Bacteria
Q.10. Describe the economic importance of bacteria. (2005, 07,15)
Related Questions -
Q. Write an essay on beneficial and harmful activities of bacteria. (2012)
Ans. The economic importance of bacteria derives from the fact that bacteria are exploited by humans in a number of beneficial ways. Despite the fact that some bacteria play harmful roles, such as causing disease and spoiling food, the economic importance of bacteria includes both their useful and harmful aspects.
Useful Bacteria: -
Biotechnology and Bacteria: -
Biotechnology or Industrial microbiology is defined as the application of organisms such as bacteria, fungi and algae to the manufacturing and services industries. These include:
- Fermentation processes, such as brewing, baking, cheese and butter manufacturing, Bacteria, often Lactobacillus in combination with yeasts and molds, have been used for thousands of years in the preparation of fermented foods such as cheese, pickles, soy sauce, sauerkraut, vinegar, wine, and yogurt.
- Chemical manufacturing such as ethanol, acetone, organic acid, enzymes, perfumes etc. In the chemical industry, bacteria are most important in the production of enantiomerically pure chemicals for use as pharmaceuticals or agrochemicals.
- Pharmaceuticals, such as antibiotics, vaccines and steroids.
- Microbial mining, which is the bacteria and other microorganisms are cultured in container and then used to bring these processes e.g., copper extraction.
- Can cause deadly diseases
- Spoiling food
- Tooth decay
- Produce deadly toxins
- Body odor
- Food poisoning
- Helps make some foods
- Preserve food
- Keep gut healthy
- Break down toxins
- Make soil
Genetic Engineering and Bacteria: -
Genetic engineering is the manipulation of genes. It is also called recombinant DNA technology. In genetic engineering, pieces of DNA (genes) are introduced into a host by means of a carrier (vector) system. The foreign DNA becomes a permanent feature of the host, being replicated and passed on to daughter cells along with the rest of its DNA. Bacterial cells are transformed and used in production of commercially important products. The examples are production of human insulin (used against diabetes), human growth hormone (somatotrophin used to treat pituitary dwarfism), and infections which can be used to help fight viral diseases.
Using biotechnology techniques, bacteria can also be bioengineered for the production of therapeutic proteins, such as insulin, growth factors or antibodies.
Fibre Retting: -
Bacterial populations, especially that of Clostridium butyclicum, are used to separate fibres of jute, hemp, flax, etc, the plants are immersed in water and when they swell, inoculated with bacteria which hydrolyze pectic substance of the cell walls and separate the fibres. These separated fibres are used to make ropes and sacks.
Digestion: -
Some bacteria living in the gut of cattles, horses and other herbivores secrete cellulase, an enzyme that helps in the digestion of the cellulose contents of plant cell walls. Cellulose is the major source of energy for these animals.
Vitamin Synthesis: -
Escherichia coli that lives in the human large intestine synthesize vitamin B and release it for human use. Similarly, Clostridium butyclicum is used for commercial preparation of riboflavin, and vitamin B.
Waste Disposal: -
Aerobic and anaerobic bacteria are used to decompose sewage wastes. They break down organic matter to harmless, soluble sludge in settling tanks. The methane gas produced is used as energy source. Similarly toxic chemicals synthesized by living organisms and those present in the pesticides are disposed with the help of bacteria.
Pseudomonas putida has been created by using genetic engineering techniques and can break down xylene and camphor.
The ability of bacteria to degrade a variety of organic compounds is remarkable and has been used in waste processing, and bioremediation. Bacteria capable of digesting the hydrocarbons in petroleum are often used to clean up oil spills. Fertilizer was added to some of the beaches in Prince William Sound in an attempt to promote the growth of these naturally occurring bacteria after the infamous 1989 Exxon Valdez oil spill. These efforts were effective on beaches that were not too thickly covered in oil. Bacteria are also used for the bioremediation of industrial toxic wastes.
