Biofilms

A biofilm can be defined as a layer of microorganisms adhering to a surface and each other in an aqueous environment. This microbial colony is able to attach to organic or inorganic surfaces by excreting a sticky sugary material called extracellular polymeric substance (EPS). The strand-like structure of EPS can create a complex matrix by binding large numbers of cells together. The biofilm produced by this process may be made up of a single species of microorganism, or a number of different species. The size of the biofilm is also highly variable depending upon environmental conditions. It may be the thickness of a single cell or several centimetres thick and visible to the naked eye. The presence of these highly organised microbial colonies challenges much of the traditional thinking about microbial behaviour.

One of the most studied examples of a biofilm is dental plaque. Plaque is often comprised of a highly diverse range of microbial species. Damage to the teeth can be particularly serious when the balance shifts to species that can readily survive in an acidic environment and produce acidic metabolic by-products. The acids produced by these bacteria can damage teeth enamel once the pH falls below the critical pH for maintenance of enamel mineral content. Selection for these organisms is associated with regular periods of low pH as occurs during sugar catabolism and reduced saliva production.

Whilst dental plaque is a highly visible and accepted cause of morbidity, biofilms are now associated with many chronic and serious infections. The US National Institutes of Health suggest that 65% of all microbial infections and up to 80% of chronic infections are associated with biofilm formation. The high prevalence of biofilms and the fact that we know they behave differently to isolated organisms highlights the importance of understanding this microbial phenomenon.

Why do biofilms form?

Microbes congregated in a biofilm are better able to resist environmental stressors including host immune defences. This equates to a greater chance of survival. The first microbe to adhere to the surface can be termed an early colonist. The attachment of these early colonists is initially very weak. However, if they are not immediately separated from the surface, the attachment becomes more permanent due to a process called cell adhesion. Cell adhesion involves the excretion of substances such as proteins onto the surface of the microbe that facilitates binding of other cells.

During this period of colonisation, it is thought that the microbes are able to communicate with the colony using the phenomenon of quorum sensing. This allows microbes to regulate their gene expression based upon the cell-density of the colony. Interestingly, this communication method can be utilised within a species or between different microbial species and allows the colony to behave as a group.

Diversity within the biofilm has been shown to increase the chances of biofilm success due to reduced competition for the same resources. Studies also demonstrate that cells within a biofilm become more ordered and densely packed as time goes on. This offers additional advantages to the colony by further reducing its ability to be penetrated.

Clinical relevance

Studies demonstrate that biofilm bacteria can be up to a thousand times more resistant to antibiotic stress compared to free-swimming bacteria. There are several proposed reasons for this. Firstly, the EPS is thought to act as a physical barrier with limited permeability to antibiotics. Secondly, the bacteria within a biofilm often enter a phase of reduced metabolic activity and growth. This translates to a reduced sensitivity to antibiotics as the bactericidal activity of many antibiotics depends upon active bacterial metabolism.

The matrix also provides protection to the microbes inside as the host’s immune system is less likely to recognise it and mount an immune response. This makes treating a biofilm infection much more challenging. Microbes may periodically leave the protection of the biofilm at which point the host is likely to recognise the cell and mount an immune response. This may explain the nature of many chronic relapsing infections.

Some conditions thought to involve biofilm formation include:

• Chronic pseudomonal infections in cystic fibrosis;
• Chronic otitis media;
• Chronic sinusitis;
• Chronic prostatitis;
• Toxic shock syndrome;
• Kidney stones;
• Endocarditis; and
• Infections of medical devices (e.g. prosthetic joints, cardiac pacemakers, urinary catheters).

Treatment

Antimicrobial treatment is often insufficient to fully eradicate a biofilm infection. For some biofilm infections, such as chronic lower airways infections in cystic fibrosis, the aim of treatment may be to suppress the infection rather than complete eradication.

Selection of an appropriate antimicrobial agent should consider the sensitivity of the microbe. However, the antimicrobial must also be able to penetrate the biofilm well enough to achieve effective concentrations at the site of infection. The general ability of antibiotics to penetrate a biofilm is displayed in Table 1.

Table 1. Ability of antibiotics to penetrate biofilm

Owing to the higher minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) associated with biofilms, higher doses of antibiotics are generally required. However, reaching effective antibiotic levels may not always be possible with conventional administration methods due to the limitations of renal and hepatic function, toxicities, and side effects. Combination therapy with systemic and topical administration may help to overcome this issue and could be suitable for some patients, e.g. antibiotic inhalation for biofilms of the airways or bladder irrigations for urinary biofilms.

Other treatment options that may be used in combination include physical removal of the biofilm (e.g. wound debridement, removal of infected medical devices), use of antimicrobials from different classes, and a prolonged duration of therapy. Research is continuing into the development of anti-quorum sensing medicines to disrupt communication within the biofilm.