Thousands of years ago, when no one was safe from famine, dangerous wild animals, or diseases, some of our early ancestors got a bright idea: let’s band together! By forming communities made of people with different talents and skills, the survival rate of individuals grew tremendously. There’s strength in numbers but let’s not pat ourselves on the back too hard — we’re not the only ones who got this idea. Ants and bees do it. Microbes too — they group together into communities called biofilms.
The microorganisms that form biofilms include bacteria, fungi, and protists. Perhaps the most common biofilm familiar to most is the dental plaque — that sticky, colorless film of bacteria and sugars that constantly forms on our teeth. That slime on the surface of water, particularly ponds, is also biofilm.
According to this paper, a bacterial biofilm is defined as “a structured community of bacterial cells enclosed in a self-produced polymeric matrix and adherent to an inert or living surface.” In plain English, this means that bacteria sometimes join together, cling to essentially any surface, and form a protective matrix around the group. Indeed, we’ve found biofilms almost anywhere; on mineral, metals, inside our gut etc. In fact, biofilms have been around for at least 3.3 billion years. However, it’s in wet and moist environments that you’ll the most biofilms. They love wetness.
A vast number of pathogens are grouped as biofilms. Like humans, they’ve learned this configuration enhances their survival rate as they are better able to combat the cells of our immune system bent upon destroying them.
How biofilms form
The slimy films start forming when initially free-floating bacteria adhere to surfaces in aqueous environments and start ‘laying their roots’. To stay sticky, the bacteria excrete a glue-like substance that’s effective at anchoring them to all kinds of materials, from plastics to soil to medical implants such as pacemakers. This glue is known as an extracellular polymeric substance (EPS) and is comprised of sugars, proteins, and nuclei acids like DNA.
In time, layers upon layers of EPS are added. After a period of growth, a complex 3D structure emerges which is packed with water channels on the inside that facilitate the exchange of nutrients and waste products.
A fascinating thing about biofilm formation has to do with how the bacteria communicate. Pathogens can instruct each other where to position themselves through quorum sensing. Basically, this phenomenon allows a single-celled bacteria to sense how many other bacteria are there in its close proximity. If the bacteria senses there’s a dense population surrounding it, it will be inclined to join them. Remember, strength in numbers.
“Disease-causing bacteria talk to each other with a chemical vocabulary,” says Doug Hibbins of Princeton University.
“Forming a biofilm is one of the crucial steps in cholera’s progression,” said Dr. Bonnie Bassler, a microbiologist also at Princeton. “They [bacteria] cover themselves in a sort of goop that’s a shield against antibiotics, allowing them to grow rapidly. When they sense there are enough of them, they try to leave the body.”
Sometimes clumps of biofilm can break away from the main mass and establish themselves on a new surface. These new pioneers will continue to extend their slimy film until they form a new, bigger colony.
How big can a biofilm get
Most biofilms are very thin — just a few cell layers thick. That’s too thin to see with the naked eye. In fact, your kitchen counter almost certainly has a biofilm layer on it. You just can’t see it. Some biofilms, however, can grow many inches thick and are obviously noticeable. You’ll find these thick slime molds growing as algae on rocks in a streambed.
The thickness of biofilms depends on several environmental factors. Some organisms can produce large amounts of EPS and hence grow a thicker biofilm. Water flow is also an important factor or, to be more precise, shear stress is. If a biofilm forms in a creek where there’s a high flow of water, it ought to be fairly thin. Biofilms formed in slow flowing water, like a pond, can grow quite thick.
Why biofilms form
As mentioned, bacteria band together because as a community they enhance their chance of survival, but what threats do they face and how does living a slime mold protect them? Some of the stressors bacteria face are the lack of water, high or low pH, or the presence of ‘toxic’ substances, i.e. antibiotics or antimicrobials.
The EPS layers act as the first line of defense against these threats. It can prevent dehydration or shield the bacteria against UV light. When they come in contact with the EPS, antimicrobials, bleach or even metals become bounded and neutralized by the sticky EPS.
Antibiotics can certainly destroy biofilm but not always because biofilms employ another line of defense. For instance, despite antibiotic substances might penetrate the EPS layer, they can be met dormant bacteria. Because these bacteria lack cellular activity, the antibiotics don’t work their magic because there’s nothing to disrupt.
Another line of defense against antibiotics are the ‘persisters’ or special bacteria that do not divide. These bacteria produce substances that block the targets of many antibiotics, according to a 2010 paper. Compared to free-floating bacteria, those growing as a biofilm can be up to 1,500 times more resistant to antibiotics
Finally, living inside a community, often made of different bacterial species, means its members can reap the benefits that come with having a multi-skilled network. For instance, some biofilms are made of both autotrophic and heterotrophic microorganisms. The autotrophs produce their own food using photosynthesis and available organic material while heterotrophs don’t make their own food and require external sources of carbon. As such, in these biofilms, the microorganisms will often cross-feed. It’s a sort of division of labor.
