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The occurrence and diverse roles of Bacillus spp. and their endospores in the environment is reviewed, with particular emphasis on soil ecology, host-symbiont and host-parasite interactions, and human exploitation of spores as biological control agents and probiotics.
Rhizoctonia solani isolates varying in their virulence were tested for their ability to produce oxalic acid (OA) in vitro. The results indicated that the virulent isolates produced more OA than the less virulent isolates. In order to isolate OA-detoxifying strains of Pseudomonas fluorescens, rhizosphere soil of rice was drenched with 100 mM OA and fluorescent pseudomonads were isolated from the OA-amended soil by using King's medium B. These isolates were tested for their antagonistic effect towards growth of R. solani in vitro. Among them P. fluorescens PfMDU2 was the most effective in inhibiting the mycelial growth of R. solani. P. fluorescens PfMDU2 was capable of detoxifying OA and several proteins were detected in the culture filtrate of PfMDU2 when it was grown in medium containing OA. To investigate whether the gene(s) involved in OA-detoxification resides on the plasmids in P. fluorescens PfMDU2, a plasmid-deficient strain of P. fluorescens was generated by plasmid curing. The plasmid-deficient strain (PfMDU2P-) failed to grow in medium containing OA and did not inhibit the growth of R. solani. Both PfMDU2 and PfMDU2P- were tested for their efficacy in controlling sheath blight of rice under greenhouse conditions. Seed treatment followed by soil application of rice with P. fluorescens strain, PfMDU2, reduced the severity of sheath blight by 75% compared with the control, whereas PfMDU2P- failed to control sheath blight disease.
As a potential component in future integrated parasite-control strategies, biological control by means of predacious fungi seems to be moving from a promising possibility toward becoming a reality, and the netforming nematode-destroying fungus Duddingtonia flagrans appears to be the candidate of choice. Not only has this fungus been found in, and isolated from, fresh sheep, cattle and horse faeces, but it also appears to be the only fungus that is able to consistently and significantly reduce the number of infective trichostrongyle larvae in faeces from animals fed fungal spores. Results from the last few years have shown that D. flagrans is able to trap and destroy free-living stages of the most important and common trichostrongylid larvae with very similar external life-cycles, as well as larvae of parasites with a slightly different transmission biology (Nematodirus spp., Dictyocaulus viviparus). The introduction of microfungi for biological control could be as part of a feed supplement or incorporated in feed-blocks presented to animals which are raised under relatively intensive conditions and constant surveillance. Apart from the special niche for organic farmers, such a product would be suited for horses, small ruminants (as either milking herds or housed daily for other reasons), cattle in special situations and free-roaming pigs. The most important constraint, still, for a major breakthrough in biological control in the latter is the lack of good antagonists against the long-lived and rather resistant infective stages of parasites, being transmitted as larvae inside the egg. Since the first Conference on Novel Approaches to the Control of Helminth Parasites of Livestock in Armidale, Australia, 1995, there has been a steady evolution within the area of biological control of parasitic nematodes. Today this principle is being exploited and tested out in almost all parts of the world, under various climatic conditions and production systems. Where, in the past, a large part of the work focused on cattle and to a lesser degree horse and sheep parasites, the focus of the research in many of the newly involved countries is on small ruminants, because of their importance to primarily small-scale farmers in local communities. Today research and trials are either on-going or being planned in many developing countries, as well as in countries in transition. The involvement of multinational agencies in addition to national and industrial interests is very welcome and should increase the chances and keep up the momentum for development and implementation of biological control in future animal production around the world.
The potential utility of micrometer-sized particles as controlled-release devices for the volatilization of insect pheromones for mating disruption applications is evaluated in this study for two pheromone/model compound systems (codlemone/1-dodecanol and disparlure/1,2-epoxyoctadecane). To expedite the measurement of release rates from these particle devices, two techniques based on thermogravimetric analysis (TGA) have been exploited: isothermal TGA (I-TGA) at elevated temperatures (40-80 degrees C) with N(2) convection and volatilization temperature (VT) by dynamic TGA. A correlation between these two methods has been established. Samples that exhibit a higher VT provide a lower release rate from a particle substrate. Using these techniques, it has been demonstrated that chemical interactions between adsorbed liquids and particle surfaces may play a small role in defining release characteristics under conditions of low surface area, whereas parameters associated with total surface area and micropore structure appear to be much more significant in retarding evaporation for uncoated particles containing an adsorbed liquid. Additional regulation of release rates was achieved by coating the particle systems with water-soluble or water-dispersible polymers. By careful selection of particle porosity and coating composition, it is envisioned that the evaporation rate of pheromones can be tailored to specific insect control applications.
