Next Lesson - Innate Immunity
Core
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 start in a free-living planktonic state. 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 (which genes are switched on) 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, and this signalling is linked to the biofilm’s tolerance of antibiotics. Different microenvironments (small areas with different conditions) can evolve within the matrix, with chemical gradients (gradual changes from one place to another) in oxygen and nutrients. Bacteria deep in the biofilm receive less of both, so many switch to a slower metabolism and stop dividing, which helps them tolerate antimicrobials (drugs that kill microbes or stop them growing, including antibiotics). The different microenvironments also allow different microbial species to co-exist or different activities to take place.
5. Under certain circumstances, bacteria can detach from their matrix. This can be active (triggered by the bacteria themselves, for example when nutrients run short) or passive (forces from outside, such as flowing fluid, break up the matrix). Bacteria can be released from their colony in a planktonic state or detach as whole chunks of biofilm. These chunks then go on to colonise (settle and grow on) other surfaces.
Quorum sensing is the modulation of gene expression in response to changes in cell population density. It helps to regulate processes such as biofilm formation, sporulation (forming dormant spores) and virulence factor release (virulence factors are molecules that help bacteria cause disease). It has 4 key principles:
- Sensing the population density: the bacteria detect how many of them there are (that is, whether there are enough bacteria to carry out a process).
- A signal (an autoinducer) that the bacteria release and that builds up as the population grows. When it reaches a threshold concentration, a quorum (the critical number of bacteria needed) has been reached.
- A receptor on the bacteria that detects the signal.
- A switching mechanism that alters gene expression so that the bacteria 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. In hospital, biofilm infections are often iatrogenic (caused by medical care or treatment). Common routes include indwelling devices (devices left inside the body), mechanical ventilators, intravascular lines (tubes placed in blood vessels), urinary catheters, prosthetic heart valves or orthopaedic implants (such as joint replacements). Each gives bacteria a surface to stick to. Biofilms also cause infections that are not iatrogenic, such as chronic wounds and lung infection in cystic fibrosis. 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. Many of the organisms responsible for infection are commensal bacteria (bacteria that normally live on us without harm). They live on the surface of the skin, in the upper respiratory tract, in the lower gastrointestinal tract or in the urogenital tract. If they are carried from their usual site to another part of the body, or a medical device gives them a way in, they can cause disease (become pathogenic). Two common examples are Staphylococcus epidermidis and Staphylococcus aureus. Other bacteria, such as Pseudomonas aeruginosa, also cause biofilm infections.
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). Endocarditis is infection of the endocardium (the inner lining of the heart), and here it affects an artificial valve. The most likely causative organisms vary depending on the onset of infection relative to the time of operation (early PVE means within 1 year of surgery, late PVE means more than 1 year after):
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, such as S. aureus or coagulase negative staphylococci (staphylococci that do not make the enzyme coagulase, for example S. epidermidis), are common causes. 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, circulating bacteria are more likely to embed on the valve if the patient has a prosthetic valve or another valve problem, for example a bicuspid aortic valve (a valve with two cusps instead of three) or a damaged valve. They can then form a vegetation (a clump of bacteria and clot on the valve). Other patients of note include people who inject drugs. Their endocarditis is most often caused by S. aureus, which can be MRSA (meticillin-resistant Staphylococcus aureus, a strain that does not respond to many common antibiotics).
A coagulase negative staphylococcus is one that does not produce the enzyme coagulase (which clots blood plasma). As a rule of thumb, other than S. aureus, the staphylococci that infect humans are coagulase negative. A few animal-associated species are coagulase positive, but they rarely cause human infection.
Cardiac pacing wire endocarditis: A pacemaker is a device, placed under the skin of the chest, that sends small electrical signals to the heart to control the heartbeat. Its wires (leads) are put into a vein and guided to the heart, and they can become infected, causing endocarditis. 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. The infection usually affects the soft tissue around the joint and the joint itself, causing pain, swelling and an effusion (extra fluid in the joint). The most common causative organisms for PJIs are coagulase negative staphylococci and S. aureus. Treatment usually combines antibiotics with surgery. If the infection is acute and the implant is well fixed, the surgeon may debride the joint (remove infected and dead tissue) but keep the implant (DAIR: debridement, antibiotics and implant retention). Otherwise the infected implant is removed and replaced, in one operation or two (revision surgery). The antibiotic plan is decided by a team that includes infection specialists or microbiologists.
Bacteria in biofilms also release virulence factors (molecules that help bacteria cause disease). 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). These signals help to regulate virulence factors such as exotoxins (toxins released by bacteria). Biofilm bacteria also resist antibiotics in other ways, for example by making beta lactamases (enzymes that break down penicillin-type antibiotics), and the biofilm helps protect them from the immune system. Together, these make bacterial colonies more resilient to immune attacks and antibiotics.
The efficacy of antimicrobials to treat biofilms is limited because:
- Antimicrobials may penetrate the matrix poorly, because the matrix can hold them back.
- 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 prevention is key. Biofilm formation and persistence on indwelling devices can be reduced by using sterile equipment, hand hygiene and an aseptic technique (a way of working that keeps microbes away from the patient) when inserting and caring for devices such as urinary catheters and vascular access devices (tubes placed in blood vessels). The same applies to any prosthetic material. If an infection is already established on prosthetic material, treatment often needs the device to be removed or replaced as well as antibiotics, because antibiotics alone struggle to clear the biofilm.
Patients with a suspected infection should have samples taken so the microorganism can be identified and tested for its antimicrobial susceptibility (which antimicrobials it responds to). This allows the most effective antimicrobial to be prescribed. For endocarditis the samples include blood cultures (blood samples grown in the laboratory to find the organism), and for a prosthetic joint infection several separate samples of joint fluid or tissue are taken.
Edited by: Dr. Marcus Judgeand Dr. Ben Appleby
- 9190
