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UCSB Research
Article Explores Differences Between Wet and Dry Bacteria
Coastlines, Vol 31, No. 3
UCSB - Alumni Association
Winter 2001
by Gail Brown

Better understanding of the nature of bacteria has emerged from interdisciplinary research recently published in the Journal of Bacteriology. The article contrasts the properties of wet and dry bacteria in a study that employed the atomic force microscope in UCSB's physics department.

Most bacteria - wet and dry - occur as "biofilms" in which the bacteria are embedded in a polymer-type substance, according to the authors. Dry bacteria are of special importance to the research of one of the authors, Trish Holden, an environmental microbiologist and assistant professor in UCSB's Bren School of Environmental Science & Management.

Holden studies subsurface soil microbiology and pathogenic microbes in coastal environments, an important public health issue. She has been called upon to help in cases of California beach closures to determine the source of pathogens. Holden studies microbes in deeper soils, partly to determine the rate and extent of biodegradation of pollutants like gasoline and fuel oil.

"We're trying to understand what's different about the dry biofilms," Holden says. "There are millions of them in the soil, and when it rains they get wet, then they dry out. With food spoilage due to bacteria there is a major economic impact, so that's yet another application that gives us reasons to study dry biofilms.

Wet biofilms occur extensively in aquatic systems in the body. They are of interest to many, including industries dealing with bacteria that attach to equipment causing corrosion; doctors coping with bacterial drug resistance; dentists dealing with tooth decay; and those designing waste treatment filters Biofilms in most of these cases are constantly wet, and colonies of these bacteria have a mushroom shape, leaving room between the stalks for fluid flow.

However, bacteria in the soil, in food, or on plant leaf surfaces are often relatively dry by comparison. They may appear as patchy, unsaturated biofilms or dense microcolonies. And they grow in an environment that is only transiently wet.

"This article put the phrase 'unsaturated biofilms' solidly into the scientific literature," says Holden. "It's an area that has not been widely explored."

Another co-author, Helen Hansma, adjunct associate professor of physics at UCSB, says the study is the first time that the surface force of an unsaturated biofilm has been measured. Hansma heads an interdisciplinary research group in biophysics, combining physics and biological sciences.

Hansma, Holden, and their students Ilene D. Auerbach '98 and Cody Sorensen used the atomic force microscope, or AFM, a relatively new tool developed by Helen Hansma's husband, Paul Hansma, a UCSB physicist. The AFM uses a tiny needle to tap surfaces of very small objects. In addition to generating high-resolution pictures of the objects (the bacteria P. putida, in this case), the AFM can measure the adhesive force or 'stickiness' of the cells.

"By employing the AFM, we were not just taking pictures, but we can look at the interaction between the tip and sample," says Hansma. "We can measure the strength of the adhesive force between them. It gives us a map of the biofilm's stickiness. We found less stickiness between cells compared to the top. We found that the cells are covered with polymer structures on the surface."

The researchers found that the surface roughness grew with increasing sample size due to the crevices between the cells. They suggest that in unsaturated biofilms, outside substances must diffuse into the bacterial surfaces through external air or water films, as opposed to through the channels of wet biofilms.

Holden and Hansma state that by studying unsaturated biofilms directly and at high resolution, they strengthen the hypothesis that biofilms are shaped differently and function differently in unsaturated systems as compared with wet systems and thus merit further study and description.

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