The bacterial cell envelope

Abstract

The bacteria cell envelope is a complex multilayered structure that serves to protect these organisms from their unpredictable and often hostile environment. The cell envelopes of most bacteria fall into one of two major groups. Gram-negative bacteria are surrounded by a thin peptidoglycan cell wall, which itself is surrounded by an outer membrane containing lipopolysaccharide. Gram-positive bacteria lack an outer membrane but are surrounded by layers of peptidoglycan many times thicker than is found in the gram-negatives. Threading through these layers of peptidoglycan are long anionic polymers, called teichoic acids. The composition and organization of these envelope layers and recent insights into the mechanisms of cell envelope assembly are discussed.

Figure 1.

Transenvelope machines in the Gram-negative cell envelope. The AcrA/B proteins together with TolC form an efflux pump that expels harmful molecules such as antibiotics from the cell directly into the media (Koronakis et al. 2000; Eswaran et al. 2004; Murakami et al. 2006). The flagellar basal body hook structure connects the motor to the flagella (DePamphilis and Adler, 1971). Distances shown provide a reasonable estimate of the size of the cellular compartments shown. PE, periplasm; CYT, cytoplasm.

Figure 2.

The cellular machineries required for OM biogenesis. The Lpt pathway, together with MsbA, transports LPS from its site of synthesis to the cell surface. β-barrel proteins and lipoproteins are made initially in the cytoplasm in precursor form with a signal sequence at the amino terminus. The signal sequence directs these precursors to the Sec machinery for translocation from the cytoplasm. Chaperones like SurA deliver beta-barrel proteins to the Bam machinery for assembly in the OM. For OM lipoproteins, after the signal sequence is removed and lipids are attached to the amino-terminal cysteine residue, the Lol machinery delivers them to the OM.

Figure 3.

Depiction of Gram-positive and Gram-negative cell envelopes: CAP = covalently attached protein; IMP, integral membrane protein; LP, lipoprotein; LPS, lipopolysaccharide; LTA, lipoteichoic acid; OMP, outer membrane protein; WTA, wall teichoic acid.