History of biofilm research
Perhaps because many biofilms are thick enough to be visible to the naked eye, the microbial communities were among the first to be studied by early microbiologists. Anton van Leeuwenhoek scraped the plaque biofilm from his teeth and observed what he described as the "animalcules" inside them under his primitive microscope.
In the years which followed, researchers have concentrated primarily on planktonic (free-floating) bacteria, the kinds of microbes studied by the likes of Louis Pasteur and Robert Koch. It was not until the 1970s that scientists began to appreciate that bacteria in the biofilm mode of existence constitute such a major component of the bacterial biomass in most environments. In the 1980s and 1990s, scientists began to understand how elaborately organized a bacterial biofilm community can be.3)
Paul Stoodley of the Center for Biofilm Engineering at Montana State University attributes much of the lag in studying biofilms to the difficulties of working with heterogeneous biofilms compared with homogeneous planktonic populations. In a 2004 paper in Nature Reviews, the molecular biologist describes many reasons why biofilms are extremely difficult to culture, such as the fact that the diffusion of liquid through a biofilm and the fluid forces acting on a biofilm must be carefully calculated if it is to be cultured correctly. According to Stoodley, the need to master such difficult laboratory techniques has deterred many scientists from attempting to work with biofilms.4)
Although research on biofilms has surged in the last 20-30 years, the majority of biofilm research to date has focused on external biofilms or those that form on various surfaces in our natural environment. Better tools to analyze external biofilms has realized they cause a wide range of problems in industrial environments. For example, biofilms can develop on the interiors of pipes, which can lead to clogging and corrosion. Biofilms on floors and counters can make sanitation difficult in food preparation areas.
Since biofilms have the ability to clog pipes, watersheds, storage areas, and contaminate food products, large companies with facilities that are negatively impacted by their presence have naturally taken an interest in supporting biofilm research, particularly research that specifies how biofilms can be eliminated.
This means that many recent advances in biofilm detection have resulted from collaborations between microbial ecologists, environmental engineers, and mathematicians. This research has generated new analytical tools that help scientists identify biofilms.
Prevalence of biofilm
According to a recent public statement from the National Institutes of Health, more than 65% of all microbial infections are caused by biofilms…. If one recalls that such common diseases as urinary tract infections (caused by E. coli and other pathogens), catheter infections (caused by Staphylococcus aureus and other gram-positive pathogens), child middle-ear infections (caused by Haemophilus influenzae, for example), common dental plaque formation, and gingivitis, all of which are caused by biofilms, are hard to treat or frequently relapsing, this figure appears realistic.
Kim Lewis 5)
Diseases for which biofilm have been implicated
In just a short period of time, researchers studying internal biofilms have already determined they cause a number of chronic infections and diseases. Notable diseases include:
Atherosclerosis – Biofilm may contribute to the development of atherosclerosis. Ott et al. 's work showed a diverse group of bacterial "signatures" in atherosclerotic lesions of patients with coronary heart disease.6) In a commentary following Ott's paper, Katz and Shannon concluded that his work suggested that atherosclerotic plaques are composed of "functional biofilm." The team noted that the characteristics of a "mature" arterial wall make it well-suited for biofilm formation and explains the inefficacy of antibiotics, such as macrolides or fluoroquinolones, in clinical trials.7)
Chronic sinusitis – One study found that biofilms are present on the removed tissue of two-thirds of patients undergoing surgery for chronic inflammation of the sinuses.8)
Chronic wounds – Biofilm have been implicated in chronic wounds. Dr Randall Wolcott has published work offering strategies for managing wounds.9)
Molecular analyses of chronic wound specimens revealed diverse polymicrobial communities and the presence of bacteria, including strictly anaerobic bacteria, not revealed by culture. Bacterial biofilm prevalence in specimens from chronic wounds relative to acute wounds observed in this study provides evidence that biofilms may be abundant in chronic wounds.
