Field evaluation of biosurfactants, surfactin and di-rhamnolipid produced by Bacillus subtilis subsp. subtilis (VCRC B471) and Pseudomonas fluorescens (VCRC B426) against immature stages of the urban malaria vector Anopheles stephensi

BACKGROUND & OBJECTIVES

Bacillus subtilis subsp. subtilis (VCRC B471) and Pseudomonas fluorescens (B426) produce mosquitocidal biosurfactant, surfactin and di-rhamnolipid. The objective of the study was to carry out a small-scale field evaluation of the two biosurfactants to determine the efficacy, application dosage, residual activity and frequency of application against Anopheles stephensi immatures in selected sites in Goa, India.

METHODS

Surfactin (VCRC B471) and di-rhamnolipid (VCRC B426) were formulated as aqueous suspensions (5% AS), and were applied at the dosages of 34, 51 and 68 mL/m² and 27, 41 and 54 mL/m² respectively. Two experiments were carried out with the two formulations.

RESULTS

Surfactin (VCRC B471) formulation was effective at all the dosages and there was sustained reduction (>80%) in immature density in the treated sites up to 18 days in experiment 1 and up to 15 days in experiment 2. No pupae were found in the treated sites throughout the study. Di-rhamnolipid (VCRC B426) formulation was also found to reduce the immature density in the treated sites up to 14 days in experiment 1 and up to 15 days in experiment 2.

INTERPRETATION & CONCLUSION

For VCRC B471, the optimum application dosage determined was 51 mL/m² and for VCRC B426, 27mL/m². The formulations are to be applied fortnightly for effective control of Anopheles. The application dosage determined in the present study can be used for large scale field evaluation to assess their suitability for use in public health programmes for the control of Anopheles mosquitoes vectoring malaria.

Fly ash based Bacillus thuringiensis var. israelensis formulation for use against Culex quinquefasciatus, the vector of filariasis in natural ecosystems

BACKGROUND & OBJECTIVES

Fly ash is produced in huge quantities by the various thermal power stations in India. This thermal waste has been employed as a carrier material in the preparation of a biopesticidal water dispersible powder (WDP) formulation for use against mosquitoes. In the present investigation, this newly developed fly ash based WDP formulation was evaluated in natural breeding habitats of mosquito.

METHODS

Fly ash based WDP formulation of Bacillus thuringiensis var. israelensis (VCRC B17) was evaluated for its efficacy and residual activity in aquatic habitats supporting breeding of Culex quinquefasciatus, the vector of lymphatic filariasis in Neyveli Township, Neyveli Lignite Corporation, India for a period of one month.

RESULTS

At an application rate of 10 kg/ha, the WDP was effective for five days regardless of the habitat, and provided 80-100% reduction in larval abundance of Cx. quinquefasciatus.

INTERPRETATION & CONCLUSION

The study indicates that for continued control of immature density and prevention of adult emergence, a weekly application of this formulation is necessary. This study also showed that fly ash based formulations can be used for immediate control of mosquitoes in different types of habitats and has also brought out a new avenue for the utilization of coal ash.

Cost-effective medium for the production of mosquito pupicidal lipopeptide from Bacillus subtilis subsp. subtilis (VCRC B471)

BACKGROUND & OBJECTIVES

A cyclic lipopeptide (CLP), surfactin produced by a strain of Bacillus subtilis subsp. subtilis (VCRC B471) was found to exhibit mosquitocidal activity. The present study was carried out to enhance the surfactin level using low cost material in the production medium.

METHODS

Two carbon sources, glucose and common sugar, and two nitrogen sources, ammonium nitrate and soya were used in the study. Different concentrations of 'C' and 'N' sources were used in the production medium to enhance the production of surfactin.

RESULTS

A new medium (SS7) containing 2% sugar, 6% soya and 0.5% common salt with micronutrients was designed which was found to enhance the production of surfactin. The crude mosquitocidal metabolite (CMM) produced in this medium was 3 g/l which was two times higher than that obtained using synthetic medium NYSM. The LC50 dosage of the CMM to the pupal stages of An. stephensi (2.3 μg/ml) was comparable to that obtained with CMM from the conventional medium.

INTERPRETATION & CONCLUSION

The newly designed cost-effective medium designated as sugar soya medium (SSM) enhanced the production of surfactin and the cost of production was estimated as [symbol: see text] 6 per litre, which is six times lesser than that of the conventional medium. Replacement of sodium chloride with cooking salt further reduced the cost of the medium.

Species-diagnostic polymerase chain reaction assays for Phlebotomus argentipes and Phlebotomus papatasi, vectors of Leishmania

Species-specific differences encountered in the nucleotide sequences of a highly variable region of the 18S rRNA gene were used to design a multiplex polymerase chain reaction (PCR) assay for the identification of Phlebotomus papatasi Scopoli and Phlebotomus argentipes An-nandale & Brunetti, vectors of Leishmania. This multiplex PCR assay uses a common forward primer and two reverse primers, which are specific for the two species. Amplification of a PCR product of size 788 bp indicates the presence of P. papatasi, whereas a product of size 677 bp indicates the presence of P. argentipes. The assay was found to be highly specific and sensitive.

