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B R Levin

Publications and source records attributed to B R Levin.

At least 55 records · Page 3Linked to original sources
PubMed1986

Transitory derepression and the maintenance of conjugative plasmids.

It has been proposed that bacterial plasmids cannot be maintained by infectious transfer alone and that their persistence requires positive selection for plasmid-borne genes. To test this hypothesis, the population dynamics of two laboratory and five naturally occurring conjugative plasmids were examined in chemostat cultures of E. coli K-12. Both laboratory plasmids and three of the five wild plasmids failed to increase in frequency when introduced at low frequencies. However, two of the naturally occurring plasmids rapidly increased in frequency, and bacteria carrying them achieved dominance in the absence of selection for known plasmid-borne genes. Three hypotheses for the invasion and persistence of these two plasmids were examined. It is concluded that although these two extrachromosomal genetic elements are repressed for conjugative pili synthesis, as a consequence of high rates of transfer during periods of transitory derepression in newly formed transconjugants, they become established and are maintained by infectious transfer alone. The implications of these observations to the theory of plasmid maintenance and the evolution of repressible conjugative pili synthesis are discussed.

Conjugation, Genetic↗
PubMed1986

Difference in structure between type b and nontypable Haemophilus influenzae populations.

The extent of chromosomal genetic variability and the genetic structure of Haemophilus influenzae populations was analyzed. A total of 119 isolates from humans in Göteborg, Sweden, and Birmingham, Ala., and 16 strains from a type culture collection were characterized for capsular type, biotype, outer membrane protein profile, and enzyme electrophoretic type (ET). The results of this study indicate that the bacteria identified as H. influenzae are a genetically extremely variable array of organisms. For the six enzymes studied, the estimated mean genetic diversity was 0.57 (approximately 20% higher than the corresponding estimate for Escherichia coli). Two lines of evidence indicate that despite its ability to recombine by transformation, H. influenzae maintains a largely clonal population structure. Although there is considerable potential for generating different genotypes, there were only 88 distinct ETs among the 135 strains, and isolates of the same ET and biotype were recovered at frequencies greater than would be anticipated at random. This evidence for a clonal population structure holds for uncapsulated as well as capsulated strains. However, these data also suggest that the stability of H. influenzae clones (clone persistence time) may be less than that of the nontransforming species E. coli. The ET data indicate that there is somewhat less variability among H. influenzae strains that express the same capsular antigens, biotype, and outer membrane proteins than among randomly chosen isolates. Nevertheless, there is substantial genetic variation among isolates within each of these classes and combinations thereof. There is also variation in these typing characteristics among strains of the same ET. These observations and those on genetic variability and population structures have implications for the characterization of H. influenzae isolates in clinical and epidemiological studies.

Bacterial Outer Membrane Proteins↗
PubMed1985

Genetic diversity in relation to serotype in Escherichia coli.

The extent of chromosomal-gene diversity among 261 isolates of Escherichia coli sharing single O, K, or H antigens and various combinations thereof was estimated by multilocus enzyme electrophoresis, which detects allelic variation in structural genes. The results of this study indicate that the genetic diversity among isolates sharing single antigenic determinants can approach or equal that observed among randomly chosen strains; that the magnitude of the diversity varies among antigens; and that the genetic diversity is reduced, but not eliminated, among isolates sharing two antigenic determinants. With one exception, isolates of the same O:K:H serotype were of identical or closely related electrophoretic types (ETs). Isolates of the same ET generally shared the same combination of antigenic determinants, but some ETs included isolates of different serotypes. The implications of these findings for epidemiological research and the clone hypothesis of population structure are discussed, and possible evolutionary mechanisms causing antigenic divergence and convergence are considered.

Antigens, Bacterial↗
PubMed1984

The population biology of bacterial viruses: why be temperate.

