6 Special Groups of Gram-Negative Bacteria

List of six special groups of bacteria:- 1. Anoxygenic Photosynthetic Bacteria 2. Oxygenic Photosynthetic Bacteria (Cyanobacteria) 3. The Methylotrophic Bacteria 4. The Nitrifying Bacteria 5. The Stalked and Prosthecate Bacteria 6. The Cyanomorphic Bacteria.

Gram-Negative Bacteria: Group # 1. Anoxygenic Photosynthetic Bacteria:

In the first edition of Bergey’s Manual of Systematic Bacteriology (1984), the anoxygenic photosynthetic bacteria were placed in a single class, called the An-oxy-photo-bacteria under the Gram-negative bacteria, Gracilicutes. But due to their phylogeneitc differences revealed by r-RNA sequence analysis, they have been scattered in different phyla and have been placed with non-photosynthetic relatives in the second edition of the Manual (2001).

The new taxonomic positions are summarized in Table 4.5:

The anoxygenic photosynthetic bacteria are morphologically diverse. They may be spherical (Thiocystis, Thiocapsa), ovoid (Chromatium, Rhodomicrobium), strongly curved to nearly circular (Rhodocyclus), spiral (Rhodospirillum), prosthecate (Rhodomicrobium) or even filamentous (Chloroflexus).

Morphological characteristics and some other features of the four major groups (shown in Table 4.5) are shown in Table 4.6:

 

 

In culture or in thick cell suspension, the bacteria are coloured in various shades of purple, violet, red to olive green and brown, because they contain various bacteriochlorophylls and carotenoids. The proportion of these two classes of pigment determines the shade of colour.

The bacteriochlorophylls, their absorption maxima and occurrence are given in Table 4.7:

 

 

Morphological features of some representative genera of anoxygenic photosynthetic bacteria are presented in Fig. 4.20:

 

 

The anoxygenic phototrophic bacteria are, in general, anaerobic. Only some of the non-sulfur purple or green bacteria (Chloroflexus) can grow in dark as heterotrophs. Under such conditions, they are colourless, because oxygen inhibits bacteriochlorophyll synthesis. In nature, photosynthetic bacteria grow in deeper layers of fresh water and marine habitats where H₂S, organic acids etc. are available.

At the same time they require light. Interestingly, bacteriochlorophylls absorb maximally beyond the red wave lengths, i.e. more than 700 nm and some even beyond 1,000 nm, i.e. infra-red rays. Light of such long wave lengths is not utilized by the eukaryotic photosynthetic pigments.

The phototrophic bacteria do riot possess the membrane-bound photosynthetic cell organelles, the chloroplasts found in green plants and algae. The pigment-bearing structures in bacteria are called chromatophores. These structures contain the entire complements of photosynthetic pigments and the enzymes of photosynthetic electron transport.

In sectioned cells, electron microscopy reveals the presence of vesicular or lamellar chromatophores varying in size from 70 to 300 nm. They are variously shaped and may be spherical to oblong which are dispersed evenly or restricted near the cell membrane. They originate by invagination of the cytoplasmic membrane and by pinching off from a membrane system.

The chemical composition of chromatophores in the different groups, like non-sulfur purple bacteria, sulfur purple bacteria and green bacteria is slightly variable. But they are essentially composed of lipids and proteins.

The composition of isolated chromatophores in three representative genera is shown in Table 4.8:

The oblong pigment-bearing chromatophores of green sulfur and green non-sulfur bacteria are known as chlorobium vesicles or chlorosomes. Electron microscopic study reveals that they are 40- 100 x 70-260 nm in size. They are surrounded by a 3 nm thick envelope containing within 10 to 30 closely packed rod elements extending to the full length of the chlorosome. A crystalline base plate connects the chlorosomes to the cytoplasmic membrane.

