Germination, growth, and sporulation of Bacillus thuringiensis subsp. israelensis in excreted food vacuoles of the protozoan Tetrahymena pyriformis

Dissecting the Environmental Consequences of Bacillus thuringiensis Application for Natural Ecosystems

Bacillus thuringiensis (Bt), a natural pathogen of different invertebrates, primarily insects, is widely used as a biological control agent. While Bt-based preparations are claimed to be safe for non-target organisms due to the immense host specificity of the bacterium, the growing evidence witnesses the distant consequences of their application for natural communities. For instance, upon introduction to soil habitats, Bt strains can affect indigenous microorganisms, such as bacteria and fungi, and further establish complex relationships with local plants, ranging from a mostly beneficial demeanor, to pathogenesis-like plant colonization. By exerting a direct effect on target insects, Bt can indirectly affect other organisms in the food chain. Furthermore, they can also exert an off-target activity on various soil and terrestrial invertebrates, and the frequent acquisition of virulence factors unrelated to major insecticidal toxins can extend the Bt host range to vertebrates, including humans. Even in the absence of direct detrimental effects, the exposure to Bt treatment may affect non-target organisms by reducing prey base and its nutritional value, resulting in delayed alleviation of their viability. The immense phenotypic plasticity of Bt strains, coupled with the complexity of ecological relationships they can engage in, indicates that further assessment of future Bt-based pesticides' safety should consider multiple levels of ecosystem organization and extend to a wide variety of their inhabitants.

Modeling the impact of biolarvicides on malaria transmission

Biolarvicides are in use in several parts of the world for malaria vector control. We propose a five compartment dynamical systems model to study malaria transmission when biolarvicides are administered, to study the impact of this environmentally safe method on malaria spread. A comprehensive analysis of the model is presented. Model analysis shows that the basic reproductive rate R is larger in the absence of biolarvicides as compared to their presence. Theoretical analysis is corroborated by data from field studies. We show that there exist intermediate parameter regimes that separate disease-free and endemic states, which can in turn be modulated by biolarvicide use. Using Latin hypercube sampling we study the sensitivity of the model to parameter value changes. Calibration of our model to mosquito population and biolarvicide data for indoor and outdoors scenarios, yield parameter values hitherto not available or measurable. We validate our model with malaria incidence data from a region in India and provide predictions for malaria incidence in the presence and absence of biolarvicide. This model provides a prognostic tool to field work involving biolarvicide use in control of malaria.

Effects of Bacillus cereus Endospores on Free-Living Protist Growth

We studied the predator-prey interactions between heterotrophic protists and endospores of Bacillus cereus group bacteria, in order to gain insight on survival and dispersal of B. cereus endospores in the environment. It has been hypothesised that the spore stage protects against digestion by predating protists. Therefore, experiments were carried out to investigate the impact of B. cereus endospores and vegetative cells, as the only food source, on individual amoeboid, flagellated and ciliated protists. The presence of fluorescent-labelled intracellular bacteria confirmed that B. cereus endospores as well as vegetative cells were ingested by protists and appeared intact in the food vacuoles when observed by epifluorescence microscopy. Furthermore, protist growth and bacterial predation were followed by qPCR. Protists were able to grow on vegetative cells as well as endospores of B. cereus, despite the lower cell division rates observed for some protists when feeding on bacterial endospores. Survival and proliferation of ingested bacteria inside protists cells was also observed. Finally, B. cereus spore germination and growth was observed within all protists with higher abundance in the amoeboid protist after antibiotic treatment of the protist surface. These observations support that protists can act as a potential breeding ground for B. cereus endospores.

