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Antimicrobial Agents and Chemotherapy, April 2003, p. 1443-1446, Vol. 47, No. 4
0066-4804/03/$08.00+0 DOI: 10.1128/AAC.47.4.1443-1446.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Antibiotic Resistance in Bacteria Isolated from Subgingival Plaque in a Norwegian Population with Refractory Marginal Periodontitis
Trude Handal,1* Dominique A. Caugant,1,2 and Ingar Olsen1
Institute of Oral Biology, Dental Faculty, University of Oslo,1
Division of Infectious Disease Control, Norwegian Institute of Public Health, Oslo, Norway2
Received 23 September 2002/
Returned for modification 14 November 2002/
Accepted 23 January 2003

ABSTRACT
The prevalence of ß-lactamase-producing bacteria in
subgingival plaque from patients with refractory periodontitis
in Norway was assessed by the chromogenic nitrocefin method.
ß-Lactamase activity was detected in 68% of the patients.
Enzyme-producing strains belonged predominantly to the genus
Prevotella; others were staphylococci, enteric gram-negative
rods, and
Bacillus spp.

TEXT
Microorganisms of dental plaque may cause marginal periodontitis,
a disease characterized by loss of attachment around the teeth,
which is common in the Western world and even more so outside
it (
6). Most putative periodontal pathogens are gram-negative
anaerobic rods, e.g.,
Porphyromonas gingivalis,
Prevotella intermedia,
Bacteroides forsythus, and
Treponema denticola (
24,
31). However,
superinfecting organisms such as enteric gram-negative rods,
pseudomonads, staphylococci, and yeasts have been isolated from
refractory periodontitis lesions (
22), as well as taxa not considered
a common part of the oral microbiota (
5). There are reports
of increasing ß-lactamase production in subgingival
periodontal pathogens in several countries (
7,
12,
17,
27).
In Norway, however, this situation has not been studied. The
aims of this study were (i) to investigate the extent of ß-lactamase
production in subgingival bacteria isolated from patients with
refractory marginal periodontitis in Norway, (ii) to characterize
the ß-lactamase-producing bacteria using enzymatic
and/or biochemical methods and partial sequencing of the 16S
rRNA gene, and (iii) to determine their susceptibility to various
antimicrobial agents.
Subgingival plaque samples from 25 consecutive cases of refractory marginal periodontitis received from specialists in periodontics were examined. The patients were aged 39 to 66, and none had responded to conventional periodontal treatment. One patient (no. 14 [Table 1]) had received a combination of an aminopenicillin and metronidazole as an adjunct to conventional treatment within 3 months prior to sampling. The other patients had not received antibiotics in the past 3 months. Bacterial samples were collected from the bottom of the periodontal pocket and were immediately transferred to prereduced anaerobically sterilized Dental Transport Medium (Anaerobic System, Morgan Hill, Calif.). Appropriately diluted samples were plated onto nonselective Trypticase soy agar plates supplemented with 5% defibrinated human blood, hemin (5 mg/ml), and menadione (0.05 mg/ml). In addition, selective agar plates (wolinella medium, CVE medium, clindamycin blood agar, TSBV agar, mitis salivarius agar, mannitol salt agar, MacConkey agar, Trypticase soy agar plates supplemented with 5% defibrinated human blood, hemin (5 mg/liter), and N-acetyl muramic acid (10 mg/liter) and blood agar plates supplemented with 3 µg of amoxicillin/ml were inoculated with 0.1 ml of undiluted sample for recovery of periodontopathogens. Nonselective and selective agar plates were incubated anaerobically (90% N2, 5% H2, 5% CO2) at 37°C for up to 14 days in anaerobic jars (Anoxomat WS9000; Mart, Lichtenvoorde, The Netherlands). All morphotypes of bacteria on plates were subcultured and tested for ß-lactamase production.
ß-Lactamase production was assessed by using chromogenic
nitrocefin-impregnated disks (BBLDrySlideNitrocefin; Becton
Dickinson) and a chromogenic nitrocefin solution (
27). ß-Lactamase-positive
and -negative strains of
Staphylococcus aureus, provided by
the Microbiology Laboratory at the National Hospital, Oslo,
Norway, were included as controls. Enzymatic and/or biochemical
profiling relied on commercial diagnostic kits designed for
identification of a number of different microorganisms (Analytab
Products [API]; bioMérieux, Marcy-l'Etoile, France).
The preparation, incubation, and reading of the kits were carried
out according to the manufacturer's recommendations. To confirm
species identification with the API system, partial sequencing
of the 16S rRNA gene was also performed as previously described
(
26).
Bacterial susceptibility to ampicillin, amoxicillin, amoxicillin-clavulanic acid, tetracycline, minocycline, doxycycline, clindamycin, metronidazole, cefotaxime, and ceftazidime were assessed by using the Etest (AB Biodisk, Solna, Sweden). Etest extended-spectrum ß-lactamases (strip of cefotaxime-cefotaxime plus clavulanic acid and strip of ceftazidime-ceftazidime plus clavulanic acid) were used to confirm the presence of clavulanic inhibitable extended-spectrum ß-lactamase enzymes. For anaerobes, brucella agar plates were streaked with a McFarland standard no. 1 inoculum in brucella broth. For facultative organisms, PDM agar plates were streaked with a McFarland standard no. 0.5 inoculum suspended in 0.85% NaCl. The Etest MIC was read according to the recommendations of the manufacturer. Appropriate quality controls were included in each run.
