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
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