Evaluation of antibiotic susceptibility of Bacteroides, Prevotella and Fusobacterium species isolated from patients of the N. N. Blokhin Cancer Research Center, Moscow, Russia

• The 122 gram-negative anaerobic rods were isolated from cancer patients.
• All isolates were identified to the species level by MALDI-TOF MS.
• Three B. fragilis isolates proved to be multidrug-resistant.
• Three strains of Prevotella showed reduced susceptibility to metronidazole.

In total 122 non-duplicate Bacteroides, Prevotella and Fusobacterium spp isolated from cancer patients between 2004 and 2014 were involved in this study. Most of the strains belonged to the B. fragilis group (55%), followed by Prevotella strains (34.4%) and Fusobacterium spp (10.6%). The species identification was carried out by matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS), and they were identified on species level with a log (score) >2.0. The most common isolates were B. fragilis, B. thetaiotaomicron, B. ovatus and B. vulgatus. Among Prevotella species, the most frequently isolated species were P. buccae, P. buccalis, P. oris, P. denticola and P. nigrescens, and most of the Fusobacterium spp. were F. nucleatum. Susceptibilities of the strains were determined by the E-test methodology. The percentage of the susceptibility of B. fragilis group isolates were: metronidazole (MIC ≤4 μg/ml), 97%; imipenem (MIC ≤2 μg/ml), 95.5%; amoxicillin/clavulanate (MIC ≤4 μg/ml), 95.5% and clindamycin (MIC ≤4 μg/ml), 77.6%. Three B. fragilis isolates proved to be multidrug-resistant (parallel resistance to imipenem, amoxicillin/clavulanate and metronidazole or clindamycin was observed). All Prevotella strains tested were susceptible to imipenem and amoxicillin/clavulanate, whereas 78.6% of the pigmented Prevotella species and 46.4% of the non-pigmented species were resistant to penicillin (MIC >0.5 μg/ml). The susceptibility to metronidazole and clindamycin were 93% and 88%, respectively. All Fusobacterium strains were sensitive to all tested antibiotics, including penicillin.