The specific growth rates at various temperatures of 12 bacterial species were measured and plotted as Arrhenius profiles. Temperature characteristics and optimum temperatures for maximum specific growth rates were estimated from these curves. The data reveal that one of two forms of the Arrhenius profile is characteristic of each bacterium: one curve is a simple smooth curve with a single predominant slope at sub-optimum temperatures; the other is a more complex curve with two distinct slopes at sub-optimum temperatures. The simple curves describes bacteria across the entire biokinetic range whereas the more complex curve occurs only with organisms which have optimum temperatures for replication above 37 degrees C. Describing bacteria in terms of these forms of the Arrhenius profile is less arbitrary than is categorization based on optimum temperatures.
The DbtS ؉ phenotype (which confers the ability to oxidize selectively the sulfur atom of dibenzothiophene [DBT] or dibenzothiophene sulfone [DBTO 2 ]) of Rhodococcus erythropolis N1-36 was quantitatively characterized in batch and fed-batch cultures. In flask cultures, production of the desulfurization product, monohydroxybiphenyl (OH-BP), was maximal at pH 6.0, while specific productivity (OH-BP cell ؊1) was maximal at pH 5.5. Quantitative measurements in fermentors (in both batch and fed-batch modes) demonstrated that DBTO 2 as the sole sulfur source yielded a greater amount of product than did DBT. Specifically, 100 M DBT maximally yielded Ϸ40 M OH-BP, while 100 M DBTO 2 yielded Ϸ60 M OH-BP. Neither maintaining the pH at 6.0 nor adding an additional carbon source increased the yield of OH-BP. The presence of SO 4 2؊ in growth media repressed expression of desulfurization activity, but SO 4 2؊ added to suspensions of cells grown in DBT or DBTO 2 did not inhibit desulfurization activity.
Recent progress in studies on the bacterial chromosome is summarized. Although the greatest amount of information comes from studies on Escherichia coli, reports on studies of many other bacteria are also included. A compilation of the sizes of chromosomal DNAs as determined by pulsed-field electrophoresis is given, as well as a discussion of factors that affect gene dosage, including redundancy of chromosomes on the one hand and inactivation of chromosomes on the other hand. The distinction between a large plasmid and a second chromosome is discussed. Recent information on repeated sequences and chromosomal rearrangements is presented. The growing understanding of limitations on the rearrangements that can be tolerated by bacteria and those that cannot is summarized, and the sensitive region flanking the terminator loci is described. Sources and types of genetic variation in bacteria are listed, from simple single nucleotide mutations to intragenic and intergenic recombinations. A model depicting the dynamics of the evolution and genetic activity of the bacterial chromosome is described which entails acquisition by recombination of clonal segments within the chromosome. The model is consistent with the existence of only a few genetic types of E. coli worldwide. Finally, there is a summary of recent reports on lateral genetic exchange across great taxonomic distances, yet another source of genetic variation and innovation.
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