Phylum Proteobacteria

We start our survey of the Bacteria with the Proteobacteria. This is by far the biggest single phylum, and occupies the whole of volume 2 in the second edition of Bergey. The size of the group is matched by its diversity, both morphological and physiological; most forms of metabolism are represented, and the wide range of morphological forms gives rise to the group's name. (Proteus was a mythological Greek god who was able to assume many different forms.) The reason such a diverse range of organisms

Table 7.2 Phyla of domain Bacteria

Phylum

Aquificae

Phylum

Thermotogae

Phylum

Thermodesulfobacteria

Phylum

'Deinococcus-Thermus' *

Phylum

Chrysiogenetes

Phylum

Chloroflexi

Phylum

Thermomicrobia

Phylum

Nitrospira

Phylum

Deferribacteres

Phylum

Cyanobacteria

Phylum

Chlorobi

Phylum

Proteobacteria

Phylum

Firmicutes

Phylum

Actinobacteria

Phylum

Planctomycetes

Phylum

Chlamydiae

Phylum

Spirochaetes

Phylum

Fibrobacteres

Phylum

Acidobacteria

Phylum

Bacteroidetes

Phylum

Fusobacteria

Phylum

Verrucomicrobia

Phylum

Dictyoglomi

* This Phylum has not yet been assigned a formal name.

Those phyla discussed in the text are shown in bold print.

* This Phylum has not yet been assigned a formal name.

Those phyla discussed in the text are shown in bold print.

have been assigned to a single taxonomic grouping is that their 16S rRNA indicates a common ancestor (thought to be photosynthetic, though few members now retain this ability). At the time of writing more than 460 genera and 1600 species had been identified, all of them Gram-negative and representing almost half of all accepted bacterial genera. These include many of the best known Gram-negative bacteria of medical, industrial and agricultural importance. For taxonomic purposes, the Proteobacteria have been divided into five classes reflecting their presumed lines of descent and termed the Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria and Epsilonproteobacteria (Figure 7.4). It should be stressed that because classification is based on molecular relatedness rather than shared phenotypic traits, few if any morphological or physiological properties can be said to be characteristic of all members of each class. Equally, organisms united by a particular feature may be found in more than one of the proteobacterial classes, for example nitrifying bacteria are to be found in the a, / and 7 Protobacteria. For this reason, in the following paragraphs we describe the Proteobacteria in terms of their phenotypic characteristics rather than attempt to group them phylogenetically.

Photosynthetic Proteobacteria

The purple sulphur and purple non-sulphur bacteria are the only members of the Pro-teobacteria to have retained the photosynthetic ability of their presumed ancestor. The type of photosynthesis they carry out, however, is quite distinct from that carried out by plants, algae and cyanobacteria (see later in this chapter), differing in two important respects:

• it is anoxygenic - no oxygen is produced by the process

• it utilizes bacteriochlorophyll a and/or b, which have different absorbance properties to chlorophylls a and b.

Like organisms that carry out green photosynthesis, however, they incorporate CO2 into carbohydrate by means of the Calvin cycle (see Chapter 6). All are at least facultatively a (3 y 8 e

Figure 7.4 The phylogenetic relationships of the Proteobacteria, based on 16S rRNA sequences

anaerobic, and are typically found in sediments of stagnant lakes and salt marsh pools, where they may form extensive coloured blooms. Because the absorption spectrum of bacteriochlorophylls lies mostly in the infrared part of the spectrum, they are able to utilise light energy that penetrates beyond the surface layers of water.

The coloration, ranging from orange/brown to purple, is due to the presence of carotenoid pigments such as lycopene and spirillixanthin, which mask the blue/green colour of the bacteriochlorophylls. The photosynthetic pigments are located on highly folded extensions of the plasma membrane. Photosynthetic proteobacteria include rods, cocci and spiral forms.

Under anaerobic conditions, the purple sulphur bacteria typically utilise hydrogen sulphide or elemental sulphur as an electron donor for the reduction of CO2.

