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.     

Sustainability of Virulence in a Phage-Bacterial Ecosystem

Virulent phages and their bacterial hosts represent an unusual sort of predator-prey system where each time a prey is eaten, hundreds of new predators are born. It is puzzling how, despite the apparent effectiveness of the phage predators, they manage to avoid driving their bacterial prey to extinction. Here we consider a phage-bacterial ecosystem on a two-dimensional (2-d) surface and show that homogeneous space in itself enhances coexistence. We analyze different behavioral mechanisms that can facilitate coexistence in a spatial environment. For example, we find that when the latent times of the phage are allowed to evolve, selection favors “mediocre killers,” since voracious phage rapidly deplete local resources and go extinct. Our model system thus emphasizes the differences between short-term proliferation and long-term ecosystem sustainability.The replication strategies of phages fall into two major categories: virulent and temperate. A temperate phage has the ability to integrate its DNA into the host chromosome, where it is then replicated along with the bacterial DNA during cell division. This strategy allows the phage to slow down or completely stop exploitation of the bacteria, thus reducing the risk of driving its host to extinction. A virulent phage lacks this ability, and it is not fully understood how they manage to coexist with their bacterial prey (4, 19). Consider, for example, the highly effective T4 phage. For the sake of argument, let us assume a burst size of 100 offspring upon lysis. On average, not more than a single phage out of each burst of 100 should survive to infect another bacterium, or else the phage would rapidly outgrow the bacteria and drive them to extinction. The half-life (t_1/2) of a free T4 phage particle has been measured to be approximately 10 days in LB at 37°C (6). Therefore, on average, at least t_1/2 × log₂(100) ≈ 2 months should pass between infections to prevent runaway phage growth—a time span that seems highly unreasonable for many of the environments where phage and bacteria interact, such as soil or biofilm. Even a more considered calculation, inserting the above half-life measurement into more realistic Lotka-Volterra-like predator-prey models (9) does not change the conclusion that T4 and other virulent phages appear to be far too effective predators for coexistence to be feasible. It is, however, an undisputed fact that virulent phages and bacteria have coexisted for eons and do so still, everywhere around us and inside us. One possible explanation for this puzzle is that bacteria constantly evolve resistance to existing phages and that the phages evolve to attack resistant bacteria in a continuous arms race. This “Red Queen” argument (23) has, however, been criticized on the grounds that the rates of evolution of phages and bacteria are not symmetric (17, 12). Recent measurements support this: in soil, phages appear to be “ahead of the bacteria in the coevolutionary arms race” (24). We therefore wish to explore mechanisms other than bacterial resistance that may promote coexistence between virulent phages and bacteria.Historically, phage-bacterial ecosystem models have ignored the issue of space, utilizing zero-dimensional approaches, such as ordinary differential equations (e.g., see references 1, 5, 13, 14, 15, and 21). However, many real phage-bacterial ecosystems are found in environments with a complex spatial structure, such as soil, biofilms, or wounds in animal and plant tissue. Schrag and Mittler (20) showed that coexistence between virulent phage and bacteria is feasible in a chemostat but not in serial cultures, due to the heterogeneous nature of the environment in the chemostat. Further, experiments done by Brockhurst et al. (3) indicate that reduced phage dispersal can prolong coexistence for virulent phage and bacteria in spatial environments by creating ephemeral refuges for the bacteria. Kerr et al. (10) introduced a simple cellular automaton to model fragmented populations of phage and bacteria in which coexistence was more easily achieved when migration was spatially restricted. Thus, the main extension to the simple predator-prey framework that we examine will be to add a spatial dimension.We construct and compare two phage-bacterial ecosystem models: one model where the phage and bacteria exist in a two-dimensional space, such as the surface of an agar gel (referred to as the “spatial model”), and the other model where the phage and bacteria are repeatedly mixed, mimicking serial cultures or a well-mixed broth (referred to as the “well-mixed model”). We show that space does indeed enhance coexistence. We then move on to explore other mechanisms that phage could incorporate into their behavior to further enhance coexistence. These can broadly be classified as “hardwired” (where every phage follows the same deterministic strategy) versus “adaptive” (where each phage potentially behaves differently, thus allowing the population to explore different options).We have chosen to look at three specific mechanisms as examples of these categories: (i) phage effectiveness would be reduced if they were unable to register whether they were infecting live, infected, or dead bacteria (a hardwired behavior); (ii) phage could prolong their latent time, concurrently increasing burst size, depending on the number of multiple infections (also a hardwired behavior, but a more “active” sort, where each phage senses and responds to information from the environment; T4 is known to use such a lysis inhibition strategy), and (iii) phage offspring could have altered latent times due to mutations in the holin genes (an adaptive behavior). We will compare each of these mechanisms in the spatial and well-mixed models to investigate whether the heterogeneity possible in a spatial environment affects the outcome.     

