The cytoplasm of bacteria is the colloidal matter containing water, mineral compounds, proteins, lipids, carbohydrates, etc.
Multiple ribosomes are present in cytoplasm being the sites of protein synthesis. Bacterial ribosomes have 70S sedimentation constant in ultra-centrifugation (30S for small subunit and 50S for large one). A cluster of 50 to 55 closely related ribosomes is known as polysome. The ribosomes and polysomes are linked to the cellular membrane.
The cytoplasm also contains autonomous genetic structures – small circular DNA molecules of plasmids and episomes. They encode the synthesis of variety of substances (toxins, microbial enzymes and many others).
Various kinds of inclusions are stored in the cytoplasm. Among them are volutin and starch granules, lipoprotein bodies, pigments, accumulations of sulfur, microcristals of calcium, etc.
Volutin granules are of sizes to 0.1-0.5 μm. They are located in cytoplasm on the ends of Corynebacterium diphtheriae rods; also they can be found in the yeasts and other microorganisms.
Volutin granules contain metaphosphates. According to Neisser’s staining method volutin granules stain blue, vegetative part of bacterial cell stains brown-yellow. Lipoprotein droplets are accumulated in cytoplasm, when bacteria grow on rich nutrient media. They are actively used by microbial cells in starvation conditions. Microscopical methods for their detection include Sudan stain.
Similar manner starch granules can be stored and next consumed by microbial cell in case of unfavourable situations. They are detected by stain with Lugol’s iodine solution.
The cytoplasmic membrane encases the whole contents of bacterial cell. It is the innermost layer of bacterial envelope usually of 5-10 nm in thickness. The membrane is generally organized like phospholipid bilayer with embedded membrane proteins. Bacterial membranes lack sterols except mycoplasma cells.
Cytoplasmic membrane produces a selectively permeable barrier that provides controlled transportation of water, electrolytes, and various nutrients into bacterial cell for its vital activity. The transfer of water and ions creates osmotic pressure within bacterial cell.
The membrane carries the vast number of receptors with diverse functions. They recognize the variety of signals from the environment; signal transmission results in changes of bacterial metabolism.
Multiple membrane enzymes perform biological oxidation in bacteria. Also they support biosynthetic reactions for building of bacterial components (proteins, nucleic acids, etc.) and actively govern bacterial trans-membrane transportation.
Membrane invaginations or mesosomes are the nearest mitochondria analogs of bacterial cells. They harbor oxidative enzymes of electron transport chain thus providing energy for bacteria. Mesosomes actively participate in the growth of bacterial cells and their reproduction.
The best method for study of bacterial cytoplasmic membrane is electron microscopy.