Many store sulphur in the form of intracellular granules. The purple sulphur bacteria all belong to the y-Protobacteria. They are typically found in surface muds, and sulphur springs, habitats that provide the right combination of light and anaerobic conditions.

Representative genera: Thiospirillum, Chromatium

The purple non-sulphur bacteria were distinguished from the above group because of their apparent inability to use H2S as an electron donor. It is now known, however, that the majority can do this, but are able to tolerate much lower concentrations in comparison with the purple sulphur bacteria. The purple non-sulphur bacteria are facultative anaerobes able to grow as photoheterotrophs, that is, with light as an energy source and a range of organic molecules such as carbohydrates and organic acids as sources of both carbon and electrons. In addition, many are able to grow aerobically as chemoheterotrophs in the absence of light. Under present classification systems, purple nonsulphur bacteria are divided between the a- and j-Proteobacteria.

Representative genera: Rhodospirillum, Rhodopseudomonas Nitrifying Proteobacteria

This group comprises aerobic Gram-negative chemolithoautotrophs that derive their energy from the oxidation of inorganic nitrogen compounds (either ammonia or nitrite), and their carbon from CO2. The nitrifying bacteria are represented in both the a- and j-Proteobacteria.

The oxidation of ammonia through to nitrate is a two-stage process, with specific bacteria carrying out each stage (ammonia to nitrite and nitrite to nitrate). This is reflected in the generic names of the bacteria, bearing the prefix Nitroso- or Nitro-according to whether they carry out the first or second reaction. Nitrifying bacteria play an essential role in the cycling of nitrogen in terrestrial, marine and freshwater habitats. Nitrite, which is toxic to many forms of life, rarely accumulates in the environment, due to the activity of the Nitrobacteria. As was the case with the purple photosynthetic bacteria, several cell forms are represented among the nitrifiers.

Representative genera: Nitrosomonas (NH4+-> NO2 )

Iron- and sulphur-oxidising Proteobacteria

Two further groups of environmentally significant chemolithoautotrophs derive their energy through the oxidation of reduced iron and sulphur respectively.

Among the sulphur oxidisers, perhaps the best studied are members of the genus Acidithiobacillus*, which includes extreme acidophiles such as A. thiooxidans that are capable of growth at a pH as low as 1.0! These may utilise sulphur in its elemental form, as H2S, metal sulphides, or other forms of reduced sulphur such as thiosulphate:

The result of all these reactions is the production of sulphuric acid and a lowering of the environmental pH. Bacteria such as these are responsible for the phenomenon of acid mine drainage. Their environmental impact is discussed in Chapter 16, whilst Chapter 17 describes their use in the extraction of valuable metals from intractable mineral ores. A particularly valuable organism in the latter context is A. ferrooxidans, due to its ability to use not only reduced sulphur compounds as energy sources, but also reduced iron (see below).

A second group of sulphur oxidisers are bacteria that exist not as single cells, but join to form filaments, the best known of which is Beggiotoa. These are typically found in sulphur springs, marine sediments and hydrothermal vents at the bottom of the sea.

Acidithiobacillus ferrooxidans is also an example of an iron oxidiser. At normal physiological pH values and in the presence of oxygen, reduced iron (iron II, Fe2+) is spontaneously oxidised to the oxidised form (iron III, Fe3+). Under very acidic conditions, the iron remains in its reduced form, unless acted on by certain bacteria. A. ferrooxidans; is an obligate aerobe able to use iron II as an energy source, converting it to iron III at an optimum pH range of around 2:

Gallionella ferruginea, on the other hand, grows around neutrality in oxygen-poor environments such as bogs and iron springs. Ferric hydroxide is excreted from the cell and deposited on a stalk-like structure projecting from, and much bigger than, the cell itself. This gives the macroscopic impression of a mass of red/brown twisted filaments.

Representative genera: Acidithiobacillus, Beggiotoa (sulphur oxidisers) Leptospirillum, Gallionella (iron oxidisers)

* Note: A. thiooxidans and A. ferrooxidans formerly belonged to the genus Thiobacillus. In 2000, several species where assigned to new genera, however you may still find them referred to by their old names.