The effect of a bacteriophage on diversification of the opportunistic bacterial pathogen, Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic human pathogen that colonizes the lungs of cystic fibrosis (CF) patients. CF lungs often contain a diverse range of P. aeruginosa phenotypes, some of which are likely to contribute to the persistence of infection, yet the causes of diversity are unclear. While the ecological heterogeneity of the lung environment and therapeutic regimes are probable factors, a role for parasitic bacteriophage cannot be ruled out. Parasites have been implicated as a key ecological variable driving the evolution of diversity in host populations. PP7 drove cycles of morphological diversification in host populations of P. aeruginosa due to the de novo evolution of small-rough colony variants that coexisted with large diffuse colony morph bacteria. In the absence of phage, bacteria only displayed the large diffuse colony morphology of the wild-type. Further assays revealed there to be two distinct types of resistant bacteria; these had very different ecological phenotypes, yet each carried a cost of resistance.     

Susceptibility of Staphylococcus epidermidis planktonic cells and biofilms to the lytic action of staphylococcus bacteriophage K

AIMS: To evaluate differences in biofilm or planktonic bacteria susceptibility to be killed by the polyvalent antistaphylococcus bacteriophage K. METHODS AND RESULTS: In this study, the ability of phage K to infect and kill several clinical isolates of Staphylococcus epidermidis was tested. Strains were grown in suspension or as biofilms to compare the susceptibility of both phenotypes to the phage lytic action. Most strains (10/11) were susceptible to phage K, and phage K was also effective in reducing biofilm biomass after 24 h of challenging. Biofilm cells were killed at a lower rate than the log-phase planktonic bacteria but at similar rate as stationary phase planktonic bacteria. CONCLUSIONS: Staphylococcus epidermidis biofilms and stationary growth phase planktonic bacteria are more resistant to phage K lysis than the exponential phase planktonic bacteria. SIGNIFICANCE OF STUDY: This study shows the differences in Staph. epidermidis susceptibility to be killed by bacteriophage K, when grown in biofilm or planktonic phenotypes.     

Bacteriophages drive strain diversification in a marine Flavobacterium: implications for phage resistance and physiological properties

Genetic, structural and physiological differences between strains of the marine bacterium Cellulophaga baltica MM#3 (Flavobacteriaceae) developing in response to the activity of two virulent bacteriophages, ΦS_M and ΦS_T, was investigated during 3 weeks incubation in chemostat cultures. A distinct strain succession towards increased phage resistance and a diversification of the metabolic properties was observed. During the incubation the bacterial population diversified from a single strain, which was sensitive to 24 tested Cellulophaga phages, into a multistrain and multiresistant population, where the dominant strains had lost susceptibility to up to 22 of the tested phages. By the end of the experiment the cultures reached a quasi steady state dominated by ΦS_T‐resistant and ΦS_M + ΦS_T‐resistant strains coexisting with small populations of phage‐sensitive strains sustaining both phages at densities of > 10⁶ plaque forming units (pfu) ml^?1. Loss of susceptibility to phage infection was associated with a reduction in the strains' ability to metabolize various carbon sources as demonstrated by BIOLOG assays. This suggested a cost of resistance in terms of reduced physiological capacity. However, there was no direct correlation between the degree of resistance and the loss of metabolic properties, suggesting either the occurrence of compensatory mutations in successful strains or that the cost of resistance in some strains was associated with properties not resolved by the BIOLOG assay. The study represents the first direct demonstration of phage‐driven generation of functional diversity within a marine bacterial host population with significant implications for both phage susceptibility and physiological properties. We propose, therefore, that phage‐mediated selection for resistant strains contributes significantly to the extensive microdiversity observed within specific bacterial species in marine environments.     

