Basic morphology by Gram stain

• Gram-positive are purple; in this picture, the bacteria are round so they are cocci

• Gram-negative are pink

• there are also rodlike bacteria

How the Gram stain works

• Gram-positive bacteria have a thick peptidoglycan cell wall. you stain the cell with purple and it sticks

• Gram-negative: you stain the cells with purple. the dye doesn’t stick. when you wash and stain with red, the red color sticks

Peptidoglycan

• protects these bacteria

• N-acetylglucosamine and N-acetylmuramic acid are crosslinked by transpeptidase

• transpeptidase is targeted by all penicillins and cephalosporins; resistance to these antibiotics is what made MRSA methicillin-resistant

• β 1-4 linkage is broken by lysozyme in tears. this is a natural defense

Category: Microbiology Notes