The main difference in gram positive and gram negative bacteria resides in their :

(1)	Cell envelope
(2)	Flagella
(3)	Cell membrane
(4)	Cell wall

(4) Cell Wall

Gram positive bacteria stain blue-purple with gram stain while Gram negative does not. Gram positive bacteria contain a thick peptidoglycan layer while gram negative bacteria contain an extra outer lipid layer with a very thin peptidoglycan layer. Most pathogenic bacteria are Gram negative. Gram negative cell walls contain a thin peptidoglycan layer (without teichoic acids) that is surrounded by a thick plasma membrane. Gram positive bacteria will stain purple because of their thick peptidoglycan cell wall.

Cell Wall	Cell Wall is 20-30 nm thick.	Cell Wall is 8-12 nm thick.
Cell Wall	The wall is Smooth.	The wall is wavy.
Peptidoglycan Layer	Thick (multilayered)	Thin (single-layered)
Teichoic Acids	Present in many	Absent
Periplasmic Space	Absent	Present
Outer Membrane	Absent	Present