Bacillus thuringiensis subsp. israelensis and its dipteran-specific toxins

Bacillus thuringiensis subsp. israelensis (Bti) is the first Bacillus thuringiensis to be found and used as an effective biological control agent against larvae of many mosquito and black fly species around the world. Its larvicidal activity resides in four major (of 134, 128, 72 and 27 kDa) and at least two minor (of 78 and 29 kDa) polypeptides encoded respectively by cry4Aa, cry4Ba, cry11Aa, cyt1Aa, cry10Aa and cyt2Ba, all mapped on the 128 kb plasmid known as pBtoxis. These six δ-endotoxins form a complex parasporal crystalline body with remarkably high, specific and different toxicities to Aedes, Culex and Anopheles larvae. Cry toxins are composed of three domains (perforating domain I and receptor binding II and III) and create cation-selective channels, whereas Cyts are composed of one domain that acts as well as a detergent-like membrane perforator. Despite the low toxicities of Cyt1Aa and Cyt2Ba alone against exposed larvae, they are highly synergistic with the Cry toxins and hence their combinations prevent emergence of resistance in the targets. The lack of significant levels of resistance in field mosquito populations treated for decades with Bti-bioinsecticide suggests that this bacterium will be an effective biocontrol agent for years to come.

Persistence of the spores of B. thuringiensis subsp. kurstaki from Foray bioinsecticide in gleysol soil and on leaves

The aim of this study was to determine how long the spores of Bacillus thuringiensis subsp. kurstaki HD-1 from Foray bioinsecticide persist in soil and on leaf surface after application of the bioinsecticide in an oak forest. Foray 04 UL was sprayed over a 195-hectare oak forest on the Krotoszyn Plateau in Poland. B. thuringiensis was isolated from soil samples and tree leaves taken from randomly chosen sites. B. thuringiensis subsp. kurstaki HD-1 in the samples was identified upon clonal analysis of the cultured isolates by using the RAPD method. One month after Foray spraying, the number of B. thuringiensis increased in soil and decreased on leaf surface comparing to the number estimated two days after the application. The reduction in the number of B. thuringiensis was noted six months after the pesticide application and the number was decreasing during the following months. No B. thuringiensis was noted on leaf surface one year after Foray spraying and in soil after one and a half years. The study showed that B. thuringiensis spores from biopesticide can survive in the forest environment; however, relatively short persistence time does not pose environmental risk.

Bacillus thuringiensis Is an Environmental Pathogen and Host-Specificity Has Developed as an Adaptation to Human-Generated Ecological Niches

Bacillus thuringiensis (Bt) has been used successfully as a biopesticide for more than 60 years. More recently, genes encoding their toxins have been used to transform plants and other organisms. Despite the large amount of research on this bacterium, its true ecology is still a matter of debate, with two major viewpoints dominating: while some understand Bt as an insect pathogen, others see it as a saprophytic bacteria from soil. In this context, Bt's pathogenicity to other taxa and the possibility that insects may not be the primary targets of Bt are also ideas that further complicate this scenario. The existence of conflicting research results, the difficulty in developing broader ecological and genetics studies, and the great genetic plasticity of this species has cluttered a definitive concept. In this review, we gathered information on the aspects of Bt ecology that are often ignored, in the attempt to clarify the lifestyle, mechanisms of transmission and target host range of this bacterial species. As a result, we propose an integrated view to account for Bt ecology. Although Bt is indeed a pathogenic bacterium that possesses a broad arsenal for virulence and defense mechanisms, as well as a wide range of target hosts, this seems to be an adaptation to specific ecological changes acting on a versatile and cosmopolitan environmental bacterium. Bt pathogenicity and host-specificity was favored evolutionarily by increased populations of certain insect species (or other host animals), whose availability for colonization were mostly caused by anthropogenic activities. These have generated the conditions for ecological imbalances that favored dominance of specific populations of insects, arachnids, nematodes, etc., in certain areas, with narrower genetic backgrounds. These conditions provided the selective pressure for development of new hosts for pathogenic interactions, and so, host specificity of certain strains.

