MESOSOMES: CHARACTERISTICS, TYPES AND FUNCTIONS - BIOLOGY - 2024

The mesosomes are invaginations on the plasma membrane of Gram positive bacteria and some Gram - negative, being observed only in cells chemically fixed for electron microscopy observation.

Microbiologists originally suggested that they were multi-functional organelles. Among these possible functions were that they could participate in the synthesis of cell membranes, in the formation of endospores, in the replication and segregation of DNA, in respiration and in redox metabolism, among other functions.

Diagram of a prokaryotic cell with mesosome. Taken and edited from Mariana Ruiz LadyofHats, via Wikimedia Commons. For a time it was recognized that the mesosome system was connected in a complex way with nuclear material and was related to its replication.

Additionally, since they are considered extensions of the cytoplasmic membrane, they were assigned functions in enzymatic processes, such as electron transport.

Mesosomes were present in all Gram positive bacteria but were rare in Gram negative ones. In the latter they only appeared if they were cultivated under particular conditions.

The substitution of chemical fixation methods for electron microscopy studies by cryofixation techniques (fixation at low temperatures) proved that mesosomes were actually membrane malformations due to chemical fixation.

History

The first mentions of mesosomal structures date back to the early 50's of the last century. However, the structure was christened several years later by Fitz-James (1960). This researcher described mesosomes from chemically fixed Bacillus species.

During the 1970s, numerous researchers began to show evidence that the appearance, number and type of mesosomes depended on the chemical fixation of the bacteria.

In 1981, Ebersold et al. Experimentally demonstrated the artificial nature of these structures by studying chemically and cryogenically fixed bacteria.

Recent findings indicate that similar membrane damage, with the consequent appearance of mesosomes, can be observed in bacteria that have been exposed to antibiotics.

General characteristics

A bacterium, organisms where mesosomes have been reported.

Source: NIAID, via Wikimedia Commons

Mesosomes were described as invaginations, in the form of cytoplasmic pockets containing clusters of vesicles and tubules. They were also described as membranous spir ales, or as a combination of both types of structures.

Mesosomes appeared in all Gram positive bacteria and only in a few Gram negative species. In the latter, they only appeared when the bacteria grew in the presence of, and fixed with, osmium tetroxide.

The content of lipids, proteins and carbohydrates was considered similar to that of the plasma membrane. However, there were occasionally significant differences in the phospholipid, carotenoid, carbohydrate and menaquinone content of both structures. RNA and traces of DNA were also found in the chemical composition of mesosomes.

Types

Two types of mesosomes were described according to their location and function:

Septals

Those that participated in the formation of the septum in cell division and were involved in the formation of spores.

Side

These mesosomes were attributed synthetic and secretory functions.

Features

Energy and respiratory metabolism

Many cytochemical studies suggested that the in vivo redox reactions of the bacteria resided in the mesosomes. These studies included staining with vital stains such as Janus Green B and tetrazolium compounds.

However, biochemical studies suggested that specific oxidases, dehydrogenases and cytochromes, were totally absent or in reduced concentrations in mesosomal preparations.

Nuclear coupling to the membrane

It was suggested that the mesosome attracted the nucleus to the cell surface after a process called extrusion.

In freshly prepared protoplasts, fragments of mesosomal tubules attached externally to the membrane were often observed. This union occurred opposite to the point on the inner surface where the core made contact with the membrane.

Nuclear division

According to the results obtained from various studies, it was indicated that, at the beginning of the division, the two nuclei were each connected to a mesosome.

As the nuclear volume increases, the mesosomes divided in two and then separated, presumably carrying daughter nuclei. For this reason, mesosomes were believed to act as a primitive analog of the mitotic spindle in plant and animal cells.

Septum formation

The results about the participation of mesosomes in the formation of the septum (septum) were ambiguous. According to some authors, the association of the mesosome with the septum in some species of growing bacteria was a well established fact.

However, many experimental results suggested that mesosomes were unnecessary for the normal functioning of the cell division mechanism.

Cell wall synthesis

Because the mesosome was considered to be associated with the growing septum, it was suggested that it could also be involved in cell wall synthesis.

Membrane synthesis

The mesosome was also proposed to be the membrane synthesis site due to differential incorporation of lipids and protein precursors into mesosomal vesicles. However, there was no conclusive evidence to support this hypothesis.

Synthesis and secretion of exocellular enzymes

Some antibiotics cause malformations similar to those caused by the chemicals used to bind bacteria. Due to this, the presence of mesosomes was associated with the possibility that these structures had a secretory function of enzymes to degrade antibiotics. However, the evidence obtained was contradictory.

Place of attachment of the episome to the membrane

The episome is a bacterial replicating extrachromosomal unit that can function autonomously or with one chromosome. One of the supposedly best proven functions of the mesosome was to act as a site for cell attachment of episomes to the bacterial membrane.

DNA uptake site during transformation

The mesosome was believed to act as a DNA uptake organ during the transformation process. However, this assumption was based on indirect data and not direct evidence.

Evidence for the artificial nature of mesosomes

Among the evidence pointed out by the researchers to show that mesosomes are not organelles but artifacts caused by fixation techniques are:

1.- The number and size of the mesosomal structures varies with the fixation technique.

2.- Mesosomes are only observable in chemically fixed samples for electron microscopy.

3.- Mesosomes do not appear in cryogenically fixed bacteria.

4.- These structures appear in bacteria treated with some types of antibiotics, which cause damage similar to chemical fixers.

Formation of "mesosomes" by different conditions of attachment of bacterial cells. Adapted from Nanninga N. (1971). «The Bacillus subtilis mesosome affected by chemical and physical fixation. Taken and edited from: Original file made by TimVickersVectorized by Kkairri, via Wikimedia Commons.

Other meanings of the term mesosome

The term mesosome has other meanings in zoology:

Anatomy

Mesosome is one of the three tagmata into which the body of some arthropods is divided, the other two are prosoma and metasoma.

Taxonomy

Mesosoma is a genus of crustaceans described by Otto, 1821.

References

1. HR Ebersold, JL Cordier, P. Lüthy (1981). Bacterial mesosomes: method dependent artifacts. Archives of Microbiology.
2. VM Reusch Jr, MM Burger (1973). The bacterial mesosome. Biochimica et Biophysica Acta.
3. MRJ Salton (1994). Chapter 1. The bacterial cell envelope - a historical perspective. In: J.-M. Ghuysen, R. Hakenbeck (Eds.), Bacferiol cell wall. Elsevier Science BV
4. T. Silva, JC Sousa, JJ Polónia, MA Macedo, AM Parente (1976). Bacterial mesosomes. Real structures or artifacts ?. Biochimica et Biophysica Acta.
5. Mesosome. On Wikipedia. Recovered from
6. Mesosome. On Wikipedia. Recovered from