Introduction
Introductory microbiology deals with two groups of microscopic organisms that sit at the boundary of the living world: bacteria (kingdom Monera), which are true prokaryotic cells, and viruses, which are acellular particles that behave as living entities only inside a host cell. Both groups are ecologically and economically important, and both are central tools of modern biotechnology.
Monera: general introduction
Monera is the kingdom of prokaryotes, organisms whose cells lack a membrane-bound nucleus and membrane-bound organelles. It includes:
- Eubacteria (true bacteria): the common bacteria found in soil, water, air, food and living bodies.
- Archaebacteria: bacteria adapted to extreme habitats (high salt, high temperature hot springs, marshy methane-producing habitats).
- Cyanobacteria (blue green algae): photosynthetic prokaryotes, historically grouped with algae but structurally prokaryotic like bacteria.
Bacteria are the most abundant and widely distributed organisms on earth, found in soil, water, air, extreme habitats, and as symbionts or parasites of plants and animals.
Structure of a bacterial cell
A typical bacterial cell has the following parts:
- Capsule / slime layer: a mucilaginous outer covering (present in some bacteria) that protects the cell from desiccation and from the host's immune cells, and helps attachment to surfaces.
- Cell wall: a rigid wall made of peptidoglycan (murein), a polymer of amino sugars cross linked by short peptide chains. It gives shape and protects the cell from bursting. Bacteria are broadly classed as Gram positive (thick peptidoglycan wall, retains crystal violet stain) or Gram negative (thin peptidoglycan wall with an outer lipopolysaccharide membrane, does not retain crystal violet).
- Cell (plasma) membrane: a phospholipid bilayer with embedded proteins lying just beneath the cell wall; controls entry and exit of substances and carries the respiratory enzymes, since bacteria have no mitochondria.
- Cytoplasm: a semifluid matrix containing ribosomes and storage granules (glycogen, lipid, volutin), and, in photosynthetic forms, pigments.
- Nucleoid: the region of cytoplasm holding a single, circular, double stranded DNA molecule (the bacterial chromosome). It is not bound by a nuclear membrane, so it is called a nucleoid rather than a true nucleus.
- Plasmid: small, circular, extra-chromosomal DNA molecules that often carry genes for antibiotic resistance; they replicate independently of the main chromosome and are the main vector used to carry foreign genes in genetic engineering.
- Ribosomes: smaller (70S) than eukaryotic ribosomes, scattered free in the cytoplasm; the site of protein synthesis.
- Flagella: whip-like protein appendages used for locomotion, present in motile bacteria; may be single (monotrichous), tufted (lophotrichous), at both ends (amphitrichous), or all over the surface (peritrichous).
- Pili / fimbriae: fine hair-like projections used for attachment to surfaces or host cells; the sex pilus is used for transfer of DNA during conjugation.
Bacteria show three basic shapes: coccus (spherical, e.g. Streptococcus), bacillus (rod shaped, e.g. Bacillus subtilis), and spirillum (spiral, e.g. Spirillum); a short curved rod is called vibrio (e.g. Vibrio cholerae).