Phylum
Actinobacteria: The high GC Gram-positive bacteria
The high GC gram-positive bacteria make up volume 4
of the second edition of Bergey. The
actinomycetes are aerobic, filamentous bacteria that form branching mycelia superficially similar to those
of the Fungi. Remember, however, that the actinomycetes are procaryotes and the
fungi are eucaryotes, so the mycelia formed by the former are appreciably
smaller. In some cases, the mycelium extends clear of thesubstratum and bears
asexual conidiospores at the hy-phal
tips. These are produced by the formation of cross-walls and pinching off of
spores, which are often coloured. The best known actinomycete genus is Strepto-myces, which contains some 500
species, all with a char-acteristically high GC content (69–73 per cent). Strepto-myces are very prevalent in
soil, where they saprobicallydegrade a wide range of complex organic substrates
by means of extracellular enzymes. Indeed, the characteris-tic musty smell of
many soils is due to the production of a volatile organic compound called geosmin. A highproportion of
therapeutically useful antibiotics derive from Streptomyces species, in-cluding well-known examples such as
streptomycin, erythromycin and tetracycline.
Most actinomycetes, including Streptomyces, are aerobic; however, members of the genus Actinomyces are facultative anaerobes.
The coryneform bacteria are morphologically half way
between single celled bacilli and the branching filamentous actinomycetes. They
are rods that show rudimentary branching, giving rise to characteristic ‘V’ and
‘Y’ shapes. Among the genera in this group are Corynebacterium, Mycobacterium,
Propionibacterium and Nocardia.
Corynebacterium
species are common in soil, and are also found in the mouths of
avariety of animals. C. diphtheriae
is the causative agent of diphtheria; it only becomes pathogenic when it has
been infected by a bacteriophage that carries the gene for the diphtheria
exotoxin.
Members of the genus Mycobacterium are characterised by their unusual cell wall
structure; they include unusual complex lipids calledmycolic acids. This causes
the cells to be positive for the acid-fast
staining technique, a useful way of identi-fying the presence of these bacteria.
Mycobacteria are rod shaped, sometimes becoming filamentous; when fil-aments
are formed, propagation is by means of fragmen-tation. M. leprae and M. tuberculosis
cause, respectively, leprosy and tuberculosis in humans.
Propionibacterium
species ferment lactic acid to propionic acid. Some species
areimportant in the production of Swiss cheeses, whilst P. acnesis the main cause of acne in humans.
Representative genera: Corynebacterium, Propionibacterium
Related Topics
Privacy Policy, Terms and Conditions, DMCA Policy and Compliant
Copyright © 2018-2024 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.