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Chapter: Essential Microbiology: Procaryote Diversity

Phylum Actinobacteria: The high GC Gram-positive bacteria

The high GC gram-positive bacteria make up volume 4 of the second edition of Bergey.

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


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