## Growth and Reproduction in Bacteria This section outlines the processes of bacterial growth, reproduction (both asexual and sexual mechanisms of genetic recombination), and the overall importance of bacteria to ecosystems and human society. ## Bacterial Growth Bacteria reproduce by **binary fission**, leading to **exponential (logarithmic) growth**. This means that with each generation, the population size doubles. | Number of cells: | 1 | 2 | 4 | 8 | 16 | | :--- | :--- | :--- | :--- | :--- | :--- | | **Exponential** | **$2^{0}$** | **$2^{1}$** | **$2^{2}$** | **$2^{3}$** | **$2^{4}$** | The rate of growth is influenced by species, nutrient availability, temperature, pH, and other environmental factors. For example, *Escherichia coli* can have a generation (doubling) time of just 20 minutes, while *Mycobacterium tuberculosis* can take over 24 hours. ### Phases of Growth The growth of a bacterial population in a closed system (like a lab culture) follows a predictable pattern represented by a growth curve with four distinct phases. - **Lag Phase**: No growth. Bacteria are adapting to the new environment. They are metabolically active, synthesizing new enzymes and molecules needed for growth, but not yet dividing at their maximum rate. - **Log Phase (Exponential Phase)**: Rapid growth. The population increases exponentially as cells divide at their maximum, constant rate. The number of new cells produced is greater than the number of cells dying. - **Stationary Phase**: No net growth. The growth rate slows and becomes zero. The rate of cell division equals the rate of cell death. This is caused by: - Exhaustion of essential nutrients. - Accumulation of toxic waste products. - Depletion of oxygen (for aerobes). - **Death Phase**: Population decline. The number of dying cells exceeds the number of new cells being formed, leading to a decline in the viable population. ## Reproduction in Bacteria ### Asexual Reproduction: Binary Fission This is the primary method of reproduction in bacteria, resulting in two genetically identical daughter cells. It is a form of . **Key Steps:** 1. **DNA Replication**: The single, circular bacterial chromosome is replicated, starting at the origin of replication. 2. **Cell Elongation**: The cell grows in size, increasing its cell wall and membrane material to accommodate the duplicated components. 3. **Cell Division**: The plasma membrane pinches inward at the cell's midpoint, and a new cell wall (septum) forms, separating the two chromosomes. 4. **Formation of Daughter Cells**: The septum is completed, and the cell divides into two separate, identical daughter cells, each with a full copy of the chromosome. ### Sexual Reproduction: Genetic Recombination Bacteria do not undergo true sexual reproduction, but they can exchange genetic material, a process called **genetic recombination**. This creates new combinations of genes, increasing genetic diversity. Because it lacks meiosis and zygote formation, it is often called **parasexuality**. **Types of Genetic Recombination:** - **Conjugation**: - Direct transfer of DNA from a **donor** cell to a **recipient** cell through a physical bridge-like connection (pilus). - Often involves the transfer of a **plasmid** (a small, circular piece of DNA). Sometimes, part of the main chromosome can also be transferred. - Discovered by Joshua Lederberg and Edward Tatum in 1946 using *E. coli*. - **Transduction**: - Transfer of bacterial DNA from one bacterium to another via a **bacteriophage** (a virus that infects bacteria). - The phage accidentally packages bacterial DNA instead of its own viral DNA and injects it into a new host bacterium. - **Transformation**: - The uptake and incorporation of "naked" DNA fragments from the surrounding environment. - This DNA is often released from dead or lysed bacteria. The recipient cell is known as a *transformed cell*. ### Mutations and Genetic Variation - **Mutations**: Random errors during DNA