Genetically modified probiotics should be banned

Abstract

Beneficial bacteria living in the human gut are now subject to extensive genetic modification that could turn them into pathogens. In view of our vast ignorance of gut ecology, we cannot allow genetically modified probiotic bacteria to be used.

Probiotics for health

‘Probiotics’ are naturally occurring bacterial strains belonging to species found in the human gut, and are being added to food for their health-promoting effects. The probiotics studied most extensively are Bifidobacterium and Lactobacillus, both derived from fermented milk products. The efficacy of probiotics has been clearly established in recent years. For example, double-blind, randomized trials with probiotics added to milk reduced respiratory infections and the severity of illness among children in a day-care setting [1]. Another study showed that probiotic treatment relieved diarrhoea in children [2].

This success has attracted the attention of genetic engineers, who want to ‘improve’ on the successful applications, which probably date back to the beginning of written history.

The cross-talk between the human host and the gut bacteria has evolved over millions of years. Its contributions to the health of the human host depend on an intricate network of bacteria–bacteria and bacteria–host interactions that, if thrown out of balance, will very likely result in disease [3].

Can GM ‘improve’ probiotic bacteria without turning them into dangerous pathogens?

Probiotic bacteria modulate the immune system and provide an ecological balance in the gut that excludes disease-causing microbes. Germ-free mice bred in the laboratory have less immune cells, and tend to leak more food antigen across the intestinal barrier. These conditions improve after about a month of exposure to bacteria [4]. However, probiotic bacteria must not be pathogenic, and it is essential for probiotic treatments to be tested for safety. The vast majority of applications have been free of pathological outcomes; but there has been one case of local infection from a rogue Lactobacillus strain [5]. The prospect that genetic modification might ‘improve’ probiotic microbes must be seriously balanced against the potential of turning harmless, beneficial microbes into dangerous pathogens (‘No biosecurity without biosafety’, Sis 26 http://i-sis.org.uk/isinews.php), particularly in the case of bacteria that naturally inhabit the human gut.

The complete genome sequence of the probiotic Lactobacillus acidophilus has been determined and features contributing to survival in the gut and promoting interactions with the intestines have been identified [6]. The genome sequence of Bifidobacterium longum, similarly, reflects its adaptation to the human gastrointestinal tract including potential immunomodulating proteins [7]. Milk-fermenting bacteria harbour bacteriophages (viruses), including those that cause diseases, and ‘temperate phages’ capable of integrating their viral genome into the bacterial genome [8]. Temperate bacteria phages play an important role in horizontal gene transfer among bacteria residing in the same environment, in this case, the human gut.

Genetic modification of bacteria can be done by DNA transformation (direct uptake of DNA), transduction (transfer of genes by temperate bacterial phage) or by the use of plasmids (small circular DNAs that replicate with the bacterial cell but stay outside the bacterial chromosome). Normally, transgenes are propagated in bacteria in plasmids because DNA transformation is not successful unless the DNA shares homology (sequence similarity) with the bacterial chromosome.

Lactic acid bacteria (Lactobacillus spp.) have been genetically modified to increase proteolytic activity, to resist viruses, to metabolize complex carbohydrates or to enhance metabolism. The only modified lactic acid bacterium approved under the European Directive for Food Supplements (Directive 2002/46/EC) [9] so far is a strain with a modified luciferase gene to detect antibiotic residues in milk, but that strain does not enter the food chain because it is used on a small test sample of milk that is then destroyed. This same Directive currently lays down rules on vitamins and minerals, and is expected to be extended to cover amino acids, essential fatty acids, fibre, various plants and herbal extracts and probiotics.

Dangerous experiments with probiotics

It has been suggested that a random ‘gene shuffling’ technique should be employed to improve lactic acid bacteria for use as probiotics [10]. Gene shuffling is an inherently hazardous procedure that can generate millions of recombinant bacteria in a matter of hours; it will be impossible to predict how many of those might be lethal pathogens (‘Death by DNA shuffling’, SiS 18 http://www.i-sis.org.uk/isisnews.php).

A US patent application for recombinant lactic acid bacteria for treating allergy includes fermented milk product (yogurt) containing lactic acid bacteria modified with synthetic genes specifying epitope IgE antibodies (allergy antibodies) on the surface of the bacterium. Allergy therapy would include eating the recombinant yogurt to suppress the allergy as the natural allergen is encountered [11]. This kind of ‘therapy’ must be treated with extreme caution. Experience tells us that interfering with the immune system can lead to nasty surprises, as in the case of the harmless mousepox virus that turned into a lethal pathogen when a gene that was supposed to boost antibody production was inserted into it [12]. In another experiment, a Lactobacterium strain of human origin was modified with a gene for tetanus toxin to produce antigen to immunize against tetanus. The recombinant lactic acid bacterium was delivered as a nasal spray to provide a strong immunization [13]. No consideration has been given to the distinct possibility that the tetanus toxin gene could easily be passed along to a pathogen.

Genetic engineers are also identifying Bifidobacterium probiotic strains and thinking of ‘enhancing’ them by genetic modification. Plasmid vectors belonging to bifidobacteria or shuttle plasmid vectors for transferring genes between Escherichia coli and bifidobacteria are being used, so far, to study the role of bifidobacteria in the gut ecosystem rather than in the production of modified probiotic strains [14]. The instability of recombinant plasmids has proved an obstacle to industrial exploitation of GM bifidobacteria [15]. Furthermore, gene transfer was observed in the digestive system of previously germ-free mice between lactobacteria and bifidobacteria [16], suggesting that GM probiotic strains would alter the entire microbial ecology of the digestive tract in an unpredictable manner.

A recent review stressed the huge market for probiotics in Europe, pointing to the value of molecular genetic technology in characterizing and identifying many probiotic microbes [17]. An earlier review discussed bacterial replacement therapy as a form of ‘germ warfare’ to prevent and control infections of skin, oral cavity, ears and uro-genital tract. The friendly probiotic bacteria are used to colonize the gut microflora to eliminate or minimize pathogens from establishing themselves. That approach has proved successful in controlling dental caries, ear infections and streptococcal diseases. In some rare instances, the ‘friendly’ bacteria had antibiotic resistance markers or were genetically modified [18].

No GM bacteria must be allowed for probiotic use

The study of bacteria colonizing the human gut has only just begun. There are 10 times more bacteria than there are cells in the intestine, consisting of more than 400 different species; the overwhelming majority of the species still unknown. Comparative studies in germ-free and conventional animals have demonstrated the importance of a continuous cross-talk between the mammalian organisms and its intestinal bacteria for a normal development of many anatomical structures and physiological, biochemical and immunological functions [19]. So far, it seems honest to conclude that we know only some few letters in this cross-talk alphabet. In view of our vast ignorance of gut ecology, we cannot allow genetically modified probiotic bacteria to be used, unless and until we fully understand the intricate ecological balances that have co-evolved with the human species. There should be a ban on the use of any GM probiotic bacteria in human subjects.