1. Bacteria divide by binary fission, meaning one cell splits into two daughter (progeny) cells. This leads to logarithmic (exponential) growth.
2. Doubling time varies between bacterial species. For example, in E. coli it’s about 20 minutes, while in Mycobacterium tuberculosis it’s more than 24 hours.
3. Growth is affected by:
   • The presence or absence of nutrients in the medium
   • Temperature
   • pH
   • Characteristics of the bacteria themselves (for example, the species)
4. When you plot bacterial growth over time, the curve shows four 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.

5. 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 can ferment sugars through glycolysis to produce ATP. An exception is aerobic bacteria like Pseudomonas aeruginosa, which do not rely on fermentation.

If oxygen is present, pyruvate enters the TCA cycle, generating more ATP.

This difference is used in microbial identification. In this test, bacteria are grown in a culture medium containing a pH indicator such as phenol red:

• Fermentative bacteria produce acids (for example, from pyruvate and lactate), which lower the pH. Phenol red turns yellow in acidic conditions.
• If there is no fermentation, acids are not produced, the pH does not drop, and phenol red stays red.

Iron metabolism

Iron is an essential nutrient for bacteria. In humans, innate immunity limits iron availability to invading pathogens through a process called nutritional immunity.

Iron is sequestered in the body in forms such as hemoglobin, transferrin, ferritin, and lactoferrin, making it difficult for pathogens to access.

To obtain iron from the host, bacteria produce specialized iron-chelating compounds called siderophores.