Bacterial Structure, Growth, and Binary Fission

Bacteria are single-celled prokaryotes with a cell wall, a cell membrane, cytoplasm with ribosomes and a circular chromosome, and they reproduce by binary fission, in which one cell divides into two identical cells. In brief: under ideal conditions, a population of E. coli can double about every 20 minutes. Growth follows a typical curve with four phases: lag, exponential (log), stationary and death. Starting from one cell, after 10 hours of unrestricted doubling every 20 minutes (30 generations), there would be over a billion cells.

Bacterial growth explains how infections develop, how food spoils and how antibiotics and disinfectants work. This guide describes the structure of a bacterial cell, how binary fission works, the phases of the growth curve and the factors that influence growth, works through a doubling calculation and corrects common mistakes.

What is the structure of a bacterial cell?

A typical bacterium is about 1 to 5 micrometers long. It is a prokaryotic cell with these structures.

• Cell wall: made of peptidoglycan, a polymer of sugars and amino acids. It gives shape and prevents bursting. Penicillin works by blocking its synthesis.
• Cell membrane: a phospholipid bilayer that controls what enters and leaves, and the site of respiration and ATP production, since bacteria have no mitochondria.
• Cytoplasm: contains enzymes, ribosomes (70S) and storage granules.
• Nucleoid: the region with the circular chromosome, with no nuclear membrane.
• Plasmids: small rings of DNA that often carry genes for antibiotic resistance.
• Capsule or slime layer: a sticky outer layer, in some species, that protects against drying and the immune system and helps attachment.
• Flagella: whip-like structures that rotate to move the cell.
• Pili (fimbriae): hair-like projections for attachment and for gene transfer between cells.

The Gram stain divides bacteria into Gram-positive (thick peptidoglycan wall, which stains purple) and Gram-negative (thin wall with an outer membrane, which stains pink). This helps to choose antibiotics.

How does binary fission work?

Binary fission is asexual reproduction in which one cell divides into two genetically identical cells. It occurs in these steps.

1. The cell grows and the circular chromosome is replicated, starting at one origin of replication. The two copies move to opposite ends of the cell.
2. The cell elongates, and the cell membrane begins to grow inward at the middle.
3. A new cell wall (septum) forms across the middle of the cell, guided by a ring of the protein FtsZ.
4. The cell divides into two daughter cells, each with a complete chromosome.

There is no mitosis or spindle. The daughter cells are clones, except for rare mutations. Variation arises mainly by mutation and by horizontal gene transfer.

What are the phases of bacterial growth?

When bacteria are placed in a fresh nutrient medium in a closed container (batch culture), the number of living cells follows a characteristic curve.

1. Lag phase: the cells adjust to the new environment, make enzymes and grow in size, but there is little division. The number stays almost constant.
2. Exponential (log) phase: cells divide at a constant rate, so the population doubles every generation time. A graph with a logarithmic scale gives a straight line.
3. Stationary phase: growth slows as nutrients run out and waste products build up. The rate of cell division equals the rate of cell death, so the number of living cells is constant.
4. Death (decline) phase: death rate exceeds division rate, and the number of living cells falls.

What affects bacterial growth?

• Temperature: most human pathogens grow best at 37 °C. Refrigeration at 4 °C slows growth but does not stop it, and heating above 70 °C kills most bacteria.
• Nutrients: carbon and nitrogen sources, minerals and sometimes vitamins.
• pH: most prefer a neutral pH (6.5 to 7.5). Acidic foods such as pickles resist spoilage.
• Oxygen: aerobes need it, anaerobes are killed by it, and facultative anaerobes such as E. coli can grow with or without it.
• Water availability: salt and sugar preserve food by drawing water out of bacteria by osmosis.

Structures of a bacterial cell

Worked example: exponential growth

A culture of bacteria starts with 500 cells per milliliter, and the generation time is 30 minutes. Assume that the cells are in the exponential phase from the start.

1. Number of generations in 4 hours: 4 h = 240 min; 240 ÷ 30 = 8 generations.
2. Population: N = N₀ × 2ⁿ = 500 × 2⁸ = 500 × 256 = 128 000 cells per mL.
3. After 6 hours (12 generations): 500 × 2¹² = 500 × 4 096 = 2 048 000 cells per mL.
4. Time to reach 1 000 000 cells per mL: 1 000 000 ÷ 500 = 2 000 = 2ⁿ. Since 2¹¹ = 2 048, n is about 10.97 generations (log₂ 2 000 ≈ 10.97). Time ≈ 10.97 × 30 min ≈ 329 minutes, or about 5.5 hours.

Real populations cannot grow forever in a closed container. The calculation holds only in the exponential phase. If a single bacterium doubled every 20 minutes without limit, then after 24 hours there would be 2⁷² cells, more than 4.7 × 10²¹, with a mass greater than that of the Earth within a few days, which obviously does not happen because of nutrient limits. This is why the stationary phase occurs.

Do bacteria have other ways to survive and exchange genes?

Some bacteria, such as Bacillus and Clostridium, form endospores: tough, dormant structures that resist heat, drying and chemicals, and can survive for years. This is why sterilization in an autoclave (121 °C under pressure) is needed to kill them. Bacteria can also exchange genes by conjugation (transfer of a plasmid through a pilus), transformation (uptake of free DNA) and transduction (transfer by bacteriophages). These processes spread antibiotic resistance.

How was bacterial growth studied?

Robert Koch developed solid media with agar in the 1880s, allowing the growth of single colonies from single cells, and Julius Petri invented the Petri dish in 1887. Jacques Monod studied the growth curve of bacteria in the 1940s. The idea of binary fission was recognized in the nineteenth century, and the molecular details of the division machinery, such as FtsZ, were discovered in the 1990s.

Which mistakes are common?

Mistake 1: Saying that bacteria reproduce by mitosis

Bacteria divide by binary fission. They have no nucleus or spindle.

Mistake 2: Thinking that the lag phase is a time of no activity

Cells are metabolically active in the lag phase. They are preparing for growth.

Mistake 3: Using the wrong power of 2

The number of generations is time divided by generation time. The population is the starting number times 2 to the power of the number of generations.

Mistake 4: Assuming that growth continues exponentially

Limited resources cause the population to level off in the stationary phase.