RESUMEN
Herein, we report a colorimetric immunosensor for T7 bacteriophage based on gold nanoparticles modified with covalently bonded anti-T7 antibodies. The new immunosensor allows for a fast, simple, and selective detection of T7 virus. T7 virions form immunological complexes with the antibody modified gold nanoparticles which causes them to aggregate. The aggregation can be observed with the naked eye as a color change from red to purple, as well as with a UV-vis spectrophotometer. The aggregate formation was confirmed with SEM imaging. Sensor selectivity against the M13 bacteriophage was demonstrated. The limit of detection (LOD) is 1.08 × 10(10) PFU/mL (18 pM) T7. The new method was compared with a traditional plaque test. In contrast to biological tests the colorimetric method allows for detection of all T7 phages, not only those biologically active. This includes phage ghosts and fragments of virions. T7 virus has been chosen as a model organism for adenoviruses. The described method has several advantages over the traditional ones. It is much faster than a standard plaque test. It is more robust since no bacteria-virus interactions are utilized in the detection process. Since antibodies are available for a large variety of pathogenic viruses, the described concept is very flexible and can be adapted to detect many different viruses, not only bacteriophages. Contrary to the classical immunoassays, it is a one-step detection method, and no additional amplification, e.g., enzymatic, is needed to read the result.
Asunto(s)
Anticuerpos/química , Anticuerpos/inmunología , Bacteriófago T7/aislamiento & purificación , Oro/química , Nanopartículas del Metal/química , Bacteriófago T7/inmunología , Colorimetría , InmunoensayoRESUMEN
The morphological changes of gold nanoparticles induced by T7 virus (bacteriophage) and the determination of its femtomolar concentration by a plasmonic method are presented. Carboxymethyl chitosan capped gold nanoparticles (CMC-AuNPs) are used as plasmonic probes and are synthesized by a simple one pot wet chemical method. HR-TEM images show that the spherical structure of the CMC-AuNPs is changed into chain-like nanostructures after the addition of T7 virus due to the strong coordination of CMC-AuNPs with T7. Since T7 capsids comprise a repeating motif of capsomers built from proteins that bind to the acid groups of chitosan, the conjugation of carboxymethyl chitosan-linked AuNPs with T7 virions enables colorimetric biosensing detection. The absorbance intensity (â¼610 nm) increases in the concentration range of T7 from 2 × 10(-15) M to 2 × 10(-13) M and the detection limit is found to be 2 × 10(-15) M (2 fM). The present work demonstrates eco-friendly biopolymer stabilized AuNPs as potential nanomaterials for biosensing of viruses. Our method is very simple, low cost, selective and highly sensitive, and provides new insight into virus induced chain-like morphology of AuNPs.
Asunto(s)
Bacteriófago T7/aislamiento & purificación , Oro/química , Nanopartículas del Metal/química , Nanopartículas del Metal/ultraestructura , Resonancia por Plasmón de Superficie/métodos , Quitosano/análogos & derivados , Quitosano/química , Límite de DetecciónRESUMEN
Herein, we describe indole-based analogues of oroidin as a novel class of 2-aminoimidazole-based inhibitors of methicillin-resistant Staphylococcus aureus biofilm formation and, to the best of our knowledge, the first reported 2-aminoimidazole-based inhibitors of Streptococcus mutans biofilm formation. This study highlighted the indole moiety as a dibromopyrrole mimetic for obtaining inhibitors of S. aureus and S. mutans biofilm formation. The most potent compound in the series, 5-(trifluoromethoxy)indole-based analogue 4b (MBIC50 = 20 µM), emerged as a promising hit for further optimisation of novel inhibitors of S. aureus and S. mutans biofilms.