A model of the chlorosome structure of Chlorobium limicola is shown in Fig. 4.21:

Gram-Negative Bacteria: Group # 2. Oxygenic Photosynthetic Bacteria (Cyanobacteria):

Cyanobacteria constitute a large and morphologically diverse group of prokaryotic organisms which carry out a plant-like photosynthesis using H₂O as exogenous electron donor for photosynthetic reduction of NADP producing O₂ as a by-product. In rare instances, e.g. Oscillatoria limnetica can also utilize H₂S as electron donor like the anoxygenic sulfur purple and green bacteria.

Cyanobacteria also differ from other photosynthetic bacteria in possessing chlorophyll a like green plants, but they do not have chlorophyll b. An exception is the genus Prochloron. Prochloron and the allied genera, Prochlorothrix and Prochlorococcus, possess both chlorophylls a and b.

However, prochlorons do not have the phycobilin pigments which are the main light-harvesting accessory pigments present in all other cyanobacteria (and also in the red algae). More recently, a unicellular prokaryotic alga, named Acaryochloris marina has been found that contains chlorophyll d in addition to chlorophyll a and phycobilins.

Cyanobacteria exhibit a great diversity of form. They may be unicellular, the cells being generally associated to form colonies, or filamentous. Filaments may be single or in bunches, may show false branching or rarely true branching. Diameter of cells may vary from 1 to 10 µm. Cell mass generally has a blue-green colour due to the presence of blue phycocyanin and green chlorophyll pigments. But they may also appear red due to the presence of phycoerythrin. Both phycocyanin and phycoerythrin are phycobilins.

Morphological variations of cyanobacteria are shown in Fig. 4.22:

The organisms reproduce by several means, like binary fission, budding, fragmentation of trichomes into hormogonia which are short chains of 5 to 15 cells. Some cyanobacteria reproduce by multiple fission, a process in which a cell undergoes several internal divisions producing a number of small cells called baeocytes within the mother cell.

Baeocytes are liberated by rupture of the mother cell. Many filamentous cyanobacteria form also thick walled resting spores, known as akinetes. The akinetes are desiccation resistant and can germinate to produce new filaments under favourable conditions.

Another very characteristic structure formed in many filamentous cyanobacteria is a heterocyst. It is produced by differentiation of a vegetative cell of the trichome. During differentiation, the cytoplasm becomes less granular appearing transparent under light microscope.

Additional layers of wall are laid down. The pigments are changed to eliminate phycobilins and photosystem II, though photosystem I remains unchanged. Within the heterocyst, the nitrogen-fixing enzyme system, nltrogenease, is synthesized. Due to the absence of photosystem II, heterocyst’s cannot evolve O₂, though they can harness the light energy with the help of photosystem I. The heterocyst’s are the sites of N₂-fixation of cyanobacteria.

All cyanobacteria have a Gram-negative type cell wall. The peptidoglycan layer is comparatively thick which is bound inside to the cytoplasmic membrane and outside to the outer lipopolysaccharide membrane. Many cyanobacteria possess another layer outside the outer membrane which is variously known as sheath, capsule or glycocalyx.

This layer is mainly composed of highly hydrophilic polysaccharides and may have a micro-fibrillar structure. In filamentous forms, the cross walls are provided with single or numerous pores through which micro-plasmodesmata connect adjacent cells. Like other bacteria, cyanobacterial cells may possess pili and fimbriae.

Though cyanobacteria do not possess any kind of flagella, many forms exhibit a gliding motility on solid surface e.g. Oscillatoria filaments exhibit an oscillating movement. The photosynthetic apparatus of cyanobacteria is contained in thylakoids which are flattened sac­like structures connected with the cytoplasmic membrane. Thylakoids are arranged concentrically or may be disposed radially. Sometimes, the thylakoids form a convoluted structure.

They contain chlorophyll a, accessory carotenoids, the photosynthetic electron transport system as well as the photosynthetic reaction centre. On the surface of the thylakoids, numerous hemispherical bodies measuring 20 to 70 nm in diameter are orderly arranged.

These are known as phycobilisomes and they are complexes of proteins and phycobilin pigments. In a unicellular cyanobacterium, called Gloeobacter, thylakoids are absent, but rod-shaped phycobilisomes are present and the cytoplasmic membrane contains the other photosynthetic components. Phycobilisomes are absent in Prochloron and the related genera.