Distribution of Bacillus thuringiensis subsp. israelensis in Soil of a Swiss Wetland reserve after 22 years of mosquito control

Recurrent treatments with Bacillus thuringiensis subsp. israelensis are required to control the floodwater mosquito Aedes vexans that breeds in large numbers in the wetlands of the Bolle di Magadino Reserve in Canton Ticino, Switzerland. Interventions have been carried out since 1988. In the present study, the spatial distribution of resting B. thuringiensis subsp. israelensis spores in the soil was measured. The B. thuringiensis subsp. israelensis concentration was determined in soil samples collected along six transects covering different elevations within the periodically flooded zones. A total of 258 samples were processed and analyzed by quantitative PCR that targeted an identical fragment of 159 bp for the B. thuringiensis subsp. israelensis cry4Aa and cry4Ba genes. B. thuringiensis subsp. israelensis spores were found to persist in soils of the wetland reserve at concentrations of up to 6.8 log per gram of soil. Continuous accumulation due to regular treatments could be excluded, as the decrease in spores amounted to 95.8% (95% confidence interval, 93.9 to 97.7%). The distribution of spores was correlated to the number of B. thuringiensis subsp. israelensis treatments, the elevation of the sampling point, and the duration of the flooding periods. The number of B. thuringiensis subsp. israelensis treatments was the major factor influencing the distribution of spores in the different topographic zones (P < 0.0001). These findings indicated that B. thuringiensis subsp. israelensis spores are rather immobile after their introduction into the environment.

Modular columns to study depth-dependence behavior of mosquito larvae and toxicity of Bacillus thuringiensis subsp. israelensis

Modular transparent column system was designed to study depth-dependence behavior of mosquito larvae. The system was used in preliminary experiments to evaluate the effect of water depth on the larvicidal activity of Bacillus thuringiensis subsp. israelensis de Barjac against bottom feeder larvae of Aedes aegypti (Linn.) (Diptera: Culicidae), and suggestions for increasing the efficiency of the device are discussed.

Interactions between food-borne pathogens and protozoa isolated from lettuce and spinach

The survival of Salmonella enterica was recently shown to increase when the bacteria were sequestered in expelled food vacuoles (vesicles) of Tetrahymena. Because fresh produce is increasingly linked to outbreaks of enteric illness, the present investigation aimed to determine the prevalence of protozoa on spinach and lettuce and to examine their interactions with S. enterica, Escherichia coli O157:H7, and Listeria monocytogenes. Glaucoma sp., Colpoda steinii, and Acanthamoeba palestinensis were cultured from store-bought spinach and lettuce and used in our study. A strain of Tetrahymena pyriformis previously isolated from spinach and a soil-borne Tetrahymena sp. were also used. Washed protozoa were allowed to graze on green fluorescent protein- or red fluorescent protein-labeled enteric pathogens. Significant differences in interactions among the various protist-enteric pathogen combinations were observed. Vesicles were produced by Glaucoma with all of the bacterial strains, although L. monocytogenes resulted in the smallest number per ciliate. Vesicle production was observed also during grazing of Tetrahymena on E. coli O157:H7 and S. enterica but not during grazing on L. monocytogenes, in vitro and on leaves. All vesicles contained intact fluorescing bacteria. In contrast, C. steinii and the amoeba did not produce vesicles from any of the enteric pathogens, nor were pathogens trapped within their cysts. Studies of the fate of E. coli O157:H7 in expelled vesicles revealed that by 4 h after addition of spinach extract, the bacteria multiplied and escaped the vesicles. The presence of protozoa on leafy vegetables and their sequestration of enteric bacteria in vesicles indicate that they may play an important role in the ecology of human pathogens on produce.

Packaging of live Legionella pneumophila into pellets expelled by Tetrahymena spp. does not require bacterial replication and depends on a Dot/Icm-mediated survival mechanism

The freshwater ciliate Tetrahymena sp. efficiently ingested, but poorly digested, virulent strains of the gram-negative intracellular pathogen Legionella pneumophila. Ciliates expelled live legionellae packaged in free spherical pellets. The ingested legionellae showed no ultrastructural indicators of cell division either within intracellular food vacuoles or in the expelled pellets, while the number of CFU consistently decreased as a function of time postinoculation, suggesting a lack of L. pneumophila replication inside Tetrahymena. Pulse-chase feeding experiments with fluorescent L. pneumophila and Escherichia coli indicated that actively feeding ciliates maintain a rapid and steady turnover of food vacuoles, so that the intravacuolar residence of the ingested bacteria was as short as 1 to 2 h. L. pneumophila mutants with a defective Dot/Icm virulence system were efficiently digested by Tetrahymena sp. In contrast to pellets of virulent L. pneumophila, the pellets produced by ciliates feeding on dot mutants contained very few bacterial cells but abundant membrane whorls. The whorls became labeled with a specific antibody against L. pneumophila OmpS, indicating that they were outer membrane remnants of digested legionellae. Ciliates that fed on genetically complemented dot mutants produced numerous pellets containing live legionellae, establishing the importance of the Dot/Icm system to resist digestion. We thus concluded that production of pellets containing live virulent L. pneumophila depends on bacterial survival (mediated by the Dot/Icm system) and occurs in the absence of bacterial replication. Pellets of virulent L. pneumophila may contribute to the transmission of Legionnaires' disease, an issue currently under investigation.

