Susceptibility of viridans group streptococci isolated from dento-alveolar infections to eight antimicrobial agents.

Sir, Many studies have demonstrated a trend towards decreased antimicrobial susceptibility in viridans group streptococci isolated from blood cultures.1,2 This raises concerns when this group of organisms cause more serious infections, such as infective endocarditis, when penicillin resistance can create therapeutic difficulties.3 However, few routine data are available on the susceptibility of viridans streptococci isolated from dental infections. The microflora of an acute dental abscess is usually polymicrobial, involving three or four species, with a predominance of strict anaerobes.4 A previous UK study5 reported penicillin resistance in four out of 22 isolates of Streptococcus mitis, Streptococcus sanguis and Streptococcus oralis from 78 patients. We report susceptibility data on viridans streptococci that were recovered from 41 consecutive aspirates of pus from dento-alveolar infections over a 3 year period (1998–2000) submitted to the Microbiology Department, Glasgow Dental Hospital and School. The isolates were identified by colony morphology, Gram’s stain, optochin test and catalase reaction. Species identification was carried out using the Rapid ID32 Strep system (API System; bioMérieux, Marcy l’Étoile, France). All isolates were stored at –70°C in trypticase soy broth (BBL Microbiology Systems, Cockeysville, MD, USA) with 15% glycerol until testing. Antimicrobial susceptibility testing was determined by Etest (AB Biodisk, Solna, Sweden) for penicillin, amoxicillin, erythromycin, clindamycin, ceftriaxone, rifampicin, vancomycin and teicoplanin in accordance with the manufacturers’ instructions. Susceptibility testing was performed on IsoSensitest agar supplemented with 5% horse blood and incubated in 5% CO2 at 37°C for 24 h. Breakpoints for susceptibility were derived from values given by the BSAC.6 The MIC range, MIC50 and MIC90 of each antibiotic tested and the number of resistant strains is given in Table 1. The majority of the viridans group streptococci isolated were S. mitis (n = 35) and S. oralis (n = 12). The remainder comprised Streptococcus parasanguis (n = 2) and S. sanguis (n = 1). Strains of S. mitis had the highest levels of antimicrobial resistance to β-lactams, with a penicillin MIC range of 0.016–3.0 mg/L and with three isolates (9%) demonstrating penicillin resistance (MIC ≥ 2.0 mg/L). The S. mitis erythromycin MIC range was <0.016–>256 mg/L, with 10 isolates (29%) demonstrating erythromycin resistance (MIC ≥ 1.0 mg/L). Strains of S. oralis had a penicillin MIC range of 0.023–2 mg/L, and for erythromycin the MIC range was <0.016–4.0 mg/L. For strains of S. oralis, one isolate was penicillin resistant and two isolates were erythromycin resistant. The S. parasanguis isolates (n = 2) were susceptible to all the antimicrobials tested except for one isolate, which was resistant to erythromycin (MIC = 3 mg/L). The S. sanguis isolate was