Introductory Microbiology

Classification based on O2 requirement

• Although O₂ is found as a cellular component, most organisms need O₂ for respiration. In these organisms O₂ serves as the terminal electron acceptor and such organisms are referred to as obligate aerobes e.g. Nitrobacter. As opposed to this, there are organisms which do not use molecular O₂ as terminal electron acceptor although O₂ is a component of their cellular material. Infact molecular O₂ is toxic to these organisms and these are called as obligate anaerobes e.g. Clostridium. In these organisms NO_3-, SO₄₌ or organic compounds serve as electron acceptors.
• Some microorganisms can also grow either in the presence or absence of molecular O2 are termed as facultative anerobes e.g.E.coli. In addition to these major classes, there are organisms which grow best at reduced O₂ pressure but are obligate aerobes and these are called microaerophillic e.g. most lactobacilli.