Catch-bonds Hold on Tighter When Pulled Apart

a micrograph of salmonella bacteria represented by yellow and white cells that are curled together

Biological Strategy

Salmonella

AskNature Team

Image: NIAID / Flickr / Public Domain - No restrictions

Attach Temporarily

Living systems must sometimes, temporarily, stay in one place, climb or otherwise move around, or hold things together. This entails attaching temporarily with the ability to release, which minimizes energy and material use. Some living systems repeatedly attach, detach, and reattach for an extended time, such as over their lifetimes. Despite being temporary, these attachments must withstand physical and other forces until they have achieved their purpose. Therefore, living systems have adapted attachment mechanisms optimized for the amount of time or number of times they must be used. An example is the gecko, which climbs walls by attaching its toes for less than a second. Other examples include insects that attach their eggs to a leaf until they hatch, and insects whose wings temporarily attach during flight but separate after landing.

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Manage Tension

When a living system is under tension, it means there is a force pulling on it, like a person pulling on a rope tied to a horse. When applied to a living system, unless the system is completely rigid, the result is that it gets stretched. If stretching exceeds the strength of the living system’s material, it can damage it. Living systems manage tension using materials that are flexible and stretchable enough to survive most tension that occurs in their environment. The ocean’s intertidal zone offers a good example. The waves and incoming and outgoing tides put tension on soft-bodied organisms. Mussels resist tension with flexible threads that hold them onto rocks; in contrast, large algae have stretchy fronds.

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Manage Shear

The effect of shear stress on a living system is parallel internal surfaces sliding past each other. Slippage occurs in parallel with the force. Think about holding two wooden boards on top of each other and sliding one to the right and the other to the left. This may be easy until you add glue, which increases their shear strength and makes them harder or impossible to slide. Shear can occur in solids, liquids, and gases. Living systems must increase their shear strength to overcome these types of forces. For example, darkling beetles lock their wings together in flight to prevent lateral movement by using many small hairs on each wing. These hairs interlock to provide shear strength, just as two hair brushes put together would be difficult to slide past each other.

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Bacteria

Domain Bacteria (“small staff”): L. acidophilus, Staphylococcus

From inside of our own bodies to the deepest parts of the ocean, bacteria have adapted to almost every possible environment. All bacteria are unicellular, microscopic, and lack a membrane-bound nucleus and organelles, and come in three basic shapes: spherical (cocci), rod (bacilli), and spiral (spirilla). Some strains can be deadly, like Staphylococcus aureus (MRSA) while others, like Lactobacillus acidophilus, found in our intestinal microbiome, keep us healthy. Cyanobacteria, also known as blue green algae, caused the Great Oxidation Event 2.4 billion years ago, producing Earth’s first oxygenated atmosphere.

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Salmonella bacteria grip their host more tenaciously when pulled apart because complex adhesive proteins change their structure as stronger force is applied.

Pathogens must adhere to host tissue in order to survive and reproduce. They produce many adhesive s on their surface most of which exhibit slip-bond type binding with host compounds; in other words, the bonds slip and let go when pulled apart. Salmonella bacteria have evolved special binding proteins that exhibit catch-bond type adhesion to host cells—these bonds actually become stronger when pulled apart, up to a certain threshold. FimH is one of Salmonella’s multi-domain adhesive proteins. The binding domain is a lectin pocket that binds to mannose on the surface of the host cell. This domain is linked to another part of the adhesive protein (a long pilin domain) which anchors FimH to the bacteria. When the bonds between the bacteria and its host are pulled apart by weak forces, the pilin domain holds tightly to the binding domain which in turn binds mannose with low force. But when the bonds between the bacteria and its host are pulled apart by stonger forces, the pilin domain is stretched away from the binding domain causing the structure of the binding pocket to shift so that it binds mannose with 100 times more force. In short, strong mechanical forces pull a binding inhibitor away from the binding domain increasing adhesion: a catch-bond. There is an optimum range within which catch-bonding occurs; when that range is exceeded, catch-bonds revert to slip-bonds.