that usage iron as acofactor, such as tricarboxylic mountain cycle enzyme or a finish electrontransfer chain comprise cytochromes. Iron has actually been detected in themembrane, and also mycoplasmas room thought to use a truncated electron transportsystem to create energy." Ureaplasma urealyticum is a mycoplasmal varieties that have the right to causenon-gonococcal urethritis (NGU) in men and possibly in women(Krause and also Taylor-Robinson, 1992).It can additionally cause respiratory condition innewborns. The ureaplasmas space "unique in their capability to metabolize ureathrough the enzyme urease." Mycoplasma genitalium was at first recovered indigenous the urethra of maleswith NGU. Subsequently, it has actually been uncovered in specimens from therespiratory tract. Its possible role in urogenital condition remains poorlydefined(Krause and also Taylor-Robinson, 1992).
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Mycoplasma genitaliumThe finish sequence the the M. Genitalium genome(Fraser et al., 1995)shows the it is a one duplex DNA of 580,070 bp. The presumptiveorigin of replication is in one A+T rich an ar between dnaA and also dnaN. Atotal the 470 open up reading frames (ORFs) were identified, with about halfthe genome transcribed mostly in one direction native the beginning andthe other fifty percent transcribed primarily in opposing direction. Thusthis genome reflects a symmetrical strand prejudice for transcription reminiscentof part circular viral genomes. Of the 470 ORFs, 374 were established bysequence matches come entries in a non-redundant bacter protein database orin the complete collection of interpreted sequences from the Haemophilus influenzagenome, or by GeneMark. 96 ORFs have no sequence matches in the databases.The predicted coding regions that could be compared with both Eschericiacoli and also Bacillus subtilis showed greater similarity to the B. Subtilissequence, strongly sustaining the deduced evolutionary partnership betweenMycoplasma and also the Lactobacillus-Clostridium branchof the gram-positive eubacteria. The 470 ORFs in M. Genitalium display an mean size that 1040 bp and also comprise88% of the genome, providing an average of one gene every 1235 bp. This issimilar to the gene size and also density seen with the 1727 suspect codingregions in H. Influenza, with an average gene size of 900 bp consisting of 85%of the genome in ~ a density of one gene every 1042 bp. Hence the reductionin genome size for M. Genitalium did not an outcome from a increase in genedensity or palliation in gene size. One major factor in to reduce genomesize that this parasitic microbe is the considerable loss of genes encodingenzymes necessary for countless anabolic pathways, in particular biosynthesis ofamino acids, purines and also pyrimidines, and also fatty acids. Also, the mycoplasmahave lost the capacity to do a cell wall. No surprisingly for a parasitic organism the must gain most the itscellular structure blocks from its host, a substantial variety of transportproteins space encoded, with a portion of total genes in this categorysimilar to that of H. Influenza. Together expected, gene encoding enzymesneeded for replication and also repair the DNA, transcription, translation and"cellular processes" such as cell division, cell killing, and also proteinsecretion space present, again at a portion of the total genes similar to(or better than) that of H. Influenza. Catabolic metabolism come produceenergy is greatly anaerobic, with an undamaged glycolytic pathway however nottricarboxylic acid cycle or electron transport system(Fraser et al., 1995). It has been argued that the "degenerative" setting of development ofM. Genitalium by palliation in genome dimension from that of an ancestralgram-positive bacterium has actually led come the minimal set of genes essential for aliving organism. One can expect the the "necessary" gene that areretained will carry out a similar role and therefore be conserved in otherbacteria or other organisms. If so, then the removed of "unnecessarygenes" will boost the fraction of proteins conserved in various other species.The similar fraction of total genes devoted to replication, transcriptionand various other retained processes in both H. Influenza and also M. Genitalium(Fraser et al., 1995)shows the the enhance of proteins committed tothese tasks might be decreased by about the same amount in M. Genitalium. Bysearching because that "bacterial conserved regions" (or BCRs) conserved in distantlyrelated bacteria and for "ancient conserved regions" (or ACRs) shared witheukaryotic or archaeal homologs,Koonin et al. (1996)showed the thefraction of proteins containing BCRs and ACRs is close come the same in E. Coli,H. Influenza, and M. Genitalium. This refutes the model that only acore of critical genes is maintained in M. Genitalium, however rather suggeststhat in all three bacteria over there is a balance in between highly conservedgenes and an ext variable genes. This has actually been rationalized together possiblyreflecting one equilibrium in between the security of major physiologicalprocesses and also need for ecological adaptability(Koonin et al., 1996).
Mycoplasma pneumoniaeThe complete sequence the the M. Pneumoniae genome(Himmelreich et al., 1996)shows that it is a one duplex DNA that 816,394 bp. Thepresumptive origin of replication is in one A+T rich an ar between dnaA anddnaN. A full of 677 open up reading frames (ORFs) and 39 genes coding forvarious RNAs were identified. Just 67 (or 9.9% of the total) ORFs had nosignificant similarity come sequences in the databases.The predicted coding areas that could be contrasted with both Eschericiacoli and Bacillus subtilis showed higher similarity to the B. Subtilissequence, strongly supporting the deduced evolutionary connection betweenMycoplasma and also the Lactobacillus-Clostridium branchof the gram-positive eubacteria. Similar to the case for M. Genitalium , one would expect the evolution ofM. Pneumoniae as a helminth bacterium to enable the lose of biosyntheticpathways the were no longer needed, when retaining the transfer proteinsneeded for acquisition the essential building blocks because that macromolecularsynthesis (e.g., amino acids, purines, pyrimidines, fatty acids) native thehost. Indeed, the reductive advancement of M. Pneumoniae indigenous ancestralbacteria caused a smaller genome. The main reasons of this reductionin genome dimension are (1) the ns of several anabolic pathways, such asbiosynthesis of amino acids, purines and pyrimidines, and fatty acids, (2)the lack of a capacity to make a cell wall, and (3) a reduction of thenumber the proteins affiliated in necessary processes such together DNA replication,repair, recombination, cell division, and protein secretion. A complete of 44transport proteins have been predicted native the genome sequence. Thismodest number, compared to the variety of substrates expected to betransported, suggests that countless of this transporters may not it is in highlyspecific. The coding regions in M. Pneumoniae comprise a full length of 724,174 bp,or 88.7% the the genome. The mean size the a gene is 1011 bp, offering anaverage of one gene every 1140 bp. This is comparable to the gene size anddensity seen with both the smaller sized M. Genitalium genome and also theH. Influenza genome, which is more than twice as large.
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