Bacterial Reproduction and Growth of Microorganisms
Preview to Chapter 10
Bacterial Reproduction 287
Alternate Means of Bacterial Reproduction
Bacterial Spore Formation Bacterial Growth 288
Bacterial Growth Curve
Batch and Continuous Growth
Bacterial Growth on Solid Media Enumeration of Bacteria 292
Viable Count Procedures
Newsbreak: Unsafe Ice Cream Goes Undetected
Direct Count Procedures
Most Probable Number (MPN) Procedures Factors Influencing Bacterial Growth 296
Methodology: Enriching for Specific Bacteria
Historical Perspective: Deep Sea Thermal Vent Bacteria
Methodology: Growing Cultures of Aerobic and Anaerobic Bacteria
Acidity and pH
In this chapter we will:
• Study the reproduction of bacteria.
• See that bacterial reproduction results in a characteristic growth curve.
• Learn that a consequence of bacterial reproduction by binary fission is a high reproductive capacity.
• Examine the factors that influence bacterial growth rates.
• Learn the following key terms and names:
acidophiles generation time
alkalophiles growth curve
barotolerant lag phase
batch culture log phase
binary fission mesophiles
chemostat most probable number colony forming units enumeration procedure
(CFUs) obligate aerobes
continuous culture obligate anaerobes
cysts optimal growth death phase temperature
direct counting osmophilic
doubling time psychrophiles
exponential phase salt tolerant
facultative anaerobes stationary growth phase
Most bacteria reproduce by binary fission.Each bacterial cell divides exactly in half to form two equal-size progeny (daughter) cells (FIG. 10-1). Since bacteria typically are single celled, the reproduction of a single cell accomplishes the reproduction of the entire organism. Binary fission is an asexual process — meaning that a single cell divides to form genetically identical progeny and that genetic recombination does not occur in the process.
During the reproduction of a bacterial cell, the parent cell elongates and the cell wall grows inward, dividing the cell in half. This establishes two progeny cells, each surrounded by a cell wall and a plasma membrane. Each of the progeny cells receives a complete set of hereditary information. Replication of the bacterial chromosome is a prerequisite for reproduction of a duplicate bacterial cell.
Binary fission is the most common means of bacterial reproduction.
FIG. 10-1A, Colorized micrograph of Escherichia coli dividing by binary fission. B, Cell growth occurs at specific sites so that the cell elongates prior to division. The cell wall and plasma membrane are growing inward to separate the cells and the replicated bacterial chromosomes.
During cell division the bacterial chromosome appears to be attached to the plasma membrane and cell wall. Formation of a crosswall or septumby the inwardly moving cell wall and plasma membrane physically separates the bacterial chromosomes and distributes them to the two daughter cells. Septum formation pinches off and separates the two complete bacterial chromosomes, providing each progeny cell with a bacterial chromosome (genome) containing a complete set of genetic information. This process requires active protein synthesis to move the bacterial chromosomes to the proper positions. On completion of the crosswall there are two equal-size cells that can separate. Repeating the process results in the multiplication of the bacterial population.
Alternate Means of Bacterial Reproduction
Binary fission is the most common mode of bacterial reproduction. Some bacteria multiply by other means. The various modes of replication differ in how the cellular material is apportioned between the daughter cells and whether the cells separate or remain together as part of a multicellular aggregation. For example, bacteria in the genus Hyphomicrobium attach to solid surfaces in fresh and saltwater environments and reproduce by budding. Buddingis a type of division characterized by an unequal division of cellular material. Similarly, Caulobacter cell division is unequal; cell division is by elongation of a stalked cell, followed by fission (FIG. 10-2). The daughter cell develops when a crosswall forms, segregating a small portion of the cytoplasm containing
FIG. 10-2Colorized electron micrograph of Caulobacter crescentus showing mother cell with stalk (pink) and daughter cell forming by fission.