4. Direct Microscopic Count A specific volume of a bacterial suspension(0.01 ml) is placed on a microscope slide witha special grid. Stain is added to visualize bacteria. Cells are counted and multiplied by a factor toobtain concentration. 52
4. Direct Microscopic CountAdvantages:• No incubation time required. 53
4. Direct Microscopic CountDisadvantages:• Cannot always distinguish between live anddead bacteria.• Motile bacteria are difficult to count.• Requires a high concentration of bacteria(10 million/ml). 54
Petroft Hausser HaemacytometerDirect Microscopic Count 55
Direct Microscopic Count 56
57
Indirect Methods of Measurement 1. Turbidity 2. Metabolic Activity 3. Dry Weight 58
1. Turbidity As bacteria multiply in media, it becomes turbid. Use a spectrophotometer to determine% transmission or absorbance. Multiply by a factor to determineconcentration. 59
1. Turbidity Advantages:• No incubation time required. 60
1. Turbidity Disadvantages:• Cannot distinguish between live and deadbacteria.• Requires a high concentration of bacteria(10 to 100 million cells/ml). 61
spectrophotometerA spectrophotometer is used to determine turbidity bymeasuring the amount of light that passed through asuspension of cells. 62
spectrophotometer 63
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2. Metabolic Activity As bacteria multiply in media, they produce certain products: • Carbon dioxide • Acids 65
2. Metabolic Activity Measure metabolic products. Expensive 66
Fermentation product 67 67
Measurement of Bacterial Growth 68
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