Four aqueous extracts of different plant organs are the following: Artemisia herba-alba, Opuntia ficus-indica, Camellia sinensis and Phlomis crinita were evaluated against two bacterial strains: Porphyromonas gingivalis and Prevotella intermedia, which are implicated in periodontal diseases. By using a disc method, these plant extracts demonstrated powerful bacterial activity against these Gram-negative strains. The minimum inhibitory concentration values of the four plant extracts varied between 0.03 and 590.82 mg/ml for the microbes. Another assay using commercial antibiotics and antibacterials as positive controls was also conducted. Values obtained after statistical analysis of inhibition diameters of all plant extracts demonstrated that for P. gingivalis, the aqueous extracts of A. herba-alba and O. ficus-indica were most effective, followed by those of C. sinensis and P. crinita. For P. intermedia, aqueous extracts of O. ficus-indica and C. sinensis appeared to be more efficient with significantly different (P > 0.05) inhibition diameters, followed by those of O. ficus-indica and P. crinita. In summary, the statistical results reveal that these plant extracts exert stronger antibacterial activity on P. intermedia germ as compared to P. gingivalis.