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INFECTION AND IMMUNITY, Sept. 1978, p. 821-829 0019-9567/78/0021-0821$02.00/0 Copyright i 1978 American Society for Microbiology

Vol. 21, No. 3

Printed in U.S.A.

Bacteriology of Human Experimental Gingivitis: Effect of Plaque Age
S. A. SYED AND W. J. LOESCHE* Dental Research Institute, Department of Oral Biology, Department of Microbiology-Medical School, University of Michigan Schoolof Dentistry, Ann Arbor, Michigan 48109

Received for publication 28 June 1978

Twenty-five subjects with previously excellent hygiene and healthy gingiva developed heavy plaque accumulations and bleeding or nonbleeding gingivitis about certain papilla after 21 days of no oral hygiene. Gingival marginal plaque about a single papilla was collected at 0, 1, 2, and 3 weeks of no oral hygiene.ineach subject. The plaque was dispersed, serially diluted, and plated on MM10 sucrose agar in an oxygen-free atmosphere. From 50 to 100 colonies from a single high-dilution plate were characterized for each sample. Over 8,500 isolates were partially characterized and placed into one of 29 taxonomic species or groups. The flora was predominantly gram-positive at all time periods. Streptococcal speciesdominated in the 0- and 1-week-old plaques, i.e. 62 and 43% of the colonyforming units (CFU), but dropped to 26 to 32% of the CFU in the 2- and 3-weekold plaques. Actinomyces species dominated in the older plaques, i.e., 40 to 50% of the CFU. Actinomyces israelii was the most prominent species in the older plaques. Veillonella accounted for 15 to 20% of the CFU at all time periods. Although theother gram-negative species increased with time, collectively they averaged less than 5% of the CFU at week 3. The shift from a Streptococcusdominated plaque to an Actinomyces-dominated plaque was the most striking microbial change observed as the plaque aged.

Dental plaque has been associated with the human periodontal disease on the basis of epidemiological observations (28, 30, 34) andclinical observations (13, 17). However, it was not until 1965 that conclusive evidence was obtained which correlated plaque accumulation with gingivitis, the earliest stage of periodontal disease (17). Volunteers were brought into a state of excellent oral health by a supervised program of mechanical cleaning. Once optimal health was achieved, all mechanical procedures were withdrawn and plaque wasallowed to accumulate. The amount of plaque and the degree of gingivitis were monitored during a 3-week period. When the plaque was removed at day 21, and oral hygiene procedures reinstituted, the gingivitis disappeared within 5 to 7 days. Bacteriological smears of the plaque made at different time intervals showed that gram-negative organisms increased in number and proportions at about the timethat gingivitis occurred (17, 37). No cultural studies were performed, nor were the bacteriological smears associated with either plaque or gingivitis scores about a discrete site. In the present investigation the experimental gingivitis model was repeated, and extensive bacteriological analyses of plaque removed from discrete sites were performed at 0, 1, 2, and 3

weeks on the no-brushingregimen. The effect of plaque accumulation and degree of gingivitis upon the plaque flora will be presented in a separate report (22).

MATERIALS AND METHODS Subjects. Twenty-four male dental students and one female dental student suspended all oral hygiene procedures for a 3-week period. These students had been selected from a larger pool of candidates on the basis of having a minimal number of dentalrestorations in their mouths and having a generally high level of gingival health. Pilot studies had indicated that the comprehensive bacteriological studies could only be performed on about four students at a time. Accordingly the students were sorted into six groups of four or five students, and each group was consecutively brought through the no-hygiene experimental period. Sample sites. The...