Mycoplasma hominis

Introduction

Mycoplasma hominis viewed under an electron microscope [15]. The coccoid shape of the microorganism can be observed.

By [Jimmy Chapman 2013]

Classification Higher Order Taxa: Bacteria; Firmicutes; Mollicutes; Mycoplasmatales; Mycoplasmataceae; Mycoplasma
Species: M. hominis
Cluster: M. bovis; M. pulmonis; M. hominis

Description and Significance:
M. hominis is a pathogen in humans commonly found as part of urogenital tract flora especially of women and sexually active adult males [14]. This bacteria cause a variety of infections which may lead to pelvic inflammatory disease, post-abortal fever, post-partum fever and extragenital infections for immunodepressed humans [14]. It also can cause meningitis, pneumonia and abcesseses in newborn children [20]. M. hominis lives parasitically and saphrophytically with hosts [25].

Genome Structure
M. hominis’s circular chromosome has been studied and sequencing has taken place to help determine its pathology in humans [4;8;14;20]. The genome of M. hominis has 665,445 base pairs with a G-C content of 27.1% and an A-T content of 72.9% [20]. There are 537 DNA coding sequences, 345 of which the function has been established and 40 RNA genes [20]. Other factors include AUG being the most prevalent start codon at 95.1% with UAA comprising the most commonly used stop codon at 83% [20]. 106 hypothetical proteins and 14 pseudo genes were identified [20]. It has been determined that M. hominis holds two duplicates of rRNA genes [19]. Data on the sequenced genome is found at RefSeq under Project 41875 [25]. It has been found that M. hominis’s genome lacks the sequence which codes for a cell wall [25]. M. hominis most likely underwent horizontal gene transfer and gained genes from Ureaplasma parvum which have aided in the bacterium’s Arginine hydrolysis energy yielding pathway [20]. This has most likely occurred due to both M. hominis and U. parvum occupying the urogenital region of humans [20].

Cell Structure and Metabolism

M. hominis are a pleomorphic, gram negative bacteria with an average diameter of 0.2 to 0.3 µm [10]. The pleomorphic nature of M. hominis has resulted in observations of coccoid, filamentous and irregular shapes [5]. Since they lack the genes coding for a cell wall the bacteria has a membrane composed of three layers of sterol which has been integrated into the bacterium from the environment [16]. M. hominis has been found to have a widely varying structure dependent upon age [1]. The smallest cells roughly have an 80 to 100 mμ diameter while the largest are around 0.5 to 1 μ [10]. Many different internal structures have been observed [1]. These components include ribosome like granules, irregular dark regions, and mesh-like strands in nuclear regions, filamentous bodies and possibly vacuolated tiny organisms in the cytoplasm [1]. M. hominis is capable of a couple energy producing pathways including Embden-Meyerhoff-Parnas (EMP), arginine dihydrolase [20] and Riboflavin metabolism.

The Arginine Dihydrolase Energy-Yielding Pathway
The M. hominis genome was found being capable of coding for the three proteins necessary for the arginine dihydrolase pathway [20]. The enzymes are ornithine carbamoyltransferase, carbamate kinase and arginine deiminase [20]. M. hominis combines arginine and N-dimethyl-arginine utilizing arginine deaminase and N-dimethylarginine dimethylaminohydrolase, respectively, to form citrulline [20]. The enzyme ornithine carbamoyl transferase alters citrulline into ornithine and carbamoyl-phosphate [20]. Carbamate kinase changes carbamoyl-phosphate into NH3 and CO2 while ATP is generated as a by-product [20].