The genome size of cyanobacteria varies widely between 1.6 x 10⁹ to 8.6 x 10⁹. In general, unicellular forms have a smaller genome size. The complete genome has been sequenced in a species of Synechocystis and its size is 2.1 x 10⁹.

Besides the thylakoids and nuclear material, the cytoplasm contains various reserve materials in granular form, like glycogen, PHB, volutin, cyanophycean granules consisting of polymers of arginine and aspartic acid, etc. In addition, 70S ribosomes, carboxysomes containing mainly rubisco (ribulose bisphosphate carboxylase oxygenase) and gas-filled membrane-bound vesicles, called gas-vacuoles are sometimes present.

Cyanobacteria show a remarkable property of growing in a wide variety of habitats and tolerance to a wide range of environmental conditions. Terrestrial microbial mats in the Antarctica are mostly cyanobacterial. Some species can tolerate a temperature of 75°C or even more and grow in association with thermophilic bacteria.

Many cyanobacteria can grow in symbiotic association with other organisms, like fungi, protozoa, bryophytes, gymnosperms and angiosperms. The earliest fossils showing presence of microorganisms date back to the Precambrian (3.5 x 10⁹ years). These fossil microorganisms have remarkable similarity with some living cyanobacteria. It is believed that cyanobacteria were among the earliest colonizers of this planet.

Classically, the cyanobacteria have been classified in five orders, two of which—Chroococcales and Chaemesiphonales—include the unicellular forms and the rest—Pleurocarpales, Nostocales and Stigonematales—include the filamentous genera. In the second edition of the Manual, the Cyanobacteria have been tentatively placed in a single Class which has been divided into five Sections. Altogether 56 genera (designated as form-genera) have been recognized.

The major characteristics of the five sections and some representative genera of each section are described in Table 4.9.

 

 

Gram-Negative Bacteria: Group # 3. The Methylotrophic Bacteria:

All methylotrophic bacteria are aerobic, Gram-negative, morphologically and phylogenetically diverse organisms. Most of them are obligately methane oxidizers and some are able to oxidize one- carbon compounds like methanol, methylamine, formaldehyde and formic acid, but are unable to utilize methane. Both methane oxidizers as well as those oxidizing one-carbon compounds named above are designated as methylotrophs. The bacteria may be simple rods, cocci and vibrios. In the second edition of the Manual, they have been divided into 4 families belonging to 4 different orders of the Phylum Proteobacteria.

The classification of methylotrophic bacteria is presented in Table 4.10:

 

 

Methylotrophic bacteria possess intracytoplasmic membrane structures which are arranged in bundles of vesicular disks in some genera, like Methylococcus, Methylomonas, Methylobacter etc.—all belonging to the class Gammaproteobacteria. Some other genera, like Methylosinus and Methylocystis belonging to the class Alphaproteobacteria, have membrane system running parallel to the cytoplasmic membrane.

This ultra-structural difference in the organization of intracytoplasmic membrane system has been found to be associated with the pathway of methane utilization. The bacteria with the first type of organization utilize methane or other one-carbon compounds via the ribulose monophosphate pathway, and those having the second type use the serine pathway for methane utilization.

All methylotrophic bacteria can utilize methane or one-carbon compounds as sole source of carbon for production of energy (ATP) and cell constituents. In all of them, methane is oxidized via methanol, formaldehyde and formic acid to produce CO₂ and energy. Part of formaldehyde is diverted to form cell materials via either the ribulose monophosphate pathway or the serine pathway, depending on the genus.

A simplified representation of the pathway of methane oxidation is shown in Fig. 4.23:

Members of the order Methylococcales utilize the ribulose monophosphate (RuMP) pathway for synthesis of cell constituents. Three molecules of RuMP are condensed with 3 molecules of formaldehyde (Fig. 4.24) to yield 3 molecules of arabino-3-hexulose-6-phosphate, a unique intermediate of this pathway. The latter is next converted to fructose-6-phosphate and triose phosphate.

