Unit 1 Biology: Gram Positive + Negative Bacteria

• How are gram-positive and gram-negative bacteria differentiated?


  Gram-staining with crystal violet
• Describe gram-positive bacteria;


  They contain a thick, mesh-like cell wall containing peptidoglycan. They are not surrounded by an outer membrane. They stain purple.
• What is cell wall synthesis?


  Upkeep of the cell wall to keep it rigid
• Describe gram-negative bacteria;


  Thinner peptidoglycan cell walls. However, the have two lipid outer membranes. They stain pink.
• Why do antibiotics kill gram-positive bacteria easy?


  Gram-positive bacteria have a thick cell wall made of peptidoglycan. It is mesh-like and allows materials into the cell. They do not contain a protective outer membrane. This means the antibiotic can enter the cell easily. The antibiotic stops the interlinking of proteins in the cell wall, and therefore cell wall synthesis. Cell wall synthesis is needed for the upkeep of the cell wall, to keep it rigid. This means the cell wall becomes less stable. Cells are reproduced abnormally with this defect. As the cell wall is less rigid, water moves in excess by osmosis, causing the cell the lyse (burst). This means the antibiotic stops the growth of the gram-positive bacteria.
• Why do antibiotics find it harder to kill gram-negative bacteria?


  Gram-negative bacteria contain a thinner peptidoglycan cell wall, however are surrounded by two lipid protective membranes. The antibiotic cannot cross the membrane or the cell. The antibiotic cannot get into the cell to work. This results in no reduction in cell wall synthesis. The cell wall remains rigid so water does not come in the cell in excess. The cell does not lyse (burst). This results in no death of bacterial cells due to the antibiotic. The gram-negative bacteria grows as normal.
• Gram Staining Technique Procedure


  Prepare a smear of bacterial culture on a slide with a inoculated loop.

  Add 5 drops of crystal violet

  Wash of excess crystal violet with deionised water

  Add about 5 drops of iodine solution

  Decolourise with ethanol until no purple residue

  Add five drops of safranin (used in case it is gram negative). We then used a light microscope to observe.