6.Microbial Grow 6.1 The Growth Curve 115 Table 6.1 An Example of Exponential Growth ary ph Time 29 esult from eral fa ors operating in 0= have see 2 well bco ironments hav be a p what in o rall size se linking and ce damaged protein As a result of the and many othe ls become harder to kill and more re osmotic dar edical and industrial microbiology.The e is even evidence tha m more hr pathogcns be Death Phase Detrimental environme ental changes like nutrient deprivation and crobial population,like its growth during the exponential pha otal cell number mains constant because the cels mply fail to no eTowandrprodcc.itisasmcdiocd lead.That is, eath is de Although most of microbial population lydies ina Minutes of incub n,the the po s.For this and other reasons, Figure 6.3 Exp ential Microbial Gr wth.The data from table 6.1 The Mathematics of Growth ity of the log plo y and nat a cu will be ninutes. cells after 40 minutes drngspecific length of mcaed the generation time or dou ting pop ulation increase is exponential or logarithmic (figure 6.3) Prescott−Harley−Klein: Microbiology, Fifth Edition II. Microbial Nutrition, Growth, and Control 6. Microbial Growth © The McGraw−Hill Companies, 2002 growth is inhibited. Streptococcal cultures also can enter the sta￾tionary phase due to depletion of their sugar supply. Finally, there is some evidence that growth may cease when a critical popula￾tion level is reached. Thus entrance into the stationary phase may result from several factors operating in concert. As we have seen, bacteria in a batch culture may enter station￾ary phase in response to starvation. This probably often occurs in na￾ture as well because many environments have quite low nutrient lev￾els. Starvation can be a positive experience for bacteria. Many do not respond with obvious morphological changes such as endospore formation, but only decrease somewhat in overall size, often ac￾companied by protoplast shrinkage and nucleoid condensation. The more important changes are in gene expression and physiology. Starving bacteria frequently produce a variety of starvation pro￾teins, which make the cell much more resistant to damage in a va￾riety of ways. They increase peptidoglycan cross-linking and cell wall strength. The Dps (DNA-binding protein from starved cells) protein protects DNA. Chaperones prevent protein denaturation and renature damaged proteins. As a result of these and many other mechanisms, the starved cells become harder to kill and more re￾sistant to starvation itself, damaging temperature changes, oxidative and osmotic damage, and toxic chemicals such as chlorine. These changes are so effective that some bacteria can survive starvation for years. Clearly, these considerations are of great practical importance in medical and industrial microbiology. There is even evidence that Salmonella typhimurium and some other bacterial pathogens be￾come more virulent when starved. Death Phase Detrimental environmental changes like nutrient deprivation and the buildup of toxic wastes lead to the decline in the number of viable cells characteristic of the death phase. The death of a mi￾crobial population, like its growth during the exponential phase, is usually logarithmic (that is, a constant proportion of cells dies every hour). This pattern in viable cell count holds even when the total cell number remains constant because the cells simply fail to lyse after dying. Often the only way of deciding whether a bacte￾rial cell is viable is by incubating it in fresh medium; if it does not grow and reproduce, it is assumed to be dead. That is, death is de￾fined to be the irreversible loss of the ability to reproduce. Although most of a microbial population usually dies in a logarithmic fashion, the death rate may decrease after the popu￾lation has been drastically reduced. This is due to the extended survival of particularly resistant cells. For this and other reasons, the death phase curve may be complex. The Mathematics of Growth Knowledge of microbial growth rates during the exponential phase is indispensable to microbiologists. Growth rate studies contribute to basic physiological and ecological research and the solution of applied problems in industry. Therefore the quantita￾tive aspects of exponential phase growth will be discussed. During the exponential phase each microorganism is dividing at constant intervals. Thus the population will double in number during a specific length of time called the generation time or dou￾bling time. This situation can be illustrated with a simple example. Suppose that a culture tube is inoculated with one cell that divides every 20 minutes (table 6.1). The population will be 2 cells after 20 minutes, 4 cells after 40 minutes, and so forth. Because the popu￾lation is doubling every generation, the increase in population is always 2n where n is the number of generations. The resulting pop￾ulation increase is exponential or logarithmic (figure 6.3). 6.1 The Growth Curve 115 Table 6.1 An Example of Exponential Growth Division Population Timea Number 2n (N0
2n ) log10Nt 0 020 1 1 0.000 20 1 21 2 2 0.301 40 2 22 4 4 0.602 60 3 23 8 8 0.903 80 4 24 16 16 1.204 100 5 25 32 32 1.505 120 6 26 64 64 1.806 a The hypothetical culture begins with one cell having a 20-minute generation time. 90 80 70 60 50 40 30 20 10 0 1.500 1.000 0.500 0.000 Log10 number of cells ( ) Number of cells ( ) 0 20 40 60 80 100 120 Minutes of incubation Figure 6.3 Exponential Microbial Growth. The data from table 6.1 for six generations of growth are plotted directly (•–•) and in the logarithmic form ( °–° ). The growth curve is exponential as shown by the linearity of the log plot.