(i) Useful bacteria: Most of the bacteria act as a decomposer. They breakdown large molecules in simple molecules or minerals.

Examples of some useful bacteria: 

Lactobacillus: It helps in curdling of milk. Azotobacter. It helps to fix nitrogen for plants. 

Streptomyces: It is used in antibiotic production such as streptomycin. 

Methanogens: These are used for production of methane (biogas) gas from dung. Pseudomonas spp. and

Alcanovorax borkumensis: These bacteria have the ability to destroy the pyridines and other chemicals. Hence, used to clear the oil spills.

(ii) Harmful bacteria: This includes disease causing bacteria. They cause various diseases like typhoid, cholera, tuberculosis, tetanus, etc. 

Examples of some harmful bacteria:

Salmonella typhi: It is a causative organism of typhoid. 

Vibrio cholerae: It causes cholera. Mycobacterium tuberculosis’. It causes tuberculosis.

Clostridium tetani: It causes tetanus.

Clostridium spp: It causes food poisoning. Many forms of mycoplasma are pathogenic. Agrobacterium , Erwinia, etc are the pathogenic bacteria causing plant diseases. Animals and pets also suffer from bacterial infections caused by Brucella, Pastrurella, etc.