Bacteria use a bridge of fungi to cross a "canyon." Photo: Colin Ingham.

Bacteria and fungi are remarkably mobile. Now researchers at Tel Aviv University have discovered that the two organisms enjoy a mutually beneficial relationship to aid them in that movement — and their survival.

Fungal spores can attach themselves to bacteria, “hitching a ride” wherever the bacteria travel. And while this allows them to travel further than they would on their own, says Prof. Eshel Ben-Jacob of TAU’s Raymond and Beverly Sackler School of Physics and Astronomy, it’s certainly not a one-way street. Bacteria live largely in the rhizosphere — the environment that surrounds plant roots — where air pockets can interrupt their progress, he explains. When faced with a gap, the bacteria can drop the fungal spores to form a bridge, and continue across the chasm.

The research, which was recently published in PNAS, was done in collaboration with Dr. Colin J. Ingham of Wageningen University and JBZ Hospital in the Netherlands, the paper’s lead author; post-doctoral fellow Dr. Alin Finkelshtein; and graduate student Oren Kalishman working in Prof. Ben-Eshel’s TAU lab.

This discovery contributes to our understanding of the way bacteria and fungi spread. Confirmation that the two organisms work in collaboration will help scientists fight disease-causing bacteria, or promote the spread of “good kinds” of bacteria or fungi, such as those that contribute to the health of plants. “In addition we now know that when you fight fungi, you are also fighting bacteria — and vice versa,” notes Prof. Ben-Jacob.

A bridge to mutual survival

Mobile or “motile” bacteria, such as Paenibacillus vortex, are known to be able to carry cargo. With this in mind, the researchers were motivated to test whether P. vortex would be able to carry non-motile fungi, aiding in its dispersion. In fact, they observed that not only can the bacteria transport the fungi over long distances, like humans being carried by air travel, but they are also able to recover fungal spores from life-threatening locations, moving them to new and more favorable places where they can germinate and start new colonies. “The bacteria entrap the spores and wrap them in their flagella, which are like arms,” explains Prof. Ben-Jacob. “This is similar to the way the Lilliputians moved the giant Gulliver by trapping him in a mesh of ropes.”

But the bacteria’s services aren’t free. In an experiment, the researchers created air gaps or “canyons” too large for bacteria to cross. When confronted with this challenge, the bacteria used the fungi’s mycelia — branch-like structures on the spores — as natural bridges, enabling them to cross otherwise impenetrable gaps, notes Dr. Ingham.

“We see that upon encountering impossible terrains, the bacteria can bring fungal spores to help,” Prof. Ben-Jacob continues. “The bacteria allow the fungi to germinate and form a colony, and then use the mycelia to cross obstacles.”

Taking over new territories

Ultimately, this collaboration helps both the bacteria and the fungi to spread and thrive in highly competitive habitats. It’s a sophisticated survival strategy, say the researchers, and contributes to our understanding of bacteria as smart organisms with an intricate social life. “The bacteria never let us down,” Prof. Ben-Jacob says with a smile. “Just present them with a new challenge and you can be sure they’ll provide new surprises.”

These observations can also be applied to agriculture and medicine, showing new mechanisms by which bacteria and fungi can help one another to invade new territories in the rhizosphere — as well as in hospitals and within our own bodies.

Simulated interacting agents collectively navigate towards a target

Much to humans’ chagrin, bacteria have superior survival skills. Their decision-making processes and collective behaviors allow them to thrive and even spread efficiently in difficult environments.

Now researchers at Tel Aviv University have developed a computational model that better explains how bacteria move in a swarm — and this model can be applied to man-made technologies, including computers, artificial intelligence, and robotics. Ph.D. student Adi Shklarsh — with her supervisor Prof. Eshel Ben-Jacob of TAU’s Sackler School of Physics and Astronomy, Gil Ariel from Bar Ilan University and Elad Schneidman from the Weizmann Institute of Science — has discovered how bacteria collectively gather information about their environment and find an optimal path to growth, even in the most complex terrains.

Studying the principles of bacteria navigation will allow researchers to design a new generation of smart robots that can form intelligent swarms, aid in the development of medical micro-robots used to diagnose or distribute medications in the body, or “de-code” systems used in social networks and throughout the Internet to gather information on consumer behaviors. The research was recently published in PLoS Computational Biology.

