Science & Technology (Commonwealth Union) – Sujit Datta, a professor of chemical engineering, bioengineering, and biophysics at Caltech together with former graduate student Sebastian Gonzalez La Corte investigated how bacteria grow in liquid crystal fluids, in which elongated molecules are uniformly oriented in the same direction. Such molecular alignment can occur in biological environments, including certain biofilm matrices and the mucus lining of the airways and digestive tract. However, most laboratory studies of bacteria use fluids whose molecules are randomly oriented. The researchers once again discovered that changing this physical environment can produce surprising bacterial behavior.
The findings are reported in a paper published in the journal PNAS.
Earlier research by Datta’s team focused on bacterial growth in polymer-based fluids containing molecules arranged without a specific orientation. Their experiments and models showed that several widely studied bacterial species—including *Escherichia coli*, *Vibrio cholerae*, the bacterium responsible for cholera, and Pseudomonas aeruginosa, which can cause infections particularly in hospitalized or immunocompromised individuals—can develop into elongated structures several cells thick. These structures intertwine with one another, creating what the researchers described as a kind of “living gel.”
The latest study revealed a markedly different pattern in an aligned liquid crystal environment. Rather than forming thick, intertwined cables, the bacteria produced chains just one cell wide. These chains extended in relatively straight paths as they grew, before abruptly bending and buckling in an unexpected manner.
When a steel beam is squeezed from both ends, it eventually buckles, forming a gradual, arc-shaped curve that extends across much of its length. Bacterial “beams” growing inside a liquid crystal fluid, however, behave quite differently. Instead of bending along the entire structure, the buckling is concentrated in a small region where the cells form an extremely tight bend. What causes these biological beams to respond so differently?
Datta indicated that this presents a rather unusual mechanics problem. Scientists have studied the buckling of thin, slender beams for decades. He further indicated that what makes this case different is that the beam consists of living cells that are capable of reproducing.
To investigate the phenomenon, Datta and his collaborators worked with applied mathematicians from the University of Wisconsin–Madison and the University of North Carolina at Chapel Hill to develop a mathematical model describing how bacteria grow within liquid crystals.
Datta uses an analogy involving a box of matches to explain the system. Picture the matches neatly arranged in the same direction, representing the aligned molecules of the liquid crystal. Now place a bacterium into the box. Because the bacterial cell is far larger than the individual liquid crystal molecules, it can be thought of as a pencil placed inside a very large matchbox filled with aligned matches.
Datta says “The matches don’t want to be bent out of their aligned orientation, so they push back. They force the pencil to be aligned with them,”.
Researchers of the study stated that when continuing with the pencil analogy, picture the pencil growing and splitting lengthwise to create additional pencils. These new pencils remain aligned with one another, forming a continuous chain—the arrangement that requires the least energy, according to Datta. At the growing end of the pencil inside the box, however, the matches need to curve slightly to make room for the pencil’s width, and that deformation requires energy. The energy cost becomes even greater when several pencils stack together to create a wider chain.
The new model captured this behavior clearly. It demonstrated that the liquid crystal molecules exert forces on the bacteria that help keep them arranged in a chain just one cell wide. By combining these findings with established principles describing liquid crystal behavior, the researchers could also quantitatively predict how strongly bacterial chains would align within a liquid crystal fluid.
