β-lactamase-producing strains in the species Prevotella intermedia and Prevotella nigrescens

A total of 96 strains were collected that included laboratory strains and clinical isolates classified Prevotella intermedia sensu lato and the type strains of the species P. intermedia sensu stricto and Prevotella nigrescens. Susceptibility to amoxicillin and amoxicillin-clavulanic acid was determined by the Etest. PCRDNA probe assays were used to speciate each strain as P. intermedia sensu stricto or P. nigrescens. By Etest, 71 strains (74%) were susceptible to both amoxicillin and amoxicillin-clavulanic acid with minimum inhibitory concentrations in the 0.016–0.064 μg/ml range. In contrast, amoxicillin minimum inhibitory concentrations of 25 strains (26%) were in the range of 1.5–96 μg/ml with concomitant amoxicillin-clavulanic acid minimum inhibitory concentrations in the low range 0.016–0.38 μg/ml, indicating a production of β-lactamase as confirmed by nitrocefin tests. Of these β-lactamase-producing strains, 20% (5/25) were identified as P. intermedia sensu stricto by the PCR-DNA probe assay and 72% (18/25) as P. nigrescens. Our results provide support for the major role of P. nigrescens in the failure of therapy using β-lactam antibiotics.