Cells which carry out fermentation fall into every category of anaerobic cell type, obligate anaerobes, facultatively anaerobes, and aerotolerant cells. Obligate anaerobes denote bacteria that only survive in the absence of oxygen. Facultative anaerobes include other bacteria, yeast, and animal muscle tissue cells. Facultative anaerobic cells carry out fermentation in low oxygen conditions, possessing the ability for more efficient respiration with greater oxygen concentration. Aerotolerant cells are strictly fermentative.