Microorganisms: Friend and Foe Study Notes

DEFINITION AND SCOPE OF MICROBIOLOGY

Microorganisms (Microbes): These are a large number of living organisms found around or inside us that are too small to be seen with the naked eye. They can only be visualized through a microscope.
Impact on Human Life: Microorganisms are varied in type and affect humans in diverse ways. They can be highly beneficial in various biological and industrial processes, but they can also be harmful by causing diseases or spoiling food.
Microbiology: This is the branch of biology dedicated to the study of microbes.
Microbiologists: Scientists who specialize in the study of microorganisms.

HABITAT AND ADAPTABILITY OF MICROORGANISMS

Terminology:
- Habitat: The specific place where an organism lives.
- Habit: The general form or mode of growth of an organism.
Ubiquity: Microbes are ubiquitous, meaning they are present everywhere, including air, water, soil, on or inside plants, and inside the bodies of animals and humans.
Extreme Environments: Certain microbes can survive in harsh environments where most other life forms cannot. Examples include:
- Hot springs.
- Deserts.
- Extremely saline (salty) waters.
- Ice-cold water or water at temperatures below $0^\circ\text{C}$ .
- Marshlands and deep oceans.
Ecological Role: Microbes facilitate the decay of dead organic matter, which releases minerals back into the soil, thereby restoring soil fertility.
Symbiotic and Parasitic Relationships:
- Endoparasites: Parasites living inside the body of a host.
- Ectoparasites: Parasites living on the surface of the host body.
- Intestinal Flora: The intestines of many animals, including humans, are inhabited by numerous microorganisms.
- Pathogens: Disease-causing microbes. Examples include Plasmodium (causes malaria) and Trypanosoma (causes sleeping sickness). Plasmodium specifically lives in human blood.

CLASSIFICATION OF MICROORGANISMS

Microorganisms are categorized into five primary groups:

Bacteria (Singular: bacterium)
Algae (Singular: alga)
Protozoa (Singular: protozoan)
Fungi (Singular: fungus)
Viruses (Singular: virus)

BACTERIA: CHARACTERISTICS AND STRUCTURE

General Features: Bacteria are the simplest, smallest, and single-celled prokaryotic organisms. They are ubiquitous; for instance, one gram of soil can contain approximately $2,500,000,000$ bacteria. While single-celled, they often form colonies by clinging together.
Size: Bacterial cell size varies significantly, ranging from $0.2$ to $100\mu\text{m}$ (microns). One micron is equal to $\frac{1}{1000}\text{th}$ of a millimeter ( $mm$ ).
Classification by Shape: Bacteriologist Ferdinand Cohn divided bacteria into four groups based on shape:
1. Coccus (pl. cocci): Spherical. Examples: Micrococcus, Streptococcus.
2. Bacillus (pl. bacilli): Rod-shaped. Examples: Bacillus, Lactobacillus, Pseudomonas.
3. Spirillum (pl. spirilla): Spiral or corkscrew-shaped. Example: Spirillum.
4. Vibrio: Comma-shaped (incomplete spiral). Example: Vibrio.
5. Sarcinia: A group of cells with a fixed number in a colony.
Gram Staining: Developed by Christian Gram, this process categorizes bacteria based on their ability to retain Gram stain:
- Gram-positive: Retain the stain (e.g., Salmonella, Spirillum, Escherichia coli).
- Gram-negative: Do not retain the stain (e.g., Staphylococcus, Streptococcus).
- Reason for Difference: Variation in the chemical composition of the cell wall.
Anatomical Structure:
- Prokaryotic: Lacks a well-organized nucleus and membrane-bound organelles.
- Genetic Material: DNA is present in the cytoplasm without a nuclear membrane (nucleoid area).
- Cytoplasm: Viscous, granular, colloidal, and contains approximately $85\%$ water.
- Glycocalyx: A mucilaginous coating outside the cell wall secreted by the cell. It can be a loose slime layer or a tough capsule. Capsules are generally found in pathogenic bacteria.
- Other Structures: Flagella (for movement), Fimbriae, Ribosomes, Plasmids, and Mesosomes.

BACTERIAL NUTRITION, RESPIRATION, AND REPRODUCTION

Nutrition:
- Heterotrophs: Most bacteria obtain food from other organisms. This includes Saprophytes (from dead matter), Parasites (from a living host), and Symbionts (mutual benefit, e.g., Rhizobium in leguminous root nodules).
- Autotrophs: Some synthesize their own food.
  - Phototrophs: Use sunlight. Examples include Purple sulphur bacteria (use bacteriochlorophyll) and Green sulphur bacteria (use bacterioviridin). These leave sulphur deposits.
  - Chemotrophs: Use energy from the oxidation of inorganic chemicals. Examples include Sulphur bacteria, Iron bacteria, and Nitrifying bacteria.
Respiration:
- Aerobes: Require oxygen (e.g., bacteria in milk fermentation obtain oxygen from milk sugar).
- Anaerobes: Cannot survive in the presence of oxygen.
Reproduction: Primarily through Binary Fission. Under favorable conditions (nutrition, temperature, moisture, pH), they divide every $10$ to $40$ minutes.
- Pseudomonas divides every $9.5$ minutes.
- E. coli divides every $20$ minutes.

