A microbe is any organism that is not visible with the naked eye. The unaided resolution of the eye is about 0.1mm

Bacteria are classified according to their shape:

 1) Cocci: spherical bacteria

  • Cocci – smallest bacteria, occur as single spheres
  • Diplococci – pairs of spheres, e.g. pneumonia
  • Staphylococci – clusters of spheres, e.g. food poisoning
  • Streptococci – chains of spheres, e.g. sore throat

2) Bacilli: rod-shaped bacteria:

  • Individual rods – e.g. typhoid fever
  • Chains of rods – e.g. anthrax

3) Spirilla – large, spiral-shaped bacteria - e.g. syphilis

4) Vibrio – crescent-shaped bacteria - e.g. cholera

Reproduction of Bacteria

Cell elongation results in the synthesis of additional cytoplasm & nuclear material

DNA replication takes place (there is no mitotic spindle), & the nuclear material attaches to the plasma membrane or mesosome

A septum begins to develop, & the nuclear material is distributed to both sides

The septum is completed, & a cell wall develops to divide the cell into two

The two daughter cells grow to a critical size, & then repeat this process

Culturing Bacteria

New genetic material can be inserted into a bacterium in three main ways:

1) Conjugation: bacteria link together by their pili.

  • Donor passes a plasmid called the F-factor (fertility) to the recipient cell.
  • The F-factor may be in a plasmid (replicating independently), or incorporated into the main bacterial chromosome

2) Transformation: one bacterium releases DNA which is absorbed by a second bacterium, allowing it to acquire new characteristics

3) Transduction: new genes can be inserted into the bacterial chromosome by a bacteria phage (a virus acting as a vector)

The bacterial population growth curve occurs in four main phases:

1) Lag phase: cells are active, but there is little increase in number.

  • The cells accommodate to the new conditions, take in water & synthesise ribosomes & enzymes

2) Log phase: Nutrients & space are in plentiful supply, so there is little competition, & the bacteria multiply at their maximum rate

3) Stationary phase: carrying capacity (maximum number of bacteria that the environment can support) is reached, so intraspecific competition takes place between bacteria.

Hence the death rate balances the population growth rate, & the number of bacteria remains roughly constant

4) Death phase: nutrient supply is running out & waste products accumulate resulting in increased toxicity of environment.

Organisms are killed & population size eventually falls to zero.

Spores may be produced during stationary phase that are resistant to the adverse conditions


Bacterial growth can be controlled using physical methods (gamma irradiation or in an Autoclave using high temperatures) or by chemical means:

  • disinfectants
  • antiseptics

Antibiotics can be either:

  • narrow-spectrum (affecting a few types of bacterium)
  • broad-spectrum (affecting a wide range of bacteria)
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