beta-Lactamase production and in vitro antimicrobial susceptibility of Porphyromonas gingivalis.

beta-Lactamase production by 98 Porphyromonas strains was investigated by the nitrocefin (chromogenic cephalosporin) test. Human isolates of P. gingivalis (91), P. endodontalis (2), and P. asaccharolytica (1) were tested, with four closely related Porphyromonas spp. of animal origin and four reference strains. The in vitro susceptibility of 64 P. gingivalis strains was investigated on Brucella blood agar by the E test. None of the human Porphyromonas isolates tested produced beta-lactamase, but one Porphyromonas strain of animal origin, most closely resembling P. endodontalis, produced beta-lactamase. P. gingivalis was susceptible to almost all of the drugs tested: benzylpenicillin, ampicillin, cefaclor, cefuroxime, erythromycin, clindamycin, tetracycline, doxycycline, metronidazole and ciprofloxacin; all strains were inhibited at 0.016 microgram/ml, 0.023 microgram/ml, 0.315 microgram/ml, 0.064 microgram/ml, 0.19 microgram/ml, 0.016 microgram/ml, 0.094 microgram/ml, 0.047 microgram/ml, 0.023 microgram/ml, and 0.75 microgram/ml of these drugs, respectively. Cotrimoxazole exhibited variable efficacy against P. gingivalis; the range of MICs was 0.1095-32.0 micrograms/ml. The results indicate that beta-lactamase production is currently not a problem amongst clinical isolates of P. gingivalis and strains are susceptible to most antimicrobial agents.