Structure and Function of Bacterial Cell Membranes

Abstract

The limiting plasma membrane, mesosomes and intracellular lamella membrane systems of the bacterial cell possess the typical, triple-layered profile. The external component of the cell envelope of gram-negative bacteria has the "double-track" profile although this layer possesses a macromolecular organization which is uncommon in cell membranes. The membranes and particles from them appear to conform to those of other types of cells in their gross chemical composition. They usually contain up to about 75% protein and approximately 20-30% lipid. There is generally 1 major type of phospholipid in the bacterial membrane. In gram-positive bacteria, the cell membranes are differentiated into the plasma membrane and the mesosome. Different cell functions are in the 2 types of membranes. The mesosomes appear to be centers of the electron transport systems as indicated by the distribution of cytochromes and succinic dehydrogenase and their ability to reduce tellurite and tetrazolium dyes. They may assist in the formation of the cell wall septum. The precise chemical, biochemical and functional differences between plasma membranes and mesosomes are not clearly defined. The mesosomes may be active in genome separation during cell division and be associated with the incorporation of transforming DNA. Episomes do not appear to be lost on expulsion of the mesosomes although the distribution of DNA to daughter cells is severely disrupted when the organisms are propogated in the L form. Even in gram-negative organisms such as Escherichia coli, small membrane intrusions appear to be involved in nuclear separation suggesting that there is only one linkage point between nucleus and membrane in this organism and Bacillus subtilis. Gram-negative bacteria possess an additional external barrier which can exclude certain low molecular weight compounds from penetrating the cell and have an important function in the retention of some enzymes. Although the plasma membrane on gram-negative bacteria is less accessible for isolation, important transport function can be investigated with suitable mutants and by specific labeling methods. Association of polyribosomes with membranous fractions from both gram-positive and gram-negative groups of bacteria suggest that they are active sites of protein biosynthesis.