Biofilms, humans, and disease
Biofilms seem to be able to form and cling to just about any external surface as long as it’s wet. This may naturally beg the question — does that mean they can form inside the human body as well? It certainly is wet enough and, indeed, we find that the answer is ‘yes’. According to the National Institutes of Health, more than 65% of all microbial infections are caused by biofilms. That might strike you as a lot but you need to keep in mind that the vast majority of infections are common like urinary tract infections, catheter infections, common dental plaque formation and so on.
However, biofilms can be involved in a range of diseases and medical problems. One example is kidney stones which are caused by biofilms. Some 15 to 20 percent of kidney stones form as a result of urinary tract infections, produced by the interplay between infecting bacteria and mineral substances from the urine.
Then there’s endocarditis, a disease that involves inflammation of the inner layers of the heart. Endocarditis seems to be triggered by a complex biofilm made from bacterial and host component located on a cardiac valve. This type of biofilm is known as a vegetation. The vegetation can disrupt valve function, produce a near-continuous infection of the bloodstream, and can block blood circulation through a process known as embolization.
Pathogenic biofilms also plague prostheses and various medical implants like artificial joints and heart valves or pacemakers. This first came to the medical community’s attention in the 1980s when bacterial biofilms were found on intravenous catheters and pacemakers.
“When people think of infection, they may think of fever or pus coming out of a wound,” explains Dr. Patel from the Mayo Clinic. “However, this is not the case with prosthetic joint infection. Patients will often experience pain, but not other symptoms usually associated with infection. Often what happens is that the bacteria that cause infection on prosthetic joints are the same as bacteria that live harmlessly on our skin. However, on a prosthetic joint they can stick, grow and cause problems over the long term. Many of these bacteria would not infect the joint were it not for the prosthesis.”
Biofilms have been poorly understudied until recently but evidence suggests they’re involved in many human diseases, including debilitating chronic infections. According to Dr. Trevor Marshall, a biomedical researcher at Murdoch University, Australia, some large microbiota of chronic biofilm like L-shaped bacteria can evade the immune system because, long ago, they evolved the ability to reside inside macrophages. Ironically, these are the very white blood cells of the immune system which are supposed to kill the invading pathogens. Marshall also says that biofilm infections occur with great ease in immunocompromised hosts.
Targeting biofilm infections
Research carried out over the past three decades suggests that biofilms are either extremely difficult or impossible to eradicate from the human body. What’s certain is that administering antibiotics in a standard manner (high dose, constant) does not work.
After high doses of antibiotics are administered, it may seem the biofilm infection has disappeared. However, it will reappear because the biofilm was not destroyed, only weakened. It seems that while antibiotics can penetrate the biofilm matrix and kill bacteria, a number of cells called ‘persisters’ are left behind. These are able to survive the onslaught of antibiotics and gradually allow the biofilm to form again.
Dr. Kim Lewis of Tulane University, however, says that it is possible to destroy some biofilms. His treatment involves using pulsed, low dose antibiotics to break up the biofilm. For instance, research suggests this technique is effective at destroying P. aeruginosa biofilm bacteria in a manner that is indistinguishable when the same antibiotic concentrations are administered to single planktonic cells.
When the low, pulsed dosing of antibiotics is applied, the first application eradicates the bulk of the biofilm cells, leaving the persisters behind. Because the antibiotics are stopped, the survival of the persisters is not enhanced. Lewis believes this causes the cells to lose their shape and biochemical properties, making them unable to restart the biofilm formation process. A second application of the antibiotic after a certain time should then completely eliminate the persister cells.
The efficacy of this method depends on the ability to manipulate the antibiotic concentration. Furthermore, not all biofilms can be broken down this way.
Biofilms can cause serious medical conditions and, as we’ve seen, they can be very difficult to get rid of. But there are instances when biofilms can be useful, for bioremediation purposes. Biofilms are used, for instance, in treating waste water or contamination with heavy metals or radioactive substances. Another practical use for biofilms is in microbial fuel cells. In such fuel cells, microbes that live on the surface of an electrode break down nutrients and transfer electrons through a circuit, providing electricity. Microbial fuel cells can be very useful if you need to remotely generate power for sensors in wastewaters or landfills.
Biofilms are right now the subject of intense research. Biofilms cause billions in damage every year due to disease, equipment damage, energy loss or contaminations, and as such finding ways to get rid of them is a priority. The resilience of biofilms is a big challenge and requires contributions from different sciences such as biochemistry, engineering, mathematics, and microbiology.
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