Pest Control: -
Bacteria can also be used in the place of pesticides in the biological pest control. This commonly uses Bacillus thuringiensis (also called BT), a Gram-positive, soil dwelling bacterium. This bacteria is used as a Lepidopteran-specific insecticide under trade names such as Dipel and Thuricide. Because of their specificity, these pesticides are regarded as Environmentally friendly, with little or no effect on humans, wildlife, pollinators, and most other beneficial insects.
Harmful Bacteria: -
Some bacteria are harmful and act either as disease-causing agents (pathogens) both in plants and animals, or may play role in food spoilage.
Plant Pathogenic Bacteria: -
Plant disease caused by bacterial plant pathogens is a major problem worldwide for agriculture. Besides bacterial pathogens that are already established in many areas, there are many instances of pathogens moving to new geographic areas or even the emergence of new pathogen variants. In addition, bacterial plant pathogens are particularly difficult to control because of the shortage of chemical control agents for bacteria, apart from antibiotics. However, the use of antibiotics is restricted in many countries due to the potential for evolution of antibiotic resistance and the transmission of antibiotic resistance to bacteria that can cause human disease.
Agents of Disease: -
Organisms which cause disease are called pathogens. Some bacteria are pathogenic and cause diseases both in animals and plants. However, pathogenic bacteria more commonly affect animals than plants. Certain bacteria that exist in the normal flora on skin and in the mouth and human intestine are also know to cause disease when imbalances have weakened the immune system.
Saprotrophic bacteria attack and decompose organic matter. This characteristic has posed a problem to mankind as food such as stored grains, meat, fish, vegetable and fruits are attacked by saprotrophic bacteria and spoiled. Similarly milk and products are easily contaminated by bacteria and spoiled.
Q.11. Give an account of the ultrastructure of a bacterial cell. (2006)
Related Questions -
Q. Write a short note on ultrastructure of bacterial cell. (2007)
Q. Write short note on Endospore. (2013)
Q. Give a detailed account of structure of bacteria. (2013)
Q. Give a detailed account of structure of bacteria. (2013)
Ans. Perhaps the most elemental structural property of bacteria is cell morphology (shape). Typical examples include:
- coccus (spherical)
- bacillus (rod-like)
- spirillum (spiral)
- filamentous
Cell shape is generally characteristic of a given bacterial species, but can vary depending on growth conditions. Some bacteria have complex life cycles involving the production of stalks and appendages (e.g. Caulobacter) and some produce elaborate structures bearing reproductive spores (e.g. Myxococcus, Streptomyces). Bacteria generally form distinctive cell morphologies when examined by light microscopy and distinct colony morphologies when grown on Petri plates. These are often the first characteristics observed by a microbiologist to determine the identity of an unknown bacterial culture.
The Bacterial Cell Wall: -
As in other organisms, the bacterial cell wall provides structural integrity to the cell. In prokaryotes, the primary function of the cell wall is to protect the cell from internal turgor pressure caused by the much higher concentrations of proteins and other molecules inside the cell compared to its external environment. The bacterial cell wall differs from that of all other organisms by the presence of peptidoglycan (poly-N-acetylglucosamine and N-acetylmuramic acid), which is located immediately outside of the cytoplasmic membrane. Peptidoglycan is responsible for the rigidity of the bacterial cell wall and for the determination of cell shape. It is relatively porous and is not considered to be a permeability barrier for small substrates. While all bacterial cell walls (with a few exceptions e.g. intracellular parasites such as Mycoplasma) contain peptidoglycan, not all cell walls have the same overall structures. There are two main types of bacterial cell walls, Gram positive and Gram negative, which are differentiated by their Gram staining characteristics. For both Gram-positive and Gram-negative bacteria, particles of approximately 2 nm can pass through the peptidoglycan.
