Seventy-four strains representing the four species of the genus Dialister were isolated from various clinical samples. Dialister pneumosintes and Dialister micraerophilus were the two mainly encountered species. Fifty-five isolates were tested against 14 antimicrobial agents. Decreased susceptibilities to piperacillin, metronidazole, macrolides, fluoroquinolones, and rifampin were demonstrated. The clinical impact of these decreased susceptibilities remains to be investigated but should prompt microbiologists to perform antimicrobial susceptibility testing for clinically important Dialister spp.Four species are currently described in the genus Dialister: Dialister pneumosintes, Dialister invisus, Dialister micraerophilus, and Dialister propionicifaciens, the last three being described since 2003 (5, 11). They are small, anaerobic or microaerophilic gram-negative coccobacilli that grow as small, circular, tiny, and transparent colonies on Columbia blood agar, making their recovery in mixed anaerobic cultures relatively difficult. Besides, strains may be difficult to distinguish from tiny, gram-negative anaerobic cocci. Usually, Dialister spp. could be distinguished from members of the genus Veillonella by both the absence of nitrate reduction and the pattern of susceptibility to special-potency disks. Indeed, they display susceptibility to kanamycin (500 g), bile (1 mg), and metronidazole (50 g) and resistance to vancomycin (5 g) and colistin (10 g) disks, whereas Veillonella species are susceptible to the colistin disk, except for Veillonella montpellierensis and Veillonella ratti (10, 11). However, due to their biochemical characteristics (i.e., asaccharolytic and nonreactive in conventional biochemical tests), identification of members of the genus Dialister often requires molecular methods such as 16S rRNA gene sequencing (11). The involvement of Dialister spp. in human infections is now clearly established. D. pneu-