Surfactin: a novel mosquitocidal biosurfactant produced by Bacillus subtilis ssp. subtilis (VCRC B471) and influence of abiotic factors on its pupicidal efficacy

AIM

The rpoB gene of the mosquito pupicidal isolate Bacillus subtilis (VCRC B471) was amplified to confirm the subspecies as subtilis. The mosquito pupicidal activity expressed by the biosurfactant surfactin is novel, and hence, the influence of abiotic factors like pH, temperature of water and sunlight on its efficacy was studied under laboratory conditions.

METHODS AND RESULTS

The rpoB gene amplicon of the bacterium (c. 570 bp of) was sequenced (accession number: EU057603). The relatedness of the bacterium to other members of the genus Bacillus was studied by tree construction, and the identity of VCRC B471 was confirmed as B. subtilis ssp. subtilis. The mosquito pupicidal activity exhibited by surfactin was found to be unaffected between pH 3-9, temperatures 25 and 37 °C and exposure to sunlight/UV radiation. Further, the pupicidal activity of surfactin was not diminished after exposure to 121 °C for 15 min, indicating its thermostable nature.

CONCLUSIONS

VCRC B471 is confirmed as a strain of B. subtilis ssp. subtilis. The mosquitocidal toxin, surfactin produced by this bacterium being stable to UV and varied temperature, active at acidic and basic pH and temperatures between 25 and 42 °C renders this molecule an interesting lead to be developed as a mosquitocidal agent.

SIGNIFICANCE AND IMPACT OF THE STUDY

The mosquitocidal toxin, surfactin produced by B. subtilis ssp. subtilis (VCRC B471), being a biodegradable biosurfactant, exhibiting high stability to varied environmental conditions, can be used year round in breeding habitats and will be a prospective microbial toxin for use against mosquitoes.

Utility of the rDNA-ITS2-PCR assay in detecting the life stages of species S of Anopheles fluviatilis complex

BACKGROUND & OBJECTIVE

Anopheles fluviatilis, which ranks second among the major malarial vectors in India occurs as a complex of three morphologically identical species (species S, T and U) of which only species S is a vector. Hence, it becomes pertinent to have a method for the detection of this vector species under field conditions to map the distribution of this vector. An rDNA-ITS2-PCR assay has been developed earlier for species S of this complex using female adult specimens. In order to widen the range of samples on which this technique can be employed, the utility of this PCR assay in detecting different life stages/gender/parts of the vector species was studied. Also, its reliability in detecting a single species S in pools of species T was studied.

METHODS

Mosquitoes were collected from Malkangiri and Koraput districts of Orissa State where species S and T of this complex are reported. The wild caught fed females, after egg laying were subjected to PCR assay for species identification. The F1 progeny of a few PCR identified specimens was raised and samples at larval, pupal and adult stages were used for PCR assay. Single adult specimen of species S was added to pools containing different numbers of adults of species T and the pools were subjected to DNA extraction and PCR assay.

RESULTS

The PCR assay could detect species S from pure DNA extracts of the immature stages and crude DNA extracts of parts of adult/whole adult mosquito of either gender. Crude DNA extracts of pools of mosquitoes had to be diluted and used in order to obtain the species diagnostic fragment.

INTERPRETATION & CONCLUSION

The rDNA-ITS2-PCR assay producing an amplicon of 350 bp. diagnostic for species S, could detect all stages/gender. Any part of the adult can be used for species identification. Further, a single adult of species S in pools of as many as 99 adults of species T could be detected. Application of this PCR assay will be useful in mapping the distribution of species S, an important malarial vector.

rDNA-ITS2-PCR assay for grouping the cryptic species of Anopheles culicifacies complex (Diptera: Culicidae)

Anopheles culicifacies, a predominant vector of malaria in India exists as a complex of five sibling species A, B, C, D and E, of which, except species B, all the rest are vectors with varying vectorial capacities. With a combination of PCR assays, it is possible to identify all the five members of this species complex. These assays include amplification of the rDNA-ITS2 region followed by digestion of the ITS2 amplicon using restriction enzyme, Rsa I which groups the five members of the An. culicifacies complex into two categories: species A and D forming one category and species B, C and E forming another. The samples grouped thus are then subjected to two allele-specific PCR assays (AD-PCR and BCE-PCR), which has been designed using sequence differences in the mitochondrial cytochrome oxidase II (CO II) subunit. The AD-PCR assay distinguishes species A and D, whereas the BCE-PCR assay distinguishes species B, C and E. In the present study, the differences in the ITS2 region of the five species was used to design a PCR assay which groups the five members into the same two categories as obtained after digestion of the ITS2-PCR product. This assay uses a common forward primer based on the 5.8S region and two reverse primers, which is specific for the two categories. Amplification of a PCR product of size 253bp indicates the presence of species A/D, while a product of size 409bp indicates the presence of species B/C/E. By using this ITS2 PCR assay, the three-step procedure is reduced to two cutting down the time and cost involved. The ITS2 PCR assay has been validated on specimens collected from different regions of India and the results confirm to the earlier reports on the distribution of the members of the An. culicifacies complex.

