Economic Importance of Bacteria and Fungi Chapter Notes | Biology Class 9 ICSE PDF Download

Introduction

Bacteria are simple, single-celled prokaryotes lacking a true nucleus. They do not contain chlorophyll and do not have differentiated structures such as roots, stems, leaves, or flowers. According to the modern Five Kingdom classification, bacteria are not classified as plants because they lack chlorophyll and rely on heterotrophic nutrition rather than autotrophic nutrition.

Bacteria - A General Study

Bacteria are the most primitive unicellular organisms, each containing a single chromosome (nuclear material) not enclosed within a nuclear membrane. They are ubiquitous, found in air, water, soil, food, and within or on living organisms, including the human body, where they inhabit the mouth, skin, intestines, and genital tracts. Some intestinal bacteria are beneficial, synthesizing vitamins like vitamin B, while others cause diseases. Notably, the number of bacteria on human skin surpasses the global human population, often preventing harmful bacteria growth.

Size and Shape
Bacteria are the smallest living organisms, typically measuring 2 micrometers in length and 0.5 micrometers in thickness (1 micrometer = 1/1000 millimeter). They exhibit four main shapes:

1. Cocci (spherical)
2. Bacilli (rod-shaped)
3. Spirilla (spiral or twisted)
4. Vibrio (comma-shaped)

Bacteria may exist singly, in pairs (diplococci), chains (streptococci), or clusters (staphylococci).

Structure
A bacterial cell is simple, consisting of protoplasm (cytoplasm and a single circular chromosome) enclosed by a thin cell membrane and a rigid cell wall made of peptidoglycan, not cellulose. The chromosome, containing DNA, is located centrally without a nuclear membrane and is attached to the cell membrane at a point. The cytoplasm may contain vacuoles, granules, and, rarely, green pigments. Some bacteria have an outer slimy capsule for protection.

Movement
Most bacteria lack independent movement and are passively transported by wind, water, or contact. However, some possess flagella—whip-like structures that enable active movement in liquid environments through lashing motions.

Nutrition
Lacking chlorophyll, most bacteria are heterotrophic, relying on external food sources. They may be:

• Saprotrophic, feeding on decaying organic matter, or
• Parasitic, deriving nutrients from living hosts.
  Bacteria secrete powerful enzymes to break down surrounding food into soluble forms for absorption.

Respiration
Bacteria exhibit two types of respiration:

• Aerobic, requiring atmospheric oxygen, or
• Anaerobic, functioning without free oxygen and often killed by air exposure.

Bacterial Reproduction

• Bacteria primarily reproduce asexually through binary fission, a process where the cell divides into two. The circular DNA replicates, the cell elongates, and the two DNA molecules are pulled apart. 
• The cell then constricts in the middle, separating into two daughter cells, which may remain attached or become independent, depending on the bacterial species.

Rapid Bacterial Growth:

• Under optimal conditions, some bacteria divide every 30 minutes. 
• Starting with one bacterium, this rapid division could result in over 281 trillion bacteria after 24 hours.

Primitive Sexual Reproduction:

• In rare cases, a basic form of sexual reproduction called conjugation occurs in certain bacteria. 
• Two bacteria of different strains but the same species connect, and the donor transfers plasmid DNA to the recipient through a tube-like extension.

Spore Formation for Survival:

• To survive harsh conditions, such as drought, some bacteria form spores. The cell’s contents condense into a spherical mass encased in a thick, protective wall. 
• These spores, contained within the original cell wall, are released when the wall ruptures. Spores are highly resilient, capable of enduring extreme dryness, boiling or freezing temperatures, and even toxic chemicals. They remain dormant until conditions improve.

Spore Dispersal and Germination:

• Spores can be dispersed by wind, water, or contact. When favorable conditions return, the spore’s dormant protoplasm activates, breaking through the spore wall to form a new bacterium. 
• Note that spore formation is not a reproductive process but a survival mechanism to endure unfavorable conditions.

Useful Role of Bacteria in Medicine

Antibiotics

• Antibiotics are substances produced by microorganisms that stop or kill disease-causing bacteria and fungi.
• Examples include streptomycin, chloromycetin, tetracycline, and penicillin (from a fungus, Penicillium notatum).
• Penicillin, discovered by Alexander Fleming in 1929, was the first antibiotic, produced by a mould contaminating a bacterial culture.
• Streptomycin, from Streptomyces griseus, was the next antibiotic discovered by Selman Waksman for tuberculosis.
• Other antibiotics: Chlorotetracycline (from Streptomyces aureofaciens, treats typhoid) and Erythromycin (from Streptomyces erytheraeus, treats rickettsial fevers).
• A good antibiotic kills a wide range of pathogens (broad-spectrum), has minimal side effects, and spares normal host bacteria.
• Antibiotics are used as food preservatives for meat and fish, in animal feed, and to control plant pathogens.

