華中農(nóng)業(yè)大學(xué)微生物英文版課件2-資料下載頁
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【正文】 wmolecularweight substances while excluding large molecules. Paracrystalline Surface Layers (SLayers) Bacterial endospores ? Certain species of bacteria produce special structure called endospores. ? They are very resistant to heat and can not be destroyed easily, even by harsh chemicals. Endospores are also resistant to other harmful agents such as drying, radiation, acids and chemical disinfectants. Sporulating cell Central core Cortex Spore coat / membrane exosporium Micrograph of a endospore Vegetative cell Containing abundant DPA (dipicolinic acid) which is bined with calcium ions. ? Lower water content – only 1030% of the water content of the vegetative cell. ? Low pH value, and contains small acidsoluble spore proteins (SASPs) Structure of DPA Properties of endospore and its resistance Sporulation involves a very plex series of events in cellular differentiation. Bacterial sporulation does not occur when cells are dividing exponentially but only when growth ceases owing to the exhaustion of an essential nutrient. Thus, cells of Bacillus cease vegetative growth and begin sporulation when a key nutrient such as the carbon or nitrogen source bees limiting. Endospore formatiom 1, Axial filament formation 2, Septum formatiom 3, Engulfment of forespore 4, Cortex formation 5, Coat synthesis 6, Completion of coat synthesis, Increase in refractility and heat resistance 7, Lysis of sporangium, spore liberation Stages in endospore formation Spore germination 1. Activation – Usually results from treatments like heating. 2. Germination – Breaks spore’s dormant state. This process is characterized by spore swelling, loss of resistance to heat and other stresses, loss of refractility and increase in metabolic activity. 3. Outgrowth – The spore protoplast makes new ponents and develops once more into an active bacterium. Parasporal Crystal (Sporepanioned crystal) Several Bacillus species, most notably and B. thuringiensis, produce intracellular crystals of toxic glycoproteins when they sporulate. Toxic crystals Please carefully read and be familiar with the contents of table on the page 100. Comparison of the prokaryotic and eucaryotic cell Their DNA is associated with histone proteins. membranebounded anelles 9+2 type flagella Eucaryotic cell The chief distinguishing characteristics of procaryotic cells are: 1. Their geic material (DNA) is not enclosed within a membrane. 2. They lack other membranebounded anelles. 3. Their DNA is not associated with histone proteins. 4. Their cell walls almost always contain the plex polysaccharide peptidoglycan. 5. They usually divide by binary fission. During this process, the DNA is copied and the cell splits into two cells. Summary 1, Prokaryotic geic material is located in an area called the nucleoid and is not enclosed by a membrane. 2, Most bacteria have a cell wall outside the plasma membrane to give them shape and protect them from osmotic lysis. Bacterial walls are chemically plex and usually contain peptidoglycan or murein. 3, Bacteria often are classfied as either gram positive or gram negative based on differences in cell wall structure and their response to Gram staining. Grampositive walls have thick,homogeneous layers of peptidoglycan and teichoid acid. Gramnegative bacteria have a thin peptidoglycan layer surrounded by a plex outer membrane containing lipopolysaccharides and other ponents 4, Some bacteria are motile, usually by means of flagella, and bacterial species differ in the number and distribution of their flagella. 5, Structures such as capsules, fimbriae, and sex pili are found outside of some bacterial cell wall. 6, Some bacteria survive adverse environmental conditions by forming endospores, dormant structures resistant to heat,desiccation and many chemicals
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