Six independent isolates of an unusual black-pigmented Corynebacterium species (strains CN-1, CN-2, CN-3415, W70124, 91-0032, and 92-0360) were recovered from the human female urogenital tract. Four of the six source patients had complications of pregnancy, including spontaneous abortion, preterm labor, and low amniotic fluid volume at the time of the pathogen isolation. One isolate was recovered from a vaginal ulcer. All six strains yielded black-pigmented colonies on sheep blood agar, chocolate agar, and colistin-nalidixic acid agar after 24 to 48 h of incubation at 35°C. The dry, adherent colonies pitted the agar surface. The cells were coccobacillary to rod-shaped, catalase positive, nonmotile, and nonlipophilic. Only five of six isolates were available for characterization. Biochemical and chemotaxonomic studies revealed that the strains belong to the genus Corynebacterium but differ from known corynebacterial species. Comparative 16S rRNA gene sequence analysis showed that the strains are closely related and form a new subline within the genus Corynebacterium. We propose the name Corynebacterium nigricans sp. nov. for this group of coryneforms. The type strain of Corynebacterium nigricans is CN-1. It is deposited in the American Type Culture Collection (assigned strain number ATCC 700975) and in the Institute Pasteur collection (assigned strain number CIP 107346).The female urogenital tract is a natural habitat for a multitude of commensal microbes, including diphtheroids. In many instances, these microbes and their metabolites serve as barriers against colonization by harmful pathogens that may cause different diseases. The reported commensal organisms of the female urogenital tract are species of Lactobacillus, Staphylococcus, Streptococcus, Neisseria, Mycoplasma, Ureaplasma, enteric organisms, Acinetobacter, Capnocytophaga, Enterococcus, Bacteroides, Porphyomonas, Prevotella, Fusobacterium, Mobiluncus, Peptostreptococcus, Propionibacterium, Actinomyces, Bifidobacterium, Candida, and coryneform bacteria (8). Like other resident microbes of the human body, some of the vaginal microbiota also are opportunistic pathogens, which become pathogenic when the host immune status is compromised, suppressed, or under immunologic stress. A number of bacterial species are associated with urogenital-related diseases, and among the Corynebacterium species, C. lipophiloflavum, C. amycolatum, C. glucuronolyticum, C. riegelii, and C. urealyticum have been implicated (4, 6-8). The CDC coryneforms group 4 and C. lipophiloflavum are reported to cause urinary tract infections and bacterial vaginosis, respectively (5, 9).Through the use of nucleic acid-based detection and characterization methods, such as 16S rRNA gene sequence analysis, many new pathogenic organisms are continually being identified, characterized, and linked to diseases of known and unknown etiology. This approach has been successfully applied in our laboratory to identify and characterize a number of fastidious microorganisms (12,(15)(16)(17)). An unusual ...