Those bacteria that retain the dye (blue, purple) when washed with crystal violet dye are called gram positive bacteria. Their cell walls are thick. Its cell wall consists of peptidoglycan, teichoic acid and thin lipopolysaccharide. Such as Clostridium, Staphylococcus, Streptococcus, Bacillus, Actinobacteria.

Scientist Hans Christian Gram (1853-1938) invented the dyeing method. In this method bacteria are divided into two groups.
1. Gram positive bacteria: Those bacteria that retain the dye (blue, purple) when washed with crystal violet dye are called gram positive bacteria. Their cell walls are thick. Its cell wall consists of peptidoglycan, teichoic acid and thin lipopolysaccharide. Such as Clostridium, Staphylococcus, Streptococcus, Bacillus, Actinobacteria.
2. Gram negative bacteria: Those bacteria which cannot retain the dye after washing with crystal violet dye are called gram negative bacteria. Their cell walls are thin. Its cell wall consists of peptidoglycan and thick lipopolysaccharide. E. coli, Salmonella typhi, Shigella, Proteus, Rhizobium, Vibrio cholerae, Neisseria meningitidis, Cyanobacteria.

Bacteria that survive at high temperatures are called thermophilic bacteria. A temperature of 45-600C is best for their best growth. For example – Thermus aquaticus, Bacillus coagulans.

Bacteria that survive at normal temperature are called Mesophilic bacteria. A temperature of 30-40 0C is best for their best growth. Such as Azotobacter, Rhizobium.

Bacteria that survive at low temperature are called Psychrophilic bacteria. All these bacteria can survive at or below zero temperature. A temperature of 25-300C is best for their best growth. For example- Psychrobacter, Acinetobacter, Arthrobacter, Pseudomonas.

Bacteria can be divided into three groups based on heat tolerance.
1. Psychrophilic: Bacteria that survive at low temperature are called Psychrophilic bacteria. All these bacteria can survive at or below zero temperature. A temperature of 25-300C is best for their best growth. For example- Psychrobacter, Acinetobacter, Arthrobacter, Pseudomonas.
2. Mesophilic: Bacteria that survive at normal temperature are called Mesophilic bacteria. A temperature of 30-40 0C is best for their best growth. Such as Azotobacter, Rhizobium.
3. Thermophilic: Bacteria that survive at high temperatures are called thermophilic bacteria. A temperature of 45-600C is best for their best growth. For example – Thermus aquaticus, Bacillus coagulans.

Bacteria that can survive without the presence of oxygen are called anaerobic bacteria. For example, Clostridium tetani.

Bacteria that cannot survive without the presence of oxygen are called aerobic bacteria. For example- Bacillus anthracis, Azobacter beijerinckii.

Bacteria are divided into two groups based on the presence or absence of oxygen.
1. Aerobic Bacteria: Bacteria that cannot survive without the presence of oxygen are called aerobic bacteria. For example- Bacillus anthracis, Azobacter beijerinckii.
2. Anaerobic Bacteria: Bacteria that can survive without the presence of oxygen are called anaerobic bacteria. For example, Clostridium tetani.

Bacteria that are long hyphae with thin walls are called microscopic bacteria. Such as Candidatus savagella, Streptomyces.