The
Gram-positive bacteria: phylum Firmicutes and phylum Actinobacteria
The Gram-positive bacteria are divided into two large
phyla, the Firmicutes and the Actinobacteria. Some 2500 species are known, but
a substantial proportion of these belong to just a handful of genera.
Gram-positive bacteria mostly have a chemoheterotrophic mode of nutrition and
include among their number several important human pathogens, as well as
industrially significant forms.
The base composition of an organism’s DNA can be
expressed as the percentage of cytosine and guanine residues (per cent GC
content); the technique is used widely in microbial taxonomy, and the
Gram-positive bacteria are divided into those whose GC content is significantly
over or under 50 per cent. It is convenient to consider groupings within the
high GC and low GC forms as follows:
Phylum Firmicutes (low GC): spore-forming, non-spore forming, mycoplasma
Phylum Actinobacteria (high GC): actinomycetes, coryneform bacteria
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