VIRGINIA BEACH, AUG. 19 -- Bacteria commonly found in ocean water caused the deaths of at least 200 bottlenose dolphins along the mid-Atlantic coast this summer, a team of marine scientists announced today.

But the researchers said they remain puzzled by the question of why the dolphins have succumbed to bacteria that normally do not cause them serious illnesses.

"The organisms we are isolating are normally present in the marine environment and in the terrestrial environment," said Joseph L. Geraci, the marine pathologist heading the federally supported team. "What we are dealing with is a condition in which these animals are presensitized to invasion by bacteria that are otherwise rather innocuous.

"In some animals, we have obtained clear cultures of organisms such as streptococcus, and in other cases the animals are dying of what seems to be bacteria that are simply passers-by in the marine environment -- those with which the dolphins normally live in harmony," Geraci, a professor at the University of Guelph in Ontario, Canada, said at a news conference in Norfolk.

Each animal "seems to have its own sweep of bacteria that is killing it," said Geraci, in announcing the results of autopsies and blood and tissue studies that scientists have performed since Aug. 6.

No evidence has been found that the disease presents a danger to humans or other animals, the scientists said.

Geraci said that initial tests showed no abnormal levels of pesticides or heavy metal, such as compounds used on ships.

But researchers said they had not ruled out the possibility that a natural or synthetic pollutant, or a biotoxin such as those that cause paralytic shellfish poisoning, could have caused the condition.

They will also examine the oceanic and atmospheric changes -- among them the unusually warm water temperatures of this summer -- that might have produced an unusual flourishing of microorganisms.

"We've begun to assemble some bits and pieces from which we can now hope to develop a story," he said.

Since early July an estimated 200 dolphins have washed ashore from Virginia to New Jersey, many showing signs of ulcerated skin. The skin disorder -- peeling skin, dorsal fins often completely stripped of skin, mouth ulcers and lesions -- has been a common feature of the dolphin carcasses that have washed ashore.

That condition was initially a matter of particular concern, Geraci said, because the condition had "not been seen previously in any of the stranded animals that I've encountered," and does not exist in the literature.

Now the scientists understand the skin problem as a function of the bacteria-invaded dolphins: "The bacteria destroy the vessels that normally supply blood and therefore oxygen to the skin. And in the absence of those nutrients, the skin becomes devitalized and simply delaminates," Geraci said.

"The skin disorder is not primary. And this was a concern that we all had. If it is primary -- might this be something that is infectious? Do we have reason to be concerned about our own health? Can this manifestation occur in other species? . . . . We can now attribute the disorder we are seeing in the skin to these bacteria," Geraci said.

With the breakdown of the veins and arteries, blood leaks into the body cavities -- the chest and abdominal area -- producing the characteristic dark fluid that was found in "virtually every animal" examined, Geraci said.

Two weeks ago, the federal Marine Mammal Commission asked Geraci to assemble a team of specialists to investigate the problem, which seems particularly acute in Virginia waters.

Geraci and his colleagues have examined 15 of the 45 animals that have been stranded in the Virginia Beach area since the team's arrival. Tissue samples have been sent to the National Veterinary Services Laboratory in Ames, Iowa; to the Eastern Virginia Medical College in Norfolk; and to Virginia Beach General Hospital for a full range of bacteriological, toxicological and virological tests.

The dolphins found in New Jersey, Geraci said, "do not seem to be as critically involved with the organisms that we are dealing with . . . . There seems to be a milder form of the skin disorder and perhaps of the hemorrhaging, which would lead me to think that we are dealing with a more virulent organism or perhaps that we are dealing with a different organism."

Geraci added that because the water temperatures are higher here, the invading organisms "would be expected to behave somewhat differently."

Still to be determined, Geraci said, is the question of how long the condition will continue and whether it will affect other stocks of dolphins, spreading beyond the range in which it now occurs.