Serums

• Serum is blood plasma without fibrinogen, containing antibodies or antitoxins to neutralize bacterial toxins.
• Serums are prepared by injecting bacterial toxins into animals (e.g., horses), which produce antitoxins; blood is then collected and processed into serum.
• Serums are used preventively, like anti-venom for snake bites.
• Genetic engineering uses bacteria like Escherichia coli to produce substances like insulin, blood clotting factor VIII (for haemophilia A), and factor IX (for haemophilia B).

Vaccines

• Vaccines are preparations of weakened or dead germs that stimulate immunity without causing severe disease.
• They induce mild infections, prompting the body to produce antitoxins for future protection.
• Examples: TAB vaccine (killed bacteria for typhoid) and BCG vaccine (living weakened bacteria for tuberculosis).

Toxoids

Toxoids are inactivated bacterial toxins that stimulate antibody production, used for immunity against diseases like diphtheria and tetanus.

Bacteria - Role in Agriculture

Nitrogen-Fixing Bacteria

• Rhizobium bacteria live in nodules on leguminous plant roots (e.g., beans), converting atmospheric nitrogen into soluble nitrates for plant use.
• Nitrates enrich soil, benefiting other plants grown later.
• Free-living soil bacteria like Azotobacter and Clostridium convert nitrogen to ammonia, then to amino acids and nitrates.

Nitrifying Bacteria

• Plants require nitrates for protein synthesis, which they absorb from soil since they cannot use atmospheric nitrogen.
• Nitrification involves bacteria converting nitrogenous wastes and dead remains into ammonia, then ammonium compounds, nitrites (by Nitrosomonas), and nitrates (by Nitrobacter).

Denitrifying Bacteria

• These bacteria break down soil nitrates, releasing nitrogen gas back into the atmosphere.

Extra - Not in Syllabus: Decay and Putrefaction by Bacteria:

• Bacteria break down dead plants, animals, and excreta into simpler compounds like nitrates, sulphates, and carbon dioxide, used by plants.
• Decay is complete breakdown without foul smell (e.g., cow dung to soil).
• Putrefaction is incomplete breakdown with a foul smell.
• Sewage treatment plants use bacteria to decompose human excreta, producing gas for cooking and manure for fields.
• Biogas plants ferment cow dung to produce methane gas for fuel and manure.
• Intestinal bacteria synthesize B-complex vitamins and vitamin K, and aid cellulose digestion in herbivores.

Bacteria - Role in Industry

• Bacteria contribute to tea curing, producing different tea flavors.
• They assist in leather tanning by breaking down soft, perishable parts of animal hides.

Spoilage of Food by Bacteria

Bacteria cause food spoilage (e.g., milk, fruits, vegetables) through fermentation, especially in summer. Spoilage can lead to food poisoning, like botulism from Clostridium botulinum in tinned foods, causing gas-filled cans.

Food preservation methods include:

• Boiling/sterilization: Kills bacteria (except spores) at 100°C; higher temperatures (110°C) in autoclaves kill spores.
• Salting: Prevents biodegradation in foods like fish and pickles.
• Dehydration: Removes water to inhibit microbial growth (e.g., dried grains, meat, milk powder).
• Irradiation: Uses radioactive or UV light to sterilize food or kill mould spores.
• Pasteurization: Heats milk to 60°C for 30 minutes, then chills it to kill most bacteria, though not fully sterilizing.
• Refrigeration: Slows microbial growth at 0-5°C (short-term) or -20 to -30°C (long-term).
• Chemical preservatives: High sugar/salt in jams/pickles or chemicals like sodium benzoate prevent spoilage.

Bacterial Diseases in Plants and Animals

Plant Diseases
Two prevalent bacterial diseases affecting plants are black rot, which impacts mustard and cauliflower, and bacterial blight, which affects cowpea (commonly known as "lobia").

Animal Diseases
Two common bacterial diseases in cattle include:

1. Anthrax: Causes swelling on the body and reduced milk production.
2. Tuberculosis: Affects the lungs, leading to a dry, husky cough.

Human Diseases
Common bacterial diseases in humans include whooping cough, cholera, tuberculosis, diphtheria, typhoid, pneumonia, and tetanus. (Refer to Chapter 18.5 for detailed information on bacterial diseases in humans.)

Bioweapons

There are concerns that some countries may be developing "germ bombs" designed to release pathogens like anthrax bacteria upon detonation. These could trigger widespread epidemics, causing persistent and generational harm through disease outbreaks.