A model of the interactions between populations of temperate and virulent bacteriophage with sensitive, lysogenic, and resistant bacteria is presented. In the analysis of the properties of this model, particular consideration is given to the conditions under which temperate bacteriophage can become established and will be maintained in bacterial populations. The effects of the presence of resistant bacteria and virulent phage on these "existence" conditions for temperate viruses are considered. It is demonstrated that under broad conditions temperate phage will be maintained in bacterial populations and will coexist with virulent phage. Extrapolating from this formal consideration of the population biology of temperate bacteriophage, a number of hypotheses for the conditions under which temperate, rather than virulent, modes of phage reproduction are to be anticipated and the nature of the selective pressures leading to the evolution and persistence of this "benign" type of bacterial virus are reviewed and critically evaluated. Two hypotheses for the "advantages of temperance" are championed: (1) As a consequence of the allelopathic effects of diffusing phage, in physically structured habitats, lysogenic colonies are able to sequester resources and, in that way, have an advantage when competing with sensitive nonlysogens. (2) Lysogeny is an adaptation for phage to maintain their populations in "hard times," when the host bacterial density oscillates below that necessary for phage to be maintained by lytic infection alone.

Bacteriophages↗
PubMed1984

Distribution of multilocus genotypes of Escherichia coli within and between host families.

Isolates from the intestinal Escherichia coli flora of 28 members of five families (including parents, children, and household pets) in Amherst, Massachusetts, and Rochester, New York, were characterized by the electrophoretic mobilities of 12 enzymes to estimate the extent of sharing of strains among associated and unassociated hosts. Among the 655 isolates examined, 60 different combinations of electromorphs (electrophoretic types or ETs), each representing a distinctive multilocus genotype, were identified, of which 85% were recovered from only a single individual. On average, 11% of the ETs isolated from the same family were shared by two or more members; 4.9% of ETs were shared among members of unassociated families living in the same city; and only 2% were shared by families in different cities. All three ETs that were recovered from multiple hosts in the present study are widespread clones that have been isolated from many other host individuals in North America and Sweden.

Animals↗
PubMed1981

Structured habitats and the evolution of anticompetitor toxins in bacteria.

We demonstrate that in liquid cultures, defined in this study as a mass habitat, the outcome of competition between Escherichia coli that produce an antibacterial toxin (colicin) and sensitive E. coli is frequency dependent; the colicinogenic bacteria are at an advantage only when fairly common (frequencies in excess of 2 X 10(-2)). However, we also show that in a soft agar matrix, a structured habitat, the colicinogenic bacteria have an advantage even when initially rare (frequencies as low as 10(-6)). These contrasting outcomes are attributed to the colicinogenic bacteria's lower intrinsic growth rate relative to the sensitive bacteria and the different manner in which bacteria and resources are partitioned in the two types of habitats. Bacteria in a liquid culture exist as randomly distributed individuals and the killing of sensitive bacteria by the colicin augments the amount of resource available to the colicinogenic bacteria to an extent identical to that experienced by the surviving sensitive bacteria. On the other hand, the bacteria in a soft agar matrix exist as single-clone colonies. As the colicinogenic colonies release colicin, they kill neighboring sensitive bacteria and form an inhibition zone around themselves. By this action, they increase the concentration of resources around themselves and overcome their growth rate disadvantage. We suggest that structured habitats are more favorable for the evolution of colicinogenic bacteria.

Biological Evolution↗
PubMed1981

Genetic diversity and temporal variation in the E. coli population of a human host.

Electrophoretic techniques were employed to study variation in chromosomal genes encoding enzymes and in the distribution of cryptic plasmids in the E. coli population of a human host over an 11-month period. Thirteen of the 15 enzymes studied were polymorphic, and mean genetic diversity per locus was 0.39. Among 550 clones isolated from fecal samples, protein electrophoresis revealed 53 distinct electrophoretic types (ETs). Most ETs appeared on only one or a few days and were considered transients, but two (ET-12 and ET-13) were observed many times over extended periods and represented residents. Complete turnover in the transient ETs in the population occurred in periods of from two weeks to a month. ETs appearing in one month showed no particular genetic similarity to those of the previous month. - All but 4 of the 53 ETs carried one or more "cryptic" plasmids with molecular weights ranging from 1 to 80 megadaltons. With few exceptions, the plasmid composition of each ET was unique. In the course of the 11-month sampling period, there were changes in the plasmid profiles of the resident strains ET-12 and ET-13, and also in the profile of a recurrent strain, ET-2, which was isolated on four days. Modification of the plasmid profile of ET-12 involved the sequential addition of relatively high molecular weight bands. For ET-2 and ET-13, the changes in the plasmid profiles were radical, suggesting invasions of new cell types rather than merely the addition and deletion of plasmids. - The results of this study provide three lines of evidence that recombination plays a minor role in the generation of genetic diversity in the E. coli population of a single host. (1) Several pairs of loci were in strong linkage disequilibrium; compared to a randomly generated array of genotypes, the sample of ETs contained an excess of pairs differing at one or two loci and too many pairs with highly distinctive combinations of electromorphs. (2) In most cases where pairs of ETs differed at a single locus and, therefore, could reasonably have been generated by phage- or plasmid-mobilized gene transfer, the plasmid profiles of the pair members were radically different and/or the potentially transmitted alleles were not present in other ETs in the population. (3) Although ET-12 was abundant, being represented by 252 of the 550 clones sampled, the electrophoretic type most similar to ET-12 different from it at six loci, and ET-12 carried two unique alleles. We conclude that most of the genetic diversity observed in this human host is a consequence of successive invasions of E. coli genotypes.

