Characteristics and structure of prokaryote / bacteria

Prokaryotae e.g. bacteria

In this article we shall see the main features of a prokaryote (bacteria), its structure and organelles, shape, nutrition, bacterial reproduction and the growth form of bacteria.

The kingdom prokaryotae is made up of organisms commonly known as bacteria. They are the most ancient group of organisms, having appeared about 3.5billion years ago, and are the smallest organisms with a cellular structure.

They are mainly single cells, although the blue-green bacteria (cyanobacteria) may form singles rows of cells called filaments. Some bacteria stick together in characteristic patterns, forming chains or clusters like bunches of grapes, but the cells are totally independent of each other.

Individual bacteria cells can only be seen with the aid of a microscope, which is why they are known as microorganisms. The study of bacteria is called bacteriology and is an important branch of microbiology.

Bacteria range in length from about 0.1 to 10µm.  Their average diameter is about 1µm, enough room for 200 average-sized globular protein molecules (of 5nm diameter) to fit across the cell. Such a molecule in solution can diffuse about 60µm per second; thus no special transport mechanisms are needed for these organisms.

Bacteria occupy many environments, such as soil, dust, water, air, in and on animals and plants. Some are found in hot springs where temperatures may reach 78oC or higher. Others can survive very low temperatures and periods of freezing in ice. Some have been found in deep cracks in the ocean floor, at very high pressures and temperatures of 360oC. They form a starting point of unique food chains of these areas of the ocean.

Numbers of bacteria are enormous; one gram of fertile soil is estimated to contain 2.5 billion; 1cm3 of fresh milk may contain more than 3 billion. Together with fungi their activities are vital to all other organisms because they cause the decay of organic material and the subsequent recycling of nutrients.

In addition, they are of increasing importance to humans, not only because they cause disease, but because their very diverse biochemistry can be used in many biotechnological processes.

Structure of bacteria; a prokaryote

Bacteria; a prokaryote
structure of bacteria

The bacteria structure is majorly studied with a common rod-shaped bacterium, Escherichia coli, which lives in the gut of humans and other vertebrates. It is normally completely harmless. Its presence in water can be used as an indicator of contamination by faeces.

E. coli has been studied more than any other bacterium and is one of the few organisms whose entire genetic code has been determined.

Bacterial Cell wall

The bacterial cell wall is strong and rigid due to the presence of murine, a molecule that consists of parallel polysaccharide chains cross-linked at regular intervals by short chains of amino acids. Each cell is thus surrounded by a net-like sac which is really on huge molecule. The wall prevents the cell from bursting when it absorbs water (as a result of osmosis). Tiny pores allow the passage of water, ions and small molecules.

In 1884 a Danish biologist, Christian Gram, developed a stain which revealed that bacteria can be divided into two natural groups. We now know that this is due to the differences in their wall structure. Some bacteria stain with Gram’s stain and are called Gram positive, others do not and are called Gramnegative.

In Gram positive bacteria such as Staphylococcus, Bacillus and Lactobacillus, the murein net is filled with other components, mainly polysaccharides and proteins, to form a relatively thick wall.

The walls of Gram negative bacteria, such as Salmonella, E. coli and Azotobacter, are thinner but more complex. Their murein are coated on the outside with a smooth, thin, membrane-like layer of lipids and polysaccharides. This protects them from lysozyme, an antibacterial enzyme found in tears, saliva and other body fluids and egg white. Lysozyme digests the polysaccharide backbone of murein. The wall is thus punctured and lysis (osmotic swelling and bursting) of the cell can occur.

The same outer layer also gives resistance to penicillin, which attacks Gram positive bacteria by interfering with the cross-linking in the murein of growing cells so making the walls weaker and more likely to burst when water enters by osmosis.

Bacterial Cell surface membrane, mesosomes and photosynthetic membranes

Like all cells, the living material of bacterial cells is surrounded by a partially permeable membrane. The structure and functions of the cell surface membrane are similar to those in eukaryotic cells. It is also the site of some respiratory enzymes. In addition, in some bacteria it forms mesosomes and/or photosynthetic membranes.

Mesosomes are infoldings of the cell surface membrane. They appear to be associated with DNA during cell division, organizing the separation of the two daughter molecules of DNA after replication and helping in the formation of new cross-walls between the daughter cells.

Among photosynthetic bacteria, sac-like, tubular or sheet-like infoldings of the cell surface membrane contain the photosynthetic pigments, always including bacteriochlorophyll. Similar membranes are associated with nitrogen fixation.

Genetic material (bacterial chromosome)

Bacterial DNA is a single circular molecule of about 5 million base pairs and of length 1mm (much longer than the cell). The total DNA (the genome), and hence the amount of information it contains, is much less than that of a eukaryotic cell; typically it contains several thousand genes, about 500 times fewer than a human cell.

