The
nomenclature of the group comprising the branching Gram positive rods is complicated.
Considerable morphological diversity is not only seen among genera but also
among strains of the same taxon.
Most
actinomycetes are typically Gram positive, filamentous, partially acid-fast,
branched bacteria that have many microbiological characteristics in common with
members of the genera Mycobacterium
and Corynebacterium. The major groups
of the order Actinomycetales, are actinoplanetes, maduromycetes, nocardioform
actinomycetes, and streptomycetes.
Although
the aerobic actinomycetes are infrequently encountered in clinical practice,
they are important potential causes of serious human and animal infections.
The
pathogenic genera within the aerobic actinomycetes are Nocardia, Actinomadura, Streptomyces, Rhodococcus, Gordonia, Tsukamurella and Tropheryma whipplei.
The
genus Nocardia currently contains 100
species that have been characterised by phenotypic and molecular methods, and
over 30 species are associated with humans. A few of these species have also
been recently assigned to other genera. It comprises several species that are
known to be unusual causes of a wide spectrum of clinical diseases in both
humans and animals. While the majority of nocardial infections have been
attributed to Nocardia asteroides,
other pathogenic Nocardia species that have been described include Nocardia brasiliensis, Nocardia
otitidiscaviarum, and Nocardia
transvalensis. In a recent taxonomic revision of the N. asteroides taxon, two new species- N. farcinica and Nocardia nova were separated from it.
by Tim Sandle
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