Characterization of a T5-like coliphage, SPC35, and differential development of resistance to SPC35 in Salmonella enterica serovar typhimurium and Escherichia coli

The potential of bacteriophage as an alternative biocontrol agent has recently been revisited due to the widespread occurrence of antibiotic-resistant bacteria. We isolated a virulent bacteriophage, SPC35, that can infect both Salmonella enterica serovar Typhimurium and Escherichia coli. Morphological analysis by transmission electron microscopy and analysis of its 118,351-bp genome revealed that SPC35 is a T5 group phage belonging to the family Siphoviridae. BtuB, the outer membrane protein for vitamin B(12) uptake, was found to be a host receptor for SPC35. Interestingly, resistant mutants of both E. coli and S. Typhimurium developed faster than our expectation when the cultures were infected with SPC35. Investigation of the btuB gene revealed that it was disrupted by the IS2 insertion sequence element in most of the resistant E. coli isolates. In contrast, we could not detect any btuB gene mutations in the resistant S. Typhimurium isolates; these isolates easily regained sensitivity to SPC35 in its absence, suggesting phase-variable phage resistance/sensitivity. These results indicate that a cocktail of phages that target different receptors on the pathogen should be more effective for successful biocontrol.     

Elevated abundance of bacteriophage infecting bacteria in soil

Here we report the first direct counts of soil bacteriophage and show that substantial populations of these viruses exist in soil (grand mean = 1.5 x 10(7) g(-1)), at least 350-fold more than the highest numbers estimated from traditional viable plaque counts. Adding pure cultures of a Serratia phage to soil showed that the direct counting methods with electron microscopy developed here underestimated the added phage populations by at least eightfold. So, assuming natural phages were similarly underestimated, virus numbers in soil averaged 1.5 x 10(8) g(-1), which is equivalent to 4% of the total population of bacteria. This high abundance was to some extent confirmed by hybridizing colonies grown on Serratia and Pseudomonas selective media with cocktails of phage infecting these bacteria. This showed that 8.9 and 3.9%, respectively, hybridized with colonies from the two media and confirmed the presence of phage DNA sequences in the cultivable fraction of the natural population. Thus, soil phage, like their aquatic counterparts, are likely to be important in controlling bacterial populations and mediating gene transfer in soil.     

Effects of antagonistic coevolution on parasite‐mediated host coexistence

Parasites can promote diversity by mediating coexistence between a poorer and superior competitor, if the superior competitor is more susceptible to parasitism. However, hosts and parasites frequently undergo antagonistic coevolution. This process may result in the accumulation of pleiotropic fitness costs associated with host resistance, and could breakdown coexistence. We experimentally investigated parasite‐mediated coexistence of two genotypes of the bacterium Pseudomonas fluorescens, where one genotype underwent coevolution with a parasite (a virulent bacteriophage), whereas the other genotype was resistant to the evolving phages at all time points, but a poorer competitor. In the absence of phages, the resistant genotype was rapidly driven extinct in all populations. In the presence of the phages, the resistant genotype persisted in four of six populations and eventually reached higher frequencies than the sensitive genotype. The coevolving genotype showed a reduction in the growth rate, consistent with a cost of resistance, which may be responsible for a decline in its relative fitness. These results demonstrate that the stability of parasite‐mediated coexistence of resistant and susceptible species or genotypes is likely to be affected if parasites and susceptible hosts coevolve.     

Dynamics of the interaction between Pseudomonas aeruginosa bacteriophage phimF81 and host bacteria]

The population interactions of Pseudomonas aeruginosa virulent bacteriophage phi mF81 with host bacterial cells were studied in dynamics under the conditions of continuous cultivation in the chemostat regime with glucose limitation. It was detected that a maintenance of the bacterium and its specific bacteriophage in the population was realized due to the successive appearance of bacterial mutants resistant to the phage and of phage mutants overcoming this resistance.     

Bacteriophage phi297, a new species of <Emphasis Type="Italic">Pseudomonas aeruginosa</Emphasis> temperate phages with a mosaic genome: Potential use in phage therapy

The genome of halo-forming temperate Pseudomonas aeruginosa phage phi297 and lytic activity of its virulent mutant were studied. A mosaic structure was revealed for phi297 genome by its complete sequencing. The phi297 genome was partly homologous to the genomes of phages D3 and F116. High lytic activity was assumed for temperate P. aeruginosa bacteriophage phi297 on the basis of morphological features of negative colonies. Virulent mutant phi297vir, which was capable of lysing the wild-type phage bacteria, was isolated. Lytic activity was compared for phi297 and the phages from commercial mixtures of two manufacturers (facilities of Nizhnii Novgorod and Perm’). Phage phi297 caused lysis of the mutant PAO1 bacteria that were resistant to the phages from commercial preparations, but the lytic activity spectrum of phi297 was narrower that the spectra of the commercial phages. The use of nonreverting virulent mutants of certain temperate bacteriophages was proposed for the treatment of P. aeruginosa infections.     

