The
discovery of Mycoplasmas occurred during the end of the last century
and the first isolation was in 1898 of a species that was a bovine pathogen.
They are prokaryotes that are characterized mainly by their physical size,
genome size, and the absence of a cell wall. Within the genus, there are
over 70 species that range in size from 0.125 – 0.82 microns and
are the smallest known cell capable of self-replication. Their general
characteristics and cellular organization are similar to most bacteria.
Instead of a cell wall, their external covering is like that of other
organism's lipoprotein based cytoplasmic membrane, and becomes a very significant feature to their existence. It is the bacterial
surface that determines what interactions may take place and if the bacteria
have no wall, they must be capable of responding directly
to the environment.
It is important to note that the lack of a cell wall allows the organism
to be plastic, which means that its morphology is questionable and exists
in several ways. Different species of Mycoplasma in different media might
be found as filamentous, coccoidal, spherical, or granular (See figure at
right).
It is the filamentous and branching forms that the name Mycoplasma came
from, meaning “fungus form.” This plasticity serves many purposes,
including aiding in the physical stability of the organism. However, this
same feature can make them a difficult organism to study and in the past,
there were several erroneous estimates to even the size of Mycoplasmas because
of their flexibility, which allowed them to pass through filters with pore
sizes smaller than the true diameter of the cell. Their ability to survive
is also aided by the osmotically protected habitat of an animal’s
body. Also making them unique from other bacteria is their genome size,
which is up to 1/5 the size of E.coli. Ranging between 440 and 540 Megadaltons,
their molecular weight and chromosomes are smaller than that of any free-living
prokaryote. It is not surprising that some of the sequences missing are
those that code for proteins related to cell wall functions. The G-C content
of Mycoplasma is another interesting feature at 23 – 40% of the DNA.
Other aspects of Mycoplasma structure include ribosomes and unbounded DNA.
They are capable of a gliding motility, where the cell attaches to various
matrices and slides on them. This activity is mediated by cytadherance associated
proteins. Chemotactic response to sugars and amino acids in some species
also comes into effect with motility. How movement is achieved has also
been studied. In the head region, cytoskeletal-like filaments exist. Actin-like
filaments bind to the head structure to
create a network in the cell. Accessory proteins enable them to bind to
solid surfaces and animal cells. This ability to adhere and move is critical
for Mycoplasma so it can move to ideal locations within the host and colonize
various tissues. This is one of the things that makes them such an effective
and specific pathogen.
The outer membrane of these cells plays an obviously important role and deserves to be looked at in further detail. Most bacterial membranes do not contain sterols, but for most Mycoplasma, it is required. Two types of cholesterol exist in the cell – those for transport and those in the membranes, which may make up to 70% of the total. Sterols are rigid molecules that tend to make the membranes less flexible or leaky. The presence of sterols in the membrane of Mycoplasma is likely then for stability. They have also been observed facilitating attachment to the cell surface receptors of animal cells. The sterols contain lipoglycans, which stimulate the production of animal antibodies, embedded in the membrane. These lipoglycans can also aid in movement by their adhesive properties with host cells. Other things in the membrane have a stabilizing effect, including polyamines, which reduce hydrophilia and increase resistance to osmotic shock. Mycoplasmas may also contain carotenoids that aide in structure, as well as in transport.
The division, growth, and nutrient requirements of Mycoplasma is also an important aspect to their nature. Division occurs by budding, with the cells remaining attached or connected by thin hyphae-like material. The method of gene transfer is likely cell fusion or conjugation. As their structures are dynamic, so is their pattern of growth. It is highly dependent on the media it is growing in. In fibrous tissue, the colony tends to become embedded in the center, creating a dense core that lightens as it spreads out. Mycoplasma cultures on agar have this characteristic “fried egg” appearance. The ideal pH for growth is 7.6 – 8.0, but it can vary depending on the species. For growth, they also require proteins, ~9 amino acids, including purine and pyrimidine, nucleic acids, pentose, lipids, various vitamins, and several inorganic materials. The ideal temperature is 37° and certain osmotic and gaseous conditions must exist. The Mycoplasmas usually have 17 amino acids and contain a variety of enzymes for things like metabolizing glucose, electron transport, and respiration. For energy, like most bacteria, carbohydrates are most often used. Mycoplasmas are facultative aerobes, meaning that they are able to grow either in the presence or absence of oxygen, depending on the condition and species. Those that are oxidative possess the cytochrome system and make ATP by electron transport phosphorylation. Those that are fermentative produce energy by substrate phosphorylation and yield lactic acid as the final product. Glucose, fructose, mannose, maltose, starch, and glycogen can be utilized. Enzymes like oxidases, dehydrogenases, quinones, cytochromes, and catalases are used by individual species with the help of cofactors like FAD, FMN, and NAD. This is the way they grow, metabolize, and create energy. Most species obtain energy through glycolysis and synthesize ATP by substrate phosphorylation.