The efficacy of the pupal parasitoid Spalangia cameroni Perkins as a biological control agent was tested against house flies Musca domestica Linnaeus and stable flies Stomoxys calcitrans (Linnaeus) in one dairy cattle and two pig installations in Denmark. Weekly releases of S. cameroni from April through to September-October 1999 and 2000 resulted in significant suppressions of house fly populations to below nuisance level, whereas no effect on stable flies was found. Parasitism was significantly higher in the release years compared to the control years, but was below 25% averaged over the fly season for each farm. A statistical model based on a functional relationship between the innate capacity of increase of the two fly species and three explanatory variables (air temperature, fly density and parasitism) provided a fairly good fit to data with the abundances of house flies and stable flies explained mostly by temperature, but intra- and interspecific competition, and parasitism had a significant effect as well. Overall, the model was capable of explaining 14% and 6.6% of the total variation in data for house fly and stable fly, respectively. Spalangia cameroni was the predominant parasitoid to emerge from exposed house fly pupae, but from mid summer onwards Muscidifurax raptor Girault & Sanders (Hymenoptera: Pteromalidae) was also quite common. The study indicated that biological control of house flies can be an efficient alternative to chemical control.
1. Newly-hatched broiler chicks were treated orally with a commercial competitive exclusion product (Broilact) in 3 replicate trials 2. After 24 h the treated chicks and untreated control chicks were challenged orally with approximately 10(4) cfu of Campylobacter jejuni. 3. The caeca of the birds were examined quantitatively for campylobacter 12 d after the challenge. 4. In 3 separate trials, the treatment prevented or reduced colonisation of the challenge organisms in the caeca. The percentage of colonised birds varied from 0% to 62% in the treated groups and was 100% in the control groups. The average number of campylobacter was considerably lower in the treated groups than in the control groups.
The copepod Mesocyclops aspericornis Daday and the larvivorous fishes Gambusia affinis (B. & G.) and Poecilia reticulata R. & B., were released into mosquito breeding sites in Tuherahera village, Tikehau atoll, French Polynesia, to control larvae of Aedes aegypti (L.), Ae.polynesiensis Marks, Culex annulirostris Skuse and Cx quinquefasciatus Say. Treatments were completed within a week, in January 1990. Fish quickly eliminated mosquito larvae from the open breeding sites (ponds, wells). The impact of copepods in water tanks, drums and covered wells was inconsistent, apparently depending on the availability of microfaunal diet for growth of copepod nauplii. As the biting rate of adult Ae.aegypti seemed to be unaffected by the biological control of larvae, this village-scale experiment was judged to be unsuccessful as a means of vector control.
AIMS: To determine the influence of soil-borne fungus Trichoderma harzianum on the biocontrol performance of Pseudomonas fluorescens strain CHA0 and its 2,4-diacetylphloroglucinol (DAPG) overproducing derivative CHA0/pME3424 against Meloidogyne javanica. METHODS AND RESULTS: Amendment of the culture filtrate (CF) or methanol extract of the CF of a T. harzianum strain Th6 to P. fluorescens growth medium enhanced the production of nematicidal compound(s) by bacterial inoculants in vitro. In addition, bacteria overwhelmingly expressed phl'-'lacZ reporter gene when the medium was amended with CF of T. harzianum. Pseudomonas fluorescens and T. harzianum applied together in unsterilized sandy loam soil caused greater reduction in nematode population densities in tomato roots. CONCLUSIONS: Trichoderma harzianum improves root-knot nematode biocontrol by the antagonistic rhizobacterium P. fluorescens both in vitro and under glasshouse conditions. SIGNIFICANCE AND IMPACT OF THE STUDY: The synergistic effect of T. harzianum on the production of nematicidal compound(s) critical in biocontrol may improve the efficacy of biocontrol bacteria against plant-parasitic nematodes. Considering the inconsistent performance of the biocontrol agents under field conditions, application of a mixture of compatible T. harzianum and P. fluorescens would more closely mimic the natural situation and might broaden the spectrum of biocontrol activity with enhanced efficacy and reliability of control.