Asunto(s)
Alcaloides/farmacología , Antibacterianos/farmacología , Biopelículas/efectos de los fármacos , Indoles/química , Staphylococcus aureus Resistente a Meticilina/efectos de los fármacos , Pirroles/farmacología , Alcaloides/síntesis química , Alcaloides/química , Antibacterianos/síntesis química , Antibacterianos/química , Relación Dosis-Respuesta a Droga , Pruebas de Sensibilidad Microbiana , Estructura Molecular , Pirroles/síntesis química , Pirroles/química , Relación Estructura-ActividadRESUMEN
The exo-xis region, present in genomes of lambdoid bacteriophages, contains highly conserved genes of largely unknown functions. In this report, using bacteriophage λ and Shiga toxin-converting bacteriophage Ï24Β, we demonstrate that the presence of this region on a multicopy plasmid results in impaired lysogenization of Escherichia coli and delayed, while more effective, induction of prophages following stimulation by various agents (mitomycin C, hydrogen peroxide, UV irradiation). Spontaneous induction of λ and Ï24Β prophages was also more efficient in bacteria carrying additional copies of the corresponding exo-xis region on plasmids. No significant effects of an increased copy number of genes located between exo and xis on both efficiency of adsorption on the host cells and lytic development inside the host cell of these bacteriophages were found. We conclude that genes from the exo-xis region of lambdoid bacteriophages participate in the regulation of lysogenization and prophage maintenance.
Asunto(s)
Bacteriófago lambda/genética , Toxina Shiga/metabolismo , Escherichia coli Shiga-Toxigénica/patogenicidad , Escherichia coli Shiga-Toxigénica/virología , Activación Viral , Secuencia de Aminoácidos , Bacteriófago lambda/fisiología , Datos de Secuencia Molecular , Plásmidos , Profagos/genética , Profagos/fisiología , Alineación de Secuencia , Toxina Shiga/genética , Escherichia coli Shiga-Toxigénica/genética , Escherichia coli Shiga-Toxigénica/fisiologíaRESUMEN
Bacteriophage T4 is able to adjust its development to the growth parameters of the host cell. Here, we present evidence for the production of two different subpopulations of phage particles, which differ in their ability to infect starved Escherichia coli cells. The ability of phage T4 to produce a fraction of virions unable to infect starved cells is linked to the functions of genes rI and rIII, as well as rIIA. This may represent the adaptation of phage T4 in order to persist in unfavourable environmental conditions.
Asunto(s)
Bacteriófago T4/clasificación , Bacteriófago T4/aislamiento & purificación , Escherichia coli/virología , Viabilidad Microbiana , Bacteriófago T4/genética , Bacteriófago T4/crecimiento & desarrollo , Regulación Viral de la Expresión Génica , Genes ViralesRESUMEN
In Escherichia coli hosts, hydrogen peroxide is one of the factors that may cause induction of lambda prophage. Here, we demonstrate that H2O2-mediated lambda prophage induction is significantly enhanced in the oxyR mutant host. The mRNA levels for cI gene expression were increased in a lambda lysogen in the presence of H2O2. On the other hand, stimulation of the p(M) promoter by cI857 overproduced from a multicopy plasmid was decreased in the DeltaoxyR mutant in the presence of H2O2 but not under normal growth conditions. The purified OxyR protein did bind specifically to the p(M) promoter region. This binding impaired efficiency of interaction of the cI protein with the OR3 site, while stimulating such a binding to OR2 and OR1 sites, in the regulatory region of the p(M) promoter. We propose that changes in cI gene expression, perhaps in combination with moderately induced SOS response, may be responsible for enhanced lambda prophage induction by hydrogen peroxide in the oxyR mutant. Therefore, OxyR seems to be a factor stimulating lambda prophage maintenance under conditions of oxidative stress. This proposal is discussed in the light of efficiency of induction of lambdoid prophages bearing genes coding for Shiga toxins.
Asunto(s)
Bacteriófago lambda/fisiología , Proteínas de Escherichia coli/metabolismo , Escherichia coli/virología , Peróxido de Hidrógeno/farmacología , Proteínas Represoras/metabolismo , Activación Viral , Bacteriófago lambda/efectos de los fármacos , Secuencia de Bases , Sitios de Unión , Escherichia coli/genética , Escherichia coli/metabolismo , Proteínas de Escherichia coli/genética , Regulación Viral de la Expresión Génica , Datos de Secuencia Molecular , Estrés Oxidativo , Regiones Promotoras Genéticas , Profagos/efectos de los fármacos , Profagos/fisiología , Proteínas Represoras/genética , Respuesta SOS en GenéticaRESUMEN
Lysis inhibition (LIN) is a known feature of the T-even family of bacteriophages. Despite its historical role in the development of modern molecular genetics, many aspects of this phenomenon remain mostly unexplained. The key element of LIN is an interaction between two phage-encoded proteins, the T holin and the RI antiholin. This interaction is stabilized by RIII. In this report, we demonstrate the results of genetic experiments which suggest a synergistic action of two accessory proteins of bacteriophage T4, RI.-1, and RI.1 with RIII in the regulation of LIN.
