The existence of commensal or antagonistic relationships between microorganisms in the root canals of teeth with apical periodontitis was investigated. Samples were taken from 65 infected human root canals and were analysed according to species, frequency of occurrence and proportion of the total isolated flora. The most frequent species were Fusobacterium nucleatum, Prevotella intermedia, Peptostreptococcus micros, Peptostreptococcus anaerobius, Eubacterium alactolyticum, Eubacterium lentum and Wolinella recta. An odds ratio system was used to calculate positive or negative associations between the isolated bacteria. Strong positive associations were found between F. nucleatum and P. micros, Porphyromonas endodontalis, Selenomonas sputigena and W. recta. There was also a positive association between P. intermedia and P. micros, P. anaerobius and the eubacteria. In general, species of streptococci, Propionibacterium propionica, Capnocytophaga ochracea and Veillonella parvula showed no or negative associations with the other bacteria. The results are consistent with the concept of a special and selective environment occurring in the root canal that is due, in part, to the cooperative as well as antagonistic nature of the relationships between bacteria in the root canal.
Twelve male mice, 3 weeks old, were caged together with caries-active hamsters and the twin brothers of the mice served as controls. The hamsters harboured Streptococcus mutans and Streptococcus salivarius in their mouths, while neither of these streptococcal species could be recovered from the mice at the beginning of the experiment. At the end of the experiment after nine weeks Strep. mutans could be isolated in a high number from the mouth of six mice reared in the company of hamsters. Five of these mice had a moderate caries activity (2–19 lesions) while one was caries-free. Of the six mice from which Strep. mutans was not recovered, one mouse had five caries lesions while the other five animals were caries-free. In the 12 control mice Strep. mutans could not be isolated and only one animal had one caries lesion. This experiment demonstrates the transfer of Strep. mutans from hamster to mouse and a possible relation between the presence of Strep, mutans and caries activity in the mouse.
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