Cell envelope!

The cell envelope is a protective layer of armor that surrounds the bacterium and allows it to survive in diverse and extreme environments.

The cell envelopes of some bacteria consist of a cytoplasmic membrane surrounded by a tough and rigid mesh called a cell wall ; these bacteria are referred to as gram-positive bacteria.
 In contrast, the cell envelope of a gram-negative bacterium consists of a cytoplasmic membrane surrounded by a thin cell wall that is itself surrounded by a second lipid membrane called the outer membrane.
 The outer membrane contains large amounts of lipopolysaccharide (LPS), a molecule that is very toxic to humans. The space between the outer membrane and the cytoplasmic membrane, which contains the cell wall, is called the periplasmic space or the periplasm.

Whether a bacterium is gram- positive or gram-negative can usually be determined by a technique called Gram stain- ing, which colors gram-positive bacteria blue or purple and gram-negative bacteria pink.
 Gram staining is often the first step used by a hospital microbiology laboratory in identifying an unknown bacterium from a clinical specimen.

As in human cells, the cytoplasmic membrane prevents ions from flowing into or out of the cell itself and maintains the cytoplasm and bacterial components in a de- fined space.
 The cell wall is a tough layer that gives a bacterium its characteristic shape and protects it from mechanical and osmotic stresses.

 In gram-negative bacteria, the outer membrane acts as an additional protective barrier and prevents many substances from penetrating into the bacterium. This layer, however, does contain channels called porins that allow some compounds such as molecules used in metabolism by the bac- terium to pass through.

Since human cells do not possess a cell wall, this structure is an ideal target for antimicrobial agents. To appreciate how these agents work, we must first understand the structure of the cell wall. This complex assembly is made up of a substance called peptidoglycan, which itself consists of long sugar polymers. The polymers are repeats of two sugars: N-acetylglucosamine and N-acetylmuramic acid
. If the cell wall were to consist of these polymers alone, it would be quite weak.

 However, peptide side chains extend from the sugars in the polymers and form cross-links, one peptide to another. These cross-links greatly strengthen the cell wall, just as cross-linking of metal loops strengthened the chain mail armor used by medieval knights. 

The cross-linking of peptidoglycan is mediated by bacterial enzymes called penicillin-binding proteins (PBPs). (The reason for this nomenclature will become apparent in later chapters.) These enzymes recognize the terminal two amino acids of the peptide side chains, which are usually d-alanine–d-alanine, and either directly cross-link them to a second peptide side chain or indirectly cross-link them by forming a bridge of glycine residues between the two peptide side chains.

The formation of a tough cross-linked cell wall allows bacteria to maintain a char- acteristic shape. For example, some bacteria are rod shaped and referred to as bacilli. Cocci are spherical in shape. Coccobacilli have a morphology that is intermediate between that of bacilli and cocci. Finally, spirochetes have a corkscrew shape.