Hydrogen-oxidising Proteobacteria

This diverse group of bacteria are united by their ability to derive energy by using hydrogen gas as a donor of electrons, and oxygen as an acceptor:

Nearly all the members of this group are facultative chemolithotrophs, i.e. they can also grow as heterotrophs, utilising organic compounds instead of CO2 as their carbon source, and indeed most grow more efficiently in this way.

Representative genera: Alcaligenes, Ralstonia

Nitrogen-fixing Proteobacteria

The a-Proteobacteria includes certain genera of nitrogen-fixing bacteria. These are able to fix (reduce) atmospheric N2 as NH4+ for subsequent incorporation into cellular materials, a process that requires a considerable input of energy in the form of ATP:

Nitrogen-fixing bacteria may be free-living in the soil (e.g. Azotobacter), or form a symbiotic relationship with cells on the root hairs of leguminous plants such as peas, beans and clover (e.g. Rhizobium). The nitrogenase responsible for the reaction (actually a complex of two enzymes) is highly sensitive to oxygen; many nitrogen fixers are anaerobes, while others have devised ways of keeping the cell interior oxygen-free. Nitrogen fixation is discussed further in Chapters 15 and 16.

Closely related to Rhizobium, but unable to fix nitrogen, are members of the genus Agrobacterium. Like Rhizobium, these enter the tissues of plants, but instead of forming a mutually beneficial association, cause cell proliferation and tumour formation. A. tumefaciens has proved to be a valuable tool in the genetic engineering of plants, and is discussed further in Chapter 12.

Representative genera: Rhizobium, Azotobacter

Nitrogen fixation is limited to a few species of bacteria and cyanob-acteria. No eucaryotes are known to have this property.

Methanotrophic Proteobacteria

In discussing the Archaea earlier in this chapter, we encountered species capable of the production of methane, a gas found widely in such diverse locations as marshes, sewage sludge and animal intestines. Certain proteobac-teria are able to utilise this methane as a carbon and energy source and are known as methanotrophs.

Methanotrophs are strict aerobes, requiring oxygen for the oxidation of methane. The methanegenerating bacteria, however, as we've seen are anaerobes; methanotrophs are consequently to be found at aerobic/anaerobic interfaces such as topsoil, where they

Methane is one of the so-called greenhouse gases, responsible for the phenomenon of global warming. Its effects would be much more pronounced if it were not for the activity of metha-notrophic bacteria.

can find both the oxygen and the methane they require. The methane is firstly oxidised to methanol, then to formaldehyde, by means of separate enzyme systems. Some of the carbon in formaldehyde is assimilated into organic cellular material, while some is further oxidised to carbon dioxide.

Bacteria able to utilise other single-carbon compounds such as methanol (CH3OH) or methylamine (CH3NH2) are termed methylotrophs. Depending on whether they possess the enzyme methane monooxyge-nase (MMO), they may also be methanotrophs.

Representative genera: Methylomonas, Methylococcus

Sulphate- and sulphur-reducing Proteobacteria

Some 20 genera of anaerobic 5-Proteobacteria reduce either elemental sulphur or oxidised forms of sulphur such as sulphate to hydrogen sulphide. Organic compounds such as pyruvate, lactate or certain fatty acids act as electron donors:

Desulfovibrio

Lactate Acetate

Sulphate- and sulphur-reducers are found in anaerobic muds and play an important role in the global sulphur cycle.

Representative genera: Desulfovibrio (sulphate), Desulfuromonas (sulphur) Enteric Proteobacteria

This is a large group of rod-shaped bacteria, mostly motile by means of peritrichous flagella, which all belong to the y-Proteobacteria. They are facultative aerobes, characterised by their ability in anaerobic conditions to carry out fermentation of glucose and other sugars to give a variety of products. The nature of these products allows division into two principal groups, the mixed acid fermenters and the butanediol fermenters (Figure 6.23). All the enteric bacteria test negative for cytochrome c oxidase (see Vibrio and related genera below). In view of their similar appearance, members of the group are distinguished from one another largely by means of their biochemical characteristics. An unknown isolate is subjected to a series of tests including its ability to utilise substrates such as lactose and citrate, convert tryptophan to indole, and hydrolyse urea. On the basis of its response to each test, a characteristic profile can be built up for the isolate, and matched against those of known species (see Table 7.3).

