Public Release: 

Atomic force microscope used to measure how well live bacteria stick

Goal is to create better filters

Virginia Tech

(Blacksburg, Va., September 9, 2003) Virginia Tech researchers are using a modified form of atomic force microscopy (AFM) to observe at subatomic levels the efficiency of the attachment of bacteria to silica surfaces.

The geological scientists are simulating environments similar to ground water in sandy soils. Sticking efficiency of bacteria has not been previously measured experimentally using the AFM.

Graduate student Tracy Cail will report the research results at the 226th American Chemical Society National Meeting in New York City September 7-11.

She reported in March on her initial experiments to see if the AFM could be used to measure sticking efficiencies at the nanoscale (www.eurekalert.org/pub_releases/2003-03/vt-afm031703.php) - also the first such experiments. Virginia Tech researchers have developed a cantilever for use in the AFM that allows them to study the attractions between microparticles.

"The same technique can also be applied to natural systems," she says.

Cail is developing a new method for predicting how bacteria and other contaminants can be transported in groundwater. "If we understand how they stick to various surfaces then we can use the information to design filters," she says.

For her research, Cail is using the bacteria, Enterococcus faecalis, because they are easy to model. "They look like the carboxylated polystyrene beads I used to do the initial work with the AFM. They are spherical, hard, and smooth, and are about 1 micron."

The bacteria are also plentiful. "They thrive in the Virginia Tech duck pond. They live naturally in human intestines but are serious hospital pathogens," she says.

"I'm looking at groundwater applications, but there is an area for expansion in terms of controls in hospital environments," she adds.

She found that Enterococcus faecalis are surprisingly robust. "They survived being put in a vacuum, long periods without food, and the imaging process."

Cail and geological sciences professor Michael Hochella Jr. will present "Measured sticking efficiencies of Enterococcus faecalis using atomic force microscopy" during the Division of Geochemistry poster session, 6 to 8 p.m., Tuesday, Sept.9, in the Javits Convention Center North Pavillion.

Cail will complete her Ph.D. in geological sciences from Virginia Tech in December and work as a postdoctoral associate at Oak Ridge National Laboratory in the area of contaminant transport. A native of Moncton, New Brunswick, Canada, she did her undergraduate work at St Francis Xavier University and her master's degree work at the University of Nevada, Las Vegas.

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Abstract:
Measured sticking efficiencies of Enterococcus faecalis using atomic force microscopy

Tracy L. Cail, and Michael F. Hochella Jr., Department of Geological Sciences, Virginia Tech, 4044 Derring Hall, Blacksburg, VA 24061.

Sticking efficiency is the probability of attachment of a particle to a collector and is a function of the forces of attraction and repulsion between these surfaces at the nanoscale. In this study, the sticking efficiencies of live Enterococcus faecalis cells in different aqueous solutions were measured using atomic force microscopy (AFM). Individual bacterial cells were attached to the ends of silicon nitride cantilevers and brought into contact with a silica glass collector surface in aqueous solution. Contact-mode AFM was used to measure the interaction forces with picoNewton resolution as the surfaces were moved together and subsequently separated at a controlled rate. Intersurface potential energies were determined by integrating force data with respect to cell-collector separation distance and sticking efficiencies were calculated from potential energy using Spielman and Friedlander's (1974) method. Results were compared with results of a previous study that measured the sticking efficiency of carboxylated polystyrene microspheres to a glass collector surface.

Researchers' contact information: Tracy Cail at tcail@vt.edu or 540-231-8575, or her major professor and co-author Michael F. Hochella Jr. at 540-231-6227 or hochella@vt.edu.

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