Washington ? Silently, invisibly, vast miniature armies are waging a fight to the death on land and sea.

The defenders are bacteria, the one-celled microbes that infest every cranny on Earth, from the seafloor to garden soil to the human gut. The aggressors are a class of viruses known as bacteriophages – literally “bacteria-eaters” – that happily slaughter their far bigger foes.

“Every two days, half the bacteria on Earth are killed” by bacteriophages, said Vincent Fischetti, head of the Laboratory of Bacterial Pathogenesis at Rockefeller University in New York.

Scientists say people should be rooting for the “phages,” as they’re known for short, since these tiny viruses are harmless to humans, can be enlisted in the fight against disease-causing germs and perform other useful functions.

Despite the bacteria’s horrific losses to the phages, the war between them rages on. Many bacteria divide every 20 minutes, so they can reproduce their population as rapidly as they’re slain.

The numbers are staggering. Biologists estimate that the world contains a thousand billion billion billion – that’s a 1 followed by 30 zeroes – bacteria. If stretched end to end, they’d reach a distant galaxy.

Phages outnumber bacteria 10 to 1. They’re “not only the most abundant biological entities but probably also the most diverse ones,” Markus Weinbauer, a microbiologist at the Laboratoire d’Oceanographie in Villefranche, France, wrote in a just-published book, “Bacteriophage: Genetics and Molecular Biology.”

One way these ruthless little killers earn their combat medals is by chewing up bacteria, such as the deadly Staphylococcus aureus, that resist most antibiotic drugs.

“Antibiotic resistance is a nightmare for infectious-disease specialists, who increasingly have to stand by and watch helplessly as bacteria dupe the drugs aimed at them and people die because no drugs work,” according to Thomas Hausler, the Swiss author of another recent book about phages, “Virus vs. SuperBugs.”

As an example of their utility, the Food and Drug Administration last year approved the use of phages on ready-to-eat meats to kill bacteria.

Phages also are being used to treat open sores caused by diabetes and warriors whose wounds resist antibiotics, according to Dr. Randall Wolcott, head of the Southwest Regional Wound Care Center in Lubbock, Texas.

“I have been using bacteriophages in the management of chronic wounds for several years now,” Wolcott said.

“Numerous companies are vying for an opportunity to sell phage therapeutics to a willing public,” said Stephen Abedon, a researcher at Ohio State University in Columbus.

Viruses are the most numerous biological creations on Earth.

They come in many shapes and sizes besides phages. Many are responsible for infectious diseases from the common cold to AIDS.

Scientists disagree on whether viruses are alive, because they can’t reproduce on their own but must hijack the genetic machinery of living cells.

Phages use bacteria as factories to manufacture more of themselves.

When a phage bumps into a bacterium, it latches on to its surface and penetrates the cell wall.

Once inside, it takes control of the bacterium’s DNA to make hundreds of copies of itself.

The bacterium soon dies, releasing a horde of baby viruses to find new victims.

In this way, phages kill bacteria without the use of antibiotic drugs.