Sec 6: Microbial Growth Curve

Microbial Growth Curve Population growth is often studied by analyzing microbial growth in liquid media. When microorganisms are cultured in liquid media, they are then incubated in a closed culture vessel such as a test tube or flask with one batch of media. Fresh media is not provided during incubation, so the nutrients in the media are consumed and their concentration decreases, and waste accumulates. The growth of a population of microbes that reproduce by binary fission in a culture batch can be plotted as the logarithm of the number of healthy cells against the incubation time. The resulting curve has five distinct phases. Although this is based on what occurs in the laboratory, because microbes do encounter conditions in the natural environment they will mimic what happens in batch culture.Furthermore, routinely create artificial media for microbes (e.g., fermentation vessels in pharmaceutical plants) that constitute batch cultures. Therefore understanding the growth curve is very important. Lag Phase When microorganisms are introduced into fresh culture media, usually no immediate increase in cell number occurs. This period is called the lag phase. This is not an inactivation time; rather the cell synthesizes new components that are necessary for various reasons. Cells may be old and depleted of ATP, essential cofactors, and ribosomes; it must then be synthesized before growth can begin. The medium may be different from the one the microorganism grew in previously. In this case, new enzymes are needed to use different nutrients. Perhaps the microorganisms have been damaged and need time to recover. cells begin to replicate their DNA, increase in mass, and divide. As a result, the number of cells in the population begins to increase. Exponential Phase During the exponential phase, microbes grow and divide at the maximum rate possible given their genetic potential, the nature of the medium, and environmental conditions. The growth rate is constant during the exponential phase, i.e. completing the cell cycle and doubling in number periodically. The population is most uniform in terms of chemical and physiological properties during this phase; therefore exponential biochemical and physiological studies. Growth rates during the exponential phase depend on several factors, including nutrient availability. When microbial growth is limited by a nutrient concentration that is lower than required, the final growth or cell yield increases with the initial amount so that the nutrient can become an existing limit. Growth rate also increases with nutrient concentration but eventually saturates, as is seen with many enzymes. The shape of the curve is thought to reflect the rate of nutrient uptake by transport proteins in microbes. At sufficiently high nutrient levels, the transport system is saturated, and growth rate does. Stationary Phase (Still Phase) In closed systems such as batch cultures, growth eventually stops and the growth curve becomes horizontal. This stationary phase is achieved by some bacteria at a population level of approximately 109 cells per milliliter. Protist cultures often have a maximum concentration of about 106 cells per milliliter. The final population size depends on nutrient availability and other factors, as well as the type of microorganism. In the stationary phase, the total number of living microorganisms remains constant. This may result from a balance between cell division and cell death, or the population may stop dividing but remain metabolically active. Microbes enter a stationary phase for various reasons. One important reason is nutritional limitations, if important nutrients are depleted, population growth will slow down and eventually stop.Aerobic organisms are often limited by O2 availability . Oxygen is not very soluble and can be depleted so quickly that only the surface of the culture has an adequate O2 concentration for growth. Growth may also stop due to the accumulation of toxic waste products. This appears to limit the growth of many cultures grown in the absence of O2 .For example, streptococci can produce so much acid from fermenting sugar that their growth is stunted. There is some evidence that growth may stop when critical population levels are reached. So going into stationary phase may be the result of several factors. Nutrient levels influencing population growth must be transmitted to molecules that influence cell growth.DNA, the protein that binds to the origin of chromosomes to initiate replication, becomes less active in the stationary phase. Replication in progress is completed, but no further initiations occur. This is one of many ways cells conserve energy by eliminating processes that are not essential for survival. Death Phase Cells grown in batch culture cannot remain in stationary phase indefinitely. Eventually they enter a phase known as the death phase. During this phase, the number of cells decreases exponentially, with cells dying at a constant rate. Adverse environmental changes such as nutritional deficiencies and the buildup of toxic waste cause irreparable damage to cells. Long Term Stationary Phase (Extended stationary phase) Long-term growth experiments reveal that after a period of exponential death some microbes have long periods in which the population size remains or is more or less constant. This longterm quiescent phase (also called extended quiescent phase) can last months to years . During this time, the bacterial population continues to grow so that the cells that are actively reproducing are the ones that are best able to use the nutrients that have been released by the dying cells and are best able to tolerate the accumulation of toxins. This dynamic process is characterized by successive waves of genetically distinct variants. In this way, natural selection can be witnessed in one culture vessel.