From fructose-6-phosphate, RuMP is regenerated through the transaldolase and transketolase reactions as in case of normal pentose phosphate cycle. However, in the RuMP pathway, sedoheptulose-7- phosphate acts as an intermediate instead of sedoheptulose-1,7-diphosphate. In the RuMP pathway, there are two unique enzymes — hexulose phosphate synthetase and hexulose phosphate isomerase — which catalyse condensation of RuMP and formaldehyde, and isomerization of arabino-3-hexulose- 6-phosphate to fructose-6-phosphate, respectively.

Some methylotrophic bacteria, like those belonging to the family Methylocystaceae, synthesise cell materials from formaldehyde using the serine pathway. This pathway is more complex than the RuMP pathway and involves at least nine steps. At first, formaldehyde carried by tetrahydrofolate (THF) is condensed with glycine to form serine through the action of serine hydroxymethyl transferase (Step 1).

Next, serine undergoes oxidative deamination to yield hydroxypyruvate and the latter is reduced to glyceric acid by hydroxypyruvate reductase (Steps 2 and 3). Glyceric acid is then phosphorylated with ATP to produce 3-phosphoglyceric acid (3-PGA) by the action of glycerokinase. 3-PGA is an important intermediate of this pathway, because it is partly removed for synthesis of cell materials and the rest is utilized to regenerate glycine, the acceptor of formaldehyde for continuous operation of the cyclic pathway.

The enzymes catalysing different steps of the serine pathway are:

1.    Serine hydroxymethyl transferase

2.    Oxidative deaminase

3.    Hydroxypyruvate reductase

4.    Glycerokinase

5.    Enolase

6.    Phosphoenlol pyruvate carboxylase

7.    Malate dehydrogenase

8.    Malate thiokinase

9.    Malyl-CoA lyase

10.    Reductive aminase

For regeneration of glycine from 3-PGA, it is dehydrated by enolase to form phosphoenolpyruvic acid (PEP) which is next carboxylated (addition of CO₂) by PEP-carboxylase to form oxalacetic acid. At the next step, oxalacetic acid is dehydrogenated through the action of malate dehydrogenase to produce malic acid. Another key enzyme of the pathway, malate thiokinase, acts on malic acid transferring coenzyme A to yield malyl-CoA which is next cleaved into acetyl-CoA and glyoxylic acid by a lyase (malyl-CoA lyase).

The cycle is completed by formation of glycine from glyoxylic acid by reductive amination. Acetyl-CoA liberated from malyl-CoA is fed into the glyoxylic shunt and eventually forms glyoxylic acid which also contributes to glycine formation. The reactions of serine pathway and glyoxylic acid formation through glyoxylic pathway is shown in Fig. 4.25.

It should be noted that methylotrophic bacteria are all members of eubacteria, while the bacteria which produce methane, i.e. the methanogenic ones, are all members of archaebacteria.

Gram-Negative Bacteria: Group # 4. The Nitrifying Bacteria:

These bacteria perform an important ecological function by converting ammonia into nitrate, a process known as nitrification. Ammonia released from hydrolytic degradation of proteins of plant, animal or microbial remain is oxidized in two steps to nitric acid. The acid forms nitrate and acts as nitrogen source to plants and microbes. Thus, nitrification constitutes a very important part of the natural nitrogen-cycle.

The two-step process of nitrification consists of oxidation of ammonia to nitrous acid which is also called nitrosification and oxidation of nitrous acid to nitric acid. These two oxidation steps are performed by two different groups of bacteria. The main genera performing nitrosification are Nitrosomonas, Nitrosospira and Nitrosococcus, and those of the second step are Nitrobacter, Nitrococcus and Nitrospira.

In the first edition of the Manual all these bacteria were placed in one family called Nitro-bacteriaceae on the basis of their ecological and physiological functions. However, on the basis of 16S r-RNA homology, they have been found to be not closely related.

The taxonomic positions of these bacteria in the second edition of the Manual are shown in Table 4.11:

It is seen from Table 4.11 that the bacteria have been brought under several different families and orders of alpha-, beta- and gamma Proteobacteria. The genus Nitrospira is not even a proteobacterium, but it has been included in a separate class, Nitrospira, and a separate phylum of the same name.