Association of the level of mosquito larvicidal activity with the growth & sporulation in Bacillus sphaericus H-5a5b strains

The role of growth and sporulation in the production of mosquito larvicidal factors in B. sphaericus H-5a5b strains was investigated using 6 strains that differed in their larvicidal activity. Among these, strain B64 produced maximum biomass (15.5 g/l by 29th h) while B45 and B85 yielded the least (12.8 g/l by 41st and 37th h respectively). Strains B43 and B42 reached the peak of viable cell count (4-6 x 10(10) cells/ml) 4 h earlier than B64 and 12 h earlier than the rest of the strains. Strains B42 and B43 produced higher number of heat resistant spores (4 x 10(8) spores/ml), while strains B45 and B57 produced the lowest numbers (2-4 x 10(5) spores/ml). Mosquitocidal toxin synthesis was noticed as early as the 5th and 9th h in the cultures of the strains B42 and B64 respectively while in those of other strains it was detected by the 13th h or later. The results indicated that generally the highly and moderately toxic strains grew faster and sporulated better than the poorly toxic ones.

Longevity and migration of Wuchereria bancrofti infective larvae and their distribution pattern in relation to the resting and feeding behaviour of the vector mosquito, Culex quinquefasciatus

The longevity, migration and distribution of infective larvae (L3) of Wuchereria bancrofti within the host mosquito were studied by feeding Culex quinquefasciatus on microfilaraemic human blood and allowing the microfilariae to develop to L3. The L3 were found to remain alive and active for 46-50 days, i.e. as long as the host mosquitoes survived. The larvae started their migration to the head of the mosquito soon after their development to L3, on day 13 after the initial, infective bloodmeal. Although more L3 were usually found in the head region of the mosquitoes than in the thorax or abdomen, the larvae showed an oscillatory pattern of movement between all three regions. This movement was significant in the mosquitoes fed only on microfilaraemic blood, but not in those that took a second bloodmeal on normal human blood on day 8 after the infective bloodmeal. The distribution of the L3 in the three regions of the mosquitoes was independent of parasite density. Observations on L3 movement over a 24-h period indicated that there were always more L3 in the head than in the thorax and abdomen and that the number of L3 in the head was maximal at 18.00 hours and minimal at 06.00 hours. When infective mosquitoes were fed on human blood through a Parafilm membrane, 45.2% of the L3 present in the mosquitoes were shed into the blood or on the membrane. All the L3 present in each mosquito migrated to its head during feeding.

In vitro cultivation of third stage larvae of Wuchereria bancrofti to fourth stage: influence of some physico-chemical factors

It has been reported that third stage larvae (L3) of Wuchereria bancrofti strain from Jakarta, molted to the fourth stage (L4) in vitro, in a simple culture medium supplemented with 10% human serum. In the present study, this culture medium has been used to examine the effects of some physico-chemical parameters on larval growth, development and molting of Wuchereria bancrofti from India. Lymph at 10% concentration enhanced the in vitro survival time of larvae. Molting of larvae from L3 to L4 stage has been obtained using human fetal lung cells in cellular co-culture and as a source of conditioned medium. Given these improvements in the medium supplementation, it has been observed that the age of L3s (duration of L3s maintenance within the mosquitos) is one of the most important parameters for the development of L3s in vitro. No molting was observed when one day L3s were used whereas, molting occurred with one or two weeks old L3s. On the contrary, when more than 3 weeks old L3s were used molting failed to occur even though duration of survival of L3s was improved and in this case, most of the larvae were degenerated.

Isolation of mosquito-pathogenic Bacillus sphaericus & B. thuringiensis from the root surface of hydrophytes

Attempts were made to isolate B. sphaericus and B. thuringiensis active against mosquito larvae from the root surface of hydrophytes. Out of 139 samples processed, 86 B. sphaericus and 23 B. thuringiensis isolates were obtained. Sixty two of the B. sphaericus isolates belonged to the serotype H5a5b, 2 to H6 and 22 isolates did not agglutinate with any of the 6 antisera tested. Twenty of the B. thuringiensis isolates belonged to the H14 serotype, 1 each to the H10 and H17 serotype(s) and 1 to an unknown serotype. Fifty nine of the B. sphaericus and 20 of the B. thuringiensis isolates fall under highly toxic category with the LC50 dose of 1-50 ng/ml for Culex quinquefasciatus third instar larvae.

Establishment of a standard test method for determining susceptibility of Mesocyclops to different insecticides

A method to determine the susceptibility of Mesocylops sp. to insecticides was devised. Matured adults were used for the bioassays. Mesocyclops sp. (25 in number) were placed in 24.9 ml of sterilized paddy field water in 5 cm diameter petri dish and 0.1 ml of appropriate stock solution of the insecticide was added. Mortality was scored 24 h after treatment. Among the insecticides tested, Permethrin, Dichlorvos, Temephos, DDT, Carbendazim, Fenitrothion, Zolone and Aldrin were effective against Mesocyclops sp.