By David N. Leff
Would you believe that a bacterium by the warm, fuzzy name of Mollicutes could be all bad? Its trivial (i.e., colloquial) moniker is Ureaplasma, a genus in the taxonomic class properly known as Mollicutes.
Ureaplasma urealyticum is a widespread human pathogen, and no bacteriologist, urologist or obstetrician has a kind word to say about it. On the contrary:
"U. urealyticum inflicts a sexually transmitted disease," observed molecular biologist John Glass, a senior biologist and bacterial genomicist, now at Eli Lilly & Co., of Indianapolis, Ind. "Most adults carry Ureaplasma urealyticum," he said. "An estimated 60 to 80 percent of them are infected. In most adult humans, it's a harmless commensal, colonizing the lower urogenital tract.
"Where you have problems, though," Glass said, "it may cause non-gonococcal urethritis - urinary tract infection - and can lead to adverse pregnancy outcomes in the genital tract. It is also associated with pneumonia and meningitis in low-birth-weight newborns. This isn't a urogenital problem in neonates; it's a respiratory problem. Virtually all extremely low-weight infants are infected with U. urealyticum, acquired vertically from their mothers."
By this route, the pathogen can reach and inflame the infant's upper respiratory tract, as far as the pharynx, as well as the central nervous system. If untreated, it may be life-threatening. Medical treatment, however, is minimal.
Glass noted, "There are routine diagnostic tests in growth media that can differentiate U. urealyticum from Chlamydia - a far more pathogenic sexually transmitted bacterium. Such assays demonstrate the growth on culture broth containing urea - U. urealyticum's main nutrient - whereas Chlamydia doesn't grow as a free-living organism. Routine therapy," he said, "would be antibiotics such as erythromycin or tetracycline. They work, but take a really long time. Ureaplasmas don't need machinery for cell wall generation, because they just have a simple plasma membrane - like eukaryotic cells."
A Micro-Microorganism Like No Other
That's only the beginning of U. urealyticum's unique differences from all other prokaryotic microorganisms. These variants are just beginning to emerge, thanks to total sequencing of the bug's genome, as reported in today's issue of Nature, dated Oct. 12, 2000. Glass is lead author of that paper, which is titled: "The complete sequence of the mucosal pathogen Ureaplasma urealyticum." Its senior author is microbiologist Gail Cassell, since 1997 - when she left the University of Alabama, Birmingham - senior vice president for infectious diseases at Lilly.
As reported in Nature, U. urealyticum boasts a circular chromosome comprising 751,719 base pairs, carrying 652 genes. That makes it the second-smallest genome on the planet sequenced so far - among free-living, self-replicating microorganisms. The world title-holder for this minimality is its Mollicutes cousin, Mycoplasma genitalium. Its genome, sequenced five years ago, weighed in at 580,070 base pairs, carrying 470 genes. By extreme comparison, Pseudomonas aeruginosa, one of the biggest-time bacterial killers, runs to 6,284,403 base pairs, with 5,570 genes.
"Genitalium is barely an organism," Glass told BioWorld Today. "Biochemically, it performs glycolysis [conversion of sugars to energy]; and all of its energy comes from carbohydrate sources. It's an extremely wimpy organism, which grows very slowly.
"Ureaplasma, on the other hand," he went on, "is a very robust organism, compared to the other mycoplasmas that are human pathogens - like M. genitalium and M. pneumoniae. It grows in cell culture very rapidly, and relies almost solely for its own energetic purposes on the hydrolysis [enzymatic metabolism] of urea. This is quite similar to other bacterial ureases, but is 300 to 2,000 times more efficient at breaking down urea. So U. urealyticum," Glass added, "apparently possesses enzymes that generate energy from metabolizing urea differently than any known bacteria."
One goal of sequencing a pathogen's genome is to pinpoint the molecular factors that make it virulent. "Ureaplasma in large doses is lethally toxic, at least in mice," Glass observed, "so that's one of the virulence factors - though in humans it may not get to a toxic concentration. The organisms have been shown to be involved in suppression of immune response.
"Another possible virulence factor," he added, "is that Ureaplasma bacteria have phospholipase activity. Phospholipases can elicit a biochemical cascade that in eukaryotes could lead to premature labor. You can see why that could lead to adverse pregnancy outcomes.
"Finally," Glass said, "we assumed that part of Ureaplasma's hemolytic [blood-destroying] activity and virulence resulted from its generation of hydrogen peroxide - which is hemolytic. One of the things we were surprised to find was a gene that codes for a protein called hemolysin A. Mycobacterial species like Mycobacterium tuberculosis and M. leprae," Glass pointed out, "that have hemolysin A are virulent, whereas those that don't are not. So that may be another virulence factor."
Glass said, "It may be possible to make some better PCR-based diagnostic tools. We might identify new antigens that could be used in protein-based diagnostics. But there are no immediate benefits in that regard."
Democratizing Small-Genome Sequencing
He said the researchers are hoping "to understand the genome of this extremely intractable organism - which has no genetic systems, no mutants. For E. coli, for example," he explained, "I can transfer DNA from one organism to another, and change the phenotype of the E. coli that I'm working with. For even Mycoplasma pneumoniae and genitalium, I can go in and manipulate the genome - not very effectively, but enough to do a number of quite elegant experiments. Ureaplasma labs have been working for many years to do that and just haven't gotten it done. Nothing has worked. Now that we can see the whole genome, the hope is that our lab or other labs will be able to work more effectively with Ureaplasma."
Glass cited the view of another co-author, Ellson Chen, then of Perkin-Elmer Biosystems, in Foster City, Calif. "Chen believed it would be possible for one or two people to sequence a megabase a year, using one automated instrument. That could be done at a small academic lab, employing a different approach than the random shotgun technique taken by the large sequencing centers. This," Glass concluded, "was Chen's effort, in essence, to democratize microbial genome sequencing."