Coryneform bacteria have been increasingly recognized as human pathogens in recent years. Their emergence is related to the increased number of immunocompromised or intensively treated patients with prolonged survival rates, and also because their identification in the clinical laboratory has significantly improved. Still, an important number of organisms have not yet been identified to species level. There have been profound changes in the taxonomy of these organisms, deriving mainly from chemotaxonomic and molecular methods. This has allowed new genera and species to be recognized and previously established taxa to be re-defined. The actual medical importance and frequency of isolation of the different coryneform bacteria is barely known because of the paucity of reports considering new taxonomic advances. It seems likely, however, that a few species represent the most common clinical isolates. Corynebacterium diphtheriae has been almost eliminated from western countries but several recent outbreaks have been recognized in countries of the former Soviet Union, and systemic diseases caused by non-toxigenic strains are increasingly reported. Other important species within this genus include: C. urealyticum, C. jeikeium, the newly recognized C. amycolatum, C. striatum, C. minutissimum, and C. pseudodiphtheriticum. C. xerosis (as presently defined) is rarely isolated from clinical samples. A great variety of other Corynebacterium species have also been isolated from clinical samples, but for most of them clinical information is lacking. Other medically important genera of coryneform bacteria are Actinomyces, Brevibacterium, Rhodococcus, Rothia and Turicella. Other genera (some of them newly defined) and Centers for Disease Control and Prevention (CDC) coryneform groups have been isolated from clinical samples, but usually only microbiological and taxonomic studies have been reported. There are few data supporting therapeutic options for infections caused by coryneform bacteria other than glycopeptides, resistance to which has been described in only a few isolates. Standardized methods for evaluating the in-vitro activity of antimicrobial agents are not available, although several studies, reported during this decade, show promising results from the more convenient methods for susceptibility testing of these organisms. Clinical studies are needed to define breakpoints for clinical categorization of in-vitro data. Clinical and microbiological collaboration is essential in order to define the actual medical importance of coryneform bacteria.
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