Isolation of Bacillus sphaericus from Lombok Island, Indonesia, and Their Toxicity against Anopheles aconitus

Maize growth response to different Bacillus strains isolated from a salt-marshland area under salinity stress

Maize (Zea mays) growth performance has been hindered due to the high soil salinity. Salinity is one of the most severe abiotic stresses that has led to growth imbalance and profitability of harvests in arid and semi-arid regions. Plants have taken advantage of salt-tolerant bacteria as plant growth-promoters to enhance growth and reduce the adverse effects of salinity through the regulation of some biochemical, physiological, and molecular features. Preferences for non-chemical, eco-friendly, and economical approaches have caused the inquiry of the Bacillus genus as a joint group of plant growth-promoting rhizobacteria known to alleviate salt-stress impacts. In the present study, halotolerant Bacillus strains were isolated from salt-marshland soil and characterized for their physiological, molecular, and biochemical properties. Twenty-four bacterial isolates collected from high saline fields of salt marshland were analyzed by MALDI-TOF MS proteome analysis, which confirmed the taxonomic affiliation with Bacillus cereus, Bacillus subtilis, Bacillus atrophaeus, and Bacillus thorngiensis. Applying the isolates on maize plants as bio-inoculant bacteria obviously increased the growth parameters (P < 0.01). Pot experiments showed that isolates 74 and 90 were the most prominent strains to minimize the harmful effects of salinity. Its effects are heightening the potassium/sodium ratio and K-Na selectivity in shoots and roots measured by flame atomic absorption photometry (AAS). Accordingly, Bacillus cereus isolate 74 showed a maximum increase in dry weights of the shoot (133.89%), root (237.08%), length of the shoot (125%), and root (119.44%) compared to the control condition. Our findings suggest that bacteria isolated from marshland may be an economical and simple means to increase plant growth and resistance to high salinity soil conditions.

Bacillus sphaericus exposure reduced vector competence of Anopheles dirus to Plasmodium yoelii by upregulating the Imd signaling pathway

Background

Vector control with Bacillus sphaericus (Bs) is an effective way to block the transmission of malaria. However, in practical application of Bs agents, a sublethal dose effect was often caused by insufficient dosing, and it is little known whether the Bs exposure would affect the surviving mosquitoes’ vector capacity to malaria.

Methods

A sublethal dose of the Bs 2362 strain was administrated to the early fourth-instar larvae of Anopheles dirus to simulate shortage use of Bs in field circumstance. To determine vector competence, mosquitoes were dissected and the oocysts in the midguts were examined on day 9–11 post-infection with Plasmodium yoelii. Meanwhile, a SYBR quantitative PCR assay was conducted to examine the transcriptional level of the key immune molecules of mosquitoes, and RNA interference was utilized to validate the role of key immune effector molecule TEP1.

Results

The sublethal dose of Bs treatment significantly reduced susceptibility of An. dirus to P. yoelii, with the decrease of P. yoelii infection intensity and rate. Although there existed a melanization response of adult An. dirus following challenge with P. yoelii, it was not involved in the decrease of vector competence as no significant difference of melanization rates and densities between the control and Bs groups was found. Further studies showed that Bs treatment significantly increased TEP1 expression in the fourth-instar larvae (L4), pupae (Pu), 48 h post-infection (hpi) and 72 hpi (P < 0.001). Further, gene-silencing of TEP1 resulted in disappearance of the Bs impact on vector competence of An. dirus to P. yoelii. Moreover, the transcriptional level of PGRP-LC and Rel2 were significantly elevated by Bs treatment with decreased expression of the negative regulator Caspar at 48 hpi, which implied that the Imd signaling pathway was upregulated by Bs exposure.

Conclusions

Bs exposure can reduce the vector competence of An. dirus to malaria parasites through upregulating Imd signaling pathway and enhancing the expression of TEP1. The data could not only help us to understand the impact and mechanism of Bs exposure on Anopheles’ vector competence to malaria but also provide us with novel clues for wiping out malaria using vector control.

Impact of Bacillus sphaericus exposure on Anopheles dirus's fecundity and resistance development

Mosquitoes are important vectors of many infectious diseases. Bacillus sphaericus (Bs) is an ideal larvicide and has attracted more and more attention, recently. However, the fundamental research of its application is very limited, especially on the subsequent impact of Bs exposure on mosquito's fecundity and resistance emergence. Through bioassay, LC50 and LC95 of Bs in killing Anopheles dirus larvae were determined as 9.793 ± 1.878 IU/L and 62.4 ± 6.438 IU/L at 48 h posttreatment, 7.128 ± 0.913 IU/L and 34.385 ± 12.547 IU/L at 72 h post treatment, respectively. After being treated with a sub-lethal dose of Bs, gravidity, oviposition, hatch, pupation, and eclosion of the surviving mosquitoes were counted and analyzed to elucidate the subsequent effects of Bs exposure on the reproductive capacity of A. dirus. The result interestingly showed that the exposure of Bs significantly reduced the oviposition ability of the surviving A. dirus, without effect on egg formation/gravidity, hatch, pupation, and eclosion. The surviving mosquitoes were also maintained routinely for generations to test the sustained effect of Bs exposure on the fecundity of the offsprings. After conventional breeding for generations, the capacity of egg laying totally recovered. To explore the rules of resistance development, bioassays were performed after treatment twice with a sub-lethal dose of Bs on two continuous generations of A. dirus larvae. The killing efficacies between the Bs treated group and control group were compared. The results showed that LC50 and LC95 increased by 4.35- and 7.37-folds after treatment with the sub-lethal dose of Bs on two consecutive generations, respectively. The results indicated that A. dirus was sensitive to Bs, which could reduce oviposition of the surviving A. dirus. The subsequent effect might help to further decrease the mosquito population. However, a sub-lethal dose of Bs exposure could easily cause resistance development. Our study provides a dose standard and reference for the rational use of Bs, which will be helpful for mosquito control.