Combating bacterial infection

As bacteria slowly defeat our antibiotic arsenal, it is time to look at other ways to tackle them? Maria Anguita looks at some of the alternatives and finds the key could lie in biofilms.

Hardly a day goes by without the NHS being mentioned in the news. Unfortunately recently there has been an increase in the number of infection-acquired deaths in hospital, and the patients dying from these infections did not have them when admitted to the hospital in the first place - these infections were acquired once the patient was inside the hospital.

The most common hospital-acquired infection is methyllin-resistant Staphylococcus Aureus (MRSA), but in recent month there has been a rise in the number of Clostridium difficile and new strains of E-coli.

It is difficult to pin-point exactly why there are hospital-acquired infections, and contrary to popular belief this is not a modern problem caused by the privatisation of hospital cleaners or of decreasing hygiene standards. Throughout the ages hospital-based disease has always been the problem, whereas once upon a time people were dying of erysipelas caught in the old Victorian wards, nowadays people are dying from MRSA infection, and no amount of cleaning is going to eradicate the problem completely.

The traditional hospital-acquired infections are becoming resistant to antibiotics, and unfortunately new strains are developing which are also resistant to the few antibiotics left which could still pose an impediment to their growth. For example the emerging new E.coli strain 0157 is far more dangerous than MRSA, and will soon pose a problem due to its resistance to antibiotics.

Transmission of bacteria though the surface of medical devices is a common route of infection. For example, in 2006 there were 200,000 intravenous catheter devices inserted into patients in the UK alone and is considered one of the main routes of infection transmission in hospitals. Transmission through medical and household textiles is also an important infection route, as are medical and laboratory surfaces, ventilation systems and food processing plants.

One way of tackling infection is to engineer materials to give them antibacterial properties which will have an impact on the ability of bacterial adhesion, multiplication and transfer. The aim of surface engineering is to maintain the physical and mechanical properties of the material but provide a platform for technologies which can be applied to a range of applications - for example, develop an antibacterial coating and apply it in various different ways. Other parameters affecting bacterial colonisation onto surfaces are surface energy and the surface roughness. Both parameters can be adjusted by the deposition of a thin plasma polymer coating.

Bacteria infect a surface through biofilm formation. In this case bacteria adhere to the substrate/surface. Other bacteria will adhere to the first bacteria thereby forming a coating. The adhesion process changes the characteristics of the bacteria, which will form an external polysaccharide coating. It is this polysaccharide coating which gives bacteria its resistant properties. A rapid growth and mutation rate in the bacterial genome allows it to change this polysaccharide coating and thereby adapt itself rapidly to antibiotics

Previous technologies and antibiotic therapies have focused on how to break through this polysaccharide coating. However new technologies are focusing on treatments to stop the initial attachment of the bacteria to the surface in the first place, or devising a treatment which will allow the rapid sloughing off of the bacterial coat with a slowed down re-attachment process - that is, making it more and more difficult for the bacterial to attach to the material.

The development of surfaces which prevent the adhesion of bacteria are currently the most promising future. The aim is to create a surface with reduces the strength of the bacteria attachment to the substrate, or to assist the removal. For example, hydrophyllic PHEMA coatings to block copolymers and biomimetic polymers. Biomimetic polymers are synthetic materials which mimic naturally occurring biological materials (membranes). Natural membranes are made of a phospholipid bilayer which is hydrophilic – indeed each phospholipid head attracts 25 water molecules.

Coating of a poly-hydroxymethacrylate (PHEMA) derivative works by covering a metallic surface with a uniformly distributed layer of strongly hydrophilic groups. This prevents the attachment of the bacteria to the surface by forming a dense water-coat between the surface and bacteria, thereby bacteria cannot attach to the surface.

Biocides, for example sulphobetaines (detergents) and carboxybetaines, are much cheaper to make and can also be used to make substrates with low bacterial adhesion. However, for these the considerations below need to be taken into account:

- How clinically effective is the material?
- How can the human immune reaction be minimised?
- What are the opportunities for combining anti-adherent and biocidal surfaces to produce a material which prevents the contamination with bacteria and at the same time is easy to slough off? This would also mean that these surfaces are easier to clean.