Asunto(s)
Bacteriólisis , Bacteriófago T4/fisiología , Escherichia coli/virología , Proteínas Virales/metabolismo , Liberación del Virus , Bacteriófago T4/genética , Escherichia coli/genética , Escherichia coli/fisiología , Proteínas Virales/genéticaRESUMEN
In the study presented here, we tested, how large a fraction of lysogenic culture was undergoing filamentation, which could indicate triggering of the SOS response or SOS-independent prophage induction that is also known to cause cell filamentation. Here, antibiotic stress was triggered by adding mitomycin C and oxidative stress was induced by hydrogen peroxide. Observation of bacterial cells under an optical microscope revealed more filamenting cells for lysogenic Escherichia coli than for strains not carrying a prophage. Moreover, the amount of filamenting cells depended not only on the stress agents used and the type of the prophage, but also on the host. During induction of the 933W prophage, the resulting phage titer and the amount of elongating cells were different when using E. coli O157:H7 EDL933 clinical isolate and the E. coli MG1655 laboratory strain. The amount of filamenting cells correlates well with the observed phage titers.
Asunto(s)
Antibacterianos/farmacología , Bacteriófago lambda/fisiología , Escherichia coli/fisiología , Escherichia coli/virología , Estrés Oxidativo , Toxina Shiga/genéticaRESUMEN
Shiga toxin-producing Escherichia coli (STEC) is a group of pathogenic strains responsible for bloody diarrhea and hemorrhagic colitis, with often severe complications. Shiga toxins are the main factors causing the phathogenicity of STEC. Production of these toxins depends on the presence of stx1 and stx2 genes, which are located on lambdoid prophages, and their expression is stimulated upon prophage induction. Therefore, a transition of the phage genome from the prophage state to an extrachromosomal genetic element, and its further propagation, is crucial for the pathogenic effects. However, our knowledge on specific conditions for induction of these prophages in bacteria occurring in human intestine is very limited. In this report we present results of our studies on five different phages, originally occurring in STEC strains, in comparison to bacteriophage lambda. We found that efficiencies of induction of prophages and their further development vary considerably in response to different induction agents. Moreover, efficiency of progeny phage production might be modulated by other factors, like temperature or bacterial growth rate. Therefore, it is likely that pathogenicity of different STEC strains may be significantly different under specific conditions in their natural habitats.
Asunto(s)
Bacteriófago lambda/fisiología , Profagos/fisiología , Toxina Shiga/biosíntesis , Escherichia coli Shiga-Toxigénica/virología , Activación Viral/fisiología , Secuencia de Aminoácidos , Bacteriófago lambda/genética , Bacteriófago lambda/metabolismo , Secuencia de Bases , Regulación Viral de la Expresión Génica/fisiología , Genes Reporteros , Peróxido de Hidrógeno/farmacología , Mitomicina/farmacología , Datos de Secuencia Molecular , Norfloxacino/farmacología , Inhibidores de la Síntesis del Ácido Nucleico/farmacología , Oxidantes/farmacología , Regiones Promotoras Genéticas , Profagos/genética , Profagos/metabolismo , Secuencias Reguladoras de Ácidos Nucleicos , Toxina Shiga/genética , Escherichia coli Shiga-Toxigénica/metabolismo , Escherichia coli Shiga-Toxigénica/patogenicidad , Cloruro de Sodio/farmacología , Factores de Tiempo , Proteínas Virales/biosíntesis , Proteínas Virales/química , Proteínas Virales/genética , Activación Viral/efectos de los fármacos , Activación Viral/genéticaRESUMEN
Evolution of bacteria to selective chemical pressure (e.g. antibiotics) is well studied in contrast to the influence of physical stressors. Here we show that instantaneous physical stress in a homogeneous environment (without concentration gradient) induces fast adaptation of Escherichia coli. We exposed E. coli to a large number of collisions of around 105 per bacterium per second with sharp ZnO nanorods. The pressure exerted on the bacterial cell wall was up to 10 GPa and induced phenotype changes. The bacteria's shape became more spherical, the density of their periplasm increased by around 15% and the average thickness of the cell wall by 30%. Such E. coli cells appeared almost as Gram-positive bacteria in the standard Gram staining. Additionally, we observed a combination of changes occurring at the genomic level (mutations identified in form of single nucleotide polymorphisms) and down-regulation of expression of 61 genes encoding proteins involved in ß-oxidation of fatty acids, glycolysis, the citric acid cycle, as well as uptake of amino acids and enzyme cofactors. Thus, we show that bacteria undergo phenotypic changes upon instantaneous, acute physical stress without any obviously available time for gradual adaptation.