The Gram Positive Cell Wall: -
The Gram positive cell wall is characterized by the presence of a very thick peptidoglycan layer, which is responsible for the retention of the crystal violet dyes during the Gram staining procedure. It is found exclusively in organisms belonging to the Actinobacteria (or high %G+C Gram positive organisms) and the Firmicutes (or low %G+C Gram positive organisms). Bacteria within the Deinococcus-Thermus group may also exhibit Gram positive staining behaviour but contain some cell wall structures typical of Gram negative organisms. Embedded in the Gram positive cell wall are polyalcohols called teichoic acids, some of which are lipid-linked to form lipoteichoic acids. Because lipoteichoic acids are covalently linked to lipids within the cytoplasmic membrane they are responsible for linking the peptidoglycan to the cytoplasmic membrane. Teichoic acids give the Gram positive cell wall an overall negative charge due to the presence of phosphodiester bonds between teichoic acid monomers.
The Gram Negative Cell Wall: -
Unlike the Gram positive cell wall, the Gram negative cell wall contains a thin peptidoglycan layer adjacent to the cytoplasmic membrane. This is responsible for the cell wall’s inability to retain the crystal violet stain upon decolourisation with ethanol during Gram staining. In addition to the peptidoglycan layer, the Gram negative cell wall also contains an outer membrane composed by phospholipids and lipopolysaccharides, which face into the external environment. As the lipopolysaccharides are highly-charged, the Gram negative cell wall has an overall negative charge. The chemical structure of the outer membrane lipopolysaccharides is often unique to specific bacterial strains (i.e. sub-species) and is responsible for many of the antigenic properties of these strains.
The Bacterial Cytoplasmic Membrane: -
The bacterial cytoplasmic membrane is composed of a phospholipid bilayer and thus has all of the general functions of a cell membrane such as acting as a permeability barrier for most molecules and serving as the location for the transport of molecules into the cell. In addition to these functions, prokaryotic membranes also function in energy conservation as the location about which a proton motive force is generated. Unlike eukaryotes, bacterial membranes (with some exceptions e.g. Mycoplasma and methanotrophs) generally do not contain sterols. However, many microbes do contain structurally related compounds called hopanoids which likely fulfill the same function. Unlike eukaryotes, bacteria can have a wide variety of fatty acids within their membranes. Along with typical saturated and unsaturated fatty acids, bacteria can contain fatty acids with additional methyl, hydroxy or even cyclic groups. The relative proportions of these fatty acids can be modulated by the bacterium to maintain the optimum fluidity of the membrane (e.g. following temperature change).
As a phospholipid bilayer, the lipid portion of the outer membrane is impermeable to charged molecules. However, channels called porins are present in the outer membrane that allow for passive transport of many ions, sugars and amino acids across the outer membrane. These molecules are therefore present in the periplasm, the region between the cytoplasmic and outer membranes. The periplasm contains the peptidoglycan layer and many proteins responsible for substrate binding or hydrolysis and reception of extracellular signals. The periplasm it is thought to exist as a gel-like state rather than a liquid due to the high concentration of proteins and peptidoglycan found within it. Because of its location between the cytoplasmic and outer membranes, signals received and substrates bound are available to be transported across the cytoplasmic membrane using transport and signalling proteins imbedded there.
Other Bacterial Surface Structures: -
Fimbrae and Pili: -
Fimbrae are protein tubes that extend out from the outer membrane in many members of the Proteobacteria. They are generally short in length and present in high numbers about the entire bacterial cell surface. Fimbrae usually function to facilitate the attachment of a bacterium to a surface (e.g. to form a biofilm) or to other cells (e.g. animal cells during pathogenesis)). A few organisms (e.g. Myxococcus) use fimbrae for motility to facilitate the assembly of multicellular structures such as fruiting bodies. Pili are similar in structure to fimbrae but are much longer and present on the bacterial cell in low numbers. Pili are involved in the process of bacterial conjugation. Non-sex pili also aid bacteria in gripping surfaces.
