Beta-lactam resistance is common among salivary Prevotella melaninogenica and related organisms

Objectives: Prevotella melaninogenica (sensu lato) is a ubiquitous colonizer of the mouth and frequent producer of β-lactamase, an enzyme, which leads to resistance to beta-lactam agents. Our aim was to examine the frequency of β-lactamase production and in vitro resistance to three commonly used antibiotics in dentistry.
Methods: Salivary isolates originated from two groups of post-partum women; Group I isolates (n=147) were collected from 38 mothers between 1994-1995, and Group II isolates (n= 81) from 26 mothers between 2003-2004. Isolates were identified to the species level by partial 16S rRNA sequencing: P. melaninogenica (n=149), P. denticola (n=44), and newly described P. jejuni (n=35). β-lactamase production was performed by the nitrocefin disk test. Minimal Inhibitory Concentrations (MICs) of amoxicillin (AMO), amoxicillin/clavulanate (AMC), and metronidazole (MZ) were determined using the agar dilution method. Resistance rates were based on the breakpoints provided by the European Committee on Antimicrobial Susceptibility Testing (EUCAST) and the Clinical and Laboratory Standards Institute (CLSI).
Results: Of the 228 isolates, 161 (70.6%) produced β-lactamase: 68.7% in Group I and 74.1% in Group II. The majority of P. denticola (52.3%), P. jejuni (77.1%), and P. melaninogenica (74.5%) were positive for β-lactamase production. MIC ranges of P. denticola to AMO were <0.125-64, to AMC <0.125-4, and to MZ <0.125-16. Corresponding ranges of P. jejuni were <0.125-16, and <0.125-16, and <0.125-128, and those of P. melaninogenica <0.125-128, and <0.125-32, and <0.125-128, respectively. P. melaninogenica isolates in Group I were more resistant to AMO (28.6% vs. 17.6%) but in Group II more resistant to MZ (0.0% vs. 23.5%). The majority of MZ-resistant isolates were β-lactamase producers.
Conclusions: β-lactamase production and resistance to tested antimicrobial agents are common among P. melaninogenica and related organisms in post-partum women´s saliva. Surprisingly, the overall MZ resistance among the tested Prevotella isolates was nearly 10%.