Introduction to Microbiology

1. Introduction to Microbiology

2. Prokaryotes vs Eukaryotes Prokaryotic DNA Eukaryotic DNA • Bacterial chromosome is a single, double-stranded circle that is contained in a discrete area known as the nucleoid • Histones not required to maintain the conformation of the DNA • DNA does not form nucleosomes • Plasmids, which are smaller, circular, extra-chromosomal DNAs, may also be present. • Contained in a nucleus • Has more than 1 chromosome, numbers vary depending on species • Requires histone proteins to wind around forming a condensed structure called a nucleosome • Does not contain plasmids

3. Gram Positive vs Gram Negative Bacteria • Gram positive and Gram negative bacteria have similar internal, but very different external structures • The cytoplasm of both types of bacterial cells contain • the DNA chromosome • the mRNA & Ribosomes • Proteins & metabolites   • The structure, components, and functions of the cell wall distinguish Gram (+) from Gram (-) bacteria.

4. Gram positive and Gram negative bacteria. A Gram positive bacterium has a thick layer of peptidoglycan (left). A Gram negative bacterium has a thin peptidoglycan layer and an outer membrane (right). Structures in () are not found in all bacteria.

5. Bacterial Cell Walls • The cytoplasmic membranes of most prokaryotes are surrounded by rigid peptidoglycan (murein) layers. • The exceptions are • Archaeobacteria organisms (which contain pseudoglycans or pseudomureins related to peptidoglycan) • and mycoplasmas (which have no cell walls at all). • Because the peptidoglycan provides rigidity, it also determines the shape of the particular bacterial cell. • Gram negative bacteria are also surrounded by outer membranes.

6. Gram Positive Bacteria • A Gram + bacterium has a thick, multilayered cell wall consisting mainly of peptidoglycan surrounding the cytoplasmic membrane. • The peptidoglycan is a meshlike exoskeleton similar in function to the exoskeleton of an insect. • The peptidoglycan is essential for the structure, for replication, and for survival in the normally hostile conditions in which bacteria grow. • During infection, the peptidoglycan • Can interfere with phagocytosis, • is mitogenic (stimulates mitosis of lymphocytes), • and has pyrogenic activity (induces fever).

7. Killing Gram Positive Bacteria • The peptidoglycan can be degraded by treatment with lysozyme, an enzyme in human tears and mucus (is also produced by bacteria and other organisms) • Lysozyme degrades the glycan backbone of the peptidoglycanwithout the peptidoglycan, the bacteria succumb to the large osmotic pressure differences across the cytoplasmic membrane and lyse. • Removal of the cell wall produces a protoplast that lyses unless it is osmotically stabilized.

8. Gram Negative Bacteria • Gram negative cell walls are more complex than Gram positive cell walls, both structurally and chemically. • Structurally, a Gram negative cell wall contains two layers external to (outisde of) the cytoplasmic membrane. • Immediately external to the cytoplasmic membrane is a thin peptidoglycan layer, which accounts for only 5% to 10% of the Gram negative cell wall by weight. External to the peptidoglycan layer is the outer membrane, which is unique to Gram negative bacteria. • The area between the external surface of the cytoplasmic membrane and the internal surface of the outer membrane is referred to as the periplasmic space.

9. Periplasmic Space in Gram Negative Bacteria • This space is actually a compartment containing a variety of hydrolytic enzymes, which are important to the cell for the breakdown of large macromolecules for metabolism. • Enzymes typically include proteases, phosphatases, lipases, nucleases, and carbohydrate-degrading enzymes. • In the case of pathogenic Gram (-)species, many of the lytic virulence factors such as collagenases, hyaluronidases, proteases, and beta-lactamase are in the periplasmic space. • Some binding proteins can be components of a chemotaxis system, which senses the external environment of the cell.

10.  Outer Membranes are Unique to Gram (-) Prokaryotes • It maintains the bacterial structure and is a permeability barrier to • large molecules (e.g., proteins such as Lysozyme) • and hydrophobic molecules. • It also provides protection from adverse environmental conditions such as the digestive system of the host (important for Enterobacteriaceae organisms). • The outer membrane has an asymmetric bilayer composed primarily of lipopolysaecharides (LPS). • LPS is an amphipathic molecule, meaning that it has both hydrophobic and hydrophilic ends •  LPS is also called endotoxin, a powerful stimulator of immune responses.

11. Gram (-) Bacteria & Plasmids • A plasmid is an extra-chromosomal DNA molecule separate from the chromosomal DNA which is capable of replicating independently of the chromosomal DNA. • In many cases, it is circular and double-stranded • Plasmids are most commonly found in Gram (-) bacteria • not usually essential for cellular survival • they often provide a selective advantage: many confer resistance to one or more antibiotics.

13. Bacterial Capsules •  Some bacteria (Gram positive or Gram negative) are closely surrounded by loose polysaccharide or protein layers called capsules. • In cases in which it is loosely adherent and nonuniform in density or thickness, the material is referred to as a slime layer. • The capsule and slime layers are also called the glycocalyx.

14. Bacterial Capsules • Capsules and slimes are unnecessary for the growth of bacteria but are very important for survival in the host. The capsule is poorly antigenic and antiphagocytic and is a major virulence factor • The capsule can also act as a barrier to toxic hydrophobic molecules, such as detergents, and can promote adherence to other bacteria or to host tissue surfaces