PHAGE-MEDIATED SELECTION AND THE EVOLUTION AND MAINTENANCE OF RESTRICTION-MODIFICATION

Restriction-modification (R-M) was discovered because it provides bacteria with immunity to phage infection. But, is phage-mediated selection the sole mechanism responsible for the evolution and maintenance of these ubiquitous and multiply evolved systems? In an effort to answer this question, we have performed experiments with laboratory populations of E. coli and phage and computer simulations. We consider two ecological situations whereby phage-mediated selection could favor R-M immunity; i) when bacteria with a novel R-M system invade communities of phage-sensitive bacteria in which there are one or more species of phage, and ii) when bacteria colonize bacterial-free habitats in which phage are present. The results of our experiments indicate that in established communities of bacteria and phage, the advantage R-M provides an invading population of bacteria is ephemeral. Within short order, mutants resistant (refractory) to the phage evolve in the dominant population and subsequently in the invading population. The outcome of competition then depends on the relative fitness of the resistant states of these bacterial clones, rather than R-M. As a consequence of sequential selection for independent mutants, this rapid evolution of resistance occurs even when two and three species of phage are present. While in our experiments resistance also evolved when bacteria colonized new habitats in which phage were present, a novel R-M system greatly augmented the likelihood of their becoming established. We interpret the results of this study as support for the hypothesis that the latter, colonization selection, may play an important role in the evolution and maintenance of restriction-modification. However, we also see these results and other observations we discuss as questioning whether protection against phage is the unique biological role of restriction-modification.     

Elevated Abundance of Bacteriophage Infecting Bacteria in Soil

Here we report the first direct counts of soil bacteriophage and show that substantial populations of these viruses exist in soil (grand mean = 1.5 × 10⁷ g⁻¹), at least 350-fold more than the highest numbers estimated from traditional viable plaque counts. Adding pure cultures of a Serratia phage to soil showed that the direct counting methods with electron microscopy developed here underestimated the added phage populations by at least eightfold. So, assuming natural phages were similarly underestimated, virus numbers in soil averaged 1.5 × 10⁸ g⁻¹, which is equivalent to 4% of the total population of bacteria. This high abundance was to some extent confirmed by hybridizing colonies grown on Serratia and Pseudomonas selective media with cocktails of phage infecting these bacteria. This showed that 8.9 and 3.9%, respectively, hybridized with colonies from the two media and confirmed the presence of phage DNA sequences in the cultivable fraction of the natural population. Thus, soil phage, like their aquatic counterparts, are likely to be important in controlling bacterial populations and mediating gene transfer in soil.     

Antagonistic coevolution between a bacterium and a bacteriophage

Antagonistic coevolution between hosts and parasites is believed to play a pivotal role in host and parasite population dynamics, the evolutionary maintenance of sex and the evolution of parasite virulence. Furthermore, antagonistic coevolution is believed to be responsible for rapid differentiation of both hosts and parasites between geographically structured populations. Yet empirical evidence for host-parasite antagonistic coevolution, and its impact on between-population genetic divergence, is limited. Here we demonstrate a long-term arms race between the infectivity of a viral parasite (bacteriophage; phage) and the resistance of its bacterial host. Coevolution was largely driven by directional selection, with hosts becoming resistant to a wider range of parasite genotypes and parasites infective to a wider range of host genotypes. Coevolution followed divergent trajectories between replicate communities despite establishment with isogenic bacteria and phage, and resulted in bacteria adapted to their own, compared with other, phage populations.     

