Basic Bacteria Structure & Metabolism Flashcards
Acid Fast Stain
Spirochetes
- Stain poorly or too thin
- Darkfield microscopy
Mycoplasma
•no cell wall
Chlamydia & Rickettsia
- Obligate intracellular
- Cell walls minimized
• LPS consists of a toxic lipid A, a core polysaccharide, and polysaccharide side chains called O antigens in some bacteria.
Cell Membrane
The bacterial cell (plasma) membrane is exceptionally rich in proteins and does not contain sterols. It plays a role in segregation of daughter chromosomes at cell division, analogous to the role of the mitotic apparatus of eukaryotes. The membrane is the site of synthesis of DNA, cell wall polymers, and membrane lipids and contains the entire electron transport system of the cell. Like cell membranes of eukaryotes, it is a permeability barrier and contains proteins involved in selective and active transport of solutes. It is also involved in secretion to the exterior of proteins, including exotoxins and hydrolytic enzymes involved in the pathogenesis of disease.
Flagella
Flagella are molecular organelles of motility found in many species of bacteria, both Gram positive and Gram negative. They may be distributed around the cell (peritrichous), at one or at both ends of the cell (polar). In all cases, they made of proteins, are long (up to 20 µm) individually helical in shape and propel the cell by rotating at the point of insertion in the cell envelope. The presence or absence of flagella and their position are important taxonomic characteristics.
Pili
Pili (fimbriae) are hair-like projections up to a thousand of which are found on the surface of cells of many Gram-positive and Gram-negative species. They are composed of molecules of a protein called pilin arranged to form a tube with a minute, hollow core. They are, in many cases, adhesins, responsible for the ability of bacteria to colonize surfaces and cells.
Capsule
Many bacterial cells surround themselves with one or another kind of hydrophilic gel. This layer is often quite thick; even thicker than the diameter of the cell itself. Because it is transparent, this layer is usually not seen microscopically unless special stains are used or it is made visible by its ability to exclude particulate material, such as India ink. If the material forms a reasonably discrete layer, it is called a capsule; if it is amorphous in appearance, it is referred to as a slime layer. Most capsules or slime layers are polysaccharides made of single or multiple types of sugar residues.
Core
In contrast to the structural richness of the layers and appendages of the cell envelope, the interior seems relatively simple. There are two clearly visible regions, one granular (the cytoplasm) and one fibrous (the nucleoid).
Cytoplasm
The dense cytoplasm is bounded by the cell membrane. It appears granular because it is densely packed with ribosomes, which are much more abundant than in the cytoplasm of eukaryotic cells. This reflects the higher growth rate of bacteria. The overall subunit structure of the bacterial ribosome resembles that of eukaryotic ribosomes, but is smaller. It differs enough in function that a large number of antimicrobial agents have the prokaryotic ribosome as their target (i.e. aminoglycosides, tetracyclines, macrolides). Except for the functions associated with the cell membrane, all of the metabolic reactions of the cell take place in the cytoplasm. The cytoplasm has a cytoskeleton composed of actin and tubulin filaments which serves to localize proteins and along with the cell wall, give shape to the cell.
Nucleoid
The bacterial genome resides on a single chromosome (there are rare exceptions) which includes 600 to 64,000 genes encoded in one, large, circular molecule of double-stranded DNA. In contrast to eukaryotes, most bacterial genes have no introns. Tightly packed by supercoiling, the chromosome together with related proteins is localized in the area of the cell interior called the nucleoid. This lack of compartmentalization makes metabolic processes more efficient than in eukaryotic cells because there is no need to transport mRNA from where it is made to where it functions.
Spores
Bacterial Metabolism ATP Generation
ADP-ribosylation (ADPR)
Relatively few biosynthetic pathways are unique to bacteria, but some of them form a basis for bacterial vulnerability or bacterial pathogenicity. One of the most powerful called ADP-ribosylation (ADPR) is the mechanism for the action of multiple bacterial toxins (i.e. pertussis toxin, diphtheria toxin, exotoxin A).
- Toxin active unit binds both nicotinamide adenine dinucleotide (NAD) from body fluids and its target protein typically a human enzyme
- This catalyzes the transfer of an ADP-ribose group to the target protein
- The ADP-ribosylated protein is rendered inactive by this action.
- The biologic outcome of ADPR depends on the normal function of the now ADPribosylated target protein. If it is crucial to an essential process like protein synthesis the result is cell death. If it is a regulatory protein, the pathway it regulates may be either suppressed or stimulated.
Type III
Type III systems involve as many as 20 protein components. They cause syringe-like injection of these proteins activated by contact of the bacterial surface syringe with mammalian host cells. This leads to the direct secretion of bacterial proteins into the cytoplasm of the mammalian cell.
Type IV
Type IV systems are like type III but are also able to transfer DNA between bacterial cells or between a bacterial and a eukaryotic cell.