Some important characteristics of the above bacterial genera are tabulated in Table 4.12:

The nitrifying bacteria have a chemolithotrophic metabolism. The members of Nitrosomonas are obligate chemolithotrophs and they can grow only in purely inorganic salts containing medium. They do not tolerate the presence of any organic compound, not even glucose or agar-agar. For isolating these bacteria, the medium is solidified with silica gel.

The nitrosifiers oxidize NH₄⁺ ions to nitrous acid. The energy released by oxidation of ammonia is utilized for C0₂-fixation and ATP production. Similarly, the nitrifies oxidize nitrite to nitrate and the oxidation energy released is utilized for CO₂ fixation.

As the bacteria of both the groups produce acid as oxidation product, they have to be grown in media containing insoluble neutralizers, like MgCO₃ or CaCO₃. During growth, these insoluble compounds are solubilized due to acid production and C0₂ is liberated. The optimum pH is around 9.0. The strongly alkaline medium absorbs C0₂ and the bacteria fix it via Calvin cycle.

Some representative species of nitrifying bacteria are Nitrosomonas europaea, N. javanicus, Nitrosococcus oceanus, Nitrosospira briensis, Nitrobacter winogradskyi, Nitrococcus mobilis etc. An ultra structural feature of the nitrifying bacteria is the presence of extensive intracytoplasmic membrane system. In Nitrosomonas europaea the membrane system is in a few layers below the cytoplasmic membrane. In Nitrobacter winogradskyi it is arranged in the form of parallel lamellae in one pole of the bacterial cell. In Nitrosococcus oceanus, the lamellar membrane system runs across the cell.

A schematic representation of the membrane systems as they appear in ultra-thin sections is given in Fig. 4.26:

Gram-Negative Bacteria: Group # 5. The Stalked and Prosthecate Bacteria:

Certain bacterial genera are characterized by the presence of an appendage which may be an extension of the cell body or may consist of extracellular polysaccharide. In the first case, the appendage is known as a prostheca and in the second case, a stalk.

The well-known genera of bacteria possessing these pecularities i.e. a prostheca or a stalk and their taxonomic position are shown in Table 4.13:

(i) Caulobacter:

Caulobacter species have straight or more often curved rods with one end of the cell drawn out to form a prostheca. Under nutrient deficient conditions the prostheca may attain a considerable length. The prostheca consists of cell wall and cell membrane which are continuous with those of the cell.

It helps the bacterial cell to attach to some suitable substratum with a knob-like hold-fast. Caulobacter cells may get attached to other bacteria or organisms also. They often form beautiful micro-colonies where a large number of Caulobacter cells remain attached with the help of their hold-fasts.

A prosthecate Caulobacter cell develops a polar flagellum and divides by transverse fission to produce two dissimilar cells, one prosthecate cell and the other provided with a flagellum. The flagellate cell gets detached and becomes a swarmer, then it sheds the flagellum and develops a prostheca at the same end where the flagellum was attached. It gets attached and starts reproducing. The stalked mother cell continues to produce another flagellated cell. Under optimal conditions, the life cycle is completed within 2 to 3 hrs.

The morphological features of Caulobacter vibroides are presented:

 

(ii) Hyphomicrobium:

Hyphomicrobium is an aerobic prosthecate bacterium in which the prostheca is used for reproduction by budding and not for attachment as in case of Caulobacter. Moreover, the prostheca of Hyphomicrobium has cytoplasmic content, whereas that of Caulobacter has cell wall and membrane, but no cytoplasm.

The bacteria are about 0.5 µm wide and 1.0 µm in length and the prostheca is 0.2 to 0.3 µm in width and 1 to 5 times the length of the cell. The bacteria occur in soil, fresh water, sea water, water pipes and sewage. The organisms are oligotrophic which means that they are able to grow under nutrient deficient environments. They are often found in the laboratory water baths.

