Bacteria - Overview
Introduction
A typical bacterial cell is composed of an outer capsule, a cell wall, a cell membrane, cytoplasm containing nuclear material and ifmotile, appendages such as flagella and fimbrae or pili. Some species of bacteria are more resistant to environmental influences than others, particularly those species of bacteria that are able to produce spores which can remain inactive until the appropriate environmental conditions prevail allowing the bacteria to resist conditions such as freezing, wet, dry or hot
conditions.
The structural features of pathogenic bacteria are important in the production of disease and also very useful for the identification and diagnosis of infection in veterinary medicine.
Capsule
The outermost part of a bacterial cell is the capsule, often described as the glycocalyx. Most capsules are composed of polysaccharides, although in some species the capsule is made of polypeptides. Capsules can be visualised
under light microscopy by using staining techniques.
The main function of bacterial capsules is to provide protection from adverse environmental conditions, prolonging the period of survival in such conditions. The capsule also facilitates adherence to surfaces and interferes with host cells involved in phagocytosis.
Cell Wall
The cell wall lies between the cell membrane (inner) and the capsule (outer) and protects the bacteria from mechanical damage and osmotic lysis. Cell walls are non-selectively permeable and are only able to exclude large molecules. Species dependant differences in the structural and chemical composition of the cell wall creates variation in the pathogenicity of the cell and also influences the staining properties of the cell which is important for species identification. Peptidoglycan (a polymer unique to prokaryotic cells) provides the cell wall with rigidity.
Bacteria can be divided into two major groups on the basis of the colour of the cell wall when stained using the Gram Method. The groups are called “Gram Positive” and “Gram Negative”. Gram positive bacteria stain blue and have a thick cell wall composed mainly of peptidoglycan and teichoic acids. Gram negative bacteria stain red and their walls have a much more complex structure containing
an outer membrane, a periplasmic space and an inner membrane. For further information on the structure of both types of cell wall please see Bacterial structure
Antibiotic treatments such as penicillin interefere with the ability of the bacterial cell to produce peptidoglycan and therefore cannot produce their cell wall making them more vulnerable to the environment.
Cytoplasmic Membrane
Bacterial cytoplasmic membranes are flexible structures composed of phospholipids and proteins and are similar to the lipid bi-layer membranes found in eukaryotic cells. Only a limited number of small molecules such as water, carbon dioxide and lipid-soluable compounds can enter bacterial cells by passive diffusion. Nutrients and waste metabolites are transferred via active transport using ATP (adenosine triphosphate).
The cytoplasmic membrane is also the site of electron transport for bacterial respiration and also contains enzymes and carrier
molecules that function in the biosynthesis of DNA, cell wall polymers and membrane lipids.
Cytoplasm
The cytoplasm is enclosed by the cytoplasmic membrane and is an aqueous fluid containing nuclear material, ribosomes, nutrients and the enzymes involved in most cellular functions. Storage granules can often be seen in the cytoplasm under certain environmental conditions. Storage granules mainly contain starch and glycogen.
Nuclear Material
The bacterial genome is composed if a single haploid circular chromosome containing double-stranded DNA (dsDNA). Bacterial genomes
vary in size depending on species but often has a folded structure to form a dense body which is visible using a scanning electron microscope. During replication the DNA helix unwinds and both daughter cells (produced by binary fission) receive
a copy of the original genome.
The cytoplasm also contains Plasmids. Plasmids are small circular pieces of DNA that are separate from the genome and are capable of autonomous replication. Several different plasmids can be within the cytoplasm of a single bacteria. Plasmids
can be transferred between bacteria during binary fission or through a process called conjugation. Plasmid DNA codes for characteristics including antibiotic resistance and endotoxin production.