The genotypic diversity that occurs in natural populations of antagonistic microorganisms provides an enormous resource for improving biological control of plant diseases. In this study, we determined the diversity of indigenous 2,4-diacetylphloroglucinol (DAPG)-producing Pseudomonas spp. occurring on roots of wheat grown in a soil naturally suppressive to take-all disease of wheat. Among 101 isolates, 16 different groups were identified by random amplified polymorphic DNA (RAPD) analysis. One RAPD group made up 50% of the total population of DAPG-producing Pseudomonas spp. Both short- and long-term studies indicated that this dominant genotype, exemplified by P. fluorescens Q8r1-96, is highly adapted to the wheat rhizosphere. Q8r1-96 requires a much lower dose (only 10 to 100 CFU seed(-1) or soil(-1)) to establish high rhizosphere population densities (10(7) CFU g of root(-1)) than Q2-87 and 1M1-96, two genotypically different, DAPG-producing P. fluorescens strains. Q8r1-96 maintained a rhizosphere population density of approximately 10(5) CFU g of root(-1) after eight successive growth cycles of wheat in three different, raw virgin soils, whereas populations of Q2-87 and 1M1-96 dropped relatively quickly after five cycles and were not detectable after seven cycles. In short-term studies, strains Q8r1-96, Q2-87, and 1M1-96 did not differ in their ability to suppress take-all. After eight successive growth cycles, however, Q8r1-96 still provided control of take-all to the same level as obtained in the take-all suppressive soil, whereas Q2-87 and 1M1-96 gave no control anymore. Biochemical analyses indicated that the superior rhizosphere competence of Q8r1-96 is not related to in situ DAPG production levels. We postulate that certain rhizobacterial genotypes have evolved a preference for colonization of specific crops. By exploiting diversity of antagonistic rhizobacteria that share a common trait, biological control can be improved significantly.
The pathogenicity of 32 fungal isolates from the genera of Beauveria, Verticillium, Paecilomyces, Metarhizium, Mariannaea, and Hirsutella to second-instar tarnished plant bug, Lygus lineolaris (Palisot de Beauvois), was tested under laboratory conditions. These isolates originated from various insect hosts and substrates from France, Denmark, Austria, Italy, Turkey, Syria, and the United States. A single exposure concentration (1 x 10(7) conidia/ ml) assay for each isolate was first conducted by immersing the insects in 10 ml of a fungal suspension for 5s. These were followed by concentration-mortality assays on five of the most pathogenic isolates using four test concentrations ranging from 2 x 10(4) to 2 x 10(7) conidia/ml. B. bassiana 726 (Bemisia-passaged GHA strain) was used as a standard for comparison in all of the assays. Among the test isolates, three produced mortality not significantly different from the water control. Mortality ranged from 35 to 98% among the other 29 isolates. The LC50 values of the five most pathogenic isolates ranged from 0.8 to 5.0 x 10(5) conidia/ml. The LT50 values for these isolates ranged from 6.0 to 6.9, 3.1 to 5.1, and 2.5 to 4.0 d for concentrations of 2 x 10(5), 2 x 10(6), and 2 x 10(7) conidia/ml, respectively. Two strains of B. bassiana (ARSEF 1394,5665) and one M anisopliae (ARSEF 3540) were more pathogenic to the nymphs than the standard, having significantly lower LC50 and LT50, values. Our results demonstrated that several genera of entomopathogenic fungi have promise as microbial control agents against L. lineolaris.
Unfed adult Ixodes scapularis Say were treated with spores of the entomopathogenic fungus Metarhizium anisopliae Metschnikoff in the laboratory and in the field. An M. anisopliae suspension containing 4 x 10(9) spores per milliliter caused 96% mortality in the laboratory, versus 53% mortality among field-treated ticks. The LC50 value for unfed adult I. scapularis in the laboratory was 4 x 10(7) spores per milliliter. Our results indicate that M. anisopliae was highly pathogenic to unfed adult ticks and showed potential for controlling questing adult I scapularis.
Sudden, large-scale infestations of house mice (Mus musculus domesticus) occur irregularly in the cereal-growing regions of Australia, resulting in substantial economic losses. Mathematical modelling has been used to evaluate the use of the liver nematode Capillaria hepatica as a potential agent for the biological control of mouse populations. The models suggest that C. hepatica is unlikely to be successful as a single-release control agent: instead, the parasite would need to be released when it becomes apparent that an outbreak is likely. Stage-structured models, including time delays and seasonal mouse demographic parameters, suggest that the parasite may be successful as a control agent, provided it can be introduced into the mouse population at least one year before an outbreak occurs. The optimum time for introduction is in summer or autumn. Some generalisations of this work are discussed. A parasite which affects fecundity alone is unlikely to be a good control agent, because it will destabilize the host population. Macroparasites may be unable to spread sufficiently rapidly to control sudden rises in vertebrate populations.
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