Samacheer Class 8 Science - Micro Organisms

Bacteria are described according to the shape of their cells. They are:
Bacilli – Rod shaped bacteria.
Spirilla – Spiral shaped bacteria.
Cocci – Spherical or ball shaped bacteria.
Vibrio – Comma shaped bacteria.

Disease-causing microorganisms are called pathogens, and they enter the human body through several different routes. Pathogens can enter through cuts and wounds in the skin, mouth, or nose, directly accessing the bloodstream or internal tissues. Some pathogens are airborne and enter a healthy person by breathing contaminated air. When a patient sneezes or coughs, droplets containing microbes spread into the air and can be inhaled by others, as seen in diseases like tuberculosis and influenza. Other pathogens enter the body through consumption of food and water contaminated with the pathogen, such as cholera-causing bacteria. Additionally, pathogens can enter through blood transfusion when infected blood from a patient is transferred to a healthy person, as in the case of AIDS and other blood-borne diseases. Understanding these routes of transmission is crucial for preventing the spread of infectious diseases.

1. Natural fertilizer: Bacteria and fungi decompose plant and animal waste, releasing nutrients into the soil and forming compost.
2. Nitrogen fixation: Rhizobium, free-living bacteria and cyanobacteria such as Nostoc convert atmospheric nitrogen into nitrate salts in the soil.
3. Biocontrol: Microorganisms such as baculoviruses attack insects that harm crops.

1. Bacteria are single – celled prokaryotes (cells without nuclei).
2. Bacteria are grouped under the kingdom Monera. The study of Bacteria is called Bacteriology.
3. Bacteria are of two types based on respiration
Aerobic bacteria (requires oxygen).
Anaerobjc bacteria (Does not requires oxygen).
4. A bacterium has an outer covering known as the cell wall. Nuclear material is represented by a nucleoid without nuclear membrane.
5. An extra chromosomal DNA called plasmid is present in the cytoplasm.
6. Protein synthesis is carried out by 70S ribosomes. Other cell organelles (mitochondria, Golgi body endoplasmic reticulum etc.,) are absent. Flagella aids in locomotion.
7. Bacteria are described according to the shape of their cells. They are:
Bacilli – Rod shape bacteria
Spirilla – Spiral shaped bacteria
Cocci – Spherical or ball shaped bacteria
Vibrio – Comma shaped bacteria
Bacteria are also classified according to number and arrangement of flagella as follows:
Monotrichous – Single flagella at one end. Example: Vibrio cholera
Lophotrichous – Tuft of flagella at one end. Example: Pseudomonas.
Amphitrichous – Tuft of flagella at both ends. Example:Rhodospirillum rubrum.
Peritrichous – Flagella all around. Example: E.coli.
Atrichous – Without any flagella. Example: Corynebacterium diptherae.

Role of microbes in medicine:
We obtain antibiotics and vaccines from microbes.
1. Antibiotics:
It is a substance produced by living organisms which is toxic to other organisms. The antibiotic penicillin got from the fungus Penicillium chrysogenum is used to treat diseases like tetanus, diphtheria. Streptomycin got from streptomyces bacteria is used to cure bacterial infections like plague.
2. Vaccines:
They are prepared from dead or weakened microbes. When a vaccine is injected into the body of a patient, it produces antibodies to fight the germs. These antibodies protect the body from infections in future. Example: MMR vaccine for measles.

The beneficial bacterial count in the human body, particularly in the gut, can be improved through several dietary and lifestyle methods. Consuming fiber-rich foods provides nourishment for beneficial bacteria and promotes their growth. Eating seasonal fruits and vegetables supplies essential nutrients and promotes a healthy microbial balance. Intake of fermented foods with live microbes, such as yogurt, kimchi, and other fermented products, directly introduces beneficial bacteria into the digestive system. Consuming prebiotic foods, which are substances that selectively feed beneficial bacteria, helps increase their population. Including whole grains in the diet provides complex carbohydrates that support beneficial bacterial growth. Preferring a plant-based diet rich in vegetables, legumes, and grains creates an environment favorable for beneficial microbes. Additionally, avoiding artificial sweeteners is important because they can negatively affect the balance of gut bacteria and reduce the population of beneficial microorganisms.

Probiotics are live microorganisms, typically bacteria, that are used as food supplements to promote health and improve the microbial balance in the human gut. They are commonly found in yogurt and other fermented milk products. Common examples of probiotic bacteria include Lactobacillus acidophilus and Bifidobacterium bifidum. These beneficial bacteria work by improving the microbial spectrum in the gut, meaning they increase the diversity and population of helpful microorganisms in the digestive system. Probiotics provide several important health benefits: they decrease the risk of colon cancer by producing protective compounds and reducing harmful bacteria, they decrease cholesterol absorption by breaking down bile salts and reducing cholesterol uptake in the intestines, and they prevent diarrheal diseases by increasing the immunity power of the gut and competing with harmful pathogens for space and nutrients. Regular consumption of probiotic-rich foods helps maintain a healthy gut microbiome.