UA ENTO 446 - Natural occurrence and dispersal of Bacillus thuringiensis in the environment

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ier1.1·ativegenus,DNA-Chapter 1.2micin162-Naturaloccurrence and dispersal of Bacillusthuringiensis in the environment)f101.67,PerH. DamgaardThe Royal Veterinary and Agricultural University. Department of Ecology.Thorvaldsensvej 40. 1870 FrederiksbergC.Denmarkod;341.Key words: Natural occurrence, Soil, Phylloplane, Insects habitats, Food, Clinicalinfections, EpizootiologyAbstract: "Although Bacillus thuringiensis is widely used to control insect pests,the environmental fate of B. thuringiensis is known only in relativelygeneral terms. Until recently, environmental studies looking at the naturaldistribution and life cycle of this bacterium were conducted onlyinfrequently" [34]. Accordingly, very little is known about the naturaltransmission and behaviour of B. thuringiensis, even though the bacteriumhas been isolated from a variety of habitats, ranging from soil, phylloplane,and insects to consumables through-out the world. This chapter gives areview of the occurrence and fate of B. thuringiensis in differentenvironments, along with a discussion of the epizootiology ofB.thuringiensis.1. OCCURRENCE IN SOILThe soil habitat is characterised as an environment with extremediversity in abiotic factors such as water content, ranging fromcompletely arid to swamps, and in amount of nutrients, ranging from"hot spots" with high amounts of nutrients to nutrient-poor habitats. Inaddition, soil contains a rich fauna of invertebrates, including insects.In general, B. thuringiensis is found all over the globe and in all testedecosystems, due to its heat- and drought-resistant spore. Chaket al.[18]and Martin & Travers [60] indicate that a large fraction of thespore-forming bacteria collected from mountain regions isB.23J..F.Charles et al. (eds.),Entomopathogenic Bacteria: From Laboratory to Field Application, 23-40.©2000Kluwer Academic Publishers. Printed in the Netherlands.24Chapterl.2thuringiensis, Both references conclude that this abundance is notconnected with any specific insect population, as hardly any insects werefound in these regions.In the past, soil has been the main source for isolation of B.thuringiensis strains (e.g. [1, 29, 43, 57, 60, 68, 72, 81]). Nevertheless,information about specific numbers of B. thuringiensis in different soilshas only been published by a few authors. Martin [59] and Ishii & Ohba[46] found up to 5x 104B. thuringiensis spores g'! soil. Using colonyhybridisation and a probe directed against crylA, Hansen et al. [41] found9xI02spores go! soil containing this gene.More is known about the numbers of B. thuringiensis compared toother spore-forming bacteria from the genus Bacillus. The B.thuringiensis-index is defined as the number of colonies having B.thuringiensis / B. cereus colony morphology and containing a crystalinclusion body, divided by the total number of colonies with this colonymorphology.The highest B. thuringiensis indices reported from soil samples are0.28 [60] and 0.27 [19]. The lowest index, 0.005, was found by DeLuccaet al. [29]."Non-toxic" strains occur commonly in soil, however, this could bedue to the actual definition of "toxicity" or because of the limitednumber of insect species used in the testing regime and thereforeunresolved insect toxicity.The frequency of serotypes found in soil varies substantially. Themost abundant serotype seems to be kurstaki, but galleria has also beenvery common in one study. Of all serovars found in soil no specificserovar has been found in frequencies higher than 0.5.Although the kurstaki serotype has been found in soils all over theworld, caution should be taken before generalising the causes for thisdominance, since 80% of the serovars used for biocontrol are serovarkurstaki [64].2. OCCURRENCEONFOUAGEThe phylloplane is a heterogenous environment in which manyspecies of bacteria can be found, including many Bacillus species.Furthermore, the phylloplane is a habitat for a high diversity ofleaf-eating insects, including many lepidopteran and coleopteran species.B. cereus and B. mycoides have been isolated from the foliage ofdifferent plants, for example broadleaf forest trees and grass foliage, inquantities estimated in the range of 103_105bacteria go! foliage [85],-n.zPH Damgaardnotwereaccounting for 15-20% of the bacterial population [16, 83]. As B. cereushas been isolated from this habitat it is obvious thatB.thuringiensis alsomust be present; however, few studies have been dedicated to the naturaloccurrence of B. thuringiensis in the phylloplane.In a 3-year study, Smith&Couche [82] isolated B. thuringiensis fromfoliage of different deciduous trees, conifers, and shrubs; the numbers ofB. thuringiensis ranged from 3 to 100 B. thuringiensis spores ern". Thesenumbers are much higher than those reported by Ohba [67], although thiscould be attributed to differences in geographical locations, tree speciesanalysed, as well as technical differences.Ohba [66] examined 25 mulberry leaves and found a B. thuringiensisfrequency to vary from 0 to 0.45 on the 25 leaves analysed, with anaverage index of 0.032. This support the "hot spots" observed byDamgaard et al. [23] where the frequency of B. thuringiensis on cabbageleaves ranged from 0.02 to 0.67, with an average of 0.11.Upon serotyping 150 B. thuringiensis strains collected from cabbageleaves, Damgaard et al. [23] found the majority (64%) to belong toserovar kurstaki. Eleven other serotypes were found, but in very lownumbers. Ohba [66] found serovarpakistanito be the most abundant(46%)whereas kurstaki was only found in 7% of the strains. In asubsequent investigation by Damgaard et al. [22] ofB.thuringiensisoccurrence on grass foliage, 32 strains of B. thuringiensis were isolated,of which 75% belonged to serovar israelensis. Another study byDamgaard et al. [25] on the natural occurrence of B. thuringiensis onpine needles found that of 35 B. thuringiensis strains isolated, 31 %belongedto serovar israelensis and 20% to serovar aizawai.In the study by Smith&Couche [82], 68% of the tested strainsreacted in a Western blot with an antibody directed towards whole,purified crystals of B. thuringiensis serovar kurstaki (HD-l), indicatinglepidopteran activity. This is similar to observations by Damgaard et al.[23]who found 68% of the strains isolated from cabbage leaves to belepidopteran. In a survey of grass foliage, 84% of the strains were foundto be active against Aedes aegypti (Diptera), whereas no lepidopteranactivestrains found [22]. In contrast to this, Ohba [66] found


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