Bacterial growth
As bacterial growth is defined as the multiplication of bacteria by cell division , coupled with an increase in mass . The rate of growth is determined by the number of divisions per unit of time, the cell division rate.
procedure
Cell division is usually equivalent; H. the two daughter cells are alike. In addition, there is an inequitable division in which the daughter cells are designed differently, as in the Caulobacteraceae . Another variant is budding , in which exospores can be formed as a permanent stage . Between successive divisions, the cell content and the cell wall material are doubled.
Bacteria cultivation in the laboratory
The cultivation of bacteria in the laboratory usually takes place in liquid nutrient media or on gelled nutrient media, for example on the basis of an agar gel. Only a relatively small number of bacterial species can be cultivated in this way in the laboratory, many species need significantly more complex living conditions and also the interaction with one to numerous other living beings in order to reproduce.
Stages of growth in static cultures
The growth of bacteria in a static culture ( batch culture ) can be divided into four phases: start-up phase (lag), exponential phase (log), stationary phase, death phase.
Start-up phase (lag)
In the start-up phase, also known as the latency phase (or lag phase ), the substances available in the nutrient medium are analyzed by receptors in the bacterial cell membrane . Depending on this, the expression of genes is switched on that code for enzymes that enable the substances in the nutrient medium to be broken down. This takes a certain amount of time, which can vary depending on the bacterial species and the composition of the nutrient medium. The breakdown of nutrients forms the basis for cell growth and division. Furthermore, in many cases the metabolism improves the properties of the culture medium for cell growth and cell division, for example by lowering the redox potential .
Exponential phase (log)
| Number of divisions | Number of bacteria | time |
|---|---|---|
| 0 | 1 | 0 |
| 1 | 2 | 20 min |
| 2 | 4th | 40 min |
| 3 | 8th | 60 min |
| 4th | 16 | 80 min |
| 5 | 32 | 100 min |
| 6th | 64 | 120 min |
| 7th | 128 | 140 min |
| (...) | (...) | (...) |
| 18th | 262.144 | 6 h |
| (...) | (...) | (...) |
| 36 | 6.87 x 10 10 | 12 h |
| (...) | (...) | (...) |
| 72 | 4.72 x 10 21 | 24 hours |
| n | 2 n | nx 20 min |
After adaptation of the metabolism and possibly also of the medium, the bacterial culture changes into an exponential growth . The generation time here remains the same over several cell division cycles. Under ideal conditions in laboratory cultures, the human intestinal bacterium Escherichia coli has a generation time of around 20 minutes: The example in the table below shows that the number of bacterial cells (starting with 1) doubles every 20 minutes.
Stationary phase
If the system approaches the capacity limits of the available space and the nutrients, the number of bacterial cells in the medium no longer increases. A dynamic equilibrium is established in which the number of dying bacteria and bacteria that are added through cell division are balanced.
This phase is sometimes called the idiophase and may be of relevance in biotechnology, as it often leads to an increased synthesis of secondary metabolites (such as insulin ).
Death phase
When the nutrients in the medium are practically used up and the tolerance value for the population density of the respective bacterial species has been reached, the death phase begins. The bacterial cells starve or die from waste products of their own metabolism, which are present in high concentrations in this phase . Bacteria also secrete toxins , which reduces competition for nutrients from representatives of their own or other species. The concentration of these toxins also increases with increasing population density.
Suppression of bacterial growth
Various measures are used to slow down or stop bacterial growth. Among other things, this plays a role in food preservation . In medicine , antibiotics can be used, which often suppress important functions of bacterial anabolism .
literature
- Olaf Fritsche: microbiology . Springer Spectrum, Berlin, Heidelberg 2016, ISBN 978-3662497289 .
- Georg Fuchs (Hrsg.): Allgemeine Mikrobiologie , 10th edition. Georg Thieme Verlag, Stuttgart, New York 2017, ISBN 978-3132418851 .
- Michael T. Madigan, John M. Martinko, David A. Stahl, David P. Clark: Brock Microbiology , 13th Edition. Pearson, Munich et al. 2013, ISBN 978-3868941449 .
- Katharina Munk (Ed.): Microbiology , 2nd edition. Thieme 2018.