Bacteria

Most bacteria are not pathogenic.Bacteria are prokaryotes. They have a cell wall and no membrane-bound organelles and no nucleus. Bacteria are either classified by the type of cell wall or by their shape.Spherical bacteria are called cocci. Rod-shaped bacteria are called bacilli. Comma shaped bacteria are called vibrio, spiral shaped bacteria are called spirilla and corkscrew shaped bacteria are called spirochaetes.Bacteria may be classified as Gram-negative or Gram-positive.In the Gram stain, crystal violet and iodine are applied to the bacteria followed by alcohol. Red safranin dye is then applied as a counterstain.Gram-positive bacteria have a thick layer of peptidoglycan with substances such as teichoic acid in the net-like structure of the peptidoglycan in their cell wall.The crystal violet is trapped in the cell wall so the bacterial cell wall does not decolourise when dehydrated using alcohol and does not pick up the red counterstain.Gram positive bacteria look purple-blue under the microscope. An example is methicillin-resistant staphylococcus aureus (MRSA).Gram negative bacteria have a thin cell wall. They have a thin layer of peptidoglycan between two membranes with no teichoic acid. The outer membrane is made of lipopolysaccharide. The iodine-crystal violet is washed out when the bacteria are dehydrated with alcohol. The peptidoglycan takes up the red safranin dye when the counterstain is applied. Gram negative bacteria appear red. An example is E. coli.The two different types of cell wall react differently to antibiotics such as penicillin. Penicillin inhibits the formation of the peptidoglycan layer of the cell wall. In Gram negative bacteria the peptidoglycan is hidden between two membranes and is less vital to the wall structure.