Bacterial ribosomes

Ribosomes are the sites of protein synthesis. 70S ribosomes (smaller) with no endoplasmic reticulum present (many other details of protein synthesis differ, including susceptibility to antibiotics, e.g prokaryotes inhibited by streptomycin).

Capsules

Capsules are slimy or gummy secretions by some bacteria which show up clearly after negative staining (when the background, rather than the specimen is stained). In some cases these secretions unite into colonies. They also enable bacteria to stick to surfaces such as teeth, mud and rocks, and offer useful additional protection to the bacteria.

For example capsulate strains of pneumococci grow in their human hosts causing pneumonia, whereas non-capsulate strains are easily attacked and destroyed by phagocytes, and are therefore harmless.

Spores

Some bacteria, mainly of the genera Clostridium and Bacillus, form endospores (spores produced inside cells). They are thick-walled, long-lived and extremely resistant, particularly to heat, drought and short-wave radiations. Their position in the cell is variable and is of importance in recognition and classification.

Flagella (singular flagellum)

Many bacteria are motile due to the presence of one or more flagella. The flagellum is a simple hollow cylinder of identical protein molecules. It is rigid and wave-shaped. It propels the cell along by rotating at the base, providing a corkscrew-like motion rather than a beat. Examples of bacteria with flagella are Rhizobium (one flagellum) and Azotobacter (many flagella), both of which are involved in the nitrogen cycle.

Motile bacteria can move in response to certain stimuli, that is show tactic movements. For example, aerobic bacteria will swim towards oxygen (positive aerotataxis) and motile photosynthetic bacteria will swim towards light (positive phototaxis).

Flagellum are most easily seen with the electron microscope using the technique of metal shadowing (Where the specimen is sprayed with a heavy metal which is opaque to electrons. Sheltered areas remain uncoated, forming an electron-transparent shadow. The photograph is published as negative to make the shadow black).

Pili (singular pilus)

Projecting from the walls of some gram positive bacteria are numerous fine protein rods called pili or fimbriae. They are shorter and thinner than flagella and are concerned with attachment to specific cells or surfaces. Various types occur, but of particular interest is the F pilus. This is involved in sexual reproduction.

Plasmids

In addition to the single circular DNA molecule found in all bacteria, some species also contain one or more plasmids. A plasmid is a small self-replicating circle of extra DNA. It possesses only a few genes which generally give extra survival advantage.

Some confer resistance to antibiotics. For example some staphylococci contain a plasmid which includes a gene for the enzyme penicillinase. This breaks down penicillin, thus making the bacteria resistant to penicillin.

The spread of such gene by conjugation has important implications in medicine. Other plasmid genes are known which:

  • Confer resistance to disinfectants
  • Cause disease
  • Are responsible for the fermentation of milk to cheese by lactic acid bacteria.
  • Confer ability to use complex chemicals as food, such as hydrocarbons, with potential applications in clearing oil spills and producing protein from petroleum.

Cell shape of bacteria

the different bacteria shapes
shapes of bacteria

Bacterial shape is an important aid to classification. The four main shapes are;

  1. Cocci (singular; coccus): spherical shape
  2. Diplococci (pairs): the pneumococci (Diplococcus pneumonia) are the only member; cause pneumonia
  3. Streptococci (chains): e.g many streptococcus spp.; some infect upper respiratory tract and cause disease e.g S. pyogenes causes scarlet fever and sore throats, S. thermophiles gives yoghurt its creamy flavor, S. lactis.
  4. Staphylococci (like a bunch of grapes) e.g. Staphylococcus aureus lives in nasal passages; different strains cause boils, pneumonia, food poisoning and other diseases.
  • Bacilli (sing. Bacillus) rod-shaped
  • Single rods: e.g. Escherichia coli, common gut living symbiont; Lactobacillus, Salmonella typhi causes typhoid fever.
  • Rods in chains: e.g. Azotobacter, a nitrogen-fixer; Bacillus anthracis causes anthrax

Some bacilli with endospores with various positions, shapes and sizes of spores are some with:

  • Oval spore, central not swollen e.g. Bacillus anthracis. Causes anthrax.
  • Spherical spore, terminal swollen e.g. Clostridium tetani. Causes tetanus.
  • Subterminal swollen e.g. Clostridium botulinum (spores may also be central), causes botulism.
  • Spirilla (sing. Spirillum) spiral-shaped: helical rod with single flagellum; e.g. Spirillium, body of spirochaetes is similar in form but locomotion differs, e.g. Treponema pallidum cause syphilis.
  • Vibrios: comma shaped e.g. Vibrio cholera causes cholera; single flagellum.

Reproduction in bacteria.

