For an exponentially growing culture of micro organisms the specific growth rate (µ) is a
parameter, that gives the cell biomass (g) synthesized per gram of existing cell biomass per
unit of time (usually, per hour). This rate (µ) is inversely related to the doubling time of the
culture, td: µ = ln2/td ≈ 0.7/td. Hence, the shorter the doubling time of cells, the higher is the
specific growth rate of the culture.
Two micro organisms, A and B, were inoculated each in a fresh growth medium with an initial
optical density (OD) of 0.1. A lag phase of 1 hr duration was observed for both cultures.
Three hours after inoculation, the OD of culture A was 0.4, while that of the culture B was
1.6.
1. Estimate the specific growth rate for culture A
2. Estimate the specific growth rate for culture B
Answers
Answer:Define the generation time for growth based on binary fission
Identify and describe the activities of microorganisms undergoing typical phases of binary fission (simple cell division) in a growth curve
Explain several laboratory methods used to determine viable and total cell counts in populations undergoing exponential growth
Nataliya, a 24-year-old pregnant woman in her second trimester, visits a clinic with complaints of high fever, 38.9 °C (102 °F), fatigue, and muscle aches—typical flu-like signs and symptoms. Nataliya exercises regularly and follows a nutritious diet with emphasis on organic foods, including raw milk that she purchases from a local farmer’s market. All of her immunizations are up to date. However, the health-care provider who sees Nataliya is concerned and orders a blood sample to be sent for testing by the microbiology laboratory.
The bacterial cell cycle involves the formation of new cells through the replication of DNA and partitioning of cellular components into two daughter cells. In prokaryotes, reproduction is always asexual, although extensive genetic recombination in the form of horizontal gene transfer takes place, as will be explored in a different chapter. Most bacteria have a single circular chromosome; however, some exceptions exist. For example, Borrelia burgdorferi, the causative agent of Lyme disease, has a linear chromosome.
The most common mechanism of cell replication in bacteria is a process called binary fission, which is depicted in Figure 1. Before dividing, the cell grows and increases its number of cellular components. Next, the replication of DNA starts at a location on the circular chromosome called the origin of replication, where the chromosome is attached to the inner cell membrane. Replication continues in opposite directions along the chromosome until the terminus is reached.
A) A micrograph of two rod shaped cells attached at their ends. B) A diagram of binary fission. First a cell replicates its DNA and elongates. Then, as the cell continues to elongate, each loop of DNA travels to one end or the other. The cell then starts to constrict in the center. This results in two cells each containing a loop of DNA.
Figure 1. (a) The electron micrograph depicts two cells of Salmonella typhimurium after a binary fission event. (b) Binary fission in bacteria starts with the replication of DNA as the cell elongates. A division septum forms in the center of the cell. Two daughter cells of similar size form and separate, each receiving a copy of the original chromosome. (credit a: modification of work by Centers for Disease Control and Prevention)
The center of the enlarged cell constricts until two daughter cells are formed, each offspring receiving a complete copy of the parental genome and a division of the cytoplasm (cytokinesis). This process of cytokinesis and cell division is directed by a protein called FtsZ. FtsZ assembles into a Z ring on the cytoplasmic membrane (Figure 2). The Z ring is anchored by FtsZ-binding proteins and defines the division plane between the two daughter cells. A diagram of a cell dividing. The cell is shaped like a figure 8; each end of the cell contains a loop a DNA.
In eukaryotic organisms, the generation time is the time between the same points of the life cycle in two successive generations. For example, the typical generation time for the human population is 25 years. This definition is not practical for bacteria, which may reproduce rapidly or remain dormant for thousands of years. 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. Bacterial doubling times vary enormously. Whereas Escherichia coli can double in as little as 20 minutes under optimal growth conditions in the laboratory, bacteria of the same species may need several days to double in especially harsh environments. Most pathogens grow rapidly, like E. coli, but there are exceptions. For example, Mycobacterium tuberculosis, the causative agent of tuberculosis, has a generation time of between 15 and 20 hours. On the other hand, M. leprae, which causes Hansen’s disease (leprosy), grows much more slowly, with a doubling time of 14 days.
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.
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