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Types of bacteria

Bacteria can be divided into two categories depending on the arrangement of their cell walls.

The cell walls are made from peptidoglycan. This is a carbohydrate that forms cross-links with peptides (short polymers of amino acids). Peptidoglycan turns blue or violet in a Gram stain.

  • Gram-positive bacteria have cell walls outside the cell membranes. The cell walls are made up of about 30-40 layers of peptidoglycan.
  • Gram-negative bacteria have cell walls each sandwiched between two cell membranes. These cell walls generally consist of just a few sheets (2-3) of peptidoglycan.
These anthrax bacteria have been stained in a gram stain.
These anthrax bacteria have been stained in a gram stain.