Binary Fission

Chapter 10

Bacterial Reproduction and Growth of Microorganisms

Chapter Outline

Preview to Chapter 10

Bacterial Reproduction 287

Binary Fission

Alternate Means of Bacterial Reproduction

Bacterial Spore Formation Bacterial Growth 288

Generation Time

Methodology: Logarithms

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

Temperature

Methodology: Enriching for Specific Bacteria

Historical Perspective: Deep Sea Thermal Vent Bacteria

Oxygen

Methodology: Growing Cultures of Aerobic and Anaerobic Bacteria

Salinity

Acidity and pH

Pressure

Light Radiation

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

barophiles halophiles

barotolerant lag phase

batch culture log phase

binary fission mesophiles

budding microaerophiles

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

procedures osmotolerant

doubling time psychrophiles

exponential phase salt tolerant

facultative anaerobes stationary growth phase

BACTERIAL REPRODUCTION

Binary Fission

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. 10-1 A, 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 septum by 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. Budding is 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. 10-2 Colorized electron micrograph of Caulobacter crescentus showing mother cell with stalk (pink) and daugh­ter cell forming by fission.

Дата добавления: 2015-08-05