Growth of individuals and asexual reproduction

Bacteria have a very large surface area to volume ratio and can therefore gain food sufficiently rapidly from their environment by diffusion and active transport mechanisms. Therefore providing conditions are suitable, they can grow very rapidly. Important environmental factors affecting growth are temperature, nutrient availability, pH and ionic concentrations. Oxygen must also be present for obligate anaerobes.

On reaching a certain size, dictated by the nucleus to cytoplasm ratio, bacteria reproduce by the nucleus to cytoplasm ratio, bacteria reproduce asexually by binary fission, that is, by division into two identical daughter cells.

Cell division is preceded by replication of the DNA and while this is being copied it may be held in position by a mesosomes. The mesosomes may also be attached to the new cross-walls that are laid down between the daughter cells, and plays some role in the synthesis of cell wall material. In the fastest growing bacteria such divisions may occur as often as every 20 minutes.

Sexual reproduction

In 1946 it was discovered that bacteria can take part in a primitive form of sexual reproduction. Gametes are not involved, but the essential feature of sexual reproduction, namely the exchange of genetic material, does take place and is called genetic recombination. The process was discovered using E. coli as follows.

Normally E. coli can make all of its own amino acids, given a supply of glucose and mineral salts. Random mutations were induced by exposure to radiation and two particular mutants selected.

One could not make biotin (a vitamin) or the amino acid methionine. Another could not make the amino acids threonine and leucine.

About 108 cells of each mutant were mixed and cultured on media lacking all four growth factors. Theoretically, none of the cells should have grown but a few hundred colonies developed, each from one original bacterium, and these were shown to possess genes for making all four growth factors. Exchange of genetic information had therefore occurred, but no chemical responsible could be isolated. Eventually it was shown with the electron microscope that direct cell-to-cell contact, that is conjugation, can occur in E. coli.

Conjugation therefore involves transfer of DNA between cells in direct contact. One cell acts as the donor (male) and the other as the recipient (female). The ability to serve as a donor is determined by genes in special type of plasmid called the sex factor, of F factor (F for fertility). This codes for the protein of a special type of pilus, the F pilus  or sex pilus. This enables cells to come into contact.  The pilus is hollow and it is believed that the DNA passes through the pilus from the donor (F+) to the recipient (F).

Note that the donor retains the F factor and the recipient also becomes F+. the process is slow, so the Fcell can replicate by binary fission one or several times before the process is complete, thus maintaining Fcells in the population.

The F factor is particularly important because in a few cases, about 1 in 100 000, it becomes integrated with the rest of the DNA in the host cell. In such cases, the process of conjugation involves transfer of not only the F factor, but also the rest of the DNA. This takes about 90 min and separation may occur before exchange is complete. Such strains consistently donate all or large portions of their DNA and are called Hfr strains H = high, f = frequency, r = recombination), because the donor DNA can recombine with the recipient DNA.

Nutrition in bacteria.

how bacteria feeds
bacteria feeding

Nutrition is the process of acquiring energy and materials. Living organisms can be grouped on the basis of their source of energy or source of carbon, the latter being the most fundamental material required for growth. Only two forms of energy can be used by living organisms to synthesise their organic requirements, namely light and chemical energy. Those that use light are known as phototrophs and those that use chemical energy are called chemotrophs. Phototrophs carry out photosynthesis.

As already noted, organisms can also be described as autotrophic or heterotrophc, depending on whether their source of carbon is inorganic (carbon dioxide) or organic respectively. Thus four nutritional categories can occur as will be discussed. The largest group is the chemoheterotrophic bacteria.

Chemoheterotrophic bacteria

These bacteria obtain energy from chemical in their food. They use an enormous range of chemicals. There are three main groups, namely saprotrophs, mutualists and parasites.

a saprotroph is an organism that obtains its food from dead and decaying matter. The saprotroph secretes enzymes onto the organic matter to digest it. Thus digestion is outside of the organism. Soluble products of digestion are absorbed and assimilated within the body of the saprotroph.

Saprotrophic bacteria and fungi constitute the decomposers and are essential in bringing about decay and recycling of nutrients. They produce humus from animal and plant remains, but also cause decay of materials useful to humans, especially food.

Mutualism (or symbiosis) is the name given to any form of close relationship between two living organisms in which both partners benefit. Examples of bacterial mutualists are rhizobium, a nitrogen-fixer living in the root nodules of legumes such as pea and clover, and Escherichia coli, which inhabits the gut of humans and probably contributes vitamins of the B and K groups.

A parasite is an organism that lives in or on another organism, the host, from which it obtains its food and, suffers harm from the parasite. Parasites which cause disease are called pathogens. Some parasites can only survive and grow in living cells and are called obligate parasites. Others can infect a host, bring about its death and then live saprotrophically on the remains; these are called facultative parasites.

It is a characteristic of parasites that they are very exacting in their nutritional requirements, needing ‘accessory growth factors’ that they cannot manufacture for themselves but can only find in other living cells.

