Phylum Firmicutes: The low GC Gram-positive bacteria

Phylum Firmicutes: The low GC Gram-positive bacteria

The low GC Gram-positive bacteria form volume 3 of the second edition of Bergey. The spore-forming Gram-positive bacteria include two large genera, Clostridium and Bacillus. Although not particularly close in phylogenetic terms, they are both capableof propagation by endospores.Clostridium species are obligate anaerobes, and common inhabitants of soil. Sug-ars are fermented to various end-products such as butyric acid, acetone or butanol. Lacking an electron transport system, they obtain all their ATP from substrate-levelphosphorylation.

Several species of Clostridium are serious human pathogens including C. botulinum (botulism) and C.tetani (tetanus). C. perfringens causes gas gangrene, andif ingested, can also result in gastroenteritis. All these conditions are due to the production of bacterial exo-toxins. The resistance of spores to heating is thus highly relevant both in medicine and in the food industry. Re-lated to Clostridiumare the heliobacteria, two genera of anaerobic photoheterotrophic rods, some of which produce endospores. They are the only known photo-synthetic Gram-positive bacteria.

Bacillus species are aerobes or facultative anaerobes.They are chemoheterotrophs and usually motile by means of peritrichous flagella. Only a few species of Bacillus are pathogenic in humans, notably B. anthracis,the causative agent of anthrax. This is seen by many as a potential agent of bioterrorism, and here again the relative indestructibility of its spores is a crucial factor. Other species, conversely, are positively beneficial to hu-mans; antibiotics such as bacitracin and polymixin areproduced by Bacillus species, whilst the toxin from B. thuringiensis has been used as a natural insecticide.

Representative genera: Bacillus, Clostridium

The non-spore-forming low GC Gram-positive bacteria include a number of medically and industrially significant genera, a few of which are discussed below.

The lactic acid bacteria are a taxonomically diverse group containing both rods (Lac-tobacillus) and cocci (Streptococcus, Lactococcus), all characterised by their fermenta-tive metabolism with lactic acid as end-product. Although they are able to tolerate oxygen, these bacteria do not use it in respiration. They are said to be aerotolerant. Like the clostridia, they lack cytochromes, and are there-fore unable to carry out electron transport phosphory-lation. The lactic acid bacteria have limited synthetic ca-pabilities, so they are dependent on a supply of nutrients such as amino acids, purines/pyrimidines and vitamins. There has been growing interest in recent years in the use of certain lactic acid bacteria as probiotics.

The genus Streptococcus remains a large one, although some members have been assigned to new genera in recent years, e.g. Enterococcus, Lactococcus. Streptococciare classified in a number of ways on the basis of phenotypic characteristics, but these do not correspond to phylogenetic relationships. Many species produce haemolysis when grown on blood agar, due to the pro-duction of toxins called haemolysins. In α-haemolysis, haemoglobin is reduced to methaemoglobin, resulting in a partial clearance of the medium and a characteristic green colour. β-Haemolysis causes a complete lysis of thered blood cells, leaving an area of clearing in the agar. A few species are non-haemolytic. Streptococci are also classified on the basis of carbohydrate antigens found  in the cell wall; this system, which assigns each organism to a lettered group, is named after its devisor, Rebecca Lancefield.

Pathogenic species of Streptococcus include S. pyogenes (‘strep’ sore throat, as well as the more serious rheumatic fever), S. pneumoniae (pneumococcal pneumonia) and S. mutans (tooth decay). Cells of Streptococcus exist mostly in chains, but in S. pneu-moniae they are characteristically paired.

Lactobacillus is used very widely in the food and drink industry in the production ofsuch diverse foodstuffs as yoghurt, cheeses, pickled foods (e.g. sauerkraut) and certain beers.

The cells of staphylococci occur in irregular bunches rather than ordered chains. They also produce lactic acid but can additionally carry out aerobic respiration involving cytochromes, and lack the complex nutritional requirements of the lactic acid bacteria. They are resistant to drying and able to tolerate relatively high concentrations of salt. These properties allow Staphylococcus aureus to be a normal inhabitant of the human skin, where it can sometimes give rise to dermatological conditions such as acne, boils and impetigo. It is also found in the respiratory tract of many healthy individuals, to whom it poses no threat, but in people whose immune system has been in some way compromised, it can cause serious respiratory infections. S. aureus can also cause a type of food poisoning and is the causative agent of toxic shock syndrome. Widespread antibiotic use has been largely responsible for the development of resistant forms of S. aureus, which have become ubiquitous inhabitants of hospitals (methicillin-resistant Staphylococcus aureus: MRSA).

Representative genera: Streptococcus, Staphylococcus

The Mycoplasma (Class Mollicutes) lack a cell wall and hence have a fluid shape (pleomorphic). Since the Gram test is based on the peptidoglycan content of a cell wall, why are these organisms grouped with the Gram-positive bacteria? The answer is that although they do not give a positive Gram test, they are clearly related at the genetic level to other members of the low GC Gram-positive group. The membranes of mycoplasma contain sterols; these help in resisting osmotic lysis, and are often essential as a growth requirement. Saprophytic, commensal and parasitic forms are known, and some species are associated with respiratory diseases in animals. Mycoplasma frequently occur as contaminants in the culture of animal cells, because their small size allows them to pass through filters, and they are resistant to antibiotics directed at cell wall synthesis.

Members of the Mycoplasma are among the smallest of all known cells and have some of the smallest genomes (just over half a million base pairs).

Representative genera: Mycoplasma, Ureoplasma