NCERT-NEET-Bacteria are the sole members of the Kingdom Monera. They are the most abundant micro-organisms. Bacteria occur almost everywhere. Hundreds of bacteria are present in a handful of soil. They also live in extreme habitats such as hot springs, deserts, snow and deep oceans where very few other life forms can survive. Many of them live in or on other organisms as parasites

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Category	Description
General Characteristics	Bacteria are the sole members of Kingdom Monera. They are the most abundant micro-organisms, found almost everywhere, including extreme habitats (hot springs, deserts, snow, deep oceans). Many are parasitic.
Shapes of Bacteria	– Coccus (pl. cocci): Spherical – Bacillus (pl. bacilli): Rod-shaped – Vibrio (pl. vibrios): Comma-shaped – Spirillum (pl. spirilla): Spiral
Diversity in Shape and Size	Bacteria exhibit a wide range of shapes and sizes, from tiny mycoplasmas (0.1 µm) to large cyanobacteria (up to 60 µm in diameter).
Diversity in Number and Arrangement of Flagella	Bacteria vary in the number and arrangement of their flagella, including: – Monotrichous: A single flagellum – Lophotrichous: A cluster of flagella at one or both ends – Amphitrichous: A single flagellum at both ends – Peritrichous: Flagella distributed over the entire cell surface
Diversity in Habitat	Bacteria inhabit a wide range of environments, including: – Soil – Water (freshwater and marine) – Extreme environments (hot springs, deserts, snow, deep oceans) – Living in or on other organisms as parasites or symbionts
Metabolic Diversity	– Autotrophic Bacteria: Synthesize their own food from inorganic substrates. – Photosynthetic Autotrophs: Use light energy. – Chemosynthetic Autotrophs: Use energy from chemical reactions. – Heterotrophic Bacteria: Depend on other organisms or dead organic matter for food.
Archaebacteria	Live in harsh habitats such as: – Halophiles: Extreme salty areas. – Thermoacidophiles: Hot springs. – Methanogens: Marshy areas, gut of ruminant animals (e.g., cows, buffaloes). Have different cell wall structures enabling survival in extreme conditions. Methanogens produce methane (biogas) from animal dung.
Eubacteria	Known as ‘true bacteria’. Characterized by a rigid cell wall and, if motile, a flagellum.
Cyanobacteria	Also known as blue-green algae. Have chlorophyll a like green plants and are photosynthetic autotrophs. Can be unicellular, colonial, or filamentous. Found in freshwater, marine, or terrestrial environments. Form blooms in polluted water bodies. Some can fix atmospheric nitrogen in specialized cells called heterocysts (e.g., Nostoc, Anabaena).
Chemosynthetic Bacteria	Oxidize inorganic substances (nitrates, nitrites, ammonia) to release energy for ATP production. Play a significant role in nutrient recycling (nitrogen, phosphorus, iron, sulfur).
Heterotrophic Bacteria	Most abundant bacteria. Act as decomposers. Important in human activities (e.g., making curd, producing antibiotics, fixing nitrogen in legume roots). Some are pathogenic (e.g., cholera, typhoid, tetanus, citrus canker).
Reproduction	Mainly by fission. Under unfavorable conditions, they produce spores. Also reproduce through a primitive form of sexual reproduction involving DNA transfer between bacteria.
Mycoplasma	Lack a cell wall. Smallest living cells known. Can survive without oxygen. Many are pathogenic in animals and plants.