This small group of putative periodontal pathogens possesses certain features in common. Most of them have a Gram- negative cell wall. The outer membrane of the Gram-negative cell wall contains lipopolysaccharide (LPS) which has endotoxin activity.
Typically, LPS containing Gram- negative cell wall extracts are capable of promoting bone resorption, inhibiting osteogenesis, chemotaxis of neutrophils, and other events associated with active periodontitis. Some periodontal pathogens release a LPS that suppresses the innate immune response.
Many periodontal pathogens are strict or facultative anaerobes and are asaccharolytic, permitting survival in the restricted ecosystem of the periodontal pocket. Among the strict anaerobes is the only presumptive periodontal pathogens with a Gram-positive cell wall, Peptostreptococcus micros.
Actinobacillus actinomycetemcomitans and Porphyromonas gingivatis are the best studied and have been designated, along with Bacteroides forsythus, as true etiologic agents in periodontitis because of the host of virulence they produce and their ability to invade gingival tissues.
Prevofella intermedia/nigrescens and Fusobacterium nucleatum have been widely studied as well, and appear to satisfy all criteria for periodontal pathogenicity. Because P. intermedia and strains of F nucleatum have also been found in areas of severe gingival inflammation without evidence of attachment loss, controversy exists as to their true periodontal pathogens.
Campylobacter rectus, Elkenella corrodens, Eubacterium species, Selenomonas species, enteric rod is Pseudomonas species, and Treponema species satisfy some, but not all, criteria with any degree of confidence. Nonetheless, they remain among the list of periodontal pathogens, and microbiology testing services commonly report their presence among cultivable flora.
Relative risk values of periodontal pathogens in periodontal sites have emerged from archival reviews of data bases located in commercial testing facilities. The relative risk of a microorganism as a periodontal pathogens is often expressed as percent of total cultivable bacteria in a given culture.
For example, the cultivability of A. actinomycetemcomitans at levels at or above 0.01% indicates a periodontal site at risk for active disease. The risk for P. gingivalis, C. rectus, P. intermedia, and Pmicros in periodontal sites is 0.1%, 2%, 2.5%, and 3%, respectively.