Classification of Bacteria based on Structural, Metabolic, Genetic, Antigenic and Pathogenic Criteria

Structural Criteria – for structural classification we have some criteria such as endospores, capsule, flagella’s etc.
The presence of endospores separates bacteria in two groups: Sporogene and non-sporogene, same with the capsule, the presence of capsule classifies bacteria to capsular or non-capsular. Flagel based classification: Monotrichous Lophotrichous , Amphitrichous , Peritrichous.
Metabolic Criteria - based on the growth of bacteria in the presence of molecular oxygen bacteria are divided into: Obligate aerobes require O2 for growth; they use O2 as a final electron acceptor in aerobic respiration.
Obligate anaerobes (occasionally called aerophobes) do not need or use O2 as a nutrient. In fact, O2 is a toxic substance, which either kills or inhibits their growth. Obligate anaerobic prokaryotes may live by fermentation, anaerobic respiration, bacterial photosynthesis, or the novel process of methanogenesis.
Facultative anaerobes (or facultative aerobes) are organisms that can switch between aerobic and anaerobic types of metabolism. Under anaerobic conditions (no O2) they grow by fermentation or anaerobic respiration, but in the presence of O2 they switch to aerobic respiration.
Aerotolerant anaerobes are bacteria with an exclusively anaerobic (fermentative) type of metabolism but they are insensitive to the presence of O2. They live by fermentation alone whether or not O2 is present in their environment.

Antigenic Criteria – chemical compounds in bacterial cell which promote production of specific antibodies, and them reacts in specific way is called antigen.  So the bacteria which has a ability to cause an immune reaction is called antigenic bacteria and it’s a criteria for classification.

Pathogenic Criteria – the ability of bacteria to cause diseases in humans and animals is a criteria for classification of bacteria.