New English File Upper Intermediate Wb Key Entry Checker
New English File Upper Intermediate Wb Key Entry Checker
Bacterial biofilms are regarded to be the primary aetiological factor in the initiation of gingival inflammation and subsequent destruction of periodontal tissues (Offenbacher 1996) and three major specific pathogens have been repeatedly identified as etiologic agents, namely Aggregatibacter (Actinobacillus) actinomycetemcomitans (Aa), Porphyromonas gingivalis (Pg) and Tannerella forsythia (Tf) (Socransky et al. 1998). Although chronic exposure to bacteria and their products is a prerequisite fo