Hyphomicrobium also has a developmental cycle. The bacteria reproduce by budding and the process begins with the formation of a protuberance at the distal end of the prostheca. This bud gradually increases in size. The nuclear material of the mother cell divides and a daughter nucleus passes into the bud.

A cross-wall is laid down at the distal end of the prostheca. In the meantime, the bud develops flagella, breaks away to become a free-swimming swarmer. Eventually, the swarmer settles and begins to behave like a mother cell. The original mother cell continues to produce more buds. From a single cell, 6 to. 8 daughter cells may be produced by budding.

The reproductive cycle is diagrammatically represented in Fig. 4.28:

(iii) Rhodomicrobium:

Rhodomicrobium is a prosthecate, budding, photoheterotrophic non-sulfur purple bacterium. The prosthecae are often branched and form a network interspersed with the elliptical bacterial cells. The prostheca is used for reproduction by budding as in Hyphomicrobium, but the buds do not separate from the mother cell. Instead, they also produce buds, resulting in a network due to branching of the prostheca.

Occasionally, a daughter cell is detached from the colony, develops peritrichous flagella and becomes a swarmer. After swimming for a while, the swarmer settles and begins to reproduce by budding. Rhodomicrobium vannielii grows both in fresh-water as well as in marine habitats. An unusual feature is the formation of exospores under nutrient deficiency. Exospores are thick walled and triangular in shape. They are resistant to heat, desiccation and UV-radiation.

Morphological features of Rhodomicrobium are shown in Fig. 4.29:

(iv) Ancalomicrobium:

Ancalomicrobium, a genus belonging to the family Hyphomicrobiaceae under the order Rhizobiales, includes budding, multi-prosthecate, facultatively anaerobic, aquatic bacteria. The number of prosthecae varies from 3 to 8 per cell. The prosthecae are long and tapering. They are not used for reproduction. Under high nutrient conditions, the bacteria are without prosthecae and show a tendency to branching.

The bacteria multiply by budding and the buds are produced directly on the cell. Sometimes, knobbed motile cells with single polar flagellum are observed.

Different forms of Ancalomicrobium cells are shown in Fig. 4.30:

(v) Stalked Bacteria:

In non-prosthecate stalked bacteria, the cell produces an appendage consisting of polysaccharide which does not have any cytoplasmic connection, nor does it have any wall. The appendage may be tubular or ribbon shaped, often dichotomously branched producing an elaborate structure in which the bacterial cells may appear inconspicuous. Two well-known genera are Gallionella and Nevskia.

(vi) Gallionella:

Gallionella ferruginosa has 0.6-0.8 μm broad and 1.0-1.5 μm long curved rod-shaped bacteria occurring in fresh water springs and streams. From the concave side of the curved cells emerges a flat, twisted, ribbon-like stalk which is often impregnated with ferric hydroxide imparting a yellowish tinge.

As the bacteria multiply by binary fission, the stalk bifurcates resulting in a conspicuous dichotomously branched stalk-system at the growing tips of which the small bacterial cells are located. G. ferruginosa is microacrophilic and psychrophilic, having an optimum temperature of 6°C. The organism is capable of oxidizing Fe²⁺ to Fe³⁺ and is considered as an iron bacterium.

(vii) Nevskia:

Nevskia ramose is another stalked bacterium having relatively large cells (0.7-2.0 x 2.4-1-2 µm). The bacteria are rod to broadly elliptical in shape. They form broad repeatedly dichotomous, branched stalks giving a tree-like appearance. The organisms form floating colonies measuring up to 80 µm. Young cells are motile with one or two flagella.

The two genera, Gallionella and Nevskia, have morphological similarity, though they are phylogenetically unrelated (see Table 4.13).

The morphological characteristics of the two genera are shown in Fig. 4.31:

Gram-Negative Bacteria: Group # 6. The Cyanomorphic Bacteria:

These bacteria are phylogenetically heterogeneous but they are all filamentous, Gram-negative and often show gliding movement and resemble trichome-forming genera of cyanobacteria. They are sometimes considered as non-pigmented and non-photosynthetic mutants of cyanobacteria. Because of their morphological similarity with cyanobacteria, they have been designated as cyanomorphic bacteria (Reichenbach, 1974).