replication or damage from mutagens are the ultimate source of new genetic variation in bacteria. - **Genetic Recombination**: Along with mutation, the processes of conjugation, transduction, and transformation allow bacteria to evolve and adapt quickly. ## Importance of Bacteria ### Ecological and Environmental Importance - **Nutrient Recycling**: Bacteria are essential decomposers. They are critical in cycles like the **nitrogen cycle**: - **Nitrogen Fixation**: *Rhizobium* in legume root nodules convert atmospheric nitrogen gas (N$_2$) into ammonium (NH$_4^+$). - **Nitrification**: *Nitrosomonas* and *Nitrobacter* convert ammonia into nitrites (NO$_2^-$) and then nitrates (NO$_3^-$). - **Denitrification**: Other bacteria convert nitrates back into nitrogen gas. - **Decomposition**: Along with fungi, bacteria break down dead organic matter, creating humus which enriches soil and improves water retention. ### Economic Importance | Beneficial Roles | Harmful Roles | | :--- | :--- | | **Digestion**: Gut bacteria help digest food (e.g., cellulose in cattle) and emulsify fats. | **Food Spoilage**: Bacteria cause decomposition and decay of food materials. | | **Vitamin Synthesis**: *E. coli* synthesizes vitamins K and B12 in the human intestine. | **Decay of Materials**: Cause decay of wood, leather, and fabrics. | | **Industrial Production**: Used to make acetic acid (vinegar), acetone, lactic acid, cheese, tobacco flavorings. | **Plant Diseases**: Cause diseases like blights, wilts, soft rots, and galls. | | **Food Industry**: Crucial for making yogurt, cheese, and butter. | **Human Diseases**: Pathogenic bacteria cause numerous diseases like cholera, typhoid, and tuberculosis. | | **Antibiotics**: Many antibiotics (e.g., Streptomycin) are derived from bacteria (especially *Actinomycetes*). | | | **Biogas Production**: Decompose organic waste (sewage, dung) to produce methane gas for fuel. | | | **Biotechnology**: Used for cloning genes, producing insulin and hormones, bioremediation (cleaning oil spills), and mineral processing. | | ## Bacterial Diseases ### Important Bacterial Diseases in Humans | Disease | Causative Agent | Key Symptoms | Treatment & Prevention | | :--- | :--- | :--- | :--- | | **Cholera** | *Vibrio cholerae* | Profuse watery diarrhea, dehydration. | **Treatment**: Fluid/electrolyte replacement. **Prevention**: Clean water supply, vaccines. | | **Typhoid Fever** | *Salmonella typhi* | High fever, constipation, tender abdomen, "rosy spots" on the abdomen. | **Treatment**: Antibiotics. **Prevention**: Hygiene, pasteurization, vaccines. | | **Tuberculosis (TB)** | *Mycobacterium tuberculosis* | Low-grade fever, night sweats, weight loss, hacking cough (often with blood). | **Treatment**: Long-term, multi-drug therapy (DOTS). **Prevention**: BCG vaccine, prompt treatment of cases. | | **Pneumonia** | *Streptococcus pneumoniae* | Sudden chills, cough, chest pain, "rusty" colored sputum. | **Treatment**: Penicillin, erythromycin. **Prevention**: Vaccine available for high-risk individuals. | ### Important Bacterial Diseases in Plants | Disease | Description | Examples of Causative Agents | | :--- | :--- | :--- | | **Leaf Spot** | Discrete lesions on leaf blades, often caused by toxins. | *Xanthomonas campestris* on tomato; *Pseudomonas syringae* on tobacco. | | **Blight** | Rapidly developing necrosis that can kill entire shoots or plants. | *Xanthomonas oryzae* in rice; *Erwinia amylovora* (fire blight) on pears and apples. | | **Soft Rot** | Tissues become soft and wet due to enzymes that break down cell walls. | *Erwinia atroseptica* in potato. | | **Wilting** | Interference with water transport, causing leaves to lose turgidity and droop. | *Pseudomonas solanacearum* in potato. | | **Galls** | Localized outgrowths or tumors on the plant. | *Agrobacterium tumefaciens* (crown gall); *Rhizobium leguminosarum* (root nodules). |

6.6 Growth And Reproduction In Bacteria | Prokaryotes | Biology 11