ECONOMIC IMPORTANCE OF BACTERIA

Harmful Effects:
- Human Diseases: Cholera, diphtheria, pneumonia, tetanus, and tuberculosis.
- Plant Diseases: Black rot of cabbage, fire blight of pears.
- Food Spoilage: Meat rotting, spoilage of milk. Improperly canned food can grow anaerobic bacteria producing toxins leading to Botulism. Salmonella causes severe digestive disease through contaminated meat and eggs.
Useful Effects:
- Ecosystem: Decomposition recycles carbon, nitrogen, and minerals.
- Human Digestion: Intestinal bacteria provide essential vitamins. Grazing animals use bacteria to digest cellulose into simple substances for milk production.
- Agriculture: Nitrogen-fixing bacteria (e.g., Rhizobium) in root nodules of legumes (peas, beans) convert atmospheric nitrogen into usable nitrates and nitrites.
- Industry:
  - Retting of Fibres: Separating fibers from plant stems (e.g., jute).
  - Curing: Fermenting cheese, tobacco, and tea to enrich flavor.
  - Vinegar: Acetobacter converts ethanol into acetic acid.
  - Curd: Lactobacillus converts lactose (milk sugar) into lactic acid.
  - Biogas: Methanogens in cattle dung produce methane in biogas plants.
  - Sewage Treatment: STPs use bacteria in Primary (physical), Secondary (biological degradation), and Tertiary (complex purification) steps.

FUNGI: CHARACTERISTICS AND IMPACT

General Nature: Unicellular (Yeasts) or Multicellular (Moulds, Mushrooms). They are heterotrophic (saprophytic or parasitic) and lack chlorophyll.
Cell Wall: Composed of Chitin, distinguishing them from plants which have cellulosic cell walls.
Structure: Multicellular forms consist of long threads called hyphae, which form an interwoven mass called mycelium. Hyphae can be septate (with septums) or coenocytic (without septums).
Reproduction:
- Asexual: Formation of dust-like spores, binary fission (unicellular), budding (yeasts), or fragmentation (multicellular).
- Sexual: Various modes.
Harmful Fungi:
- Pathogens: Cause rusts (Puccinia) and smuts (Ustilago) in crops; ringworm in humans.
- Toxicity: Amanita mushrooms are deadly poisonous.
- Historical Event: The Irish Potato Famine ( $1845-1849$ ) was caused by a fungal infection (Late blight), leading to approximately one million deaths.

PROTOZOA AND VIRUSES

Protozoa: Unicellular, eukaryotic organisms.
- Examples: Amoeba, Paramecium, Euglena.
- Pathogenic Examples: Entamoeba histolytica (amoebic dysentery), Plasmodium (malaria), Trypanosoma (sleeping sickness).
- Utility: Important link in aquatic food chains; some decompose organic matter or act as symbionts.
Viruses: Non-cellular microbes considered the link between living and non-living.
- Characteristics: Behave as non-living outside a host (can be crystallized) and living inside a host by hijacking biochemical machinery for replication.
- Obligate Parasites: They must have a living host to survive.
- Structure: Outer protein coat containing genetic material (either DNA or RNA).
- Size: $15\,nm$ to $275\,nm$ (visible only under electron microscope).
- Bacteriophage: A virus that infects bacterial cells.
- Diseases: Measles, mumps, seasonal/swine flu, dengue, smallpox, rubella, ebola. Antibiotics have no effect on viruses.

FOOD PRESERVATION METHODS

Chemical Method: Using preservatives like sodium benzoate and sodium metabisulphite in jams/squashes.
Common Salt: Used for meat, fish, amla, raw mangoes, and tamarind. It extracts water from cells.
Sugar: Reduces moisture content in jams and jellies to inhibit bacterial growth.
Oil and Vinegar: Creates an acidic environment where bacteria cannot survive (vegetables, fruits, fish, meat).
Heat and Cold Treatments:
- Boiling: Kills microbes.
- Refrigeration/Freezing: Inhibits growth at low temperatures (below $18^\circ\text{C}$ for long-term).
- Pasteurization: Discovered by Louis Pasteur; milk is heated to $70^\circ\text{C}$ for $30$ minutes and then suddenly chilled.
Canning: Storing food in air-tight cans filled with inert gases like argon.
Sun Drying (Dehydration): Removing moisture from vegetables like spinach and methi.
Smoking: Dehydrating meat/fish using heat and smoke.

ANTIBIOTICS AND THE IMMUNE SYSTEM

Antibiotics: Chemicals secreted by one organism that kill or inhibit the growth of pathogenic bacteria.
- Mechanism: They target bacterial metabolic pathways, such as cell wall formation or protein synthesis, which do not exist in human cells.
- Discovery: Penicillin was the first antibiotic, discovered accidentally by Sir Alexander Fleming in $1928$ from the mould Penicillium while studying Staphylococcus aureus.
Immune System: The body's defense mechanism involving Leucocytes (WBCs) and Lymphocytes. They attack, engulf, and digest pathogens or produce inhibitory chemicals.
Immunology: The study of the body's resistance to diseases.
Vaccination/Immunization: Specific preventive measures using dead or weakened pathogens to stimulate an immune response.