The organisms are aerobic and some genera—like Leucothrix, Vitreoscilla, Herpetosiphon, Simonsiella, Saprospira etc. are chemoorganotrophic, while others—like Beggiatoa, Thiothrix, Thiospikllopsis, Thioploca etc.—can oxidize sulfide to sulfur and, eventually, sulfur to sulfate. The organisms of the latter group are at least partly chemolithotrophic.

Morphologically, Beggiatoa and Vitreoscilla resemble the cyanobacterial genus Oscillatoria. Similarly, Leucothrix and Thiothrix resemble Rivularia, and Saprospora and Thiospirillopsis bear similarity with the cyanobacteirum, Spirulina. Thioploca containing several filaments in a common sheath bears resemblance with Microcoleus.

Morphological features of some representative genera are sketched in Fig. 4.32:

In the second edition of the Manual, Thiothrix, Leucothrix, Beggiatoa, Thioploca have been placed in the class Gammaproteobacteria under the order Thiotrichales. Simonsiella and Vitreoscilla have been brought under the Order Neisseriales of the class Betaproteobacteria.

Three well-known genera — Beggiatoa, Thiothrix and Leucothrix — are briefly discussed below:

(i) Beggiatoa:

Beggiatoa comprises several species which vary widely in size, both length and width e.g. B. minima have cells 1.0 x 1.0 µm, whereas B. gigantea have 56 µm wide cells. The length of the filament may be as long as 13 mm. The organisms are colourless, filamentous, having a gliding motility like that of Oscillatoria. The filaments are unattached and free-floating in both fresh-water and marine habitats.

The filaments are generally of equal diameter throughout. When the organisms grow in sulfide containing environment, elemental sulfur in droplet form is deposited intracellularly within pockets produced by infolding of the cell membrane. The organisms are capable of utilizing intracellular sulfur by oxidation to sulfate and the electrons can be transferred to the electron transport system for generation of ATP. Autotrophic growth by CO₂ fixation has been found in some species.

But the organisms can also grow chemo-organo-trophically utilizing acetate as carbon and energy source. Fresh water species can be grown in culture. G + C content of DNA is 37 moles %. Some species are B. alba, B. minima, B. mirabilis and B. gigantea.

(ii) Vitreoscilla:

Vitreoscilla is similar to Beggiatoa, but they do not contain sulfur-granules. The filaments of Vitreoscilla tend to break-up in short few celled hormogonia which exhibit gliding movement. The organisms are chemoorganotrophic and aerobic. G + C content is 43.6 moles %. A common species is V. beggiatoades.

(iii) Leucothrix:

Leucothrix includes organisms having long, colourless, unbranched filaments which are attached to a substratum with conspicuous holdfasts. The filaments may be uniform in diameter or may be tapering towards apex. They usually grow forming microcolonies. From apical portion of a filament chains of gonidia are cut off which exhibit jerky gliding movement.

The gonidia accumulate at a spot, germinate and produce a rosette of micro-colony. The filaments are generally non-motile. The organisms are chemoorganotrophic without any intracellular sulfur granules and aerobic. G + C content varies between 46 to 51 moles %. Leucothrix is marine. Its type species is L. mucor.

(iv) Thiothrix:

Thriothrix resembles Leucothrix in having long tapering attached filaments, but differs in having sulfur granules in cells. They reproduce by formation of gonidia from apical cells. The gonidia show gliding movement. Often several gonidia stick to each other and germinate to produce microcolonies, sometimes on algae which act as substratum.

They grow under conditions where organic matter decomposes with formation of H₂S. The bacteria, like Beggiatoa, can oxidize sulfide to sulfur which deposits in the cells. Together with Beggiatoa and allied genera, they are known as filamentous sulfur bacteria. They are partly chemolithotrophic. Both marine and fresh water species of Thiothrix are known. The type species of the genus is T. nivea. It occurs in sulfur springs and sewage plants.