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What are Biofilms?
Biofilms are an important defence mechanism bacteria have developed to survive in hostile environments. Simply put, bacteria cluster together and embed themselves into a slimy extracellular matrix made of extracellular polymeric substance (EPS). This extracellular polymeric substance is made up of carbohydrate polysaccharides and proteins, possibly with DNA, lipids and other substances in the mix. Biofilms can form on a range of surfaces ranging from living to non-living and are key in allowing bacteria to embed on prostheses.
There are 5 steps used to produce a biofilm:
1. Bacteria exist in a free-living planktonic state under conditions such as starvation or limited nutrient availability. In this state, they are found as individual, freely moving cells.
2. Once they encounter an environment suitable for growth, the bacteria change their gene expression from a ‘swimming’ state to a ‘sticking’ state in which they can attach to their chosen surface.
3. Upon attachment, the bacteria start to multiply, forming colonies and adhering to each other by producing a slimy matrix of EPS.
4. This matrix allows for diffusion of nutrients between bacteria. It also allows for the transmission of molecular signals between cells. This molecular signal (an autoinducer) allows the behaviour of the entire biofilm to be regulated. This includes signalling bacteria to switch to a slower metabolism and to stop dividing which is beneficial in resisting antimicrobials and preserving resources. Different microenvironments can evolve within the matrix with varying chemical gradients, which allow for different microbial species to co-exist or different activities to take place.
5. Under certain circumstances, either voluntary (self-programmed due to stress) or non-voluntary (shear external forces breaking up the matrix), bacteria can detach from their matrix. They can be released from their colony in a planktonic state or detach as whole chunks of biofilm that then go on to colonise other surfaces.
There are 4 key principles required in this process for biofilm formation:
- Quorum sensing- quorum sensing is the modulation of gene expression in response to changes in cell population density (i.e. are there enough bacteria to carry out a process). These processes include; biofilm formation, sporulation and virulence factor release.
- Release of a signal- known as an autoinducer when a quorum (critical number of bacteria required) has been reached
- A receptor for the signal- on the bacteria to carry out the process
- A switching mechanism- to alter gene expression to carry out the process
Diagram: The formation of Biofilms on a surface, showing each step in their formation
SimpleMed original by Dr. Elena Perez
Biofilms are a major cause for concern in hospitals where they are responsible for a considerable share of healthcare-acquired infections. The cause of a biofilm based infection is usually iatrogenic. Common routes include indwelling devices, mechanical ventilators, intravascular lines, urinary catheters, prosthetic heart valves or orthopaedic implants. The issue is further compounded by the fact that many of these instruments e.g. ventilators, are used in critically unwell patients who are already more susceptible to infection. The organisms responsible for infection are usually commensal bacteria living; on the surface of the skin, in upper respiratory tract, in lower gastrointestinal tract or urogenital tract. When moved from their usual niche to a different part of the body or given opportunity to do so by a medical device they may become pathogenic. Two common examples are Staphylococcus epidermidis and Staphylococcus aureus.
Let’s go through a few common examples worth knowing about:
Prosthetic valve endocarditis: a serious complication of cardiac valve replacement surgery is prosthetic valve endocarditis (PVE). This results in an infection of the endocardium (inner lining of the heart). The most likely causative organisms vary depending on the onset of infection relative to the time of operation:
Table: Most likely causative organisms of prosthetic valve endocarditis based on time since surgery (Based on: Nataloni M et al. Journal of Cardiovascular Medicine 2010, 11:869-883)
The relative time since the operation indicates the likely route of infection. If the infection occurs soon after valve replacement, it is likely that the bacteria were introduced into the heart on the prosthetic heart valve itself. This is why bacteria which live on the skin, S. aureus or a coagulase -ve Staph eg S. epidermidis, are causative. If the PVE is late it is more likely that the source of the infection is haematogenous. This is usually from dental procedures in which bacteria situated in the oral cavity, such as the Viridans streptococci group, are introduced into the blood. In a healthy patient, the immune system usually easily clears the infection. However, if the patient has a prosthetic valve or another valvular issue, e.g. a bicuspid aortic valve or a damaged valve, circulating bacteria are more likely to embed on the valve and form a vegetation. Other patients of note include IV drug users; these patients can contract MRSA derived endocarditis.
A coagulase negative Staph is one that does not produce the coagulase enzyme. As a rule of thumb, other than S. aureus, Staphylococci are coagulase -ve.
Cardiac pacing wire endocarditis: This procedure consists of inserting a pacing wire from a pacemaker through the chest to connect it to the heart. In this case, the most common organisms are coagulase negative staphylococci and S. aureus, which may be picked up by the wire from the skin on insertion.
Prosthetic joint infections (PJI): these occur as a complication of joint replacement surgery. Common risk factors for developing a PJI include obesity, diabetes mellitus and rheumatoid arthritis. Infection usually involves the soft tissue surrounding the joint and the joint itself causing pain, swelling and effusion. The most common causative organisms for PJIs are coagulase negative staphylococci and S. aureus. Treatment for this condition is focused on sterilising the joint tissue and may include removal of the prosthesis.
One of the substances secreted by biofilms are virulence factors. As mentioned above quorum sensing is the modulation of gene expression in response to changes in cell population density. Bacteria within the matrix produce signal molecules known as autoinducers (AI). Some of the signals induced include expression of virulence factors such as beta lactamases and exotoxins. These make bacterial colonies more resilient to immune attacks and antibiotics, contributing to their virulence.
The efficacy of antimicrobials to treat biofilms is limited because:
- Most antimicrobials lack the ability to penetrate biofilms.
- Biofilms on prosthetic joints have a poor blood supply making it difficult to achieve a high concentration of antimicrobial.
- After the switching process has occurred the bacteria have a slower metabolism and do not divide as rapidly reducing antimicrobial effectiveness.
Therefore sterilisation is key to prevent biofilm formation and persistence on indwelling devices or unsterile equipment; this includes any prosthetic material.
Patients that have been infected should have a sample taken so the microorganism can be identified and tested for its antimicrobial susceptibilities to prescribe the most effective agent.
Edited by: Dr. Marcus Judgeand Dr. Ben Appleby
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