While the concept of infections has been studied for many years, our current understanding of infections is based upon studies and observations of planktonic bacteria. This is free floating bacteria that cause diseases such as pneumonia, sepsis, urinary tract infections, gas gangrene and many other examples. These types of infections often respond well to antibiotics and resolve without recurrence.
However, there are several infections that occur out of the typical sequence of planktonic bacterial infections. These infections occur postoperatively after a patient has received an implant. However, instead of presenting in the typical three- to five-day period, these infections present weeks to months later. One may see a temporary symptomatic response to antibiotics but the infection returns again at a later time despite a full course of antibiotics.
In 1978, bacteria were described as matrix-enclosed aggregates. These aggregates were immobilized on the surfaces or at interfaces in the ecosystems in which they were known to predominate.1,2 Now a new category of infection has emerged that differs radically from acute bacterial disease. Infections from biofilm present as less aggressive infections. They often persist for months or years, and they progress though periods of quiescence that alternate with periods of acute exacerbation.3
Why Biofilms Have Been A Research Dilemma
Research has demonstrated that bacteria live in matrix-enclosed communities that closely resemble the biofilms that are the predominant form of bacteria growth in industrial and environmental ecosystems. Researchers often identified and cultured bacteria. In the lab, the isolated bacterium was sensitive to the appropriate antibiotics used by researchers. However, when these antibiotics were given to humans, they were unable to resolve the infection completely. This incomplete or partial resolution of the condition led many investigators to refer to these chronic disease states as “sterile inflammatory conditions.” Furthermore, in many instances, there was no bacterium that researchers could culture from the source.3
Traditional methods for detecting bacteria usually involve mechanical removal of the organism (often by swabbing) and their propagation in liquid or on solid media. Using standard swabbing and culture (plating) techniques, researchers in a 2003 study of 3,000 patients with bacterial colonization concluded (as most similar studies have) that 10.8 percent of these individuals carried Staphylococcus aureus.4
The study authors subsequently examined subsets of these individuals, using polymerase chain reaction (PCR) to identify S. aureus by its DNA and using fluorescent in situ hybridization (FISH) probes to identify cells of this species by their 16S RNA content. They found that 100 percent of these individuals carried this organism. Then they examined whether individuals who yielded positive data in swab and plate tests carried more S. aureus as detected by PCR and FISH, and found there was no correlation.4
If certain bacteria are present on a tissue or an inert surface, the swab may or may not pick them up. They may be present in huge aggregates of hundreds of cells that will yield only one colony on plating. Alternatively, they may be expressing the set of genes that constitute the biofilm phenotype and are unable to grow in the culture conditions provided.
How Biofilm Forms And What Makes It Unique
There is a growing interest in the research of biofilms. Biofilms are estimated at 1 million nosocomial infections each year in the U.S.
Biofilm is a complex aggregation of microorganisms marked by the excretion of a protective and adhesive matrix. Biofilms are often characterized by surface attachments, structural heterogeneous diversity, complex community interactions and an extracellular matrix of polymeric substances.
Formation of a biofilm begins with the attachment of free floating microorganisms to a surface. These first colonized microorganisms adhere to the surface initially through weak, reversible van der Waals forces. If these microorganisms are not immediately separated from the surface, they can anchor themselves more permanently using cell adhesion molecules such as pili. Once colonization has begun, the biofilm grows through a combination of cell division and recruitment mediated by extracellular polysaccharides.
Understanding The Nature Of Biofilms And How It Affects The Impact Of Antibiotics
Biofilms are responsible for many device-related and chronic infections.
Bacteria that live in a biofilm usually have significantly different properties from free-floating bacteria of the same species as the dense and protected environment of the film allows them to cooperate and interact in various ways. One benefit of this environment is increased resistance to detergents and antibiotics as the dense extra cellular matrix and the outer layer of the cells protect the interior of the community. In some cases, one may see a thousand-fold increase in antibiotic resistance.
