Mesosomes - Mesosomes are complex, localized in foldings of the cell membrane.
Although also found in gram negative bacteria, they are more frequently observed in gram positive bacteria.
They are particularly well developed in bacilli. The mesosome consists of an invaginated cell membrane with many vesicles, tubules or lamellar whorls filling the in vagination.
The vesicular, type and the whorl type appear to be the most commonly seen forms.
The lipid component of the mesosome appears to be similar to that of the cell membrane, but its proteins differ.
Salton and Owen (1976) have suggested that mesosomes may arise by vesicularization of the outer half of the lipid bilayer.
Higgins et.al (1974) have observed that the tubular or vesicular imaginations may be artefacts resulting from fixation, centrifugation or filtration.
Although the mesosome represents a special cell membrane component, its true location and function in vivo is not clear.
The function of the mesosome is the subject of much controversy, and almost all the major functions of the bacterial cell have been attributed to the mesosome at some time or the other.
Mesosomes were formerly believed to be the equivalents of the mitochondria of higher cells.
They were thought to be the centres of respiratory activity.
However, it has been shown that mesosomes are not analogous to the mitochondria, since they do not have an outer membrane
Moreover, respiratory enzymes have been shown to occur throughout the cell membrane.
It is now generally agreed that the mesosomal vesicles are either devoid of many of the plasma membrane enzymes and electron transport components (e.g. ATPase, dehydrogenase, cytochromes), or else there are substantially reduced complements of these components.
Mesosomes are, therefore, unable to carry out transport processes requiring a membrane energizing system.
McLeod et. al (1973)isolated mesosomal and plasma membrane vesicles of Bacillus licheniformis, and found that only the mesosomes are unable to transport amino acids.
Because of the proximity of mesosomes to the cell wall septal region, they may be the sites of some of the wall membrane synthetic processes.
It has been suggested that mesosome formation may preced and coordinate the formation of the septum.
