10. Bacterial Physiology - Bacterial Growth and Cell Division, Growth Factors · Microbiology · MU Sofia

Bacterial Growth and Cell Division

Bacteria grow and replicate through binary fission.
Binary fission is a type of asexual cell division.
The process involves the duplication of the bacterial DNA, typically a single circular chromosome. Replication begins at a single origin of replication and proceeds bidirectionally.

1. The bacterial chromosome binds to the cell membrane.
2. The DNA and its copy are attached to the cell membrane. As the DNA is duplicated, the copies are pulled to opposite ends of the cell.
3. A new cell membrane or cross wall (septum) grows between the two newly formed DNA molecules, dividing the enlarged cell.
4. The new cell wall fully develops, resulting in the complete split into two identical daughter cells. The daughter cells contain tightly coiled DNA, ribosomes, and plasmids. Actin-like protein filaments inside the membrane help determine the shape and the site of septum formation.

The time taken for the bacteria to double in number during a specific period is known as the generation time or doubling time.
Generation time varies significantly depending on the microbial species and nutritional/environmental conditions (such as O2, temperature, and pH).
Typical generation time can range from about 8 minutes to over 24 hours.
E. coli can divide every 20 minutes in the lab but takes 12-24 hours in the intestinal tract.
Fast-growing bacteria cultivated in vitro typically have a generation time of 15–30 minutes, but this time is much longer in vivo.
Obligate anaerobes tend to grow much more slowly than aerobes in vitro.
Mycobacterium tuberculosis has an in vitro generation time of approximately 12–18 hours.

When bacteria proliferate in a nutrient broth, their growth typically follows a curve with four main phases:

Lag phase: Bacteria adapt to the growth conditions. Cells grow in size but do not divide. They synthesize proteins, RNA, enzymes, and other molecules and incorporate nutrients. There is no increase in cell number. This phase can last from 1 hour to several days. The bacterial mass per unit volume increases, but not the cell count.
Log phase (or Exponential phase): Cells divide at their maximum rate. Growth is exponential (logarithmic). Cell doubling occurs. This phase can be maintained by continuous culture (adding nutrients and removing cells). Exponential growth continues until nutrients are depleted and waste products accumulate. The generation time is typically determined during this phase.