The most thoroughly studied of all bacteria, Escherichia coli (E. coli) is a member of this group, as are a number of important pathogens of humans such as Salmonella, Shigella and Yersinia (the causative agent of plague).

Representative genera: Escherichia, Enterobacter

The methylotroph Meth-ylophilus methylotrophus was once produced in huge quantities as a source of 'single cell protein'for use as animal feed, until the low price of alternatives such as soya and fish meal made it commercially unviable.

Table 7.3 Identification of enteric bacteria on the basis of their biochemical and other properties

Some of the tests used to identify isolates of enteric bacteria are listed below. The table on the next page indicates typical results obtained for common genera; note, however, that for many cases, the result of a test may vary for different species within a genus. The symbols + and — indicate that most or all species within a genus give a positive or negative result, whilst +/— denotes that results are more variable within a genus.

Some of the tests used to identify isolates of enteric bacteria are listed below. The table on the next page indicates typical results obtained for common genera; note, however, that for many cases, the result of a test may vary for different species within a genus. The symbols + and — indicate that most or all species within a genus give a positive or negative result, whilst +/— denotes that results are more variable within a genus.

Test

Description

Indole

Tests for ability to produce indole from the amino acid tryptophan.

Methyl Red

Acid production causes methyl red indicator to turn red.

Voges-Proskauer

Tests for ability to ferment glucose to acetoin.

Citrate utilisation

Demonstrates ability to utilise citrate as sole carbon source.

Urease

Demonstrates presence of the enzyme urease by detecting rise in pH due to urea being converted to ammonia

and COi.

Gas from sugars

Production of gas from sugars such as glucose is demonstrated by collection in a Durham tube (a small inverted

tube placed in a liquid medium).

HiS production

Production of HiS from sulphate reduction or from sulphur-containing amino acids is demonstrated by the

formation of black iron sulphide in an iron-rich medium.

Ornithine decarboxylase

Growth on medium enriched in ornithine leads to pH change when enzyme is present.

Motility

Diffusion through soft agar demonstrates cellular movement.

Gelatin liquefaction

Demonstrates presence of proteolytic enzymes capable of liquefying a medium containing gelatin.

% age GC

Nucleotide composition determined by melting point measurements.

(Continued )

Table 7.3 Identification of enteric bacteria on the basis of their biochemical properties (Continued)

Escherichia

Salmonella

Shigella

Citrobacter

Proteus

Serratia

Klebsiella

Enterobacter

Eriuinia

Indole

+

+/-

+/-

+/-

+/-

Methyl Red

+

+

+

+

+

+/-

+

+/-

+

Voges-Proskauer

+/-

+

+

+

+

Citrate utilisation

+/-

+

+/-

+

+

+

Urease

+

+

+/-

Gas from glucose

+

+

+

+

+/-

+

+

HiS production

-

-

-

+/-

+

-

-

-

+

Ornithine decarboxylase

+

+

+/-

+

+/-

-

+/-

+

-

Motility

+

+

-

+

+

+

-

+

+

Gelatin liquefaction

-

-

-

-

+

+

-

+/-

+/-

% GC

48-52

50-53

49-53

50-52

38-41

53-59

53-58

52-60

50-58

Table 7.4 Differentiation between enteric bacteria, vibrios and pseudomonads

Enteric bacteria

Vibrios Pseudomonads

Oxidase test

-ve

+ve +ve

Glucose fermentation

+ve

+ve —ve

Flagella

Peritrichous

Polar* Polar

*When grown on solid media, some Nibrio species also develop lateral flagella, a unique arrangement termed mixed flagellation.

*When grown on solid media, some Nibrio species also develop lateral flagella, a unique arrangement termed mixed flagellation.

