Oral Gram-negative anaerobic bacilli as a reservoir of β-lactam resistance genes facilitating infections with multiresistant bacteria

• Prevotella and Capnocytophaga are the main sources of β-lactamases in the oral microbiota.
• The main β-lactamases in oral Gram-negative bacilli are CfxA and CSP-1.
• Spread of β-lactamase-producing bacteria is driven by various horizontal gene transfer mechanisms.
• Insertion sequences (IS), upstream of the β-lactamase gene, can facilitate its mobilisation.
• Conjugative transposons and plasmids have been transferred in vitro from oral donor to oral recipient bacteria.

Many β-lactamases have been described in various Gram-negative bacilli (Capnocytophaga, Prevotella, Fusobacterium, etc.) of the oral cavity, belonging to class A of the Ambler classification (CepA, CblA, CfxA, CSP-1 and TEM), class B (CfiA) or class D in Fusobacterium nucleatum (FUS-1). The minimum inhibitory concentrations of β-lactams are variable and this variation is often related to the presence of plasmids or other mobile genetic elements (MGEs) that modulate the expression of resistance genes. DNA persistence and bacterial promiscuity in oral biofilms also contribute to genetic transformation and conjugation in this particular microcosm. Overexpression of efflux pumps is facilitated because the encoding genes are located on MGEs, in some multidrug-resistant clinical isolates, similar to conjugative transposons harbouring genes encoding β-lactamases. All these facts lead us to consider the oral cavity as an important reservoir of β-lactam resistance genes and a privileged place for genetic exchange, especially in commensal strictly anaerobic Gram-negative bacilli.