Factors influencing the activity of mosquito control agent (Bacillus thuringiensis subsp. israelensis)

For toxicity, B. thuringiensis must be taken into the larval midgut, where a community of other bacteria is already present. The culturable flora from the Aedes aegypti mosquito midgut was analysed and its role in larval growth and insect mortality was determined. In contrast to published reports concerning B. thuringiensis subsp. kurstaki, subsp. israelensis caused toxicity and larval death even in the absence of other bacteria. The pBtoxis plasmid of B. thuringiensis subsp. israelensis encodes all the mosquitocidal toxins and a number of other coding sequences. The potential effects of selected genes on host phenotype was assessed. No evidence was found for antibiotic production from putative antibiotic synthesis genes. The plasmid also carries potential germination genes organised in a single ger operon. Comparison of the germination responses of spores from strains with and without pBtoxis revealed that this plasmid could promote activation of the spores under alkaline conditions but not following heat treatment. Introduction of the ger operon on a recombinant plasmid to the plasmidless strain established this operon as the first with an identified role in alkaline activation. Mosquito midgets provide an alkaline environment and in which enhanced germination may occur. Co-feeding experiments showed that in competition to colonise intoxicated A. aegypti larvae, B. thuringiensis carrying pBtoxis, are able to outgrow the plasmid-cured strain. This indicates a selective advantage for the presence of pBtoxis. The strain carrying the recombinant ger genes also outgrew its plasmidless parent, indicating that the ger genes may be responsible for this effect, perhaps by allowing strains a head-start by germinating more rapidly in the insect gut

Indiscriminate ingestion of entomopathogenic nematodes and their symbiotic bacteria by Aedes aegypti larvae: a novel strategy to control the vector of Chikungunya, dengue and yellow fever

Third and fourth instar larvae of Aedes aegypti actively ingested entomopathogenic nematodes (EPNs) and their symbiotic bacteria, resulting in larval mortality. All six EPN species evaluated in this study were pathogenic to Ae. aegypti but varied significantly in their virulence. Heterorhabditis bacteriophora and Steinernema carpocapsae were most virulent, H. megidis and S. kraussei showed the least virulence, whereas H. downesi and S. feltiae had intermediate virulence. Larval mortality was dose dependent for all EPN species. When using a dose of 100 infective juveniles (IJs) per larva, H. bacteriophora and S. carpocapsae caused 90%-100% mortality, whereas H. downesi and S. feltiae caused only 40%-60% mortality. Even when using 200 IJs/larva, H. megidis and S. kraussei caused a maximum of 30%-40% mortality. Some of the invasive EPNs were melanized, suggesting a strong humoral defense response by the Aedes larvae. The degree of melanization was quite variable; some EPNs were totally enveloped in a melanin sheath while others were partially coated with melanin. Melanization did not stop the EPN from multiplying and killing the Aedes larvae. IJs released from infected larvae would have the potential to infect healthy mosquito larvae. Also, both bacterial supernatant and bacterial cell suspension of Xenorhabdus nematophila caused >91% larval mortality after 48 h, whereas only the bacterial cell suspension of Photorhabdus laumondii was effective against the mosquito larvae. These data provides useful information on the potential use of EPNs and/or formulated bacterial cell suspensions in the control of the important urban and container-breeding mosquito, Ae. aegypti, and are a starting point for future simulated and actual field studies

Genetic Basis for Alkaline Activation of Germination in Bacillus thuringiensis subsp. israelensis▿

Differences in activation between spores from strains of Bacillus thuringiensis subsp. israelensis with and without the toxin-encoding plasmid pBtoxis are demonstrated. Following alkaline activation, the strain bearing pBtoxis shows a significantly greater germination rate. Expression of just three genes constituting a previously identified, putative ger operon from this plasmid is sufficient to produce the same phenotype and characterizes this operon as a genetic determinant of alkaline activation

Bacteria

This chapter helps the readers to update current knowledge of the ecology of entomopathogenic bacteria, the use of new tools to examine their interactions with the environment, and factors affecting their distribution and abundance. It includes case studies on specific bacteria and looks at how bacterial ecology affects the use and impact of bacterial entomopathogens in microbial control. Transmission is one of the key factors determining successful continuance of a bacterial entomopathogen. Most bacteria infect their hosts through horizontal transmission. Entomopathogenic cells ingested during insect feeding cause infection, the pathogen multiples within the host, and new propagules are released to the environment to infect more hosts. Virulence is considered a measurable characteristic of the ability of the microbe to cause disease and is intended for within-group or within-species pathogen comparisons. Many plant pathogens can increase their virulence through a quorumsensing mechanism mediated by the level of N-acyl homoserine lacton