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Bacterial Cell Structure; Growth and multiplication of bacteria
Typology: Lecture notes
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Cell Structure
(a) Capsule - The capsules are the outmost structures of bacterial cells. These are the gelatinous secretion of some bacteria which provides cell with additional
protection helps them in preventing phagocytosis of bacteria. Phagocytosis is a type of endocytosis in which any cells uses their plasma membrane to swallow up a large external particle. These capsules are secreted by the cell into the external environment and are highly impermeable. However, the capsules are considered to be a major virulence factor of bacteria. That means almost all the bacterial pathogens including Streptococcus pneumoniae, Klebsiella pneumonia, Neisseria meningitidis, Haemophilus influenza and Escherichia coli etc. have polysaccharide capsules on their surface. (b) Flagella - These are long (about 20 nm) hair or whip like helical filaments extending from cytoplasmic membrane to exterior of the cell. These flagella help bacteria to move towards nutrients and other stimuli. The long filament of flagella comprises of many subunits of a single protein called flagellin. This protein is synthesized within the cell and extends through the centre of flagella. Flagellin is highly antigenic and have key role in cell motility. The position of the flagella varies with the bacterial species. Functionally and structurally, it is divided into three parts,
(b) Cytoplasm - Similar to the eukaryotes, bacterial cytoplasm is also a colloidal system consisting of a variety of organic and inorganic constituents such as 80% Water and 20% Salts, Proteins. They are rich in ribosomes, DNA and fluid. Apart from chromosomal DNA, the extra choromosomal DNA is characteristically closed and circular. These extra chromosomal DNA is called Plasmids. They are highly coiled and complexed with polyamines and other support proteins. (c) Ribosomes - Ribosomes are the platform of protein synthesis whereby they receive the genetic commands and translate these in the form of specific proteins. Ribosomes are composed of ribosomal RNA and protein. The bacterial ribosomes are slightly smaller than the ribosomes of eukaryotic cells and composed of two subunits namely 50S and 30S as opposed to 60S and 40S in eukaryotes. These two subunits combine together to form complete 70S ribosomes during protein synthesis (also called translation process). Here “S” denotes a Svedberg unit which is basically a non-metric unit for the sedimentation rate or sedimentation coefficient and is considered as a measure of time defined as 10-13 seconds. The sedimentation coefficient refers to the rate at which a molecule or particle precipitate at the bottom of a test tube under the centrifugal force of an ultra-high speed centrifuge. (d) Mesosomes - They are vesicular structure produced by localized and inward folding of plasma membrane into the cytoplasm. Mesosomes are rich in respiratory enzymes and other enzymes responsible for DNA replication and cell division. (e) Nucleoid - The nucleus is not distinct in prokaryotes and hence called nucleoid. It doesn’t have a uniform shape and size as there is no nuclear membrane around it. The nucleoid is principally composed of several copies of DNA which exist in the form of closed, continuous and coiled thread. In addition, nucleoids also have some RNA and proteins.
(f) Spore - Some bacteria form highly resistant resting stage called spores, which helps them to sustain in adverse environmental conditions. They are neither a reproductive form nor a storage granule. These spores enable bacteria to be resistant against the adverse environmental conditions and bactericidal agents as well as. There exists three layers in the spore namely core, cortex and spore coat. Growth and multiplication of bacteria Bacteria multiply by binary fission where the cell divides to form two daughter cells. Nuclear division takes place before cell division and therefore, in a growing population, many cells having two nuclear bodies are commonly found. Bacterial growth may be considered as two levels, first is the increase in size of individual cells and second is the increase in number of bacterial cells. Growth in numbers of bacterial cells can be studied by bacterial counts as against that of total and viable counts. The total count indicates the number of cells either living or dead and the viable count represents the number of living cells that are capable of multiplication. Bacterial Growth Curve When bacterial cells are grown in vitro or cultured in a suitable culture media followed by incubation, their growth follows a particular pattern. If bacterial counts are carried out at regular intervals after inoculation and plotted with respect to time, a growth curve is observed. The curve shows the following phase- (i) Lag phase: Immediately following the bacterial inoculation, there is no appreciable increase in number, though there may be an increase in the size of the cells. This early period is actually the time required by the bacteria for