Asunto(s)
Escherichia coli , Mutación , Nanotubos/química , Polimorfismo de Nucleótido Simple , Estrés Fisiológico/efectos de los fármacos , Óxido de Zinc , Escherichia coli/citología , Escherichia coli/genética , Escherichia coli/metabolismo , Óxido de Zinc/química , Óxido de Zinc/farmacologíaRESUMEN
The use of low concentrations (optimally 2.5 to 3.5 microg/ml, depending on top agar thickness) of ampicillin in the bottom agar of the plate allows for formation of highly visible plaques of bacteriophages which otherwise form extremely small plaques or no plaques on Escherichia coli lawns. Using this method, we were able to obtain plaques of newly isolated bacteriophages, propagated after induction of prophages present in six E. coli O157:H(-) strains which did not form plaques when standard plating procedures were employed.
Asunto(s)
Ampicilina/farmacología , Colifagos/efectos de los fármacos , Colifagos/crecimiento & desarrollo , Escherichia coli O157/virología , Ensayo de Placa Viral/métodos , Agar , Medios de CultivoRESUMEN
BACKGROUND: Bacteriophage infections of bacterial cultures cause serious problems in genetic engineering and biotechnology. They are dangerous not only because of direct effects on the currently infected cultures, i.e. their devastation, but also due to a high probability of spreading the phage progeny throughout a whole laboratory or plant, which causes a real danger for further cultivations. Therefore, a simple method for quick inhibition of phage development after detection of bacterial culture infection should be very useful. RESULTS: Here, we demonstrate that depletion of a carbon source from the culture medium, which provokes starvation of bacterial cells, results in rapid inhibition of lytic development of three Escherichia coli phages, lambda, P1 and T4. Since the effect was similar for three different phages, it seems that it may be a general phenomenon. Moreover, similar effects were observed in flask cultures and in chemostats. CONCLUSION: Bacteriophage lytic development can be inhibited efficiently by carbon source limitation in bacterial cultures. Thus, if bacteriophage contamination is detected, starvation procedures may be recommended to alleviate deleterious effects of phage infection on the culture. We believe that this strategy, in combination with the use of automated and sensitive bacteriophage biosensors, may be employed in the fermentation laboratory practice to control phage outbreaks in bioprocesses more effectively.
Asunto(s)
Bacteriófagos/crecimiento & desarrollo , Reactores Biológicos/microbiología , Reactores Biológicos/virología , Técnicas de Cultivo de Célula/métodos , Escherichia coli/fisiología , Escherichia coli/virología , Glucosa/metabolismo , Activación Viral/fisiologíaRESUMEN
Metagenomics approaches and recent improvements in the next-generation sequencing methods, have become a method of choice in establishing a microbial population structure. Many commercial soil DNA extraction kits are available and due to their efficiency they are replacing traditional extraction protocols. However, differences in the physicochemical properties of soil samples require optimization of DNA extraction techniques for each sample separately. The aim of this study was to compare the efficiency, quality, and diversity of genetic material extracted with the use of commonly used kits. The comparative analysis of microbial community composition, displayed differences in microbial community structure depending on which kit was used. Statistical analysis indicated significant differences in recovery of the genetic material for 24 out of 32 analyzed phyla, and the most pronounced differences were seen for Actinobacteria. Also, diversity indexes and reproducibility of DNA extraction with the use of a given kit, varied among the tested methods. As the extraction protocol may influence the apparent structure of a microbial population, at the beginning of each project many extraction kits should be tested in order to choose one that would yield the most representative results and present the closest view to the actual structure of microbial population.
