Madical brain Notes 10

  Estimation of microbial numbers Several methods exist for the measurement of bacterial numbers, most of which are also applicable to the enumeration of other unicellular forms such as yeasts. Such methods fall into two main categories: those that count total cell numbers, and those that count viable cells only. Total cell counts  are generally done by direct microscopic examination. A specialisedglass slide is employed, which carries an etched grid of known area (Figure 5.1). The depth of the liquid sample is also known, so by counting the number of cells visible in the field of view, the number of cells per unit volume can be determined. The method may be made more accurate by the use of a fluorescent dye such as acridine orange, which binds to DNA, and hence avoids confusion with non-cellular debris. However, such methods cannot differentiate between living and non-living cells. Their usefulness is further limited by the fact that the smallest bacteria are difficult to res...

  Factors affecting microbial growth We discussed the nutrient requirements of microorganisms. Assuming these are present in an adequate supply, what other factors do we need to consider in order to provide favourable conditions for microbial growth? As the following section shows, growth may be profoundly affected by a number of physical factors. Temperature Microorganisms as a group are able to grow over a wide range of temperatures, from around freezing to above boiling point. For any organism, the  minimum  and  maximum growth temperatures define the range over which growth is possible; this is typically about 25–30  ◦  C. Growth is slower at low temperatures because enzymes work less efficiently and also because lipids tend to harden and there is a loss of membrane fluid-ity. Growth rates increase with temperature until the  optimum  temperature is reached, then the rate falls again (Figure 5.4). The optimum and limiting temperatures for an o...

  The kinetics of microbial growth Unicellular organisms divide by  binary fission ; each cell grows to full size, replicates its genetic material then divides into two identical daughter cells. By identical means, two cells divide into four, four into eight and so on, leading to an exponential increase in cell numbers: If we were to plot the number of cells in a population against time, we would get an exponential curve (Figure 5.8a). It is more convenient when plotting a growth curve to plot the logarithm of cell numbers of against time, giving us a straight line (Fig-ure 5.8b). Such exponential growth cannot continue indefinitely, however, and growth usually slows down due to either the supply of nutrients becoming exhausted, or be-cause metabolism leads to an accumulation of harmful waste substances. Unicellular growth usually occurs in a series of different phases (Figure 5.9): 1. Lag phase . When an inoculum of bacteria is first in-troduced into some growth medium, it wi...

  Growth in multicellular microorganisms If uninterrupted, growth in fungi proceeds radially outwards from the initiating spore, allowing the fungal colony to colonise new regions potentially rich in nutrients. Actual growth occurs solely at the hyphal tip; as this happens, the terminal cell grows longer, until eventually a new cross wall or septum is formed. Cells away from the tip do not become any longer during hyphal extension, however hyphae in this region may develop into aerial reproductive structures. Older hyphae at some distance from the tip may become completely empty of cytoplasm. Cell counts and turbidometric measurements are not appropriate to estimate growth of fungi; however total mycelial mass can be measured and its change plotted against time. A fungal growth cycle shows roughly the same phases of growth as described above for bacteria.