Introducing Spectral Structure Activity Relationship (S-SAR) Analysis. Application to Ecotoxicology

A novel quantitative structure-activity (property) relationship model, namely Spectral-SAR, is presented in an exclusive algebraic way replacing the old-fashioned multi-regression one. The actual S-SAR method interprets structural descriptors as vectors in a generic data space that is further mapped into a full orthogonal space by means of the Gram-Schmidt algorithm. Then, by coordinated transformation between the data and orthogonal spaces, the S-SAR equation is given under simple determinant form for any chemical-biological interactions under study. While proving to give the same analytical equation and correlation results with standard multivariate statistics, the actual S-SAR frame allows the introduction of the spectral norm as a valid substitute for the correlation factor, while also having the advantage to design the various related SAR models through the introduced “minimal spectral path” rule. An application is given performing a complete S-SAR analysis upon the Tetrahymena pyriformis ciliate species employing its reported eco-toxicity activities among relevant classes of xenobiotics. By representing the spectral norm of the endpoint models against the concerned structural coordinates, the obtained S-SAR endpoints hierarchy scheme opens the perspective to further design the ecotoxicological test batteries with organisms from different species.

Interactions between mosquito larvae and species that share the same trophic level

Ecological theory predicts, and empirical research shows, that species sharing the same trophic level as a target species (hereafter controphic species) can have large direct and indirect effects on the target species by sharing resources and/or by serving as alternative prey to predators. Yet, the roles of controphic species of mosquito larvae in affecting mosquito populations have received little attention. Published empirical evidence, although scarce, suggests that controphic species such as zooplankton and anuran larvae compete with mosquito larvae, can positively affect mosquito larvae by consuming bacteria that are pathogenic to mosquito larvae, reduce predation on mosquito larvae by serving as alternative prey, and ultimately cause increased predation on mosquito larvae by causing a numerical response in the predator. We conclude that more extensive theoretical and empirical studies in elucidating the roles of controphic species will better allow us to predict mosquito population dynamics and allow for better management of mosquitoes.

Bacillus thuringiensis serovariety israelensis and Bacillus sphaericus for mosquito control

Since the discovery of Bacillus thuringiensis (Berliner) serovariety israelensis de Barjac (Bti) and efficacious isolates of Bacillus sphaericus Neide, formulations of these bacteria have become the predominant non-chemical means employed for control of mosquito larvae at several locations in the United States and other countries. An overview of developments in the past 20 years is presented in this chapter regarding the toxins of Bti and B. sphaericus, their modes of action, efficacy and factors that affect larvicidal activity, development of resistance, safety, and their roles in integrated mosquito control. The efficacy of Bti formulations has been demonstrated in a variety of habitats against a multitude of species of mosquitoes. B. sphaericus formulations have been utilized predominantly in organically enriched habitats against Culex species, but they are also active in a variety of habitats having low organic enrichment, against numerous species, and across several genera. Stegomyia spp. are not susceptible to practical doses of B. sphaericus formulations. B. sphaericus has been shown to persist longer than Bti in polluted habitats and, under certain circumstances, can recycle in larval cadavers. A disadvantage of B. sphaericus has been the development of resistance in certain populations of Cx. quinquefasciatus Say and Cx. pipiens Linnaeus. Biotic and abiotic factors that influence the larvicidal activity of Bti and B. sphaericus include species of mosquito and their respective feeding strategies, rate of ingestion, age and density of larvae, habitat factors (temperature, solar radiation, depth of water, turbidity, tannin and organic content, presence of vegetation, etc.), formulation factors (type of formulation, toxin content, how effectively the material reaches the target, and settling rate), storage conditions, production factors, means of application and frequency of treatments. Due to their efficacy and relative specificity, both Bti and B. sphaericus can be ideal control agents in integrated programs especially where other biological control agents, environmental management, personal protection and the judicious use of insecticides are combined.