Vibrio and related genera

A few other genera, including Vibrio and Aeromonas, are also facultative anaerobes able to carry out the fermentative reactions described above, but are differentiated from the enteric bacteria by being oxidase-positive (Table 7.4). Vibrio and Photobacterium both include examples of marine bioluminescent species; these are widely found both in seawater and associated with fish and other marine life. The luminescence, which requires the presence of oxygen, is due to an oxidation reaction carried out by the enzyme luciferase.

Vibrio cholerae is the causative agent of cholera, a debilitating and often fatal form of acute diarrhoea transmitted in faecally contaminated water. It remains a major killer in many third world countries. Several species of Vibrio, including V cholerae, have been shown to possess two circular chromosomes instead of the usual one.

Representative genera: Vibrio, Aeromonas

The Pseudomonads

Members of this group of proteobacteria, the most important genus of which is Pseudomonas, are straight or curved rods with polar flagella. They are chemo-heterotrophs that generally utilise the Entner-Doudoroff pathway (see Chapter 6) rather than glycolysis for the oxidation of hexoses. They are differentiated from the enteric bacteria (Table 7.4) by being oxidase-positive and incapable of fermentation. A characteristic of many pseudomonads is the ability to utilise an extremely wide range of organic compounds (maybe over 100!) for carbon and energy, something that makes them very important in the recycling of carbon in the environment. Several species are significant pathogens of animals and plants; Pseudomonas aeruginosa is an effective coloniser of wounds and burns in humans, while P. syringae causes chlorosis (yellowing of leaves) in a range of plants. Because of their ability to grow at low temperatures, a number of pseudomonads are important in the spoilage of food.

Bioluminescence is the production of light by living systems

Burkholderia cepacia can utilise an exceptionally wide range of organic carbon sources, including sugars, carbo-xylic acids, alcohols, amino acids, aromatic compounds and amines, to name but a few!

Although most species carry out aerobic respiration with oxygen as the terminal electron acceptor, a few are capable of substituting nitrate (anaerobic respiration, see Chapter 6).

Representative genera: Pseudomonas, Burkholderia Acetic acid bacteria

Acetobacter and Gluconobacter are two genera of the a-Proteobacteria that convert ethanol into acetic acid, a highly significant reaction in the food and drink industries (see Chapter 17). Both genera are strict aerobes, but unlike Acetobacter, which can oxidise the acetic acid right through to carbon dioxide and water, Gluconobacter lacks all the enzymes of the TCA cycle, and cannot oxidise it further.

Acetobacter species also have the ability, rare in bacteria, to synthesise cellulose; the cells become surrounded by a mass of extracellular fibrils, forming a pellicle at the surface of an unshaken liquid culture.

Representative genera: Acetobacter, Gluconobacter Stalked and budding Proteobacteria

The members of this group of aquatic Proteobacteria differ noticeably in their appearance from typical bacteria by their possession of extracellular extensions known as prosthecae; these take a variety of forms but are always narrower than the cell itself. They are true extensions of the cell, containing cytoplasm, rather than completely extracellular appendages.

In the stalked bacteria such as Caulobacter (Figure 7.5), the prostheca serves both as a means of attaching the cell to its substratum, and to enhance nutrient absorption by

Figure 7.5 The life cycle of Caulobacter, a stalked bacterium. The stalked 'mother' cell attaches to a surface by means of a holdfast (a). It grows in length and develops a flagellum (b), before undergoing binary fission. The flagellated swarmer cell swims away (c), and on reaching a suitable substratum, loses its flagellum and develops a stalk or prostheca (d). Reproduced by permission of Dr James Brown, North Carolina State University

Figure 7.5 The life cycle of Caulobacter, a stalked bacterium. The stalked 'mother' cell attaches to a surface by means of a holdfast (a). It grows in length and develops a flagellum (b), before undergoing binary fission. The flagellated swarmer cell swims away (c), and on reaching a suitable substratum, loses its flagellum and develops a stalk or prostheca (d). Reproduced by permission of Dr James Brown, North Carolina State University increasing the surface area-to-volume ratio of the cell. The latter enables such bacteria to live in waters containing extremely low levels of nutrient. Caulobacter lives part of its life cycle as a free-swimming swarmer cell with no prostheca but instead a flagellum for mobility.