S-Layers: -
An S-layer (surface layer) is a cell surface protein layer found in many different bacteria and in some archaea, where it serves as the cell wall. All S-layers are made up of a two-dimensional array of proteins and have a crystalline appearance, the symmetry of which differs between species. The exact function of S-layers is unknown, but it has been suggested that they act as a partial permeability barrier for large substrates. For example, an S-layer could conceivably keep extracellular proteins near the cell membrane by preventing their diffusion away from the cell. In some pathogenic species, an S-layer may help to facilitate survival within the host by conferring protection against host defence mechanisms.
Capsules and Slime Layers: -
Many bacteria secrete extracellular polymers outside of their cell walls. These polymers are usually composed of polysaccharides and sometimes protein. Capsules are relatively impermeable structures that cannot be stained with dyes such as India ink. They are structures that help protect bacteria from phagocytosis and desiccation. Slime layers are somewhat looser, fibrous structures generally involved in attachment of bacteria to other cells or inanimate surfaces to form biofilms. Slime layers can also be used as a food reserve for the cell.
· An example of how a bacterial cell uses their slime layer to attach to a surface is in the Streptococcus mutans. Streptococcus mutans attaches to the teeth with a slime layer and forms a sticky film that traps food particles and other bacteria on the teeth (dental plaque). The bacteria then metabolizes the trapped food particles and release acids (thus possibly causing tooth decay).
Flagella: -
Perhaps the most recognizable extracellular bacterial cell structures are flagella. Flagella are whip-like structures protruding from the bacterial cell wall and are responsible for bacterial motility (i.e. movement). The arrangement of flagella about the bacterial cell is unique to the species observed. Common
forms include:
- Peritrichous - Multiple flagella found at several locations about the cell
- Polar - Single flagella found at one of the cell poles
- Lophotrichous - A tuft of flagella found at one cell pole
Flagella are complex structures that are composed of many different proteins. These include flagellin, which makes up the whip-like tube and a protein complex that spans the cell wall and cell membrane to form a motor that causes the flagellum to rotate. This rotation is normally driven by proton motive force and are found in the body of the cell.
Intracellular Bacterial Cell Structures: -
In comparison to eukaryotes, the intracellular features of the bacterial cell are extremely simple. Bacteria do not contain organelles in the same sense as eukaryotes. Instead, the chromosome and perhaps ribosomes are the only easily observable intracellular structures found in all bacteria. There do exist, however, specialized groups of bacteria that contain more complex intracellular structures, some of which are discussed below.
The Bacterial Chromosome and Plasmids: -
Unlike eukaryotes, the bacterial chromosome is not enclosed inside of a membrane-bound nucleus but instead resides inside the bacterial cytoplasm. This means that the transfer of cellular information through the processes of translation, transcription and DNA replication all occur within the same compartment and can interact with other cytoplasmic structures, most notably ribosomes. The bacterial chromosome is not packaged using histones to form chromatin as in eukaryotes but instead exists as a highly compact supercoiled structure, the precise nature of which remains unclear. Most bacterial chromosomes are circular although some examples of linear chromosomes exist. Along with chromosomal DNA, most bacteria also contain small independent pieces of DNA called plasmids that often encode for traits that are advantageous but not essential to their bacterial host. Plasmids can be easily gained or lost by a bacterium and can be transferred between bacteria as a form of horizontal gene transfer.
Ribosomes and Other Multiprotein Complexes: -
In most bacteria the most numerous intracellular structure is the ribosome, the site of protein synthesis in all living organisms. All prokaryotes have 70S (where S=Svedberg units) ribosomes while eukaryotes contain larger 80S ribosomes in their cytosol. The 70S ribosome is made up of a 50S and 30S subunits. The 50S subunit contains the 23S and 5S rRNA while the 30S subunit contains the 16S rRNA. These rRNA molecules differ in size in eukaryotes and are complexed with a large number of ribosomal proteins, the number and type of which can vary slightly between organisms. While the ribosome is the most commonly observed intracellular multiprotein complex in bacteria other large complexes do occur and can sometimes be seen using microscopy.
