Majority of microbes neither good nor bad

July 29th, 2008 - 3:04 pm ICT by IANS  

Washington, July 29 (IANS) Most bacteria are harmless, some even beneficial; only a handful pose threats, according to the latest research. Researchers from Universities of Wisconsin and Iowa identified a slew of microbe-induced genetic changes in a tiny squid that may hold the secrets to developing a mutually beneficial relationship.

“Interactions of animals with their microbiota have a profound impact on their gene expression, and to create a stable association with a microorganism requires a lot of conversation between the microbe and the host,” said Wisconsin microbiologist Margaret McFall-Ngai, co-author of the study.

Many studies have focussed on the bacterial side of that association. But aside from a few “professional pathogens”, like the bubonic plague-causing Yersinia pestis, most bacteria are not inherently good or bad, McFall-Ngai said.

Instead, bacterial effects are highly context-dependent: she reported in 2004 that a common bacterial “toxin” - which causes tissue damage under some circumstances - also plays a critical role in host tissue development.

She suggested that the outcome may rely on how the host itself responds to the bacterium. Problems most often arise when a normal balance is disrupted, she says. “A lot of these pathogens are just at the wrong place at the wrong time.”

To listen into the animal-microbe conversation, McFall-Ngai takes advantage of a relatively simple host-bacterium relationship: the Hawaiian bobtail squid, a colourful creature less than two inches long that nurtures a single type of light-producing bacteria, called Vibrio fischeri.

The bacteria take up residence in a specialised pouch on the squid’s belly and produce light that the squid uses as anti-predator camouflage. In return, the bacteria receive easy access to nutrients from the squid.

In the new study, a team led by former UW-Madison graduate student Carlene Chun dissected the genetics of the squid host response, including a comparison with mutant bacteria unable to successfully colonise their host.

They identified hundreds of genes affected by the establishment of a stable bacterial partnership, including some known to play a role in human responses to bacteria.

“We have thousands of bacteria that live with us, and yet there are only around 100 bacterial pathogens,” she said. Given the numbers, “it seems like these pathways and these molecules are likely to be ’symbiosis’ pathways more than ‘anti-pathogen”.

The study appears online this week in the Proceedings of the National Academy of Sciences.

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