Bacteriophages are viruses, which means they have a public-perception problem when it comes to using them to treat bacterial infections. People typically think all viruses are bad guys.
But Roger Hendrix, University of Pittsburgh distinguished professor of biological sciences, co-director of Pitt's Bacteriophage Institute and a doctor of biology described as America's "king of phage biologists," has spent time describing how phages look, act, reproduce and evolve. Perhaps his most often quoted calculation is his estimate of how many phages exist on Earth.
Twenty years ago, Norwegian scientists took a water sample from a fjord, put it through a centrifuge, then used an electron microscope to count the phages. They found 100 million per milliliter of water, about a half teaspoon. Mr. Hendrix used that information to calculate that phages are the most abundant life form on Earth whose estimated total is 10 to the 31st power -- or 10 followed by 31 zeroes. That's 10 quadrillion quadrillion phages.
If placed in a line, the parade of phages, each with its characteristic tail, would extend 200 million light years into intergalactic space. That means the line not only would reach the Andromeda Galaxy 2 1/2 million light years away and most distant object normally visible, but it would extend 80 times farther than Andromeda.
If each phage were a cockroach, Mr. Hendrix said, the entire Earth would be covered by a 30,000-mile-high layer of the hardy bugs -- a distance that would extend 1.2 times around Earth at the equator.
The point of these numbers?
Phages are everywhere around us and inside us. But not to worry. Each phage targets a specific bacterium, rather than human cells. Phages generally do not cause human illness, although bacteria can incorporate portions of phage DNA into their own to bolster their infectious power or develop immunity to the phage. Because they generally kill bacteria, phages in other nations are being used to treat and even cure dangerous bacterial infections.
Phages look like moon-landers. Its protein structure forms an icosahedron -- a geometric shape with 20 faces -- that serves as a container for its double-helix linear strand of DNA. With a tube or tail extending below the icosahedron along with insect-like legs that serve as landing gear, the phage positions itself atop a receptor on the bacterium surface and injects its DNA strand through the tube/tail. Inside the bacterium, the DNA either replicates itself until the bacterium splits apart and releases new phages, or it sits quietly there so the bacterium can incorporate phage genes into its own DNA. As a result, whenever the bacterium replicates itself, it also replicates the phage.
"One way to look at it is, the phage is paying rent to the bacterium so that the phage will be replicated," Mr. Hendrix, noting the two methods phages use to replicate themselves.
Phages can exchange genes among themselves, generating variety and improving survival. So many phages exist that bacterial immunity to one doesn't prevent other phages from targeting the same bacterium. There are 10 times more phages than bacteria. Phages are viruses; bacteria are Earth's most abundant cellular organisms.
"The conclusion is that phages have been participating in a global-scale sex orgy for a few billion years and has combined gene parts in every way imaginable. And then natural selection occurred," Mr. Hendrix said.
Because of the prevalence of phages, Mr. Hendrix alters a quote from Shakespeare's play, "As You Like It," to stress his point:
"All the world's a phage."