Despite the ubiquitous involvement of bacteria, significant progress in our understanding of specific microbial etiologies has occurred only in the past decade. Their role in periodontal disease, root canal infections, infection of the hard and soft oral tissue, as well as their importance as foci for disseminated infectious disease is well established. Anaerobes make up a significant part of the oral and dental indigenous and pathogenic flora. The clinically important anaerobes in decreasing frequency are Six genera of Gram-negative rods (Bacteroides, Prevotella, Porphyromonas, Fusobacterium, Bilophila and Sutterella spp.) Gram-positive cocci (primarily Peptostreptococcus spp.) Gram-positive spore-forming (Clostridium spp.) and non-spore-forming bacilli (Actinomyces, Propionibacterium, Eubacterium, Lactobacillus and Bifidobacterium spp.) and Gram-negative cocci (mainly Veillonella spp.) (Brook I, 2007). Anaerobic bacteria usually do not possess catalase, but some can generate superoxide dismutase which protects them from oxygen. Anaerobic bacteria can be divided into strict anaerobes that cannot grow in the presence of more than 0.5% oxygen and moderate anaerobic bacteria that are able of growing between 2 to 8% oxygen (Jousimies et al, 2003). Microaerophilic bacteria do not grow at all aerobically or grow poorly, but grow better under 10% carbondioxide or anaerobically. Anaerobic infections are caused by anaerobic bacteria Anaerobic bacteria do not grow on solid media in room air (10% carbon dioxide and 18% oxygen) facultative anaerobic bacteria can grow in the presence as well as in the absence of air.
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