The Population and Evolutionary Dynamics of Phage and Bacteria with CRISPR–Mediated Immunity

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR), together with associated genes (cas), form the CRISPR–cas adaptive immune system, which can provide resistance to viruses and plasmids in bacteria and archaea. Here, we use mathematical models, population dynamic experiments, and DNA sequence analyses to investigate the host–phage interactions in a model CRISPR–cas system, Streptococcus thermophilus DGCC7710 and its virulent phage 2972. At the molecular level, the bacteriophage-immune mutant bacteria (BIMs) and CRISPR–escape mutant phage (CEMs) obtained in this study are consistent with those anticipated from an iterative model of this adaptive immune system: resistance by the addition of novel spacers and phage evasion of resistance by mutation in matching sequences or flanking motifs. While CRISPR BIMs were readily isolated and CEMs generated at high rates (frequencies in excess of 10⁻⁶), our population studies indicate that there is more to the dynamics of phage–host interactions and the establishment of a BIM–CEM arms race than predicted from existing assumptions about phage infection and CRISPR–cas immunity. Among the unanticipated observations are: (i) the invasion of phage into populations of BIMs resistant by the acquisition of one (but not two) spacers, (ii) the survival of sensitive bacteria despite the presence of high densities of phage, and (iii) the maintenance of phage-limited communities due to the failure of even two-spacer BIMs to become established in populations with wild-type bacteria and phage. We attribute (i) to incomplete resistance of single-spacer BIMs. Based on the results of additional modeling and experiments, we postulate that (ii) and (iii) can be attributed to the phage infection-associated production of enzymes or other compounds that induce phenotypic phage resistance in sensitive bacteria and kill resistant BIMs. We present evidence in support of these hypotheses and discuss the implications of these results for the ecology and (co)evolution of bacteria and phage.     

The Molecular and Genetic Basis of Repeatable Coevolution between Escherichia coli and Bacteriophage T3 in a Laboratory Microcosm

The objective of this study was to determine the genomic changes that underlie coevolution between Escherichia coli B and bacteriophage T3 when grown together in a laboratory microcosm. We also sought to evaluate the repeatability of their evolution by studying replicate coevolution experiments inoculated with the same ancestral strains. We performed the coevolution experiments by growing Escherichia coli B and the lytic bacteriophage T3 in seven parallel continuous culture devices (chemostats) for 30 days. In each of the chemostats, we observed three rounds of coevolution. First, bacteria evolved resistance to infection by the ancestral phage. Then, a new phage type evolved that was capable of infecting the resistant bacteria as well as the sensitive bacterial ancestor. Finally, we observed second-order resistant bacteria evolve that were resistant to infection by both phage types. To identify the genetic changes underlying coevolution, we isolated first- and second-order resistant bacteria as well as a host-range mutant phage from each chemostat and sequenced their genomes. We found that first-order resistant bacteria consistently evolved resistance to phage via mutations in the gene, waaG, which codes for a glucosyltransferase required for assembly of the bacterial lipopolysaccharide (LPS). Phage also showed repeatable evolution, with each chemostat producing host-range mutant phage with mutations in the phage tail fiber gene T3p48 which binds to the bacterial LPS during adsorption. Two second-order resistant bacteria evolved via mutations in different genes involved in the phage interaction. Although a wide range of mutations occurred in the bacterial waaG gene, mutations in the phage tail fiber were restricted to a single codon, and several phage showed convergent evolution at the nucleotide level. These results are consistent with previous studies in other systems that have documented repeatable evolution in bacteria at the level of pathways or genes and repeatable evolution in viruses at the nucleotide level. Our data are also consistent with the expectation that adaptation via loss-of-function mutations is less constrained than adaptation via gain-of-function mutations.     

Linking genetic change to community evolution: insights from studies of bacteria and bacteriophage

A major goal of community ecology is to link biological processes at lower scales with community patterns. Microbial communities are especially powerful model systems for making these links. In this article, we review recent studies of laboratory communities of bacteria and bacteriophage (viruses that infect bacteria). We focus on the ecology and evolution of bacteriophage-resistance as a case study demonstrating the relationship between specific genes, individual interactions, population dynamics, community structure, and evolutionary change. In laboratory communities of bacteria and bacteriophage, bacteria rapidly evolve resistance to bacteriophage infection. Different resistance mutations produce distinct resistance phenotypes, differing, for example, in whether resistance is partial or complete, in the magnitude of the physiological cost associated with resistance, and in whether the mutation can be countered by a host-range mutation in the bacteriophage. These differences determine whether a mutant can invade, the effect its invasion has on the population dynamics of sensitive bacteria and phage, and the resulting structure of the community. All of these effects, in turn, govern the community's response to environmental change and its subsequent evolution.     

