Cell Walls of Gram-Pasitive and Gram-Negative Bacteria

The structure, chemical composition, and thickness of the

celad cell wall differ in gram-positive and gram-negative bacteria

(Table 2-2, Figure 2-4A, and "Gram Stain box).

(1) The peptidoglycan layer is much thicker in gram-positive

than in gram-negative bacteria. Many gram-positive bacteria

also have fibers offeichoic acid/that protrude outside the pepti

doglycan, whereas gram-negative bactêria do not have teichoic

acids.

(2) In contrast, the gram-negative bacteria have a complex

Outer layer consisting of fpopolvsaccharid@ ipoproteinand

phospholipDLying between the outer-membrane layer and the

cytoplasmic membrane in gram-negative bacteria is the periplasmic space which is the site, in some species, of

enzymes called 5-lactamases that degrade penicillins and other

B-lactam drugs.

species, of

The cell wall has several other important properties:

(1) In gram-negative bacteria, it contains endotoxin, a lip0-

polysaccharide (see pages 8 and 43).

(2) Its polysaccharides and proteins are antigens that are use-

ful in laboratory identification.

(3) t porir(proteins play a role in facilitating the passage of

Small, hydrophilic molecules into the cell. Porin proteins in the

outer membrane of gram-negative bacteria act as a channel to

allow the entry of essential substances such as sugars, amino

acids, vitamins, and metals as well as many antimicrobial drugs

such as penicillins.