At least one strain with ß-lactamase activity was detected in 17 of the 25 (68%) patients (Table 1). No ß-lactamase-producing bacteria were found in the subgingival plaque of the patient who had received antibiotic treatment within 3 months prior to sampling (patient no. 14 [Table 1]). The most prominent ß-lactamase-producing species belonged to the genus Prevotella (52%). Other enzyme-producing strains were enteric gram-negative rods (five patients, 20%), staphylococci (four patients, 16%), and Bacillus species (two patients, 8%). Five of the ß-lactamase-producing isolates could not be identified at the species level, two of them belonged to the genus Prevotella, and the remaining three were facultative gram-negative rods. The two Prevotella strains were closely related to strain B31FD in the GenBank database. They differed from one another by 8 nucleotides in the sequenced region and thus might represent new Prevotella species. All ß-lactamase-producing Prevotella species were recovered from the blood agar plates supplemented with amoxicillin. Nonselective agar plates, however, were necessary to detect ß-lactamase-producing bacteria other than Prevotella. The MICs of ampicillin and amoxicillin were in the range of 0.50 to 32 µg/ml and 1 to 64 µg/ml, respectively, for the Prevotella species. According to the interpretive categories of the NCCLS (18), all Prevotella isolates were susceptible to amoxicillin-clavulanate (MICs ranged from 0.023 to 0.75 µg/ml) (Table 2). The Prevotella species showed variable resistance to the tetracyclines. One of the isolates (Prevotella sp. oral strain B31FD, patient no. 2) was resistant to cefotaxime and ceftazidime (MICs = 256 µg/ml). All the Prevotella isolates were susceptible to metronidazole. Table 2 also lists the in vitro antimicrobial susceptibility of the staphylococci and the facultative gram-negative rods (19). The results of the screening for extended-spectrum ß-lactamases were negative for all bacterial isolates.
A high proportion (68%) of the patients with refractory periodontitis
in Norway harbored ß-lactamase-producing bacteria
in their subgingival plaque, as also found in other countries
(
16,
28), although higher (
12,
27) or lower (
7,
14,
25) prevalences
have been reported. In the present study API identification
correlated with that based on partial sequencing of the 16S
rRNA gene in 13 of the 24 bacterial isolates. Especially for
the
Prevotella species, the correlation was limited. Partial
sequencing of the 16S rRNA gene resulted in 97 to 100% homology
with partial 16S rRNA gene sequences in the GenBank database.
All ß-lactamase-producing
Prevotella species isolated
were resistant to ampicillin and amoxicillin. Penicillin-resistant
Prevotella species have been reported in several studies (
2,
4,
10,
13,
14,
15,
17). These isolates were all susceptible
to amoxicillin-clavulanate. Good activity of amoxicillin-clavulanate
against
Prevotella species has also been reported (
3,
4,
7,
8,
16). In accordance with previous findings (
1,
9,
10,
11,
30), 100% susceptibility to metronidazole and to clindamycin
was observed in all
Prevotella isolates. Two of the
Prevotella isolates in the present study were resistant to tetracycline,
and four showed intermediate resistance. Because of the frequent
use of tetracyclines in periodontal practice (
21,
23), patients
with refractory periodontitis often present with a history of
tetracycline therapy and a microflora that is relatively resistant
to this antibiotic (
20,
29). According to the latest published
NCCLS breakpoints (
18), all but one of the
Prevotella species
isolated in our study were sensitive to the cephalosporin cefotaxime.
The MICs were somewhat higher for ceftazidime, but no breakpoints
for that cephalosporin have been published for anaerobes. For
one
Prevotella isolate for which no species identification could
be made (B31FD), the MICs of cefotaxime (256 µg/ml) and
ceftazidime (256 µg/ml) were high, thus indicating the
presence of a different ß-lactamase. Most recent investigations
assessing ß-lactamase-producing species in the oral
cavity have focused on anaerobic gram-negative rods. Since the
majority of periodontal diseases are mixed infections, the present
study attempted to identify the range of oral species that can
produce the enzyme. Several of the enteric rods were resistant
to penicillin and showed variable resistance to tetracyclines.
Three of the isolates showed intermediate resistance towards
amoxicillin-clavulanate. These findings underscore the importance
of enteric rods in refractory periodontal diseases.
In conclusion, ß-lactamase activity in subgingival bacteria from refractory periodontitis in Norway was a common feature. A wide variety of ß-lactamase-producing bacteria, both putative pathogenic species and accompanying organisms, could be isolated from the subgingival flora of refractory periodontitis patients.

ACKNOWLEDGMENTS
This study was supported in part by SSAC grant 2000-03-0002
to D.A.C.
The excellent technical assistance of Anne-Marie Klem is acknowledged. We thank E. Arne Høiby for valuable suggestions.

FOOTNOTES
* Corresponding author. Mailing address: Institute of Oral Biology, Dental Faculty, University of Oslo, P.O. Box 1052 Blindern, 0316 Oslo, Norway. Phone: (47) 22840327. Fax: (47) 22840305. E-mail:
thandal{at}odont.uio.no.


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Antimicrobial Agents and Chemotherapy, April 2003, p. 1443-1446, Vol. 47, No. 4
0066-4804/03/$08.00+0 DOI: 10.1128/AAC.47.4.1443-1446.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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