Embden-Meyerhoff-Parnas (EMP) Energy Pathway
M. hominis utilizes a portion of the glycolytic EMP pathway to derive energy in the forms of NADH and ATP [20]. Not all the genes necessary for full all EMP processes to occur are present in M. hominis but this bacterium utilizes alternative methods to produce energy [20]. Glucose-6-phosphate is converted to fructose-6-phosphate by glucose-6-phosphate isomerase. Since 6-phosphofructokinase cannot be synthesized by M. hominis the bacterium uses transketolase to make xylulose-5-phosphate [20]. This molecule can then take one of two paths to continue the EMP pathway. The first method is to use ribulose-phosphate 3-epimerase to convert xylulose-5-phosphate to ribulose-5-phosphate. Ribulose-5-phosphate is then altered into ribose-5-phosphate by ribose-5-phosphate isomerase. Next the molecule can be converted into ribose-1-phosphate by phosphor-pentomutase, phosphoribosyl pyrophosphate (PRPP) by ribose-phosphate pyrophosphokinase or continue the glycolytic pathway by being changed into glyceraldehyde-3-phosphate by transketolase. The second method is for ribulose-phosphate 3-epimerase to be converted directly into glyceraldehyde-3-phosphate by phosphoketolase. Next, glyceraldehyde-3-phosphate dehydrogenase converts glyceraldehydes-3-phosphate into glycerate-1,3-diphosphate which yields an NADH energy molecule. An energy molecule of ATP is synthesized when glycerate-1,3-diphosphate is altered into 3-phosphoglycerate by phosphoglycerate kinase. 3-phosphoglycerate is converted into 2-phosphoglycerate and then phosphoenolpyruvate by phosphoglycerate mutase and enolase, respectively. The final energy yielding step synthesizes another ATP molecule by forming pyruvate with the aid of pyruvate kinase. The pathway ends here and is unable to create the additional NADH, ATP and CoA usually in the EMP pathway because M. hominis’s genome does not code for the enzymes pyruvate dehydrogenase E1 component, pyruvate dehydrogenase E2 component, dihydrolipoamide dehydrogenase nor phosphotransacetylase [20].

Riboflavin Metabolism
In order to generate additional energy in the form of vitamins, M. hominis utilizes riboflavin metabolism [20]. The process begins with a GTP that can take two different paths to produce 2,5-diamino-6-(5-phospho-D-ribosylamino)-pyrimidin-4(3H)-one. The first is to utilize guanosine triphosphate cyclohydrolase II while the second has an additional intermediate. This alternative path uses GTP cyclohydrolase IIa to make 2-amino-5-formylamino-6-(5-phospho-ribosylamino)-pyrimidin-4(3H)-one and 2-amino-5-formylamino-6-ribosylaminopyrimidin-4(3H)-one 5’-monophosphate deformylase to form the product, 2,5-diamino-6-(5-phospho-D-ribosylamino)-pyrimidin-4(3H)-one. This molecule is then converted into 5-amino-6-(5-phospho-D-ribosylamino) uracil by the enzyme diaminohydroxyphosphoribosylaminopyridine deaminase and 5-amino-6-(5-phospho-D-ribitylamino) uracil through the enzyme 5-amino-6-(5-phosphoribosylamino) uracil reductase. Then 5-amino-6-ribityl-aminouracil is formed through help by 5-amino-6-(5-phosphoribitylamino)uracil. The next step is to form 6,7-dimethyl-8-ribityl lumazine with help from 6,7-dimethyl-8-ribityllumazine synthase and then riboflavin with assistance from riboflavin synthase. The final energy yielding synthesis steps are to form FMN with riboflavin kinase. This molecule has two paths to form energy molecules. The first is to utilize FMN reductase to yield the molecule FMNH2. The second is to initially utilize FMN adenylyltransferase to synthesize FAD which is then turned into FADH2 by flavin reductase.

Antigenic Nature

M. hominis have lipid-associated membrane proteins on their cellular exterior [10]. These proteins are characterized by an antigenic nature [17]. This quality of the proteins makes them a specific target for antibodies as a part of a human’s immune system response to M. hominis infections [10]. This antigenic nature of M. hominis provides a potential means of identifying the bacteria’s prescence in infected hosts and which serums should be given to infected patients [10]. It has been found that there is hardly any difference in antigenic determinants or immunoreactivity characteristics [10]. If more sensitive testing methods are developed, then more distinctions can be developed and attributed to dissimilarities within the M. hominis bacterial species.

Section 2

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Section 3

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Conclusion

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References

[Sample reference] Takai, K., Sugai, A., Itoh, T., and Horikoshi, K. "Palaeococcus ferrophilus gen. nov., sp. nov., a barophilic, hyperthermophilic archaeon from a deep-sea hydrothermal vent chimney". International Journal of Systematic and Evolutionary Microbiology. 2000. Volume 50. p. 489-500.

Edited by student of Joan Slonczewski for BIOL 238 Microbiology, 2011, Kenyon College.