The Bacillus subtilis spore coat provides "eat resistance" during phagocytic predation by the protozoan Tetrahymena thermophila

Bacillus spores are highly resistant to many environmental stresses, owing in part to the presence of multiple "extracellular" layers. Although the role of some of these extracellular layers in resistance to particular stresses is known, the function of one of the outermost layers, the spore coat, is not completely understood. This study sought to determine whether the spore coat plays a role in resistance to predation by the ciliated protozoan Tetrahymena, which uses phagocytosis to ingest and degrade other microorganisms. Wild-type dormant spores of Bacillus subtilis were efficiently ingested by the protozoan Tetrahymena thermophila but were neither digested nor killed. However, spores with various coat defects were killed and digested, leaving only an outer shell termed a rind, and supporting the growth of Tetrahymena. A similar rind was generated when coat-defective spores were treated with lysozyme alone. The sensitivity of spores with different coat defects to predation by T. thermophila paralleled the spores' sensitivities to lysozyme. Spore killing by T. thermophila was by means of lytic enzymes within the protozoal phagosome, not by initial spore germination followed by killing. These findings suggest that a major function of the coat of spores of Bacillus species is to protect spores against predation. We also found that indigestible rinds were generated even from spores in which cross-linking of coat proteins was greatly reduced, implying the existence of a coat structure that is highly resistant to degradative enzymes.

The hidden lifestyles of Bacillus cereus and relatives

Bacillus cereus sensu lato, the species group comprising Bacillus anthracis, Bacillus thuringiensis and B. cereus (sensu stricto), has previously been scrutinized regarding interspecies genetic correlation and pathogenic characteristics. So far, little attention has been paid to analysing the biological and ecological properties of the three species in their natural environments. In this review, we describe the B. cereus sensu lato living in a world on its own; all B. cereus sensu lato can grow saprophytically under nutrient-rich conditions, which are only occasionally found in the environment, except where nutrients are actively collected. As such, members of the B. cereus group have recently been discovered as common inhabitants of the invertebrate gut. We speculate that all members disclose symbiotic relationships with appropriate invertebrate hosts and only occasionally enter a pathogenic life cycle in which the individual species infects suitable hosts and multiplies almost unrestrained.

Plasmid transfer between Bacillus thuringiensis subsp. israelensis strains in laboratory culture, river water, and dipteran larvae

Plasmid transfer between strains of Bacillus thuringiensis subsp. israelensis was studied under a range of environmentally relevant laboratory conditions in vitro, in river water, and in mosquito larvae. Mobilization of pBC16 was detected in vitro at a range of temperatures, pH values, and available water conditions, and the maximum transfer ratio was 10(-3) transconjugant per recipient under optimal conditions. Transfer of conjugative plasmid pXO16::Tn5401 was also detected under this range of conditions. However, a maximum transfer ratio of 1.0 transconjugant per recipient was attained, and every recipient became a transconjugant. In river water, transfer of pBC16 was not detected, probably as a result of the low transfer frequency for this plasmid and the formation of spores by the introduced donor and recipient strains. In contrast, transfer of plasmid pXO16::Tn5401 was detected in water, but at a lower transfer ratio (ca. 10(-2) transconjugant per donor). The number of transconjugants increased over the first 7 days, probably as a result of new transfer events between cells, since growth of both donor and recipient cells in water was not detected. Mobilization of pBC16 was not detected in killed mosquito larvae, but transfer of plasmid pXO16::Tn5401 was evident, with a maximum rate of 10(-3) transconjugant per donor. The reduced transfer rate in insects compared to broth cultures may be accounted for by competition from the background bacterial population present in the mosquito gut and diet or by the maintenance of a large population of B. thuringiensis spores in the insects.