Thieves, Deceivers, and Killers by William Agosta

Author:William Agosta
Language: eng
Format: epub
ISBN: 9780691092737
Publisher: Princeton University Press

ILLUSTRATION 11 Shigella flexneri

sweeps through cytoplasm propelled by its

cometlike actin tail.

There is another dangerous bacterium unrelated to Shigella flexneri that makes similar actin tails for itself. This is Listeria monocytogenes, a food-transmitted human pathogen that was responsible for a serious outbreak of disease in late 1998. Listeria can bring about severe meningitis and infrequently induces miscarriages. It turns up in processed meats and milk products, occasionally contaminating the cheese in commercial pizza. In turning actin filaments into tails, both Shigella and Listeria make use of only a single bacterial protein of their own. The proteins from the two microbes are structurally quite dissimilar and so do not share a common origin. Apparently, bacteria have independently evolved mechanisms for making actin tails at least twice.

Chemical thievery may also operate in the opposite direction, when microbes supply rather than secure chemical compounds. The organisms taking microbial chemicals are typically multicellular and harbor parasitic bacteria or have bacteria somewhere in their food chain. These contacts permit the larger organisms to appropriate the microbes’ chemicals for their own use. One creature of this sort is the dragon fish (Malacosteus niger), a sharp-toothed predator of the deep sea with an ingenious hunting technique built around bacterial chemicals. Most deep-sea fish are visually most sensitive to the blue light that penetrates the ocean depths farther than other wavelengths. Moreover, many inhabitants of this dim world are bioluminescent: they emit biologically generated light that results from certain chemical reactions. In most cases their light is blue.

Using this blue light to search the dark waters for prey would present an obvious disadvantage: A predator with a light other creatures can plainly see will very likely reveal its own presence long before sighting its prey. The dragon fish avoids this problem and remains concealed by employing a red light in searching for food. Since red light is invisible to other deep-sea animals, it gives the dragon fish’s prey no advance warning. They never realize a predator has spotted them until it is too late. For this scheme to work, the dragon fish must of course be able to see whatever its red bioluminescence illuminates. Unlike other fishes, it has to see red. There are several ways to do this, but how the dragon fish actually sees red remained an unsolved puzzle until mid-1998. It is now clear that the dragon fish’s eye contains not only the common blue-sensitive pigments but also a mixture of red-sensitive pigments absent in other fishes. Red light striking the molecules of these unique pigments excites them energetically. The excited molecules quickly shed their excess energy by passing it on to the ordinary blue-sensitive pigments, exciting them in turn. This secondhand excitation is essentially the same as the excitation the ordinary pigments experience when struck by blue light. Because the excitation appears normal, the blue-sensitive pigments respond in their normal fashion, which is to send signals along the optic nerve to the fish’s brain. In this way, the brain receives the same signals from both blue and red light, and thus the dragon fish perceives both blue and red.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.