Servicio de Microbiologia, Hospital de Mostoles, Instituto Nacional de la Salud, Calle Rio Jucar s/n, 28935 Mostoles, Madrid, Spain
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Abstract |
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Introduction |
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The SMG are an important part of normal microbial flora in humans and animals. They are indigenous to the upper respiratory tract, the female genital tract and all regions of the gastrointestinal tract. They are most prevalent in the oral cavity. This is especially true of S. anginosus, which is present in subgingival plaque. 2 SMG organisms occasionally produce abdominal infections and are frequently found in abscesses in these and other sites. 1234 Pathogenicity in these cases has been demonstrated by the recovery of the species in pure culture, or in association with other anaerobic genera. Concurrent bacteraemia is a frequent feature. 2,3
Most members of the SMG are susceptible to low concentrations of penicillin G.
Approximately 80-90% of strains have MICs of0.1 mg/L.
3,4 Many other
B-lactam antibiotics have in-vitro activity similar to that of penicillin against the SMG, but
the susceptibilities to different cephalosporins are quite variable.
5 The aim of the current study was the evaluation of the
antimicrobial susceptibilities of 100 clinically significant strains of the SMG to 16
cephalosporins, using the agar dilution method, as described by the NCCLS.
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Material and methods |
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All strains were identified as members of the SMG as designated by API 20 Strep and Rapid ID32 Strep systems (Bio-Mérieux, Marcy l'Etoile, France). After presumptive identification, they were stored in skim milk at -20°C.
Morphologic characteristics, including haemolysis, were evaluated on Columbia agar plates (Becton Dickinson, Meylan, France), supplemented with 5% sheep blood. Serologic grouping to detect A, C, F and G Lancefield antigens was carried out using a commercial antisera kit (Streptococcal Grouping, Unipath, Basingstoke, UK). Fermentation of inulin, lactose and trealose was tested in brain-heart infusion broth containing the corresponding carbohydrate (1%) and bromocresol purple (0.0015%). Acetoin production (Voges-Proskauer), aesculin and arginine hydrolysis were tested using standard methods.
Antibiotics were obtained from their manufacturers as laboratory powders of defined potency and were reconstituted in recommended diluents to yield stock solutions. These were kept frozen at -40°C at the indicated concentration ranges: penicillin, G 0.030.25 mg/L (Sigma, St Louis, MO, USA); cephalothin, 0.254 mg/L (Sigma); cefazolin, 0.254 mg/L (Sigma); cefaclor, 0.58 mg/L (Sigma); cefprozil, 0.58 mg/L (Bristol-Myers Squibb, Barcelona, Spain); cefamandole, 0.58 mg/L (Sigma); cefuroxime, 0.061 mg/L (Glaxo, Aranda de Duero, Spain); cefoxitin, 0.58 mg/L (Merck, Sharp & Dohme, Milan, Italy); cefminox, 0.58 mg/L (Tedec-Meiji Farma, Madrid, Spain); cefotaxime, 0.060.5 mg/L (Hoechst-Marion-Roussel, Romainville, France); ceftriaxone, 0.061 mg/L (Roche, Basle, Switzerland); ceftazidime, 0.58 mg/L (Glaxo Wellcome, Madrid, Spain); ceftizoxime, 0.061 mg/L (SmithKline Beecham, Toledo, Spain); cefixime 0.2532 mg/L (Merck Farma & Quimica, Barcelona, Spain); cefpodoxime, 0.061 mg/L (Hoechst-Marion-Roussel); ceftibuten 116 mg/L (Schering-Plough, Kenilworth, NJ, USA); and cefepime, 0.2532 mg/L (Bristol-Myers Squibb) were tested.
The SMG strains were taken from frozen stock cultures and grown on blood agar in 510% CO2 at 35°C for 24 h. The strains were transferred to tubes containing MullerHinton broth (Difco, Detroit, MI, USA) and incubated overnight in 510% CO2. All MICs were determined by the agar dilution method using Müller-Hinton agar (Becton Dickinson), supplemented with 5% sheep blood, containing graded concentrations of antibiotics. The inoculum used for the SMG was 104 cfu per spot, applied with a Steers replicator. All plates were incubated in 510% CO2 at 35°C for 24 h before being examined.
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Results |
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Discussion |
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Third-generation oral cephalosporins were the most varied group. Cefpodoxime had
excellent activity, with MIC50s and MIC90s of 0.25 and 0.5 mg/L,
respectively, for all strains. Cefixime was less active, with only 8% of strains having
MICs
2 mg/L. Ceftibuten was the least active, all strains having MICs >16 mg/L.
Finally, cefepime, the sole representative of the fourth-generation cephalosporins, showed
very good activity, with 98% of strains being susceptible (MIC50 0.5 mg/L
and MIC90 1 mg/L) and only two strains having MICs2 mg/L.
Although breakpoints for some cephalosporins for the SMG are not included in the most recently published NCCLS tables,6 making the interpretation of their values difficult, it is interesting to observe that some cephalosporins with similar clinical uses have very different antibacterial activities against these organisms. Therefore the selection of one of these cephalosporins, when initiating treatment of an SMG infection, should be made with knowledge of previously ascertained activity against these organisms, which are assuming increasing importance in the aetiology of human infections.
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Acknowledgments |
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Notes |
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References |
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2 . Ruoff, K. L. (1988). Streptococcus anginosus (` Streptococcus milleri' ): the unrecognized pathogen. Clinical Microbiology Reviews 1, 102-8.[Medline]
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6 . National Committee for Clinical Laboratory Standards. (1997). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow Aerobically -Fourth Edition: Approved Standard M7-A4 (and Sixth Informational Supplement M100-S7). NCCLS, Wayne, PA.
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Received 11 May 1998; returned 29 June 1998; revised 15 July 1998; accepted 9 September 1998