“Probiotics” are an enormous field and even bigger market but and as interesting as they are an, arguably, more interesting –biotic is starting to gain traction as more innovative researchers explore its possibilities. This is the field of “designer probiotics”.
The central idea is this, certain pathogenic bacteria (and I am speaking exclusively within the gut) use host sugars to facilitate binding or toxin targeting and by doing so cause disease. However, if we expressed these host sugars on something else, a harmless strain of Escherichia coli for example, we would create more sites for the attachment of pathogens and their weapons. This dilutes the effect these pathogenic bacteria can have on our insides and either prevents disease or certainly reduces the severity of it and the harmless E. coli, laden with pathogen and toxin, passes out of the body before they can cause any problems.
Additionally, there is little chance that the pathogens will evolve around this therapeutic strategy as doing so would comprimise their capacity to recognise the target receptor that has been copied and expressed on our harmless E. coli strain and therefore would reduce their capacity to cause disease as part of their lifecycle.
Then of course there is money. Currently, host sugars have been developed synthetically to be used in isolation to attempt prevention of pathogen binding but are not optimally successful as these sugar structures must survive the stomache and early gut before reaching the distal gut where much disease occurs. Making these sugars alone is not cheap but you know what is cheap, bacteria and broth. Vats of 'drug' can be grown in labs much faster than the sugars can be synthesised making it a much more cost effective approach. Of course then there is also the problem of host metabolism of the synthesised sugars. By the time the distal gut is reached the gastro-intestinal system has done what it does best and broken much of the sugar down into its component parts rendering it ineffective as a therapeutic. The probiotic approach secures the expression of the host sugars deep into the distal gut as long as the bacteria survive, which they have been shown to do. Once they get there they grow and divide increasing the amount of 'drug' in the system for free.
I am aware of three such bioengineered bugs capable of doing this work, one for Shiga toxigenic E. coli (STEC) infections, one for enterotoxigenic E. coli (ETEC) infections and one for cholera. Together these three species account for a large proportion of the 2 million deaths that occur each year due to enteric infections, not including the significant morbidity that occurs at the hands of these species.
STEC produces, as you would expect, a shiga toxin which is a very powerful toxin that causes breakdown of cell membranes leading to haemorrhagic colitis (bleeding gut) and the significantly worse haemolytic uraemic syndrome where the patient develops haemolytic anaemia (not enough blood cells because they keep popping), thrombocytopenia (not enough platelets so your blood cant clot) and renal failure (kidneys shut down). Importantly, in this case at least, the shiga toxin is made in the gut before binding to a host sugar called GB3 which facilitates absorption into the body where it does its damage. When GB3 was expressed on a harmless E. coli strain and fed twice daily to STEC infected mice it was found to be 100% effective in preventing disease as the toxin was being soaked up before reaching the gut wall. For the cautious out there the use of dead GB3 expressing E. coli was also tested and found to be just as effective if the dosage was increased to three times daily. Dead bacteria do not mutate and are not technically 'genetically modified organisms' any more so this approach has long term promise to treat STEC infection in the future.
ETEC is behind 'travellers diarrhoea' but should not be underestimated. This bacteria is endemic in developing countries and is the major killer of young children in these areas. It kills by messing around with the way your body controls water loss in the gut. The toxin made by ETEC binds to the host sugar GM1 and then is internalised by the cell. The target cells are those that line the gut surface and are responsible for absorbing nutrients, ions and water. Once inside the cell the toxin modifies a biochemical pathway to ensure a protein called adenyl cyclase is constantly stimulated which in turn causes an interruption to ion movement resulting in ions moving out of the cell into the gut but not back in again. A general rule in gut physiology is where the ions go water will follow and so water flows straight of the body into the gut causing watery diarrhoea. This diarrhoea facilitates the spread of the ETEC into water supplies and then into new hosts as they consume the contaminated water. The production of a harmless E. coli strain capable of binding the ETEC toxin was performed and the result was a bacterial strain that could bind 5% of its own weight in toxin! There is the suggestion that the administration of this strain prophylactically to travellers from developed coutries before travel to the developing world may eliminate a good proportion of disease cases and the ease in growing high quantities of 'drug' would make treating the developing world significantly easier and cheaper providing some additional hope in these areas.
Finally, cholera. Vibrio cholera is endemic to Asia and causes epidemics all over the world. Usually as a result of eating undercooked fish the pathogen enters the system, colonises the small intestine and releaases its toxin which works in the same way as the ETEC toxin. If no treatment is made available, as is the case for many where cholera is endemic, the chance of death rockets up to 50%. Treatment here is tricky as antibiotics can actually make the disease worse as toxin leeches from the dying pathogens and overwhelms the patient so most are treated with fluid therapy, keep drinking salty water (made using sachets of important salts which can be added to sterilised water) until you get better, or not. A GM1 expressing E. coli was developed and shown to be very effective in preventing disease. Mice given V. cholera infections were treated with the harmless GM1 expressing strain 1 or 4 hours post infection and 12/12 survived compared to 1/12 for the post 1 hour treatment, 8/12 compared to 2/12 for the post 4 hour treatment. In this case it was found that the GM1 producing strain could remain stable when freeze dried and so could be made, stored, then added to the oral rehydration salts as part of the current therapeutic strategy which would keep costs significantly down.
So that's it. The problem of antibiotic resistance is solved right? We just don't use antibiotics and instead use these cleverly designed genetically modified organisms that can't be evolved around without the pathogen reducing its ability to infect at all. We produce enormous quantities cheaply in vats where the 'drug' grows itself on $10 worth of ingredients and then treat the whole world.
Well not exactly. These new strains are failing to receive the commercial backing due to a perceived market place resistance to the use of genetically modified organisms as therapeutics. Soon though it may not matter as a lack of effective vaccines and the continuing threat (increasingly realised) of antibiotic resistance is placing increasing pressure on the commercial operations that underlie modern medicine who may soon have to open their minds to these new and innovative approaches.
Paton, A., Morona, R., & Paton, J. (2010). Bioengineered bugs expressing oligosaccharide receptor mimics: Toxin-binding probiotics for treatment and prevention of enteric infections Bioengineered Bugs, 1 (3), 172-177 DOI: 10.4161/bbug.1.3.10665