Top Bug

By Lori OliwensteinMar 1, 1993 6:00 AM


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Keep an eye on your E. coli when Pseudomonas strain 679-2 is around. When this unusual bacterium gets lean, it gets mean.

Bacteria are well-known killers--tuberculosis and toxic shock syndrome are just two examples of their handiwork--but most of those that do kill commit manslaughter rather than murder. That is, the death is inadvertent. It comes not because the microbes are trying to kill but because they release toxins as part of their normal activity, or because, as foreign agents in the body, they provoke an immense, damaging immune response. Bacteria have no motive for murder, because when their host dies they die, too.

Nevertheless, there are true murderers in the bacterial community. They live in soil rather than in the human body, and most of them live quietly most of the time, eating organic debris as their peers do. But when times get tough a handful of these species turn mean. When nutrient levels get low, says microbiologist Lester Casida of Pennsylvania State University, they will attack and eat their brethren.

Casida is the expert on predatory bacteria; indeed, he discovered most of the known species. And now, he says, he’s discovered the top bug of them all--a bacterium that kills not only other bacteria, including other predators, but also several different types of fungi. That’s important, says Casida, because it might be possible to harness the bug’s ferocious power as a natural fungicide and use it to protect crops from blight.

Casida came across his first bacterial predator 12 years ago. He caught it in the act of disrupting his research: it was eating what was then his research subject, a more peaceable type of soil-dwelling bacterium. Since deciding that killers are more interesting than victims, Casida has found a total of ten predatory bacteria. One is Ensifer adherens, which means sword-bearing adherer; another is Cupriavidus necator, or copper-loving slayer. Then there’s the top bug, which Casida has so far isolated from only one patch of soil in an unused field on a Penn State farm, and to which he hasn’t yet given a proper name.

Pseudomonas strain 679-2, as it’s known for the moment, has more character than its provisional moniker--in fact, it’s a habitual lawbreaker. Though the idea of predatory bacteria seems lawless enough, Casida has found that most predatory species do have standards. For instance, although they may attack one another in a petri dish, they tend to coexist peacefully in soil, focusing their aggression on nonpredators. But once strain 679-2 shows up, the truce is off. When Casida added only a few 679-2 cells to a sample of soil, they attacked all the other bacteria present--including the predatory species--as well as many of the fungi. After a single day of multiplying rapidly, they had taken over the soil.

That discovery gave Casida and Penn State plant pathologist Felix Lukezic the idea of testing Pseudomonas 679-2 as an agricultural fungicide. The two researchers sprayed the bacterium onto soil and onto the leaves of tomato and alfalfa plants in fields blighted by fungi, which had produced discolored spots on the plant leaves. The results were particularly dramatic in the case of the tomatoes: spots covered 37 percent of the leaf surface on untreated plants but only 7.9 percent on plants treated with the bacterium. In the case of alfalfa the results were also significant: the incidence of fungal infection was 5.5 percent of the leaf surface on the control plants, but only 3.4 percent on the treated ones. Clearly many fungi were getting their tickets punched.

The top bug’s murder weapon remains to be identified. In contrast to normal bacteria, some of which defend themselves by broadcasting an antibiotic that kills whatever it comes in contact with, predatory bacteria take careful aim. First they attach themselves to their prey, then they inject it with a tiny amount of some toxic substance that causes it to break into pieces small enough for a predator to absorb through its membrane. In some cases the substance is a toxic metal, but with this new bacterium it’s a new compound of some kind, an organic compound, says Casida. We’re trying to figure out chemically what it is.

Casida is also interested in a broader question. Other types of bacteria are much more widespread, he points out. These are so unusual. But how come they’re so rare if they’re so powerful?

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