Microbial Growth Lecture PDF

Microbial Growth Prof. Dr. Wael N. Hozzein  Cell Growth Microbial growth involves an increase in the number of cells. Growth of most bacteria occurs by the process of binary fission. In prokaryotes (Bacteria and Archaea), the generation time is also called the doubling time and...

Microbial Growth Prof. Dr. Wael N. Hozzein  Cell Growth Microbial growth involves an increase in the number of cells. Growth of most bacteria occurs by the process of binary fission. In prokaryotes (Bacteria and Archaea), the generation time is also called the doubling time and is defined as the time it takes for the population to double through one round of binary fission. Escherichia coli can double in as little as 20 minutes Mycobacterium tuberculosis has a generation time of between 15 and 20 hours M. Leprae grows much more slowly, with a doubling time of 14 days  Bacterial populations show a characteristic type of growth pattern called exponential growth. Calculating Number of Cells It is possible to predict the number of cells in a population when they divide by binary fission at a constant rate. As an example, consider what happens if a single cell divides every 30 minutes for 24 hours. The number of cells increases exponentially and can be expressed as 2n, where n is the number of generations. If cells divide every 30 minutes, after 24 hours, 48 divisions would have taken place. If we apply the formula 2n, where n is equal to 48, the single cell would give rise to 248 or 281,474,976,710,656 cells at 48 generations (24 hours). When dealing with such huge numbers, it is more practical to use scientific notation. Therefore, we express the number of cells as 2.8 × 1014 cells. For any number of starting cells, the formula is adapted as follows: Nn = N0 2n The Growth Curve Measuring Microbial Growth  Measuring Growth Direct methods – count individual cells Indirect Methods – measure effects of microbial growth  Direct Counting Growth is measured by the change in the number of cells over time. Cell counts done microscopically to measure the total number of cells in a population, whereas viable cell counts (plate counts) measure only the living reproducing population. Direct microscopic count: Counting chambers (slides) for microscope Does not necessarily yield an accurate count of the number of live cells because it is not always possible to distinguish between live cells and dead cells Petroff-Hausser chamber The fluorescence staining techniques make it possible to distinguish viable and dead bacteria. Additional Direct Measurement method Filtration method of choice for low counts Viable cell counts: Plate counts: Serial dilutions put on plates CFUs form colonies Fig. i7.6 Figure 6.15, step 1  Indirect Counting Turbidity measurements are an indirect but very rapid and useful method of measuring bacterial growth. However, to relate a direct cell count to a turbidity value, a standard curve must first be established. Spectrophotometry to measure turbidity OD is function of cell number Metabolic Activity Dry Weight Diameter