GA James et al. 10)
Cystic fibrosis
– The lungs of individuals with cystic fibrosis are colonized and infected by bacteria from an early age. These bacteria, which often spread amongst individuals with CF, thrive in the altered mucus, which collects in the small airways of the lungs. Over time, both the types of bacteria and their individual characteristics change in individuals with CF. In the initial stage, common bacteria such as Staphylococcus aureus and Hemophilus influenzae colonize and infect the lungs. Eventually, however, Pseudomonas aeruginosa (and sometimes Burkholderia cepacia) dominates. Once within the lungs, these bacteria adapt to the environment and develop resistance to commonly used antibiotics. Pseudomonas can develop unique characteristics that allow the formation of large colonies, known as "mucoid" Pseudomonas, which are rarely seen in people that do not have cystic fibrosis.11) Infection by the bacterium Pseudomonas aeruginosa is the leading cause of morbidity and mortality among patients with cystic fibrosis.12)
Endocarditis – Inflammation of the smooth membranes which line the inside of the heart is caused by a complex biofilm composed of both bacterial and host components.13)
Inner ear infections – The majority of ear infections are caused by biofilm bacteria.14) These infections, which can be either acute or chronic, are referred to collectively as otitis media (OM). They are the most common illness for which children visit a physician, receive antibiotics, or undergo surgery in the United States.
It appears that in many cases recurrent disease stems not from re-infection as was previously thought and which forms the basis for conventional treatment, but from a persistent biofilm…. [The discovery of biofilms in the setting of chronic otitis media represents] a landmark evolution in the medical community's understanding about a disease that afflicts millions of children worldwide each year and further endorses the emerging biofilm paradigm of chronic infectious disease.
Garth Ehrlich, PhD
Kidney stones – Biofilms also cause the formation of kidney stones.15) The stones cause symptoms of the disease by obstructing urine flow and by producing inflammation and recurrent infection that can lead to kidney failure. Approximately 15%–20% of kidney stones occur in the setting of urinary tract infection. According to Matthew Parsek, PhD, these stones are produced by the interplay between infecting bacteria and mineral substrates derived from the urine. This interaction results in a complex biofilm composed of bacteria, bacterial exoproducts, and mineralized stone material.
Leptospirosis – Biofilms also cause leptospirosis, a severe but neglected emerging disease that infects humans through contaminated water. Previously, scientists believed the bacteria associated with leptospirosis were planktonic (free-floating). One research team has shown that Leptospira interrogans can make biofilms, which could be one of the main factors controlling survival and disease transmission.16) According to the study's author, 90% of the species of Leptospira tested could form biofilms, and it takes L. interrogans an average of 20 days to make a biofilm.
Osteomyelitis – According to Parsek, biofilms may also cause osteomyelitis, a disease in which the bones and bone marrow become infected. This is supported by the fact that microscopy studies have shown biofilm formation on infected bone surfaces from humans and experimental animal models.17)
osteonecrosis and osteomyelitis of the jaw – Of 20 patients with these bone disease, all "exhibited large surface areas of bone occluded with well-developed biofilms." 18)
Periodontal disease – Perhaps the most well-known and studied biofilm bacteria. Hundreds of microbial biofilm colonize the human mouth, causing tooth decay and gum disease.
Plaque is a biofilm on the surfaces of the teeth. This accumulation of microorganisms subject the teeth and gingival tissues to high concentrations of bacterial metabolites which results in dental disease.
Matthew Parsek, PhD 19)
Dental plaque is composed of more than 500 species.20)
Prosthetic joints and heart valves – Pathogenic biofilms are also commonly found on medical devices such as joint prostheses and heart valves.21) Dr Patel of the Mayo Clinic has concluded that prosthetic joints increase the likelihood of biofilm infection.
When people think of infection, they may think of fever or pus coming out of a wound. 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.
Robin Patel, MD, EurekaAlert!
Urinary tract infections – In their 2003 Science paper, Anderson et al. reported that in the case of UTIs, intracellular Escherichia coli can mature into biofilms, creating pod-like bulges on the bladder surface. Explains how bladder infections can persist in the face of robust host defences.22) "The idea that biofilms might form inside human cells is really novel," said internist Pradeep Singh of the University of Iowa College of Medicine in Iowa City, who studies lung biofilms that plague children with cystic fibrosis.
Veterinary diseases – Biofilms have also been implicated in a wide array of veterinary diseases.23)