Bacteria are relatively simple single-celled organisms. One method of classification is by shape or morphology:

  • Cocci:
    • spherical shape
    • 0.4 - 1.5 µ m

Examples: staphylococci - form grape-like clusters; streptococci - form bead-like chains. 

  • Rods:
    • 0.25 - 1.0 µ m width by 0.5 - 6.0 µ m long

Examples: bacilli - straight rod; spirilla - spiral rod

There exists a bacterial system of taxonomy, or classification system, that is internationally recognized with family, genera and species divisions based on genetics.

Some bacteria have the ability to form resting cells known as endospores. The spore forms in times of environmental stress, such as lack of nutrients and moisture needed for growth, and thus is a survival strategy. Spores have no metabolism and can withstand adverse conditions such as heat, disinfectants, and ultraviolet light. When the environment becomes favourable, the spore germinates and giving rise to a single vegetative bacterial cell. Some examples of spore-formers important to the food industry are members of Bacillus and Clostridium generas.

Bacteria reproduce asexually by fission or simple division of the cell and its contents. The doubling time, or generation time, can be as short as 20-20 min. Since each cell grows and divides at the same rate as the parent cell, this could under favourable conditions translate to an increase from one to 10 million cells in 11 hours! However, bacterial growth in reality is limited by lack of nutrients, accumulation of toxins and metabolic wastes, unfavourable temperatures and desiccation. The maximum number of bacteria is approximately 1 X 10e9 CFU/g or ml.

Note: Bacterial populations are expressed as colony forming units (CFU) per gram or millilitre.

Hypothetical bacterial growth curveBacterial growth generally proceeds through a series of phases:

  • Lag phase: time for microorganisms to become accustomed to their new environment. There is little or no growth during this phase.
  • Log phase: bacteria logarithmic, or exponential, growth begins; the rate of multiplication is the most rapid and constant.
  • Stationary phase: the rate of multiplication slows down due to lack of nutrients and build-up of toxins. At the same time, bacteria are constantly dying so the numbers actually remain constant.
  • Death phase: cell numbers decrease as growth stops and existing cells die off.

The shape of the curve (shown on the right) varies with temperature, nutrient supply, and other growth factors. This exponential death curve is also used in modeling the heating destruction of microorganisms.