Intracellular Membranes: -
While not typical of all bacteria some microbes contain intracellular membranes in addition to (or as extensions of) their cytoplasmic membranes. An early idea was that bacteria might contain membrane folds termed mesosomes, but these were later shown to be artifacts produced by the chemicals used to prepare the cells for electron microscopy. Examples of bacteria containing intracellular membranes are phototrophs, nitrifying bacteria and methane-oxidising bacteria. Intracellular membranes are also found in bacteria belonging to the poorly studied Planctomycetes group, although these membranes more closely resemble organellar membranes in eukaryotes and are currently of unknown function.
Cytoskeleton: -
The prokaryotic cytoskeleton is the collective name for all structural filaments in prokaryotes. It was once thought that prokaryotic cells did not possess cytoskeletons, but recent advances in visualization technology and structure determination have shown that filaments indeed exist in these cells. In fact, homologues for all major cytoskeletal proteins in eukaryotes have been found in prokaryotes. Cytoskeletal elements play essential roles in cell division, protection, shape determination, and polarity determination in various prokaryotes.
Nutrient Storage Structures: -
Most bacterial habitats do not live in environments that contain large amounts of essential nutrients at all times. To accommodate these transient levels of nutrients bacteria contain several different methods of nutrient storage in times of plenty for use in times of want. For example, many bacteria store excess carbon in the form of polyhydroxyalkanoates or glycogen. Some microbes store soluble nutrients such as nitrate in vacuoles. Sulfur is most often stored as elemental (S0) granules which can be deposited either intra- or extracellularly. Sulfur granules are especially common in bacteria that use hydrogen sulfide as an electron source. Most of the above mentioned examples can be viewed using a microscope and are surrounded by a thin nonunit membrane to separate them from the cytoplasm.
Gas Vesicles: -
Gas vesicles are spindle-shaped structures found in some planktonic bacteria that provides buoyancy to these cells by decreasing their overall cell density. They are made up of a protein coat that is very impermeable to solvents such as water but permeable to most gases. By adjusting the amount of gas present in their gas vesicles bacteria can increase or decrease their overall cell density and thereby move up or down within the water column to maintain their position in an environment optimal for growth.
Carboxysomes: -
Carboxysomes are intracellular structures found in many autotrophic bacteria such as Cyanobacteria, Knallgasbacteria, Nitroso- and Nitrobacteria. They are proteinaceous structures resembling phage heads in their morphology and contain the enzymes of carbon dioxide fixation in these organisms (especially ribulose bisphosphate carboxylase/oxygenase, RuBisCO, and carbonic anhydrase). It is thought that the high local concentration of the enzymes along with the fast conversion of bicarbonate to carbon dioxide by carbonic anhydrase allows faster and more efficient carbon dioxide fixation than possible inside the cytoplasm. Similar structures are known to harbor the coenzyme B12-containing glycerol dehydratase, the key enzyme of glycerol fermentation to 1,3-propanediol, in some Enterobacteriaceae (e. g. Salmonella).
Magnetosomes: -
Magnetosomes are intracellular structures found in magnetotactic bacteria that allow them to sense and align themselves along a magnetic field (magnetotaxis). The ecological role of magnetotaxis is unknown but it is hypothesized to be involved in the determination of optimal oxygen concentrations. Magnetosomes are composed of the mineral magnetite and are surrounded by a nonunit membrane. The morphology of magnetosomes is species-specific.
Endospores: -
Perhaps the most well known bacterial adaptation to stress is the formation of endospores. Endospores are bacterial survival structures that are highly resistant to many different types of chemical and environmental stresses and therefore enable the survival of bacteria in environments that would be lethal for these cells in their normal vegetative form. It has been proposed that endospore formation has allowed for the survival of some bacteria for hundreds of millions of years (e.g. in salt crystals) although these publications have been questioned. Endospore formation is limited to several genera of Gram-positive bacteria such as Bacillus and Clostridium. It differs from reproductive spores in that only one spore is formed per cell resulting in no net gain in cell number upon endospore germination. The location of an endospore within a cell is species-specific and can be used to determine the identity of a bacterium.