Oxygen concentration
Strict aerobes: Require oxygen for growth (~20%)
Strict anaerobes: Grow in the absence of oxygen; cannot grow in the presence of oxygen
Facultative anaerobes: Grow best in the presence of oxygen, but are able to grow (at reduced rates) in the absence of oxygen
Aerotolerant anaerobes: Can grow equally well in the presence or absence of oxygen
Microaerophiles: Require reduced concentrations of oxygen (~2 – 10%) for growth