Biofilms

Biofilms for higher education

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Biofilms are aggregates of cells that stick together on a surface. We often view bacteria as relatively simplistic organisms when compared to higher eukaryotes . They do however have the ability to communicate with one another chemically, a process known as quorum sensing. The molecules that facilitate quorum sensing, can alter gene expression (among other things) causing bacterial cells to switch between different states, the formation of a biofilm being one. Biofilms serve a range of functions contributing to colonisation and persistence of bacterial populations in the host and ultimately their ability to cause disease. On-going research in the field reveals more and more factors contributing to the progression of biofilm formation every day. below we will discuss a few of these factors and how they contribute to bacterial pathogenesis.

Mechanisms of biofilm formation

Biofilms can consist of single species, or mixed communities of different bacteria. This can often be a result of initial colonisation by a single species which then change the microenvironment, paving the way for new species of bacteria. The formation of biofilm is often mediated by the release of cohesive exopolysaccharide and/or DNA in response to quorum sensing molecules indicating optimum cell density and nutrient availability. Mature biofilms can then disperse planktonic cells to spread to other parts of the human host, as shown below.

The life cycle of biofilms (source: MicrobeWiki)

Example 1: biofilms of Pseudomonas aeruginosa in cystic fibrosis.

Biofilms of the bacterium P. aeruginosa are a common problem associated with patients with compromised respiratory systems, such as those with the genetic condition cystic fibrosis. Cystic fibrosis patients overproduce mucus in the lung meaning clearage of respiratory pathogens, such as Pseudomonas is very difficult. Initially planktonic P. auruginosa attachment is mediated by its flagellar motility. The release of several quorum sensing molecules, namely those encoded in the psl and pel operons then alters gene expression to downregulate the expression of flagella express genes involved in biofilm development and maturation. These signal transduction cascades are often very complex, involving interplay between many different genes.

Review

Example 2: dental biofilms - a model for changing environments.

You may be familiar with plaques on your teeth. These are in fact examples of dental biofilms and represent an excellent model for studying how colonisation by bacteria alters the microenvironment and facilitates further colonisation by other agents. Initial colonisers (such as Streptococcus mutans and then Actinomycetes) of the mouth can bind to the surface of the oral cavity and synthesise polysaccharides such as glucan. This change in microenvironment allows for the aggregation of bridge bacteria such as Fusobacterium which alter the microenvironment further still, forming a dental plaque. Finally, pathogenic bacteria arrive and colonise, leading to the onset of infection.

Review

Role in persistence and antibiotic resistance

Bacterial biofilms have commonly been implicated as mechanisms for resistance to antibiotics and in mechanical stress resistance relating to persistence.

Antibiotics-

The outer layer of biofilms forms a physical barrier making it difficult for antibiotics to penetrate, which can lead to bacteria within the biofilm surviving antibiotic exposure and causing recurrent infection. In addition to this, due to the lack of nutrients within biofilms, the metabolism of central bacteria can be slowed down. Many antibiotics require bacteria to be rapidly dividing to be effective. The slower metabolism therefore reduces the effect of antibiotics.

Mechanical resistance-

Biofilms play a role in helping bacteria to persist on medical implements, and subsequently cause recurrent infection, often in immune-compromised patients. Studies have shown that biofilm forming bacteria are more capable of resisting mechanical stress in implements such as catheters and dialysis machines, making them more likely to re-infect the patients following treatment.

Biofilm image

Above- The picture on the left shows cells capable of forming biofilm, and the right shows cells that cannot. These cells are exposed to liquid flow and mechanical stress. You can see that biofilm formation allows cells to persist in this enviorment, which attempts to mimic medical instruments such as catheters. (Schroll et al. 2010)

 

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