Differences between Gram+ and Gram- bacteria 1.Gram+ have a thick peptidoglycan layer. Gram- have a thin layer. 2.Gram- have a second lipid membrane outside the plasma membrane, with an intermembrane space (periplasm). The outer membrane is part of the outer lipid envelope, which contains LPS (lipopolysaccharide). LPS causes fever. Gram+ do not have these structures.

Explanation of Gram staining properties 1.Gram+: Alcohol causes shrinkage of the pores in the peptidoglycan layer. Dye is trappped within the pores. 2.Gram-: Dye initially binds within the outer lipid envelope. But the alcohol dissolves this structure, so the dye washes away

Mycobacteria Cause tuberculosis (M. tuberculosis); leprosy (M. leprae); and AIDS-related pneumonia (M. avium) Have properties of both Gram+ and Gram- bacteria: Have a thick peptidoglycan, but also have an outer membrane. However, the outer membrane is structurally different from that of typical Gram- bacteria Have a waxy coat containing mycolic acid (a lipid) which prevents penetration of many dyes. But this is stained by the acid-fast dye, which will not wash away. Therefore, do not stain Gram+, despite the thick peptidoglycan.