The
evolution of Mycoplasmas is still discussed today. Two theories existed
for their origin. The first was that they are a true biological class
and that all members are phylogenetically related. The second is that
they are an assemblage of walless prokaryotes that are derived from various
bacteria. A feature of Mycoplasma is genetic heterogeneity; they are diverse
in metabolic pathways, DNA base composition, and genome size. In order
for the group to be a true class, the vast differences between species
must be explained as further evolution. But, that would not explain the
similarities between those species and walled bacteria. Of these, there
are several well-studied examples. Through comparative enzyme studies,
particular Mycoplasmas have been linked to wall containing bacteria like
Bacillus, Lactobacillus, and Streptococcus. Within the studied Mycoplasmas,
however, there were no relations found. Because of the evidence, the first
hypothesis was thrown out. The second was accepted and in fact, later
research confirmed that the genetic events leading to the formation of
subgroups occurred more than once. Degenerative evolution, by chromosome
losses in walled bacteria, created the Mycoplasma we have today. They
are agents of disease, for which there is no counterpart, suggesting that
in their transition, there was also a gain of new pathogenic capacities.
The mechanism for this evolution occurred with a large reduction in genome
size through the deletions of DNA segments. This included a significant
part that was devoted to the wall structure and processes, including several
rRNA components.
| The results... |
Perhaps one of the most important
aspects of Mycoplasma is its pathogenicity in animals, including humans.
They are most often found associated with mucous membranes, causing diseases
of the respiratory system, urogenital tract, and joints. Ligand-receptor
interactions and cytadherance are important in the binding to host
cells. The most common in humans is responsible for
10 – 20% of all
pneumonias (See Figure below left). Infection of this type is spread
with casual contact and is highly infectious. They even exist on L-arginine,
the amino acid in chocolate. Due to the lack of a cell wall, they
are a trickier organism to relate to the infection. It is usually
only after other pathogens are eliminated that the Mycoplasma are considered and treated directly. Treatment
itself must be dealt with differently. The effect of Penicillins and other
antibiotics that inhibit cell wall synthesis are obviously lost on Mycoplasma.
Drugs like tetracycline or doxycycline are usually used for months, placing
strain on the liver and kidneys. Polyene antibiotics that are made for
eukaryotes are more effective because the sterols in the membranes are
the reactive portion. For arthritic conditions, steroids are often used,
providing temporary relief, but also giving the pathogen needed components
to proliferate. In order for the organism to be brought under control
or destroyed, the membrane structure must be damaged. It has been found
that lipase detaches the fatty acids and glycerides of the
 membrane, causing
the Mycoplasma to lyse. In some circumstances, there is no cure.
Antibiotic treatment will only control
the infection.
New research may implicate the role of Mycoplasma in such things as asthma, chronic fatigue syndrome, Gulf War Syndrome, meningoencephalitis, fibromyalgia, multiple sclerosis, Guillain-Barre’ syndrome, lupus, myocarditis, pericarditis, and Crohn’s disease. These stealth organisms have the capability to cross the blood brain barrier, enter the spinal fluid, cause brain and central nervous system lesions, and may also be involved in Alzheimer’s and rheumatoid arthritis. The have a chronic nature that indicates that they can adapt to conditions that may be constantly changing within the host. They contain surface proteins that are highly specific and directly effect what host they will thrive in and the severity of the disease they will inflict. The final aspect of Mycoplasma is their genome sequencing and the studies that have stemmed from it. The entire genome has been sequenced for several of the species, including M.pulmonis and M.genitalium. These sequences are unique and have led to further research. Within the study of the genome, there has been several unique findings. In most organisms, TGA is a stop codon. In Mycoplasma, it encodes for tryptophan. They have inefficient energy pathways due to a lack of several enzymes. Instead, they produce many degridative enzymes and scavenge nutrients from their host cells. Most bacteria have 5 – 10 copies of rRNA genes, while the Mycoplasmas have only 1 –2. They have promoters similar to other bacteria, lying between –10 and –35. They also have heat-shock proteins like many bacteria. Translation occurs as it does in most bacteria also, starting with an ATG codon mainly. A ribosome hydrogen bonds to the 3’ end of the rRNA molecule and carries out translation. Since they lack a cell wall, signal peptide sequences direct proteins to a secretory pathway in the membrane. Transformation is a common process in Mycoplasmas and is similar to that of other bacteria. Mycoplasma pulmonis is the murine respiratory pathogen. It is composed of a single circular chromosome that contains 963,878 base pairs. Within the genome is a single set of rRNA genes and 29 tRNA genes. Although larger than some of the other sequenced species, M.pulmonis lacks some of the genes previously reported as essential for life, making the number even smaller than expected. The sequencing of this genome allow a better understanding of their evolution, pathophysiology, factors influencing plasticity, and identification of antigens for a vaccine. |
Mycoplasma genitalium is a pathogen of the genital tract. It is the smallest known genome, with only about 62% of its protein coding genes being essential. It contains 580,070 base pairs. Further research is being done to use those minimal genes to reveal the true genes crucial for life. The Institute for Genomic Research is actually trying to engineer a cell with these essentials to finally identify the minimal gene set for self-replicating life forms. The genome contains 250 locus accession numbers, all of which have been studied. They have been assigned 17 different codes based on their functions. Just a few are RNA, transport, metabolism, transcription, amino acid, and translation. All locus points have common names and main roles defined. There are several with unknown or presumed lacking a function, just as humans have in their genome. The number of unknown or nonfunctioning genes in Mycoplasma is much lower, though.
The
ability of this organism to exist in their state is dependent on many
factors. Their morphological heterogeneity, membrane structure, chemical
composition, nutritional and physical requirements, and metabolic and
biosynthetic activities make them a unique and prosperous pathogen. They
are not the simple organisms that they were once thought to be and they
will increasingly become a subject of study in the years to come. Perhaps,
these cellular wonders will give us the ability to unravel once and for
all the requirements for independent life. And from there, the possibilities
are endless.
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