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Science 9 April 2004:
Vol. 304. no. 5668, p. 233
DOI: 10.1126/science.304.5668.233
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Introduction to special issue

Dangerous--and Not So Dangerous--Liaisons

Stella M. Hurtley and Jean Marx

Contents
News
The Roots of Plant-Microbe Collaborations
Reviews
How Viruses Enter Animal Cells
A. E. Smith and A. Helenius
Bacterial Invasion: The Paradigms of Enteroinvasive Pathogens
P. Cossart and P. J. Sansonetti
Intracellular Parasite Invasion Strategies
L. D. Sibley
Ancient Invasions: From Endosymbionts to Organelles
S. D. Dyall et al.
See related Science Express by Horn et al.
In many biological systems, one organism, often a virus, bacterium, or other microbe, invades another, generally at the cellular level. The ramifications can be profound. For example, a virus infecting a single animal cell can ultimately lead to the demise of the whole animal. Other microbial parasites may also have catastrophic consequences for their hosts, as seen in the terrible toll taken by malarial parasites in the develoing world.

Cellular invasions can also have more benign outcomes, as when microbial symbioses are established in the roots of legumes, allowing them to gain essential nitrogen nutrients; or in our ancient past, when bacteria took up residence in prototype eukaryotic cells and evolved into mitochondria, the cell's energy powerhouses. In this special issue, we look at a variety of these cellular invasions and examine the intracellular niches established by invading microorganisms.

Smith and Helenius (p. 237) describe invasions by animal viruses, which employ multiple strategies to allow the viral nucleic acids to gain access to the host cell replication machinery and generate progeny viruses. Some bacterial pathogens can also take up residence within eukaryotic host cells. Cossart and Sansonetti (p. 242) look at infections by a group of enteroinvasive bacteria, including Salmonella, Shigella, Yersinia, and Listeria, that cause certain types of food poisoning. When these microbes invade cells, they establish a privileged intracellular niche within which they can grow and divide, protected from host defense mechanisms. In Science Express, Horn et al. (www.sciencemag.org/cgi/content/abstract/1096330) present a comparative and phylogenetic genome analysis of a chlamydia-related symbiont of free-living amoebas. They suggest that ancient chlamydiae were major inventors of mechanisms used by intracellular bacterial pathogens for survival in, and exploitation of, eukaryotic host cells. Another group of invasive organisms, the apicomplexan parasites, includes Plasmodium, which causes malaria, and the organism responsible for toxoplasmosis, which can damage multiple organs in immune-compromised individuals. Sibley (p. 248) reviews how apicomplexan parasites gain entry to susceptible host cells.

Dyall et al. (p. 253) describe how, way back in evolutionary history, endosymbionts developed into organelles such as mitochondria and chloroplasts, thus producing early eukaryotes.

In an accompanying News story (p. 234), Jean Marx looks at the molecular interactions required for the establishment of two types of intracellular symbioses that are critical for plant growth and thus for modern agriculture: In one, nitrogen-fixing rhizobial bacteria set up housekeeping in legume roots, and in the other, mycorrhizal fungi invade the roots of many plants and help them to acquire phosphate.

Keeping yourself to yourself may be the done thing in polite society, but in the cut and thrust of the real life of invasive organisms, it's simply not an option.

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Science. ISSN 0036-8075 (print), 1095-9203 (online)