Bacterial Reproduction and Growth Kinetics

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Chapter: Pharmaceutical Microbiology : Bacteria

The majority of bacterial cells multiply in number by a process of binary fission. That is, each individual will increase in size until it is large enough to divide into two identical daughter cells.


BACTERIAL REPRODUCTION AND GROWTH KINETICS

 

Multiplication And Division Cycle

 

The majority of bacterial cells multiply in number by a process of binary fission. That is, each individual will increase in size until it is large enough to divide into two identical daughter cells. At the point of separation each daughter cell must be capable of growth and reproduction. While each daughter cell will automatically contain those materials that are dispersed throughout the mother cell (mRNA, rRNA, ribosomes, enzymes, cytochromes, etc.), each must also carry at least one copy of the chromosome. The bacterial chromosome is circular and attached to the cytoplasmic membrane where it is able to uncoil during DNA replication. The process of DNA replication proceeds at a fixed rate dependent on temperature, therefore the time taken to copy an entire chromosome depends on the number of base pairs within it and the growth temperature. For Escherichia coli growing at 37 °C, replication of the chromosome will take approximately 45 minutes. These copies of the chromosome must then segregate to opposite sides of the cell before cell division can proceed. Division occurs in different ways for Gram-positive and Gram-negative bacteria. Gram-negative cells do not have a rigid cell wall and divide by a process of constriction followed by membrane fusion. Gram positive cells, on the other hand, having a rigid cell wall, must develop a cross-wall that divides the cell into two equal halves. Constriction and cross-wall formation takes approximately 15 minutes to complete. DNA replication, chromosome segregation (C-phase) and cell division (D-phase) occur sequentially in slow growing cells with generation times of greater than 1 hour and are the final events of the bacterial cell cycle. Cells are able to replicate faster than once every hour by initiating several rounds of DNA replication at a time. Thus partially replicated chromosomes become segregated into the newly formed daughter cells. In this fashion it is possible for some organisms growing under their optimal conditions to divide every 15–20 minutes. Rod shaped organisms maintain their diameter during the cell cycle and increase their mass and volume by a process of elongation. When the length of the cell has approximately doubled then the division/constriction occurs centrally. Coccal forms increase in size by radial expansion, with the division plane going towards the geometric centre. In some genera the successive division planes are always parallel. Under such circumstances the cells appear to form chains (i.e. streptococci). In staphylococci successive division planes are randomized, giving dividing clusters of cells the appearance of a bunch of grapes. Certain genera, e.g. Sarcina, rotate successive division planes by 90 ° to form tetrads and cubical octets. The appearance of dividing cells under the microscope can therefore be a useful initial guide to identification.

 

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