Bacteroides fragilis group (Bacteroides, Parabacteroides, and Phocaeicola) species belonging to the order Bacteroidales are strict anaerobic gram-negative bacilli and major constituents of the gastrointestinal tract. They are also the most frequently isolated endogenous anaerobic opportunistic pathogens, sometimes causing serious infections such as intra-abdominal abscesses, obstetric–gynecological conditions, postoperative wounds, lung and brain abscesses, skin and soft tissue infections, bacteremia, and diarrhea [1]. Among all anaerobes, B. fragilis group species are the most resistant species regarding the number of antibiotic resistance mechanisms and the antibiotic resistance levels attained. In many countries, recent studies revealed regional variations in resistance to antimicrobial agents among B. fragilis group species [2,3]. In developed countries, timely antibiotic susceptibility testing is performed to facilitate empiric therapy [4,5]. However, data on antimicrobial resistance in anaerobic pathogens in developing countries are rarely reported because of laborious processes required for susceptibility testing, and most data in the literature do not obey the guidelines from EUCAST and CLSI [6]. The most useful antibiotics for severe B. fragilis group infections are wide-spectrum β-lactams (β-lactam/β-lactamase inhibitor combinations or carbapenems), metronidazole, and tigecycline. Various molecular mechanisms are involved among B. fragilis species for β-lactams, e.g., β-lactamase production and reduced affinity of some β-lactams for penicillin-binding proteins or efflux pumps [7]. In B. fragilis group species, β-lactamase production is the most frequent cause of penicillin and cephalosporin resistance, and cepA or cepA-related gene products are responsible for this resistance. cfxA is another frequently identified β-lactamase gene that is responsible for high resistance to cefoxitin and other β-lactams, and it is carried on the mobilizable transposon MTn4555 in Bacteroides species [8].

Isolates of B. fragilis group species can be resistant to carbapenems [4,9,10]. This is usually mediated by the chromosomally located genes cfiA (also known as ccrA) and crxA, which encode class B metallo-β-lactamases in B. fragilis and B. xylanisolvens, respectively. These zinc-dependent metallo-β-lactamase gene products can confer strong resistance to almost all β-lactams including cephamycins and carbapenems as well as β-lactam/β-lactamase inhibitor combinations [[11], [12], [13]]. Based on the presence of cepA and cfiA, B. fragilis strains can be divided into distinct groups: division I (cepA-positive, cfiA-negative) and division II (cepA-negative, cfiA-positive) [12]. However, the presence of cfiA usually does not confer carbapenem resistance, but the upstream presence of an insertion sequence element harboring a promoter with high activity is necessary [14].

Information on the distribution of rare or newly described β-lactam resistance genes in B. fragilis group species is limited, which prompted us to assess the prevalence of these genes. Thus, we aimed to detect five infrequent β-lactam resistance genes, namely blaHGD1, blaOXA347, cblA, crxA, and pbbA, in B. fragilis group strains isolated from normal microbiota and clinical specimens. Moreover, the lack of knowledge on the phenotypes and genetic background of some of these resistance genes also led us to investigate these genes via various techniques.