Asunto(s)
Biota , ADN/aislamiento & purificación , Metagenómica/métodos , Microbiología del Suelo , ADN/química , ADN/genética , Metagenómica/normasRESUMEN
The aim of this study was to investigate the soil microbial communities of a phosphogypsum waste heap. The soil microbial community structures can differ over time, as they are affected by the changing environmental conditions caused by a long-term exposure to different kinds of pollutions, like is the case of soil in the post-production waste area in Wislinka (in the northern part of Poland) currently undergoing restoration. Our analyses indicated that the most abundant phyla were Proteobacteria, Acidobacteria, and Actinobacteria, and generally such an abundance is common for most of the studied soils. The most dominant class were Alphaproteobacteria, with their participation in 33.46% of the total reads. Among this class, the most numbered order was Sphingomonadales, whereas among this order the Sphingomonadaceae family was the most abundant one. The Sphingomonadaceae family is currently in the center of interest of many researchers, due to the ability of some of its members to utilize a wide range of naturally occurring organic compounds and many types of environmental contaminants. This kind of knowledge about microbial populations can support efforts in bioremediation and can improve monitoring changes in the contaminated environments.
Asunto(s)
Sulfato de Calcio , Microbiota/fisiología , Fósforo , Microbiología del Suelo , Contaminantes del Suelo , Secuenciación de Nucleótidos de Alto Rendimiento , Microbiota/genética , Polonia , ARN Ribosómico 16S , Suelo/químicaRESUMEN
Svalbard reindeer (Rangifer tarandus platyrhynchus) is a non-migratory subspecies of reindeer inhabiting the high-arctic archipelago of Svalbard. In contrast to other Rangifer tarandus subspecies, Svalbard reindeer graze exclusively on natural sources of food and have no chance of ingestion of any crops. We report the use of a non-invasive method for analysis of fecal microbiome by means of sequencing the 16S rDNA extracted from the fecal microbiota of R. tarandus platyrhynchus from a small, isolated population in Hornsund, South Spitsbergen National Park. Analyses of all samples showed that 99% of the total reads were represented by Bacteria. Taxonomy-based analysis showed that fecal bacterial communities consisted of 14 phyla. The most abundant phyla across the population were Firmicutes and Bacteroidetes, and those phyla jointly accounted for more than 95% of total bacterial sequences (ranging between 90.14 and 98.19%). Specifically, Firmicutes comprised 56.53% (42.98-63.64%) and Bacteroidetes comprised 39.17% (34.56-47.16%) of the total reads. The remaining 5% of the population reads comprised of Tenericutes, Cyanobacteria, TM7, Actinobacteria, Proteobacteria, Verrucomicrobia, Elusimicrobia, Planctomycetes, Fibrobacteres, Spirochaetes, Chloroflexi, and Deferribacteres. Differences in the fecal bacteria composition between particular reindeer were not statistically significant which may reflect the restricted location and similar diet of all members of the local population.
RESUMEN
Due to deposition of birds' guano, eggshells or feathers, the vicinity of a large seabirds' breeding colony is expected to have a substantial impact on the soil's physicochemical features as well as on diversity of vegetation and the soil invertebrates. Consequently, due to changing physicochemical features the structure of bacterial communities might fluctuate in different soil environments. The aim of this study was to investigate the bacterial assemblages in the Arctic soil within the area of a birds' colony and in a control sample from a topographically similar location but situated away from the colony's impact area. A high number of OTUs found in both areas indicates a highly complex microbial populations structure. The most abundant phyla in both of the tested samples were: Proteobacteria, Acidobacteria, Actinobacteria, and Chloroflexi, with different proportions in the total share. Despite differences in the physicochemical soil characteristics, the soil microbial community structures at the phylum level were similar to some extent in the two samples. The only share that was significantly higher in the control area when compared to the sample obtained within the birds' colony, belonged to the Actinobacteria phylum. Moreover, when analyzing the class level for each phylum, several differences between the samples were observed. Furthermore, lower proportions of Proteobacteria and Acidobacteria were observed in the soil sample under the influence of the bird's colony, which most probably could be linked to higher nitrogen concentrations in that sample.
RESUMEN
Metagenomic studies have become increasingly popular. They allow for the estimation of biodiversity in complex populations. This diversity presents an enormous but largely unexpected genetic and biological pool and can be exploited for the recovery of novel genes, entire metabolic pathways and their products. Generally metagenomic study is a genomic analysis of organisms by direct extraction and cloning of DNA from their natural environment. The most common problems of modern metagenomics are as follows: majority of the microorganisms present in the environment cannot be cultivated by standard techniques, DNA extraction methods are not very effective, isolated DNA is contaminated with various compounds, a choice for a screening method is not obvious.