The iron oxidiser Gallionella (see Nitrifying Proteobacteria above) may be regarded as a stalked bacterium, however it is not truly prosthecate, as its stalk does not contain cytoplasm.

In the budding bacteria, the prostheca is involved in a distinctive form of reproduction, in which two cells of unequal size are produced (c.f. typical binary fission, which results in two identical daughter cells). The daughter cell buds off from the mother cell, either directly, or as Hyphomicrobium spp. at the end of a hypha (stalk) (Figure 7.6). Once detached, the daughter cell grows to full size and eventually produces its own buds. Hyphomicrobium is a methanotroph and a methylotroph, so it also belongs to the methanotrophs described earlier.

Proteobacterial Methylotroph
Figure 7.6 The budding bacteria: reproduction in Hyphomicrobium. Before reproduction takes place, the vegetative cell develops a stalk or hypha, at the end of which a bud develops. This produces a flagellum, and separates to form a motile swarmer cell

In some bacteria, more than one prostheca is found per cell; these polyprosthecate forms include the genus Stella, whose name ('a star') derives from its six symmetrically arranged buds.

Representative genera: Caulobacter, Hyphomicrobium Sheathed Proteobacteria

Some genera of j Proteobacteria exist as chains of cells surrounded by a tube-like sheath, made up of a carbohydrate/protein/lipid complex. In some cases, the sheath contains deposits of manganese oxide or ferric hydroxide, which may be the product of chemical or biological oxidation. Empty sheaths encrusted with oxides may remain long after the bacterial cells have died off or been released. As with the stalked bacteria (see above) the sheath helps in the absorption of nutrients, and may also offer protection against predators.

The sheathed bacteria have a relatively complex life cycle. They live in flowing water, and attach with one end of the chain to, for example, a plant or rock. Free-swimming single flagellated cells are released from the distal end and settle at another location, where a new chain and sheath are formed (Figure 7.7).

Figure 7.7 The sheathed bacteria. The life cycle of Sphaerotilus. Free-swimming swarmer cells settle on an appropriate substratum and give rise to long filaments contained within a sheath. New locations become colonised when flagellated cells are released into the water to complete the cycle

Proteobacterial Methylotroph

Figure 7.7 The sheathed bacteria. The life cycle of Sphaerotilus. Free-swimming swarmer cells settle on an appropriate substratum and give rise to long filaments contained within a sheath. New locations become colonised when flagellated cells are released into the water to complete the cycle

Sphaerotilus forms thick 'streamers' in polluted water, and is a familiar sight around sewage outlets.

Representative genera: Sphaerotilus, Leptothrix

Predatory Proteobacteria

Bdellovibrio is a unique genus belonging to the 5-Proteobacteria. It is a very small comma-shaped bacterium, which actually attacks and lives inside other Gram-negative bacteria (Figure 7.8). Powered by its flag-ellum, it collides with its prey at high speed and penetrates even thick cell walls by a combination of enzyme secretion and mechanical boring. It takes up residence in the periplasmic space, between the plasma membrane and cell wall. The host's nucleic acid and protein synthesis cease, and its macromolecules are degraded, providing nutrients for the invader, which grows into a long

The recently-sequenced genome of Bdellovibrio bacterivorans has been shown to encode a huge number of lytic enzymes. Its ability to metabolise amino acids, however, is limited, necessitating its unusual mode of existence.

Swarmer cells

Swarmer cells

Host bacterium e.g Pseudomonas

Escherichia

Cell wall

Spiral filament undergoes septation into several flagellated daughter cells, which are released when host cell is lysed

Host bacterium e.g Pseudomonas

Escherichia

Cell wall

Spiral filament undergoes septation into several flagellated daughter cells, which are released when host cell is lysed

'Attack phase' cell attaches to host. High-speed rotation allows penetration of cell wall.

Bdellovibrio loses flagellum and becomes established in periplasmic space

Figure 7.8 The life cycle of Bdellovibrio, a bacterial predator. Once Bdellovibrio has taken up residence in the periplasmic space of its host, it loses its flagellum and becomes non-motile. In nutrient-rich environments, Bdellovibrio is also capable of independent growth

'Attack phase' cell attaches to host. High-speed rotation allows penetration of cell wall.

