Phylum Actinobacteria The high GC Grampositive bacteria

It has been suggested that camels are able to locate water sources in the desert by detecting the smell of geosmins given off by Strepto-myces spp. in the damp earth!

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 (Chapter 8). 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 the substratum 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 characteristically high GC content (69-73 per cent). Strepto-myces are very prevalent in soil, where they saprobically degrade a wide range of complex organic substrates by means of extracellular enzymes. Indeed, the characteristic musty smell of many soils is due to the production of a volatile organic compound called geosmin. A high proportion of therapeutically useful antibiotics derive from Streptomyces species, including well-known examples such as streptomycin, erythromycin and tetracycline (see Chapter 14).

Most actinomycetes, including Streptomyces, are aerobic; however, members of the genus Actinomyces are facultative anaerobes.

Representative genus: Streptomyces

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 a variety 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 called mycolic acids. This causes the cells to be positive for the acid-fast staining technique, a useful way of identifying the presence of these bacteria. Mycobacteria are rod shaped, sometimes becoming filamentous; when filaments are formed, propagation is by means of fragmentation. M. leprae and M. tuberculosis cause, respectively, leprosy and tuberculosis in humans.

The acid-fast test assesses the ability of an organism to retain hot carbol fuchsin stain when rinsed with acidic alcohol.

Propionibacterium species ferment lactic acid to propionic acid. Some species are important in the production of Swiss cheeses, whilst P. acnes is the main cause of acne in humans.

Representative genera: Corynebacterium, Propionibacterium

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