Gene Transfer among Gram Positive Bacteria

Traditionally, the Eubacteria are divided into two major groups, the gram-negative and the gram-positive bacteria. This division was originally based on their response to the gram-negative bacteria Major division of Eubacteria that possess an extra outer membrane lying outside the cell wall gram-positive bacteria Major division of Eubacteria that lack an extra outer membrane lying outside the cell wall

FIGURE 18.18 Formation of F-prime Plasmid

Integrated F-plasmids can excise from the chromosomal DNA to reform independent plasmids. In some instances, the plasmids capture pieces of chromosomal DNA during excision. In the example shown the F plasmid (purple) originally integrated using IS2 (shown flanking F). If different insertion sequences are used during excision, a novel plasmid is created. Thus, recombination between the two IS1 sequences creates a large plasmid that includes the original F-plasmid plus some chromosomal DNA. The chromosome now contains a deletion and the F-plasmid contains the corresponding chromosomal DNA. The new plasmid is termed an F-prime or F'-plasmid.

FIGURE 18.19 Differences in Envelopes of Gramnegative and Gram-positive Bacteria

The outer surfaces of gram-positive and gram-negative bacteria have different structures. A) In gramnegative bacteria, such as E. coli, there are three surface layers. The outermost layer, called the outer membrane, is a lipid-bilayer that contains various proteins embedded within the lipids, and an outer coating of lipopolysaccharide. Next, within the periplasmic space, the cell wall contains a single layer of peptidoglycan. Lipoproteins connect this cell wall to the outer membrane. The layer closest to the cytoplasm, called the inner membrane, is a lipid bilayer embedded with various proteins. B) The outer surface of gram-positive bacteria only has two layers, a thick coating of peptidoglycan and teichoic acid surrounding the inner membrane.

A) GRAM-NEGATIVE ENVELOPE

Lipopoly-saccharide

Lipopoly-saccharide

■ Inner membrane

Phospholipids

■ Inner membrane

Phospholipids

B) GRAM-POSITIVE ENVELOPE

B) GRAM-POSITIVE ENVELOPE

Several layers of peptidoglycan plus teichoic acid (red)

Inner membrane

Several layers of peptidoglycan plus teichoic acid (red)

Inner membrane

Gram-negative bacteria, including Escherichia coli, have an extra outer membrane.

Gram stain. These differences in staining are in fact due to differences in the chemical composition and structure of the cell envelope. The envelope of gram-negative bacteria consists of the following layers (from inside to outside): cytoplasmic membrane, cell wall (peptidoglycan) and outer membrane (Fig. 18.19). The envelope of gram-positive bacteria is simpler and lacks the outer membrane. Both kinds of bacteria sometimes have an extra protective layer, the capsule, on the very outside.

The gram-negative bacterium E. coli is widely used as a host for cloning and expressing genes from a variety of other organisms. The synthesis of large amounts of a purified recombinant protein, such as a human growth factor or hormone is often desirable. Secretion of a recombinant protein into the culture medium would be very convenient since this avoids purifying it away from all the other proteins inside the bacterial cell. However, the complex envelope of gram-negative bacteria is a major hindrance in the excretion of proteins into the culture medium. In contrast, secretion across the simpler gram-positive envelope is easier. Indeed, many gram-positive bacteria, such as Bacillus, excrete proteins into the culture medium naturally. As a result, outer membrane Extra membrane lying outside the cell wall in gram-negative but not gram-positive bacteria

Was this article helpful?

0 0

Post a comment