In cases of chronic bacterial diseases, diagnostic microbiology labs have reported that cultures of Pseudomonas aeruginosa from cystic fibrosis (CF) patients were sensitive to antibiotics (e.g. cloxacillin) but pulmonary clinicians saw little improvement when they utilized these antibiotics. The sera of CF patients contained very large amounts of specific antibodies against Pseudomonas but the disease persisted.
Studies at the Center for Biofilm Engineering in Montana looked at device-related infections that were recalcitrant to antibiotic therapy and insensitive to host defense mechanisms. Researchers studied and collected specimens over a 12-year period. They noted that cells of P. aeruginosa in the sputum and in the lungs (postmortem) of CF patients grew in biofilms, and the cells were surrounded by very large expanses of matrix material.5 Some researchers noted that the cells of the pathogens that caused osteomyelitis in patients and in lab animals grew in enormous biofilms, which consisted of millions of bacterial cells embedded in thick matrix material.6 Researchers studied many other chronic infections, all of which had the existence of biofilms.
Biofilms are ubiquitous. Nearly every species of bacteria have mechanisms in which they can adhere to surfaces and to each other. Biofilms grow in hot, acidic pools in Yellowstone and glaciers in Antarctica. Biolfilms can form on any solid surface implanted in the human body, such as in the case of dental plaque. Aquatic plants use biofilm to control microbial fouling of their photosynthetic surfaces. One may also
see biofilms on the interior of pipes and they can lead to clogs and corrosion.
Conventional antibiotics eradicate free-floating bacteria but do not eradicate the biofilm. There is a recurrence of symptoms followed by additional cycles of antibiotics. Researchers have studied gene expression and found that biofilm phenotypes differed from their planktonic counterparts.7
Common bacteria found in biofilms include gram-positive Enterococcus faecalis, Staphylococcus aureus, Staphylococcus epidermidis and Streptococcus viridans. Gram-negatives include Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis and Pseudomonas aeruginosa.
The biofilm concept offers possible explanations to the following unusual circumstances.
• Antibiotic coverage is not enough to eradicate the organism.
• Swabs of an infected area will only propagate a few cells.
• Blood cultures from patients with overt infectious signs are negative.
When You Suspect Biofilm Infection
Biofilms stimulate antibodies within 10 days of the initial colonization. One may use enzyme linked immunosorbent assay (ELISA) to detect antibodies against biofilm-specific epitopes common to all staphylococcal species. One can utilize phase-contrast microscopy to detect bacteria. Replacement of the implant with the use of aggressive perioperative antibiotic therapy is suggested.
Biofilm engineering has contributed to new technologies of potential interest in the control of biofilm infections. Researchers have shown that biofilms are more susceptible to conventional antibiotics in direct current electric fields or when they are treated with ultrasonic radiation.8,9
Bacteria that grow on tissue or implantable devices in the body may be well controlled by the host’s immune system. However, when the immune system is compromised, the equilibrium is tilted in favor of pathogens. Biofilms present a peculiar problem as they act as the source of disseminated planktonic bacteria that may cause a chronic inflammatory condition. Although antibiotic therapy will usually kill these planktonic bacteria, it cannot rid the body of the biofilm. For this reason, one needs to remove infected implantable devices. When devices cannot be removed, one may prescribe antibiotic therapy at low doses for the remainder of the patient’s life.
In the future, we may look at biofilm as something that changed the way we evaluate and treat infection.
Dr. Pupp (pictured) is a Fellow of the American College of Foot and Ankle Surgeons. He is the Clinical Director of the Foot and Ankle Clinic at the Southeast Michigan Surgical Hospital in Warren, Mich.
Dr. Nielson is a third-year resident at Southeast Michigan Surgical Hospital.
Dr. Burks is a Fellow of the American College of Foot and Ankle Surgeons, and is board certified in foot and ankle surgery. Dr. Burks practices in Little Rock, Ark.
For related articles, see “Key Insights For Addressing Infected Hardware” in the August 2006 issue of Podiatry Today or “How To Differentiate Between Infected Wounds And Colonized Wounds” in the July 2005 issue.
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