Photoautotrophic bacteria

Cyanobacteria or blue-green bacteria, are examples of photoautotrophic bacteria. Algae and plants are also photoautotrophic. They all carry out photosynthesis and use carbon dioxide as a source of carbon. The process of photosynthesis first evolved in bacteria, possibly process of photosynthesis first evolved in blue-green bacteria. The chloroplasts of algae and plants are thought to be descendants of what were once free-living photosynthetic bacteria that invaded heterotrophic cells.

Blue-green bacteria are common in surface layers of both fresh water and sea water and are also found as gelatinous mat-like growths on shaded soil, rocks, mud, wood and some are linked to form filaments sheathed in mucus, e.g. Anabaena and Spirulina, they differ from most bacteria, and resemble algae and plants, in producing oxygen from water during photosynthesis.

Photosynthetic membrane characteristically run throughout the cytoplasm and it is here that the photosynthetic pigments are located. The pigments include chlorophyll a, again resembling algae and plants, as well as a characteristic blue-green pigment called phycocyanin.

The cells of blue-green bacteria tend to be larger than those of other bacteria. The fact that blue-green bacteria produce oxygen in photosynthesis, have photosynthetic membranes running through the cell and contain chlorophyll a, indicate that they may be evolutionary links between the rest of the bacteria and eukaryotes.

Some blue-green bacteria, such as Anabaena, have the ability to fix nitrogen, that is, to convert nitrogen gas from the air to ammonia which can then be used in synthesis of amino acids, proteins and other nitrogen-containing organic compounds. This is done in special cells called heterocysts which develop when there is a nitrogen shortage. These cells export the nitrogen compounds to neigbouring cells in exchange for other nutrients such as carbohydrate.

Chemoautotrophic bacteria

These are more commonly known as chemosynthetic bacteria. They use carbon dioxide as a source of carbon but obtain their energy from chemical reactions. The energy is obtained by oxidizing inorganic materials such as ammonia and nitrite. Some are important members of the nitrogen cycle, carrying out a process called nitrification. This takes place in two stages, firstly ammonia is oxidized to nitrate with release of energy. This is carried out, for example, by Nitrosomonas. Secondly nitrite is oxidized to nitrate with the release of more energy. This is done, for example, by Nitrobacter.

  1. NH4+    O2             NO2+ energy
  • NO2    O2             NO3+ energy

Photoheterotrophic bacteria

A common example of photoheterotrophic bacteria is the purple non-sulfur bacteria.

Population growth in bacteria

Time (in units of 20min 1 2 3 4` 5 6 7 8 9 10
A number of bacteria                    
B Log10 number of bacteria (to one decimal place)                    
C Number of bacteria expressed as power of 2                    

The kind of growth shown in the table above is known as logarithmic, exponential or geometric. The numbers form an exponential series. This can be explained by reference to line C in the table where the number of bacteria is expressed as a power of 2. The power can be called the logarithm or exponent of 2. The logarithms or exponents form a linearly increasing series 0, 1, 2, 3, etc., corresponding with the number of generations.

The numbers in line A could be converted to logarithms to the base 2 as follows:

A number of bacteria 1 2 4 8 16 32 64 128 256 512 1024
D Log2 number of bacteria 0 1 2 3 4 5 6 7 8 9 10
                     

Compare line C with line D. however, it is conventional to use logarithms to the base 10, as in line B, Thus 1 is 100, 2 is 100.3, 4 is 100.6, etc.

The curve in graph A is known as logarithmic or exponential curve. Such growth curves can be converted to straight lines by plotting the logarithms of growth against time. Under ideal conditions, then, bacterial growth is theoretically exponential. This mathematical model of bacterial growth can be compared with the growth of a real population.  The growth curve shows four distinct phases.

  • During the lag phase the bacteria are adapting to their new environment and growth has not yet achieved its maximum rate. The bacteria may, for example, be synthesizing new enzymes to digest the particular spectrum of nutrients available in the new medium.
  • The log phase is the phase when growth is proceeding at its maximum rate, closely approaching a logarithmic increase in numbers when the growth curve would be a straight line.
  • Eventually, growth of the colony begins to slow down and it enters the stationary phase where growth rate is zero, and there is much greater competition for resources. Rate of production of new cells is slower and may cease altogether. Any increase in the number of cells is offset by the death of other cells, so that the number of living cells remains constant. This phase is a result of several factors, including exhaustion of essential nutrients, accumulation of toxic waste products of metabolism and possibly, if the bacteria are aerobic, depletion of oxygen.
  • During the final phase, the phase of decline, the death rate increases and cells stop multiplying.

Having discussed the characteristics or features of bacteria, a representative of the kingdom prokaryotae, we hope that you have gotten enough information on the topic. Do ensure to search for additional information using our search box, we are always glad to answer your questions.