288 Chapter 10 bacterial reproduction and growth of microorganisms

a duplicate genome. In the case of the actinomycetes, such as Streptomyces, reproduction involves the for­mation of hyphae. In this mode of reproduction the cell elongates, forming a relatively long and gener­ally branched filament, or hypha. Regardless of the mode of reproduction, bacterial multiplication re­quires replication of the bacterial chromosome and synthesis of new boundary layers, including cell wall and plasma membrane structures. All these modes of reproduction are asexual, like binary fission.

Bacterial Spore Formation

Spores are specialized cells produced by some bacte­ria that are involved in survival or reproduction. The production of spores represents an interesting devia­tion from vegetative cell reproduction. Some types of spores, including endospores (heat resistant spores formed within the cell) and cysts (resting or dormant cells sometimes enclosed in a sheath) are not repro-

ductive structures and their production does not i crease the number of living cells. In contra arthrospores (spores formed by the fragmentation hyphae) are produced by different bacteria as part their reproductive cycles. The fragmentation of h phae to produce arthrospores forms numerous pro eny cells. Additionally, myxobacteria form reprodu tive structures, called fruiting bodies, within whi numerous spores, called myxospores (resting cells the myxobacteria formed within a fruiting body), a formed. Myxospores are the progeny that result fro reproduction of myxobacteria; they are able to si vive transport through the air and increase the si vival capacity of myxobacteria by permitting disser ination to areas with adequate supplies of nutrien to support bacterial growth and reproduction.

Spores are specialized resistant resting cells pro­duced by bacteria. Endospores are involved in sur­vival, but other types of spores, such as arthrospores, are involved with reproduction.

BACTERIAL GROWTH

Generation Time

Bacterial growth is synonymous with bacterial cell re­production. Growing bacterial cells increase in bio-mass through their metabolism in which they convert compounds containing carbon, nitrogen, phosphorus, and other elements into the components of the cell. Most then divide into progeny of equal biomass through binary fission. By its very nature, bacterial re­production by binary fission results in doubling of the

number of viable bacterial cells. Therefore, during a tive bacterial growth, the size of the microbial populi tion is continuously doubling. Once cell division h gins, it proceeds exponentially as long as growth coi ditions permit. One cell divides to form two, each ( these cells divides so that four cells form, and so fort in a geometric progression. The time required t achieve a doubling of the population size, known a the generation time or doubling time, is the unit с measure of microbial growth rate (FIG. 10-3).

METHODOLOGY

Logarithms

To conveniently represent large numbers, particularly when the numbers may range over many orders of magnitude (multiples of 10)—as is the case for numbers of bacterial cells that may occur as a few or millions of cells—scientists use a mathematical transformation called the logarithmic transformation. The logarithm to the base 10 (log₁₀) of a number is the exponent indicat­ing the power of 10 to which a number must be raised to produce a given number. Thus the log₁₀10 = 1 be­cause the exponent to which 10 must be raised to equal 10 is 1 (10¹). Similarly the log₁₀100 = 2 because 10² = 100, and thus the exponent of 10 needed to equal 100 = 2. In a similar fashion the log₁₀l,000 = 3, log₁₀10,000 = 4, log₁₀100,000 = 5, log₁₀l,000,000 = 6, and so forth. Ob­viously in plotting the growth of a bacterial cell over the range of generations that takes 1 cell (log₁₀l = 0) to 1 billion cells (log₁₀l,000,000,000 = 9), it is far easier to

plot the logarithm of the cell number using a scale of 0 to 9 than it would be to try to plot the cell numbers on an arithmetic scale of 1 to 1,000,000,000 (see Figure).

Comparison of arithmetic number of cells and logarithmic in- j crease in numbers during bacterial growth.

BACTERIAL GROWTH 289

FIG. 10-3 A, During exponential growth the number of cells doubles each generation time. B, A graph of the log number of bacteria versus time (right) compared with number of bacteria versus time (left).