Since its discovery, this outer membrane has been used as a way to classify bacteria into those that do and do not react to a common staining technique, called a Gram stain. Bacteria with outer membranes do not react to the chemical stain are called Gram-negative. Bacteria with naked cell walls react to the stain and are classified as Gram-positive.
Both kinds of bacteria can become infectious and, when this occurs, the presence or absence of an outer membrane can also help determine how responsive they will be to antibiotics. Gram-negative bacteria tend to be more resistant to antibiotics.
“Scientists knew that outer membranes were chemical shields,” Huang said. “Thus, it was easy to relegate this third layer to an annoyance when dosing the cell with antibiotics.”
In recent years, however, researchers have picked up clues that the outer membrane is more important than they’d thought. In one study, Huang’s lab removed the cell wall of E. coli but left its outer membrane intact. Unsurprisingly, the bacteria lost their cucumber shape and became blobs. But a large fraction of these blobs survived, multiplied and ultimately reproduced new cucumber-shaped E. coli.
Rojas said that study was a clue that the outer membrane must play important structural and protective roles. “We just listened to the data. Science is about data, not dogma,” said Rojas, now an assistant professor of biology at New York University.
Over the last four years, working with collaborators from UC-San Francisco and the University of Wisconsin-Madison, the researchers tested the outer membrane’s structural powers.
They suddenly collapsed the pressure inside the bacteria, but instead of causing the cell wall to massively shrink, as prevailing assumptions would have predicted, they found that the outer membrane was strong enough to almost entirely maintain E. coli’s cucumber shape.
In other experiments, they put E. coli cells through two hours of rapid increases and decreases in pressure. E. coli cells normally shrug off these repeated insults and grow as if no changes at all had occurred. However, when the researchers weakened the outer membrane, cells died quickly.
“The presence or absence of a strong outer membrane is the difference between life and death,” Huang said.
The experiments identified a handful of components that give the outer membrane its surprising strength. Drugs that destabilize the deceptively thin outer layer could help destroy infectious bacteria, Huang said.
It’s a very exciting time to be studying biology.
Huang added that the findings are part of an emerging field of study called mechanobiology. Whereas scientists once viewed cells as sacks of chemicals to be studied by chemical means, today a confluence of tools reveal the infinitely complex structural properties that make cells and organs tick.
“It’s a very exciting time to be studying biology,” Huang said. “We are approaching the point at which our tools and techniques are becoming precise enough to discern, sometimes at almost the atomic level, the physical rules that give rise to life.”
Huang is also a member of Stanford Bio-X and a faculty fellow at Stanford ChEM-H. Other Stanford co-authors are Julie Theriot, PhD, professor of biochemistry and of microbiology and immunology, and a member of Stanford Bio-X; graduate student Amanda Miguel; postdoctoral scholar Pascal Odermatt, PhD; and undergraduate student Lillian Zhu.
This work was funded by the National Institutes of Health, the National Science Foundation, the Stanford Systems Biology Center and Simbios Center for Physics-Based Computation at Stanford, the Howard Hughes Medical Institute, the Swiss National Science Foundation, and the Allen Discovery Center program through the Paul G. Allen Frontiers Group.
Stanford’s departments of Bioengineering and of Microbiology and Immunology also supported the work. The Department of Bioengineering is jointly operated by the School of Medicine and the School of Engineering.