Antibiotic resistance testing of the total implant-associated micro-flora and its pure isolates

The aim of the present study was to examine antibiotic resistant strains among the implant-associated microorganisms in vitro, first as mixed cultures and again as pure isolates for resistance to one of five antibiotics.

Samples were taken with sterile paper points from the deepest pocket of one implant per patient (n = 24) to culture the total oral micro-flora. The samples were streaked on agar (Schaedler or BHI) and incubated for 7 d in an anaerobic atmosphere. All colonies were rinsed off the plates, aliquots were added to top-agar. Susceptibility against antibiotics (ampicillin, ampicillin + sulbactam, azithromycin and penicillin, moxifloxacin) was determined using the Etest. Resistant strains were picked, purified and characterized, and the Etests were repeated with a selection of the pure isolates.

The majority of the mixed cultures (67 - 100 %) showed complete antibiotic resistance. No association with clinical parameters like pocket depth, bleeding on probing or insertion of implants into transplanted bone could be found. Smoking and the surface of the implant also had no influence. 23 % of the 597 resistant colonies contained only yeasts, mostly isolated from irradiated tumour patients. Of the 458 resistant bacteria, the majority were Gram-positive cocci or rods. Staphylococci and M. micros were detected occasionally. The resistance for the 138 selected pure isolates was in most cases lower than for the total micro-flora, irrespective of the antibiotic.

The higher resistance of the total flora might be explained by synergistic interactions between its members.