MCB 100 1st Edition Lecture 14 Outline of Last Lecture I. Size of typical microorganisms II. The prokaryotic cell III. Structures seen in eukaryotic cells but not in prokaryotes a. Endoplasmic reticulum b. Mitochondriac. Chloroplastsd. Nuclear membrane e. Cytoskeleton f. Phagolysosomesg. snRNPs Outline of Current Lecture I. Mathematical considerations of bacterial growthA. Total count vs. viable count B. Generation time C. Growth rate II. Methods of enumerating microorganismsA. Direct microscopic count B. Membrane filter methodC. Most probable number D. Dry-weight method III. Turbidity Current LectureI. Mathematical considerations of bacterial growthA. Total bacteria cell count vs. viable cell count B. The total count includes dead bacteriaC. The viable count enumerates only those that can grow and reproduce i. A known volume of a liquid sample, usually 0.10 mL, is spread on the surface ofan agar plate that will allow bacteria to growii. After an appropriate incubation period, visible colonies of bacteria form iii. One can estimate the number of bacteria that were in the sample by counting the number of colonies that appear on the plate 1. Concentration of bacteria = (number of colonies)/(amount plated x dilution) These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.B. Bacterial growth is not so much about the growth of an individual, but the growth of the bacterial populationC. Bacteria reproduce by binary fission D. When the conditions are good for growth the bacterial population increases at an exponential rate E. Generation time: time it takes for a microbial population to doublei. Ex. 30 minutes per generation B. Growth rate: generations per unit of time [1/(generation time)] i. Ex. 2 generations per hour B. Formula to determine the number of bacteria in a growing population: i. Total number of bacteria = (initial number of bacteria) x (2^the number of generations) II. Methods of enumerating microorganisms A. Direct microscopic count: relatively fast but labor intensive. Microscopy works best for things that are larger than bacteria, like blood cells. Works best for samples with a concentration of b/w 10-200 mil cells per mL. A dead cell may look the same as a living cell, especially for bacteria i. Ex. Breed smear = direct count technique used in dairy industry in the past. A sample is smeared on a slide and examined. A count is made of the bacteria concentration and particulate matter such as dirt and straw (used to see if a sample of milk from a farm is clean enough to accept at the dairy) B. Membrane Filter method: similar to viable count except it is used for samples with a low concentration of bacteria. A known volume of water is run through asterile filter. Microbes are trapped on the filter. The filter is placed on the medium and allowed to incubate. i. Concentration of bacteria = (number of colonies)/(amount filtered) B. Most Probable Number (MPN): a statistical way of estimating the viable number of bacteria in a liquid sample by inoculating tubes of broth and counting the number of tubes that show growth. i. Results: 1. One coliform is enough to produce a positive result after incubation2. A bubble in the Durham tube indicates the presence of coliform bacteria3. The bubble is carbon dioxide produced by the fermentation of lactose4. A higher concentration of coliforms in the sample gives more positive tubes B. Dry-weight method: a liquid sample is run through a filter that has been previously weighed; microorganisms are trapped on the filter. Filter is baked dry to remove the water and reweighed. The increase in weight is proportional to the number of microorganisms that are trapped in the filter. i. Advantage: some microorganisms are difficult to count by other methods because they form clumps of cells that are hard to separateii. Disadvantage: dirt and other particulate contaminants will also be trapped by the filter II. TurbidityA. Turbid means "cloudy"B. Liquid broth culture that contains a lot of bacteria looks cloudy whereas a sample of sterile broth medium looks clearC. There's a direct relationship between the concentration of bacteria in a liquid cultureand the absorbance or scattering of light as it passes through the sample, one can use a spectrophotometer to measure the number of bacteria in a liquid
View Full Document
Unlocking...