A dash of bacterial self-confidence

Bacteria aren’t the only organisms that travel in swarms, says Shklarsh. Fish, bees, and birds also exhibit collective navigation. But as simple organisms with less sophisticated receptors, bacteria are not as well-equipped to deal with large amounts of information or “noise” in the complex environments they navigate, such as human tissue. The assumption has been, she says, that bacteria would be at a disadvantage compared to other swarming organisms.

But in a surprising discovery, the researchers found that computationally, bacteria actually have superior survival tactics, finding “food” and avoiding harm more easily than swarms such as amoeba or fish. Their secret? A liberal amount of self-confidence.

Many animal swarms, Shklarsh explains, can be harmed by “erroneous positive feedback,” a common side effect of navigating complex terrains. This occurs when a subgroup of the swarm, based on wrong information, leads the entire group in the wrong direction. But bacteria communicate differently, through molecular, chemical and mechanical means, and can avoid this pitfall.

Based on confidence in their own information and decisions, “bacteria can adjust their interactions with their peers,” Prof. Ben-Jacob says. “When an individual bacterium finds a more beneficial path, it pays less attention to the signals from the other cells. But at other times, upon encountering challenging paths, the individual cell will increase its interaction with the other cells and learn from its peers. Since each of the cells adopts the same strategy, the group as a whole is able to find an optimal trajectory in an extremely complex terrain.”

Benefitting from short-term memory

In the computer model developed by the TAU researchers, bacteria decreased their peers’ influence while navigating in a beneficial direction, but listened to each other when they sensed they were failing. This is not only a superior way to operate, but a simple one as well. Such a model shows how a swarm can perform optimally with only simple computational abilities and short term memory, says Shklarsh, It’s also a principle that can be used to design new and more efficient technologies.

Robots are often required to navigate complex environments, such as terrains in space, deep in the sea, or the online world, and communicate their findings among themselves. Currently, this is based on complex algorithms and data structures that use a great deal of computer resources. Understanding the secrets of bacteria swarms, Shklarsh concludes, can provide crucial hints towards the design of new generation robots that are programmed to perform adjustable interactions without taking up a great amount of data or memory.

Researchers at the University of Sheffield have developed polymers that fluoresce in the presence of bacteria, paving the way for the rapid detection and assessment of wound infection using ultra-violet light.

When contained in a gel and applied to a wound, the level of fluorescence detected will alert clinicians to the severity of infection. The polymers are irreversibly attached to fragments of antibiotics, which bind to either gram negative or gram positive bacteria – both of which cause very serious infections – informing clinicians as to whether to use antibiotics or not, and the most appropriate type of antibiotic treatment to prescribe. The team also found that they could use the same gels to remove the bacteria from infected wounds in tissue engineered human skin.

Professor Sheila MacNeil, an expert in tissue engineering and wound healing, explained: “The polymers incorporate a fluorescent dye and are engineered to recognise and attach to bacteria, collapsing around them as they do so. This change in polymer shape generates a fluorescent signal that we’ve been able to detect using a hand-held UV lamp.”

“The availability of these gels would help clinicians and wound care nurses to make rapid, informed decisions about wound management, and help reduce the overuse of antibiotics,” added project lead Dr Steve Rimmer.

Currently, determining significant levels of bacterial infection involves swabbing the wound and culturing the swabs in a specialist bacteriology laboratory with results taking several days to be available. The team is confident that its technology can ultimately reduce the detection of bacterial infection to within a few hours, or even less.

The research has already demonstrated that the polymer (PNIPAM), modified with an antibiotic (vancomycin) and containing a fluorescent dye (ethidium bromide), shows a clear fluorescent signal when it encounters gram negative bacteria. Other polymers have been shown to respond to S. aureus, a gram positive bacteria. These advances mean that a hand-held sensor device can now be developed to be used in a clinical setting.

The research is the result of a three-year project which started in 2006, part-funded by the Engineering and Physical Sciences Research Council (EPSRC) and the Defence Science and Technology Laboratory (Dstl) – an agency of the Ministry of Defence, interested in the medical application of the research in battlefield conditions, and a subsequent EPSRC funded PhD studentship.

The team is also investigating whether using a sophisticated technique called fluorescence Non Radiative Energy Transfer (NRET) to generate the light signal could enable a highly refined sensor technology that could have applications in other areas.

“For example, we think that NRET could be very useful in an anti-terrorist and public health capacity, detecting pathogen release or bacterial contamination, whether accidental or deliberate,” says Dr Rimmer. “NRET also allows us to learn more about how the polymers collapse around the bacteria, which is important in developing our understanding of how bacteria interact with these novel responsive polymers.”

The team is interested in talking to potential partners to take this technology forward.