Economic Importance of Bacteria and Fungi

1. Rhizobium: Lives symbiotically in root nodules of leguminous plants and fixes free atmospheric nitrogen gas ($\text{N}_2$) into soluble nitrogenous compounds (nitrates).
   2. Nitrosomonas: Nitrifying soil bacterium that oxidizes ammonia into nitrites ($2\text{NH}_3 + 3\text{O}_2 \to 2\text{NO}_2^- + 2\text{H}^+ + 2\text{H}_2\text{O}$).
   (Bonus: Nitrobacter oxidizes nitrites into nitrates: $2\text{NO}_2^- + \text{O}_2 \to 2\text{NO}_3^-$).

• Antibiotic: A chemical substance produced by a living microorganism (fungus or bacterium) that in low concentrations inhibits the growth of or destroys pathogenic bacteria.
• First Antibiotic: Penicillin.
• Discoverer: Sir Alexander Fleming in 1928.
• Source Fungus: Penicillium notatum (or Penicillium chrysogenum).

• When baker's yeast (Saccharomyces cerevisiae) is added to flour dough containing sugar and warm water, it carries out Anaerobic Respiration (Alcoholic Fermentation):

$$\text{C}_6\text{H}_{12}\text{O}_6 \xrightarrow{\text{Zymase}} 2\text{C}_2\text{H}_5\text{OH} + \mathbf{2\text{CO}_2\uparrow}$$

• The released Carbon Dioxide ($\text{CO}_2$) gas becomes trapped within the sticky gluten network of the dough, forming millions of expanding gas bubbles that cause the dough to swell and rise.
• During baking in the oven, the heat expands the trapped $\text{CO}_2$ bubbles and evaporates the alcohol, leaving behind permanent air spaces that make bread soft, light, and porous.

• Pasteurization: A thermal food preservation technique developed by Louis Pasteur that destroys all pathogenic, disease-causing bacteria (e.g., Mycobacterium tuberculosis) in milk without destroying its nutritive value or flavor.
• Methods:
1. Low-Temperature Long-Time (LTLT) / Holder Method: Heating milk to $63^\circ\text{C}$ for $30\text{ minutes}$, followed by immediate rapid cooling to below $10^\circ\text{C}$.
2. High-Temperature Short-Time (HTST) / Flash Method: Heating milk to $72^\circ\text{C}$ for $15\text{ seconds}$, followed by immediate rapid chilling to $< 5^\circ\text{C}$.

• High concentrations of salt or sugar create a strongly hypertonic environment outside the microbial cells.
• When bacterial or fungal spores land on the food, water is rapidly drawn out of their cytoplasm by Exosmosis (Plasmolysis).
• This severe cellular dehydration shrinks the protoplasm, paralyzes metabolic enzymes, and kills or prevents the reproduction of the bacteria and moulds, effectively preserving the food.

• Retting: An industrial biological process in which harvested stalks of jute or flax are submerged in slow-moving water pools for several weeks.
• Action: Anaerobic bacteria, primarily Clostridium butyricum, hydrolyze and digest the gummy pectins that bind the bark fibers to the central woody stem.
• This loosens and separates the commercial fibrous strands, allowing them to be stripped, washed, and spun into jute rope and burlap cloth.

1. Cocci (Spherical): Streptococcus pneumoniae (or Staphylococcus).
   2. Bacilli (Rod-shaped): Lactobacillus acidophilus (or Bacillus anthracis).
   3. Spirilla (Spiral / Corkscrew): Spirillum volutans (or Treponema).
   4. Vibrio (Comma-shaped): Vibrio cholerae.

• Bacterial Diseases:
1. Tuberculosis (caused by Mycobacterium tuberculosis)
2. Cholera (caused by Vibrio cholerae)
(Also Typhoid, Tetanus).
• Fungal Diseases:
1. Ringworm (caused by dermatophyte fungi Trichophyton or Microsporum)
2. Athlete's Foot (caused by Tinea pedis).