A prokaryote is a life form that has the following characteristics:
- No nucleus. The genomic DNA is found in the nucleoid.
- No membrane-bound organelles.
- DNA is typically circular (plasmid). There may be one or more plasmids in the cell. However, some prokaryotes have a plasmid and a linear chromosome.
Bacteria and archaea are both examples of prokaryotes. Bacteria are more commonly known, since they grow in a more "reasonable" range of environments. Archaea tend to like very extreme environments, meaning very high or very low temperatures, high acidity, high salinity, etc. For this reason, archaea are very difficult to grow in a lab. Bacterial cell walls usually contain peptidoglycan, which is NAG (N-acetylglucosamine)-NAM(N-acetylmuramic acid) links connected by protein cross bridges and tetrapeptide side chains. Archaeal cell walls contain pseudomurein, which is similar to peptidoglycan.
(insert pictures) Prokaryotic cells may be sphere-shaped (coccus/cocci), rod-shaped (bacillus, bacilli), or spiral-shaped.
Two cocci bound together are described as diplococci. Chains of cocci are described as streptococci. Four cocci bound together in a square shape is described as a tetrad. 8 cocci bound together in a cube shape is described as a sarcina (plural--sarcinae). Large bunches of bacteria adhered together are described as staphylococci.
Two bacilli bound together are described as diplobacilli. Chains of bacilli are described as streptobacilli. Short bacilli are described as coccobacilli.
A spiral bacterium that is like a bent rod is described as vibrio. Thick and twisted spiral bacteria are called spirilla. Thin and twisted spiral bacteria are called spirochetes.
Bacterial Cell Walls
(insert pictures) Bacteria typically fall into one of the two following categories: gram positive or gram negative. The gram positive cell wall has a thick layer of peptidoglycan surrounding a plasma membrane. Lipotechoic acids help attach the peptidoglycan to the plasma membrane, and wall teichoic acids help secure the peptidoglycan layers. When the gram staining test is done, crystal violet with iodine stays is absorbed into the peptidoglycan and is not washed out during the alcohol washing step, so the cells stain purple. Gram negative bacteria have a thin layer of peptidoglycan wedged between two phospholipid bilayer membranes, an outer membrane and an inner membrane. The outer membrane contains lipopolysaccharide, which is composed of the O polysaccharide and Lipid A. Lipid A is an endotoxin, and the O polysaccharide can serve as an antigen, as it differs between different types of bacteria. In fact, lipopolysaccharide is even used in immunological research as a method of stimulating B-cell activation. During the gram staining test, the crystal violet and iodine are easily washed off during the alcohol wash step. Then, the cells are counterstained with safranin, staining the cells red or pink. This is different from gram positive cells because since the peptidoglycan has retained the purple dye, it won't take in safranin and thus will stain purple rather than red or pink.
Some bacteria are neither gram positive or gram negative. Acid-fast bacteria, such as mycobacteria have peculiar cell walls. Outside the peptidoglycan layer, they have a large amount of waxy lipids and mycolic acids, making it difficult for crystal violet to penetrate. However, the acid-fast stain can be applied: carbolfuchsin is added with heat so that it can permeate the cell wall. Then, acid-alcohol is used as a wash, and the counterstain is methylene blue. An acid-fast bacterium will not lose the carbolfuchsin stain during the wash and thus will not take up methylene blue, so it will stain red. A non-acid-fast bacterium will stain blue.
Glycocalyx and Biofilms
The glycocalyx is a sticky protective layer found on the outside of a bacterium made of polysaccharides. It helps bacteria adhere to surfaces (and can sometimes serve as a nutrient source. It protects bacteria from dehydration and loss of nutrients. A glycocalyx that is unorganized and loose is described as a slime layer, while a glycocalyx that is neatly organized can be described as a capsule. Capsules prevent phagocytosis and increase the pathogenicity of a bacterium. An extended glycocalyx, such as extracellular polymeric substance (EPS) is used to adhere bacteria to other bacteria in a biofilm. In a biofilm, bacteria form this elaborate community where they may "help" each other distribute nutrients, communicate through quorum sensing and other chemical means, etc. and all the EPS makes it much harder for antibiotics to work against the bacteria because they're all protected. Biofilms are a large problem in hospitals, as they like to form on artificial joints and inside of catheters, posing large health risks.