LEC 5 Bacterial Growth

Bacteria divide through binary fission, an asexual reproduction process resulting in identical copies.
- DNA replication.
- Septum formation.
- Cell wall formation.
- Separation of cells.

The number of cells after g generations can be calculated using the formula: N = N_0 \times 2^g, where:
- N is the number of cells after g generations.
- N_0 is the initial number of cells.
- g is the number of generations.
The number of generations (g) can be calculated using the formula: g = (log(N) – log(N_0))/0.301, based on the fact that log(2) = 0.301.
Generation time is calculated as: t/g, where:
- t is time.
- g is the number of generations.
Example: If 5 cells divide 4 times, the number of cells would be 5 \times 2^4 = 5 \times 16 = 80.

Lag Phase: Cells are adjusting to the new environment and not actively replicating.
Log Phase: Cells are growing.
Stationary Phase: Growth slows down as death cancels out life due to limited space and nutrients.
Death Phase: Death rate exceeds the growth rate as waste products accumulate and nutrients deplete.

Biofilms are communities of bacteria surrounded by a complex polymer, usually attached to a surface.
They may contain one or more species, working together in coordinated activities, providing an environmental advantage in nature.

Temperature: Microbes grow over a range of temperatures, growing fastest at their optimal temperature.
- Colder temperatures slow down biochemical reactions; hotter temperatures denature proteins and kill cells.
pH: Bacteria grow best at neutral pH.
Osmotic pressure: High salt concentrations lead to plasmolysis. A balance in water movement is essential.

Essential elements include:
- Carbon (50% of dry weight).
- Nitrogen (14% of dry weight).
- Phosphorus & sulfur (4% of dry weight).
- Trace elements (iron, copper, molybdenum, zinc).
- Other growth factors (vitamins, amino acids, purines, pyrimidines).
Oxygen can be an essential element, but some forms of oxygen can be toxic.
- Toxic forms include singlet oxygen (^1O2), superoxide radicals (O2^-), peroxide anion (O2^{2-}), hydroxyl radical (OH\cdot), and ozone (O3).
- Some bacteria use enzymes to neutralize these compounds.

Bacteria experience all levels of oxygen. Oxygen level helps choice to need some oxygen, but only small. Oxygen gradient can exist in semi-solid medium.

Incubation conditions must consider oxygen levels and temperature.
Culture medium is a nutrient material prepared for the growth of microorganisms in the laboratory, including carbon, nitrogen, sulfur, phosphorus, trace elements, water, and adjusted pH. It must be sterile and can be solid or liquid.
Inoculum is the microbes introduced into a culture medium to initiate growth.
Culture refers to the microbes that grow and multiply in or on a culture medium after incubation.
Solid growth medium contains agar for solidification. The quadrant streak method is used to get isolated colonies, diluting bacteria over the plate surface so that each individual colony arises from a single cell.
- Colony morphology cannot be determined without isolated colonies.

Chemically defined media: Exact chemical composition is known, including the amounts of every chemical compound.
Complex media: Include extracts or digests from other organisms (e.g., peptones); exact chemical composition is not known.
Selective media: Include components to suppress the growth of unwanted organisms and encourage the growth of target organisms (e.g., high salt concentration (>1%), bile salts, antibiotics, or crystal violet).
Differential media: Include components to allow target organisms to be distinguished from other organisms.
A medium can be selective, differential, both, or neither.