| Summary: | Changes were evaluated over 10 years in the in vitro resistance of human periodontopathic strains of <i>Parvimonas micra</i> to four antibiotics. Subgingival biofilms culture positive for <i>P. micra</i> from 300 United States adults with severe periodontitis in 2006, and from a similar group of 300 patients in 2016, were plated onto anaerobically incubated enriched Brucella blood agar alone, or supplemented with either doxycycline (4 mg/L), clindamycin (4 mg/L), amoxicillin (8 mg/L), or metronidazole (16 mg/L). <i>P. micra</i> growth on antibiotic-supplemented media indicated in vitro resistance to the evaluated antibiotic concentration. <i>P. micra</i> resistance was significantly more frequent among patients in 2016, as compared to 2006, for doxycycline (11.3% vs. 0.3% patients; 37.7-fold increase), and clindamycin (47.3% vs. 2.0% patients; 23.7-fold increase) (both <i>p</i> < 0.001), whereas resistance to amoxicillin (2.3% vs. 1.0% patients) and metronidazole (0% vs. 0.3% patients) remained low and statistically unchanged between the two patient groups (<i>p</i>-values > 0.05). No <i>P. micra</i> isolates in 2006 or 2016 were jointly resistant in vitro to both amoxicillin and metronidazole. The alarming increases in subgingival <i>P. micra</i> resistance to doxycycline and clindamycin raise serious questions about the empiric use of these antibiotics, either locally or systemically, in the treatment of United States periodontitis patients harboring subgingival <i>P. micra</i>.
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