Staphylococcus aureus biofilm on the surface of a catheter

Biocides such as silver and ammonium ions have been used extensively to combat bacterial infection; however they have several undesired effects, including biological toxicity, coating instability, bacterial resistance and long-term biofilm formation. Other biocides actually attack mammalian cells and biological tissues rather than solely the bacteria thereby proving toxic to humans. Also, bacteria may develop resistance to the biocide. However one of the most common problems is that while the biocide may kill the first layer of bacteria, it does not prevent subsequent bacteria from attaching to the surface. So it is important to develop a biocide which is easy to slough off so that subsequent layers of bacteria can be killed rather than simply attaching to the biocide layer.

Other technologies include page therapy and photodynamic therapy.
Both these technologies existed before the second world war, but with the advent of antibiotics they were abandoned. However with the increasing problem of antibiotic resistance, scientists are turning back to these techniques.

Phage therapy builds on the fact that viruses infect bacteria. It is the therapeutic use of lytic bacteriophages to treat pathogenic bacterial infections. Phages are viruses that invade only bacterial cells, and in the case of lytic phages, cause the bacterium to burst and die, thus releasing more phages. Phage therapy was extensively researched before World War II, however the advent of antiobitics placed this technology in the backburner.

Phage therapy has been traditionally used for medicinal purposes, for example in the treatment of tonsillitis and some dermatological disorders. Fortunately it is now experiencing a revival as a viable alternative to antibiotics. It is becoming available for a variety of bacterial and poly-microbial biofilm infection decontamination for bacteria which do not respond to antibiotics. Phage therapy tends to be more successful where there is a biofilm covered by a polysaccharide layer that antibiotics typically cannot penetrate. Other biofilms include those on medical instruments whereby an enzyme added to the page can effectively and selectively wipe out even bacteria beneath these films.

Phages have been explored as a means to eliminate pathogens such as Campylobacter in raw food and Listeria in fresh food or to reduce food spoilage bacteria. In practice, phages are spread onto surfaces, or used during surgical procedures to prevent infection contamination through surgical instruments. Although phage therapy is generally considered safe, some bacteria for example Clostridium and Mycobacteria have no known therapeutic phages available.

Photodynamic therapy is based on the principle that upon irradiation with light corresponding to an absorption maximum of the photosensitiser, cytotoxic reactive oxygen species are produced that can cause rapid oxidation of cellular constituents and cell death. In other applications a laser probe allows pulses of light to be beamed at a specific area thereby activating a solution which would kill the bacteria. This technology has been advanced mainly in dentistry however its expansion into the materials arena is currently being investigated.

Many advances are being done in this area and so far this technology is proving to be efficient and cost effective. Recent research published in the Journal of Applied Microbiology describes research where an advanced high-power pulsed light device was constructed for the decontamination of food matrix. In this case the bacteria being used was Salmonella typhimurium, which is one of the biggest food poisoning agents in the UK. The researchers tested the viability of Salmonella typhimurium as a function of a given light does (number of pulses) and pulse frequency. It was concluded that photodynamic therapy was an effective non-thermal tool for the inactivation of salmonella, and it is suggested that novel advanced high-power pulsed light devices can be a useful tool for developing non-thermal food decontamination technologies.

It may be impossible to completely eradicate hospital-acquired infections, however there is much that we can do to improve hygiene standards, especially in the food preparation industry. However, possibly one of the most gratifying results of these technologies would be the development of cheap and cost-effective techniques which can be used in developing countries. Thousands of children are still dying as a result of diarrhoea and other gastrointestinal related disorders brought on by food poisoning. Photodynamic therapy could prove to be a very cheap and efficient method for providing clean uncontaminated water to these poorer countries.

By Maria Anguita. Maria is a freelance science and health writer. She has previously edited several science and health magazines