Bacteriophage in relation to nitrogen fixation by red clover

Summary Cultures of Rhizobium trifolii resistant to the action of a given bacteriophage, may vary appreciably in their nitrogen fixing ability in association with the proper host plant. Likewise, cultures of Rh. trifolii, sensitive to bacteriophage may or may not benefit the host plant as judged by nitrogen fixation.Since sensitive and resistant cultures may be comparable in their nitrogen fixing capacity, it appears that the behavior of a culture of Rh. trifolii toward the lytic action of a specific bacteriophage in vitro cannot be correlated with its nitrogen fixing ability in association with the host plant.If a bacteriophage is added to a sensitive strain of Rh. trifolii used for inoculation of red clover, the cultures recovered from the nudules are often only of the resistant type. When this occurs the nitrogen fixed through association of plant and bacteria is decreased in the case of a good strain, but is unaffected if the strain is of the poor type. The addition of phage to a resistant strain of Rh. trifolii used for inoculation of red clover plants does not change the resistant type of culture recovered from the nodules, nor is the fixation of nitrogen by plant and bacteria affected.     

Spatial heterogeneity and the stability of host-parasite coexistence

Spatially heterogeneous environments can theoretically promote more stable coexistence of hosts and parasites by reducing the risk of parasite attack either through providing permanent spatial refuges or through providing ephemeral refuges by reducing dispersal. In experimental populations of Pseudomonas aeruginosa and the bacteriophage PP7, spatial heterogeneity promoted stable coexistence of host and parasite, while coexistence was significantly less stable in the homogeneous environment. Phage populations were found to be persisting on subpopulations of sensitive bacteria. Transferring populations to fresh microcosms every 24 h prevented the development of permanent spatial refuges. However, the lower dispersal rates in the heterogeneous environment were found to reduce parasite transmission thereby creating ephemeral refuges from phage attack. These results suggest that spatial heterogeneity can stabilize an otherwise unstable host-parasite interaction even in the absence of permanent spatial refuges.     

Genome of a virulent bacteriophage Lb338-1 that lyses the probiotic Lactobacillus paracasei cheese strain

There is a lack of fundamental knowledge about the influence of bacteriophage on probiotic bacteria and other commensals in the gut. Here, we present the isolation and morphological and genetic characterization of a virulent narrow-host-range bacteriophage, φLb338-1. This phage was isolated from fresh sewage and was shown to infect the probiotic cheese strain Lactobacillus paracasei NFBC 338. Electron microscopy studies revealed that φLb338-1 is a member of the Myoviridae family, with an isometric head, a medium-sized contractile tail, and a complex base plate. Genome sequencing revealed a 142-kb genome with 199 open reading frames. Putative functions could be assigned to 22% of the open reading frames; these had significant homology to genes found in the broad-host-range SPO1-like group of phages which includes the Enterococcus faecalis phage φEF24C, Listeria phage A511, and Lactobacillus plantarum phage LP65. Interestingly, no significant genomic similarity was observed between the phage and the probiotic host strain. Future studies will determine if the presence of bacteriophage φLb338-1 or others in the human or animal gut plays an antagonistic role against the probiotic effect of beneficial bacteria.     

Mutants of Escherichia coli variably resistant to bacteriophage T1

Carta, Guy R. (Rutgers, The State University, New Brunswick, N.J.), and Vernon Bryson. Mutants of Escherichia coli variably resistant to bacteriophage T1. J. Bacteriol. 92:1055-1061. 1966.-Mutants resistant to bacteriophage T1 were isolated from ultraviolet (UV)-irradiated cultures of Escherichia coli B/r, a UV-resistant variant. Bacterial populations derived from some of these mutants were partially but not completely resistant to the bacteriophage. Such mutants, designated variably resistant (B/r/1v), could not be obtained from E. coli B. Phage-free mutant populations taken from different stages in growth consisted of significantly different proportions of T1-resistant and T1-sensitive cells. The growth stage-dependent range of variation exceeded 1,000-fold. In broth cultures, the highest proportion of resistant cells consistently appeared at mid-log phase, and the highest proportion of sensitive cells at lag and stationary phases. Comparable evidence for environmentally dependent changes in host-cell phenotype was obtained by efficiency of plating and cloning efficiency analysis tests. Micromanipulation showed that, in clones growing in the presence of phage T1, sensitive bacteria appeared with high frequency and underwent lysis.