The viable cell counts remain stationary as there remains equilibrium between the dying cells and the newly formed cells. (iv) Decline Phase: This is the last phase when the bacterial population declines due to cell death. The different phases of bacterial growth curve are directly related to the morphological and physiological adaptations of the cells. The maximum cell size is attained towards the end of the lag phase. In the second or log or exponential growth phase, cells are smaller in size and stained uniformly. In the third or stationary phase, bacterial cells are commonly gram variable and show uneven staining owing to the presence of intracellular storage granules. Generally spore formation takes place at this stage. Besides, many bacteria produce secondary metabolites such as exotoxins and antibiotics. Cell deaths are the common feature in the phase of decline. Nutritional diversity in bacteria On the basis of energy source organisms are designated as: Phototrophs : The organisms which can utilize light as an energy source are known as phototrophs. These bacteria gain energy from light. Chemotrophs : These bacteria gain energy from chemical compounds. They cannot carry out photosynthesis. On the basis of electron source organisms are designated as:
Lithotrophs : Some organisms can use reduced organic compounds as electron donors and are termed as Lithotrophs. They can be Chemolithotrophs and Photolithotrophs Organotrophs : Some organisms can use organic compounds as electron donors and are termed as organotrophs. Some can be Chemoorganotrophs and Photoorganotrophs. Thus, bacteria may be either: Photo-lithotrops : These bacteria gain energy from light and use reduced inorganic compounds such as H2S as a source of electrons. eg: Chromatium okeinii. Photo-organotrophs : These bacteria gain energy from light an d use organic compounds such as Succinate as a source of electrons.eg; Rhodospirillum. Chemo-lithotrophs : These bacteria gain energy from reduced inorganic compounds such as NH3 as a source of electron eg; Nitrosomonas. Chemo-organotrophs : These bacteria gain energy from organic compounds such as glucose and ammino acids as a source of electrons.eg; Pseudomonas pseudoflora. Some bacteria can live ether chemo-lithotrophs or chemo organotrophs like Pseudomonas pseudoflora as they can use either glucose or H2S as electron source. On the basis of carbon source bacteria may be: All organisms require carbon in some form for use in synthesizing cell components. All organisms require at least a small amount of CO2. However, some can use CO2 as their major or even sole source of carbon; such organisms are termed as Autotrophs (Autotrophic bacteria).
(ii) Chemoautotrophs These bacteria do not require light (lack the light phase but have the dark phase of photosynthesis) and pigment for their nutrition. These bacteria oxidize certain inorganic substances with the help of atmospheric oxygen. This reaction releases the energy (exothermic) which is used to drive the synthetic processes of the cell. Sulphomonas (Sulphur bacteria): These bacteria obtain energy by oxidation of elemental sulphur or H2S, e.g., Thiobacillus, Beggiatoa. Elemental Sulphur Oxidising Bacteria : Denitrifying sulphur bacteria oxidize elemental sulphur to sulphuric acid e.g., Thiobacillus denitrificans 2S + 2H2O + 3O2 → 2H2SO4 + 126 kcal. Sulphide Oxidizing Bacteria: These bacteria oxidizes H2S and release the sulphur e.g., Beggiatoa. 2H2S +4O2 → 2H2O + 2S + 141.8 cal Hydromonas (Hydrogen bacteria) These convert hydrogen into water, e.g., Bacillus pantotrophus, Hydrogenomonas. 2H2 + O2 → 2H2O + 55 kcal. 4H2 + CO2 → 2H2O + CH4 + Energy Ferromonas (Iron bacteria): These bacteria inhabit water and obtain energy by oxidation of ferrous compounds into ferric forms. e.g., Thomasville’s ferroxidans, Ferro bacillus, Leptothrix. 4FeCo3 + 6H2O + O2 → 4Fe (OH)3 + 4CO2 + 81 kcal. Methanomonas (Methane bacteria):
These bacteria get their energy by oxidation of methane into water and carbon dioxide. Nitrosomonas (Nitrifying bacteria): These bacteria get their energy by oxidation of ammonia and nitrogen compounds into nitrates. Nitrosomonas oxidises NH3 to nitrites. NH3 + ½O2 ® H2O + HNO2 + Energy Nitrobacter converts nitrites to nitrates. NO2 + ½O2 ® NO2 + Energy Carbon Bacteria: These bacteria oxidizes CO into CO2 e.g., Bacillus oligocarbophillous, Oligotropha carboxydovorans 2CO + O2 → 2CO2 + Energy Heterotrophic Bacteria The heterotrophic bacteria obtain their-ready made food from organic substances, living or dead. Most of pathogenic bacteria of human beings, other plants and animals are heterotrophs. Some heterotrops have simple nutritional requirement while some of them require large amount of vitamin and other growth promoting substance. Such organisms are called fastidious heterotrophs. Heterotrophic bacteria are of three types: a. Photoheterotrophs These bacteria can utilize light energy but cannot use CO2 as their sole source of carbon. They obtain energy from organic compounds to satisfy their carbon and electron requirements. Bacteriochlorophyll pigment is found in these bacteria.
iii) Symbiotic bacteria Symbiotic bacteria live in close association with other organisms as symbionts. They are beneficial to the organisms. The common examples are the nitrogen-fixing bacteria, e.g., Bacillus radicicola, B. azotobacter, Rhizobium, Clostridium, Rhizobium spp., B. radicicolaand B. azotobacter. These bacteria live inside the roots of leguminous plants. These bacteria fix free atmospheric nitrogen into nitrogenous compounds which are utilized by the plants. In return, the plant provides nutrients and protection to the bacteria.