Alleles↗
PubMed1981

Periodic selection, infectious gene exchange and the genetic structure of E. coli populations.

As a consequence of sequential replacements by clones of higher fitness (periodic selection), bacterial populations would be continually purged of genetic variability, and the fate of selectively neutral alleles in very large populations of bacteria would be similar to that in demes of sexually reproducing organisms with small genetically effective population sizes. The significance of periodic selection in reducing genetic variability in these clonally reproducing species is dependent on the amount of genetic exchange between clones (recombination). In an effort to determine the relationship between the rates of periodic selection, recombination and the genetically effective sizes of bacterial populations, a model for periodic selection and infectious gene exchange has been developed and its properties analyzed. It shows that, for a given periodic selection regime, genetically effective population size increases exponentially with the rate of recombination.--With the parameters of this model in the range anticipated for natural populations of E. coli, the purging effects of periodic selection on genetic variability are significant; individual populations or lineages of this bacterial species would have very small genetically effective population sizes.--Based on this result, some other a priori considerations and a review of the results of epidemiological and genetic variability studies, it is postulated that E. coli is composed of a relatively limited number of geographically widespread and genetically nearly isolated and monomorphic lineages. The implications of these considerations of the genetic structure of E. coli populations on the interpretation of protein variation and the neutral gene hypothesis are discussed.

Escherichia coli↗
PubMed1980

Genetic diversity and structure in Escherichia coli populations.

A survey of electrophoretic variation in 20 enzymes from 109 clones of escherichia coli from natural populations yielded an estimate of mean genetic diversity approximately twice that reported in an earlier study and four to five times larger than estimates fro most eukaryotic species. Despite this extensive variability, the number of distinctive genotypes apparently is rather limited. Identical clones were obtained from unassociated hosts, and a clone that is electrophoretically indistinguishable from the laboratory strain Escherichia coli K-12 was isolated from a human infant. The results suggest that rates of genetic recombination in natural populations of Escherichia coli are low. These findings have implications for our understanding of the genetic structure of Escherichia coli populations and the factors determining the amount of neutral gene variability in this bacterial species.

Biological Evolution↗
PubMed1980

The population biology of bacterial plasmids: a priori conditions for the existence of mobilizable nonconjugative factors.

A mathematical model for the population dynamics of nonconjugative plasmids that can be mobilized by conjugative factors is presented. In the analysis of the properties of this model, primary consideration is given to the conditions under which these nonself-transmissible extrachromosomal elements could become established and would be maintained in bacterial populations. The results of this analysis demonstrate the existence of conditions where, as a consequence of infectious transmission via mobilization, nonconjugative plasmids could become established and be maintained even when the bacteria carrying them have lower reproductive fitnesses than plasmid-free members of the population. However, these existence conditions are stringent and suggest therefore, that it is highly unlikely that plasmids of this type would become established and maintained without some direct selection favoring their carriage. The general implications of these results and limitations of the model are discussed. Brief consideration is also given to the implications of these theoretical findings to the problems of the spread of multiple antibiotic resistance plasmids (R-factors) and the risk of contaminating natural populations of bacteria with chimeric plasmids produced by work with recombinant DNA.

Conjugation, Genetic↗
PubMed1977

Probability of establishing chimeric plasmids in natural populations of bacteria.

Formulas for estimating the probability that chimeric plasmids carried by disarmed hosts will become established in natural populations of bacteria are presented and their use illustrated with a series of realistic numerical examples. The implications of these a priori probability estimates for the problem of containment for recombinant DNA research is discussed.

Conjugation, Genetic↗