Bees could put the sting back into pesticides

The natural immunisation of pests against chemicals has been unravelled , reports Bernard Dixon

There is nothing new about treating a human or other animal with a chemical to eradicate a harmful microbe from the body; it happens whenever a doctor prescribes an antibiotic to combat an infection. But scientists in India have achieved almost the opposite. They have protected bees against a chemical - an insecticide - by inoculating them with bacteria that break it down. Their discovery, the first report of the use of microbes to decontaminate a living creature, may have important applications in agriculture.

Microbes have a remarkable capacity to attack otherwise toxic chemicals, a fact already exploited in environmental cleansing. Biotechnologists have rendered several contaminated sites safe, either by introducing bacteria into the soil to break down pollutants or by stimulating the growth and activity of those already there. An example is the abandoned Greenbank gas works, near Blackburn.

In at least one very different context, microbes capable of attacking chemicals are less beneficial. Bacteria are suspected of being responsible for providing disease-carrying insects such as mosquitoes with resistance to the pesticides sprayed on breeding grounds. In some cases the insects acquire the capacity themselves, through mutation, to withstand insecticides or to produce enzymes that break them down. But bacteria on or inside insects also appear to play a significant role. One report showed microorganisms living on the surface of blowflies were capable of destroying the pesticide dieldrin.

With this in mind, ID Sharma and colleagues at the Dr YS Parmar University of Horticulture and Forestry in Solan, India, investigated further. Their first step was to discover that certain honey bees became vulnerable to the insecticide carbaryl when treated with antibiotics. Were the bees normally unaffected by carbaryl because they harboured bacteria that normally rendered the pesticide harmless? And did the antibiotics make the bees susceptible to carbaryl by killing these protective bacteria?

As reported in this month's Journal of Applied Bacteriology, both suppositions proved correct. First, the researchers studied honey bees already known to be resistant to carbaryl. They found that they contained at least three types of bacteria which, when transferred to laboratory glassware and grown in the presence of carbaryl, broke down the insecticide. One was Enterobacter aerogenes, another was a species of Citrobacter, and the third could not be identified. All were dedicated degraders of carbaryl, able to grow on it by using it as their sole source of energy and carbon.

In a second set of experiments, the researchers set out to find out what concentration of the antibiotic streptomycin would destroy the bacteria inside the bees without harming the bees themselves. The tests established that, as the bacteria were killed, so the bees became vulnerable to the insecticide. The compelling conclusion from the two sets of results was that normally the microbes were indeed protecting the bees against carbaryl.

A third group of experiments put the matter beyond question, and suggested applications of the discovery. Sharma and his colleagues grew pure cultures of the Citrobacter species, Enterobacter aerogenes and the unidentified organism in the laboratory and then inoculated them into other bees to see whether this enhanced whatever capacity they had to withstand the insecticide. In every case, introduction of the bacteria greatly increased the bees' tolerance towards carbaryl. Each of the microbes was effective, but the highest degree of protection came when the three were introduced together. This indicates that they act in concert to promote the most efficient breakdown of the insecticide and thus render the bees insensitive to its ill effects.

One possible application of these findings is in situations where crops require pollination, yet the blossom has to be treated with an insecticide to prevent insect attack. In this case, bees inoculated with strains of bacteria designed to boost their resistance to the relevant pesticide could be used to ensure pollination.

The obvious risk with an approach of this sort is that the bacteria might be transferred from beneficial bees to destructive pests, enhancing their resistance to pesticides. But Sharma believes this danger could be sidestepped by modifying the microbes so they fulfil their protective role in bees but fail to grow and thus become established in other insects.