Death of cells as a function of time is rather unpredictable and very difficult to explain. At death phase, bacteria run out of nutrients and die. This basic batch culture growth model draws out and emphasizes aspects of bacterial growth which may differ from the growth of macrofauna. It emphasizes clonality, asexual binary division, the short development time relative to replication itself, the seemingly low death rate, the need to move from a dormant state to a reproductive state or to condition the media, and finally, the tendency of lab adapted strains to exhaust their nutrients.
In reality, even in batch culture, the four phases are not well defined. Learning Objectives Examine microbial generation times. Key Points The doubling time is the generation time of the bacteria. The measurement of an exponential bacterial growth curve can be done by cell counting, colony counting, or determining the turbidity of bacterial cultures. Bacterial growth in batch culture can be modeled with four different phases: lag phase, exponential or log phase, stationary phase, and death phase.
Key Terms bacterium : A single celled organism with no nucleus. It is applied to population growth, inflation, resource extraction, consumption of goods, compound interest, the volume of malignant tumours, and many other things which tend to grow over time. October 16, October 28, Provided by : Wiktionary.
Located at : en. Provided by : Boundless Learning. Located at : www. Provided by : Wikipedia. Time measurements are in hours for bacteria with short generation times.
Lag Phase. Immediately after inoculation of the cells into fresh medium, the population remains temporarily unchanged. Although there is no apparent cell division occurring, the cells may be growing in volume or mass, synthesizing enzymes, proteins, RNA, etc. The length of the lag phase is apparently dependent on a wide variety of factors including the size of the inoculum; time necessary to recover from physical damage or shock in the transfer; time required for synthesis of essential coenzymes or division factors; and time required for synthesis of new inducible enzymes that are necessary to metabolize the substrates present in the medium.
Exponential log Phase. The exponential phase of growth is a pattern of balanced growth wherein all the cells are dividing regularly by binary fission, and are growing by geometric progression. The cells divide at a constant rate depending upon the composition of the growth medium and the conditions of incubation. The rate of exponential growth of a bacterial culture is expressed as generation time , also the doubling time of the bacterial population.
Stationary Phase. Exponential growth cannot be continued forever in a batch culture e. Population growth is limited by one of three factors: 1. During the stationary phase, if viable cells are being counted, it cannot be determined whether some cells are dying and an equal number of cells are dividing, or the population of cells has simply stopped growing and dividing. The stationary phase, like the lag phase, is not necessarily a period of quiescence.
Bacteria that produce secondary metabolites , such as antibiotics, do so during the stationary phase of the growth cycle Secondary metabolites are defined as metabolites produced after the active stage of growth. It is during the stationary phase that spore-forming bacteria have to induce or unmask the activity of dozens of genes that may be involved in sporulation process. Death Phase. If incubation continues after the population reaches stationary phase, a death phase follows, in which the viable cell population declines.
Note, if counting by turbidimetric measurements or microscopic counts, the death phase cannot be observed. During the death phase, the number of viable cells decreases geometrically exponentially , essentially the reverse of growth during the log phase. As mentioned above, bacterial growth rates during the phase of exponential growth, under standard nutritional conditions culture medium, temperature, pH, etc.
Generation times for bacteria vary from about 12 minutes to 24 hours or more. The generation time for E. For most known bacteria that can be cultured, generation times range from about 15 minutes to 1 hour. Symbionts such as Rhizobium tend to have longer generation times.
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