Nutrients from host used for elongation into spiral filament

Bdellovibrio loses flagellum and becomes established in periplasmic space

Figure 7.8 The life cycle of Bdellovibrio, a bacterial predator. Once Bdellovibrio has taken up residence in the periplasmic space of its host, it loses its flagellum and becomes non-motile. In nutrient-rich environments, Bdellovibrio is also capable of independent growth

Myxospores

Myxospores

Fruiting body b)

Cells form aggregates

Fruiting body

Vegetative growth

Figure 7.9 The Myxobacteria: a complex bacterial life cycle. When nutrients are in plentiful supply, myxobacteria divide by binary fission (a). On depletion of nutrients, they form aggregates of cells, which leads to the formation of a fruiting body (b). Within the fruiting body, some cells form myxospores, enclosed within a sporangium (c). Myxospores remain dormant until environmental conditions are favourable, then germinate into vegetative

Germinatior

Germinatior

Vegetative growth a)

Figure 7.9 The Myxobacteria: a complex bacterial life cycle. When nutrients are in plentiful supply, myxobacteria divide by binary fission (a). On depletion of nutrients, they form aggregates of cells, which leads to the formation of a fruiting body (b). Within the fruiting body, some cells form myxospores, enclosed within a sporangium (c). Myxospores remain dormant until environmental conditions are favourable, then germinate into vegetative helical cell. This eventually divides into several motile progeny cells, which are then released.

Representative genus: Bdellovibrio

Another group of bacteria that may be regarded as predatory are the Myxobacteria (Figure 7.9). These are rod-shaped bacteria lacking flagella, which yet are motile by gliding along a solid surface, aided by the excretion of extracellular polysaccharides. For this reason they are sometimes referred to as the gliding bacteria. They are heterotrophs, typically requiring complex organic nutrients, which they obtain by the lysis of other types of bacteria. Thus, unlike Bdellovibrio, they digest their prey before they ingest it. When a rich supply of nutrients is not available, many thousands of cells may aggregate to form fruiting bodies, inside which myxospores develop. These are able to resist drought and lack of nutrients for many years. Myxobacteria exhibit the most complex life cycles of any procaryote so far studied.

Representative genera: Myxococcus, Chondromyces Spirilla

Collected together under this heading are several genera of aerobic (mostly mi-croaerophilic) spiral-shaped bacteria with polar flagella. These include free-living, symbiotic and parasitic types.

Spirilla such as Aquaspirillum and Magnetospirillum contain magnetosomes, intracellular particles of iron oxide (magnetite, Fe3O4). Such magnetotactic bacteria have the remarkable ability to orientate themselves with respect to the earth's magnetic field (magnetotaxis).

Two important pathogens of humans are included in the spirilla; Campylobacter jejuni is responsible for foodborne gastroenteritis, while Helicobacter pylori has in recent times been identified as the cause of many cases of peptic ulcers.

Representative genera: Magnetospirillum, Campylobacter Rickettsia

This group comprises arthropod-borne intracellular parasites of vertebrates, and includes the causative agents of human diseases such as typhus and Rocky Mountain spotted fever. The bacteria are taken up by host phagocytic cells, where they multiply and eventually cause lysis.

The Rickettsia are aerobic organotrophs, but some possess an unusual mode of energy metabolism, only being able to oxidise intermediate metabolites such as glutamate and succinate, which they obtain from their host. Rickettsia and Coxiella, the two main genera, are not closely related phylogenetically and are placed in the a- and y-Proteobacteria, respectively

Representative genera: Rickettsia, Coxiella Neisseria and related Proteobacteria

All members of this loose collection of bacteria are aerobic non-motile cocci, typically seen as pairs, with flattened sides where they join. Some however only assume this morphology during stationary growth phase. Many are found in warm-blooded animals, and some species are pathogenic. The genus Neisseria includes species responsible for gonorrhoea and meningitis in humans.

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