Multiple-resistance to Betalactam Antibiotics, Azithromycin or Moxifloxacin in Implant-associated Bacteria

Objective: Antibiotics are more and more frequently prescribed in dentistry for prevention and treatment of oral diseases. Bacterial resistance to these agents is clearly increasing, involving even previously susceptible microorganisms, including true pathogens. In a previous study, implant-associated bacteria were tested first as mixed cultures and again as pure isolates for resistance to one of five antibiotics. The aim of the present study was to examine these resistant bacterial strains with respect to possible multiple-antibiotic resistances. Methods: Bacterial samples, taken with sterile paper points from the deepest pocket of one implant per patient (n=24), were streaked on agar plates, and sensitivity towards 4 antibiotics (ampicillin, ampicillin+sulbactam, azithromycin, penicillin) was determined using the Etest®. Colonies of resistant strains were picked and purified. The pure isolates (n=127) were then tested for resistance to all four antibiotics plus to moxifloxacin, using the Etest®. Most isolates were identified at least to the genus level. Results: Almost all resistant isolates were resistant to more than one antibiotic. 16 isolates were completely resistant (MIC >256 µg/ml for ampicillin, ampicillin+sulbactam or azithromycin, >32 µg/ml for penicillin or moxifloxacin) to all five antibiotics; half of these strains were isolated from only 3 patients with peri-implantitis. 2 multiple resistant isolates could be identified as Actinobacillus spec., 2 Staphylococcus spec., 1 Acinetobacter spec., 1 Haemophilus spec., 1 Gram-negativ coccus., 3 Lactobacillus spec. and 6 Streptococcus spec.. Three isolates were resistant to all betalactam antibiotics but sensitive to azithromycin and moxifloxacin. 9 isolates were resistant to azithromycin or moxifloxacin but sensitive to all betalactam antibiotics. Conclusion: It was shown that some of the isolates, belonging to the implant-associated microflora, were multi-resistant, even though the patients hadn't received any antibiotics six weeks prior to the sampling. The exact mechanisms that lead to these multiple resistances need to be elucidated in further studies.