Asunto(s)
ADN/aislamiento & purificación , Metagenómica , Ecosistema , Aguas del AlcantarilladoRESUMEN
Metagenomics is a powerful tool to better understand the microbial niches, especially these from extreme habitats like oceans and seas, hot springs or deserts. However, one who is going to face the metagenomic studies should realize the challenges which might occur in the course of experiments. This manuscript indicates common problems in function-driven metagenomics, especially factors that influence gene expression are taken into account. Codon usage bias, internal cell accumulation, correct protein folding or presence of proper initiation factors are discussed and possible ways to overcome these problems are proposed. Finally, the annotation process is described, including possible limitations that one should take under consideration. What is more, the most popular databases for metagenomic data are mentioned and discussed.
Asunto(s)
Metagenómica , Microbiota , CodónRESUMEN
Lambdoid bacteriophages serve as useful models in microbiological and molecular studies on basic biological process. Moreover, this family of viruses plays an important role in pathogenesis of enterohemorrhagic Escherichia coli (EHEC) strains, as they are carriers of genes coding for Shiga toxins. Efficient expression of these genes requires lambdoid prophage induction and multiplication of the phage genome. Therefore, understanding the mechanisms regulating these processes appears essential for both basic knowledge and potential anti-EHEC applications. The exo-xis region, present in genomes of lambdoid bacteriophages, contains highly conserved genes of largely unknown functions. Recent report indicated that the Ea8.5 protein, encoded in this region, contains a newly discovered fused homeodomain/zinc-finger fold, suggesting its plausible regulatory role. Moreover, subsequent studies demonstrated that overexpression of the exo-xis region from a multicopy plasmid resulted in impaired lysogenization of E. coli and more effective induction of λ and Ф24B prophages. In this report, we demonstrate that after prophage induction, the increase in phage DNA content in the host cells is more efficient in E. coli bearing additional copies of the exo-xis region, while survival rate of such bacteria is lower, which corroborated previous observations. Importantly, by using quantitative real-time reverse transcription PCR, we have determined patterns of expressions of particular genes from this region. Unexpectedly, in both phages λ and Ф24B, these patterns were significantly different not only between conditions of the host cells infection by bacteriophages and prophage induction, but also between induction of prophages with various agents (mitomycin C and hydrogen peroxide). This may shed a new light on our understanding of regulation of lambdoid phage development, depending on the mode of lytic cycle initiation.
Asunto(s)
Bacteriófago lambda/genética , Bacteriófagos/genética , Regulación Viral de la Expresión Génica , Profagos/genética , Activación Viral/genética , Escherichia coli Enterohemorrágica , Toxina Shiga/metabolismoRESUMEN
Shiga toxin-producing Escherichia coli (STEC) may cause bloody diarrhea and hemorrhagic colitis (HC), with subsequent systemic disease. Since genes coding for Shiga toxins (stx genes) are located on lambdoid prophages, their effective production occurs only after prophage induction. Such induction and subsequent lytic development of Shiga toxin-converting bacteriophages results not only in production of toxic proteins, but also in the lysis (and thus, the death) of the host cell. Therefore, one may ask the question: what is the benefit for bacteria to produce the toxin if they die due to phage production and subsequent cell lysis? Recently, a hypothesis was proposed (simultaneously but independently by two research groups) that STEC may benefit from Shiga toxin production as a result of toxin-dependent killing of eukaryotic cells such as unicellular predators or human leukocytes. This hypothesis could make sense only if we assume that prophage induction (and production of the toxin) occurs only in a small fraction of bacterial cells, thus, a few members of the population are sacrificed for the benefit of the rest, providing an example of "bacterial altruism." However, various reports indicating that the frequency of spontaneous induction of Shiga toxin-converting prophages is higher than that of other lambdoid prophages might seem to contradict the for-mentioned model. On the other hand, analysis of recently published results, discussed here, indicated that the efficiency of prophage excision under conditions that may likely occur in the natural habitat of STEC is sufficiently low to ensure survival of a large fraction of the bacterial host. A molecular mechanism by which partial prophage induction may occur is proposed. We conclude that the published data supports the proposed model of bacterial "altruism" where prophage induction occurs at a low enough frequency to render toxin production a positive selective force on the general STEC population.