Bacterial growth and metabolism
- Bacteria divide by binary fission meaning one cell divides into two progeny cells. That results in a logarithmic or exponential growth.
- The doubling time varies from bacteria to bacteria e.g. in E.coli it is just 20 minutes while in Mycobacterium tuberculosis it is more than 24 hours.
- Growth will be affected by the presence or absence of nutrients in the media, temperature, pH, characteristics of the bacteria themselves such as species etc.
- When we plot a graph of bacterial growth over time, it shows 4 phases. Following are the phases of the bacterial growth curve
- Lag phase: Cells have not yet started to divide.
- Log (logarithmic phase): Phase of rapid cell division. Beta lactam drugs like Penicillin are active during this phase.
- Stationary Phase: Growth starts to slow down due to depletion of nutrients or accumulation of toxic products.
- Death Phase: There is a decline in the number of viable bacteria.
- Bacteria can be classified as follows depending on their oxygen requirements.
| Type | Bacteria |
|---|---|
| Obligate Aerobes oxygen is essential for growth | Mycobacteria, Nocardia, Pseudomonas, Bacillus |
| Facultative Anaerobes can use oxygen but also grow in the absence of oxygen | E.coli, Staphylococcus, Yeasts etc. |
| Obligate Anaerobes cannot use oxygen as it is toxic to them | Clostridia |
| Aerotolerant Anaerobes cannot use oxygen but oxygen is not toxic to their survival. | Lactobacillus |
| Microaerophilic require oxygen at low concentrations | Campylobacter, Legionella, Helicobacter, Vibrio |
Fermentation of sugars
In the absence of oxygen,bacteria ferment sugars by glycolysis producing ATP. An exception will be Aerobic bacteria like Pseudomonas aeruginosa. If oxygen is present, pyruvate will enter the TCA cycle to generate more ATP. This characteristic is used in microbial identification. For this test, bacteria are grown in a culture medium to which a pH indicator like phenol red has been added. Fermentative bacteria will produce pyruvate and lactate which turn the medium acidic and phenol red becomes yellow. If there was no fermentation, no acids are produced and the phenol red stays red.
Iron Metabolism
Iron is an essential nutrient for bacteria. Innate immunity in humans limits iron availability to invading pathogens by a process called Nutritional Immunity. Iron is sequestered in the human body as hemoglobin, transferrin, ferritin, lactoferrin etc. so that pathogens cannot use it easily. Bacteria produce specialized iron chelating compounds called siderophores to acquire iron from the host.
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