Background: Periodontopathogenic bacteria can invade and survive within epithelial cells, but susceptibility of intracellular infection to antibiotics used in periodontitis treatment has not been studied to date.
Methods: KB cells were infected by Actinobacillus actinomycetemcomitans, strain NCTC 9710; Porphyromonas gingivalis, strains ATCC 33277 and JH16-1; or Streptococcus constellatus, strain J012b. After 2, 4, and 12 hours the bactericidal effect of antibiotics (clindamycin, doxycycline, metronidazole, and moxifloxacin) on intracellular microorganisms was tested at a concentration up to the 100-fold minimum inhibitory concentration (MIC) determined separately on planktonic bacteria.
Results: The P. gingivalis strains differed in their invasiveness and ATCC 33277 was 100-fold more invasive than JH16-1. Doxycycline and clindamycin at a concentration 10-fold MIC had no effect, but P. gingivalis intercellular infection was significantly reduced by metronidazole at 10-fold MIC after 2 and 4 hours. Moxifloxacin was effective, but a 100-fold MIC concentration was necessary to reduce P. gingivalis strains intracellular growth to 7% of the control. Other bacterial species grown inside the KB cells were more susceptible to antibiotics. Clindamycin at 10-fold MIC reduced the number of intracellular S. constellatus after 4 and 12 hours. This bacterium was eliminated by moxifloxacin at 50-fold MIC. Intracellular A. actinomycetemcomitans was killed by 10-fold MIC of doxycycline and moxifloxacin after 4 hours incubation.
Conclusions: Moxifloxacin was the most efficient antibiotic to treat intracellular infection. However, taking into account the MIC values and the levels of antibiotics in gingival fluid, elimination of intracellular bacteria by antibiotics alone seems to be questionable.