An important question that arises in probiotic usage: are they safe? This question gains all the more significance when we consider the fact that most of the probiotics are marketed in the form of foodstuffs or drugs. Traditionally, people have been using probiotics from time immemorial through their food habits and the understanding of the safety of the microbes has come from prolonged period of experience like bacteria used in food processing: Leuconostoc and Lactobacillus. Therefore, till recently the question of harmful effects of these probiotic microbes was not raised. But the extent of their viability was always in question.
Viability of the probiotics:
The viability of the probiotic bacteria are always under question when we consider the fact that they have to face diverse and extreme, external and physiological conditions before they are able to exert their beneficial effects. Oxygen toxicity is one of the major threats to the survival and activity of the bacteria and indeed a number of them die due to it according to the study of Ahn et al. e.g., the viability of both Lactobacillus and Bifidobacteria taken through yoghurt was found to be severely compromised. Some of the techniques that can ensure the protection of the probiotic lives from oxygen toxicity include microencapsulation, etc. Also, the probiotics are significantly challenged during their passage through the GI tract and on to the small intestine like low pH, bile and several other factors. Many of the probiotic products are provided in the form of lyophilized powder and they would require a congenial environment before they can grow. However, the major concern that emerges from these facts is that what the fate of these non-viable or dead bacteria is. Are they harmful to the host?
Can dead cells be used as probiotics?
The definition of WHO suggests the use of ‘live’ bacteria as probiotics. However, it has increasingly been observed that the dead or heat inactivated bacteria are also being used for their probiotic potentials. So, what can be the reason behind this? Evidences are now pouring in to suggest that the dead bacteria can provide immunological benefits as do the live ones. The live probiotic bacteria can induce increased production of immunological factors like the cytokines (IL6, IFNγ) as also enhances the activity of macrophages. The dead cells on the other hand causes alteration of the immune system functioning with different sets of targets. A similar recent study is being carried out in the department of Bioethics, Cleveland Clinic by the team of Dr. Richard Sharp and Ruth Farrell, MD in a $1.1 million research project. Already 136 patients in different study groups have enlisted themselves for the study and there seems to tremendous interest among the participants regarding the study says Dr. Sharp.
Immune-stimulation by dead probiotic cells:
The dead probiotic cells can induce the immune system in a number of ways. According to a report published in the International Journal of Immunopharmacolgy, administration of Enterococcus faecalis causes an increase in neutrophil activity and also resulted in enhancement of non-specific immunity. The studies by different groups have established that the cell walls of both Lactobacillus spp and E. Faecalis can activate the B-cells and cause IgA secretion in the intestine as also inflammatory responses are enhanced in the mammalian GI tract that involves macrophages. Marin et al. have shown that dead Bifidobacteria strains can cause several hundred fold increase in TNFα. It is an established fact that the elimination of certain pathogenic bacteria like Salmonellae, Mycobacteria, etc needs the release of IFNγ and IL-12 that is associated with T-Helper (Th1) cell based immune response. In a study by Segawa et al., it was found out that dead cells of Lactobacillus brevis could cause such immune stimulation.
Recent advances in the field has demonstrated that even the DNA or portion of the DNA sequences can provide the same beneficial effects as do the live probiotic bacteria to a large extent. The question is how can the bacterial DNA sequences be able to exert such effect? There lies specific molecular biology behind this ability. DNA from bacteria possesses non-methylated motifs of CpG islands that can bind to the Toll Like Receptor (TLR-9). Now, the TLR-9 mediated signalling depends on the adaptor protein MyD88. In essence, it may be postulated that the dead probiotic bacteria could elicit the useful traits through TLR-9 and MyD88 signalling. Genomic DNA (both methylated and unmethylated) isolated from different strains of Lactobacillus and Bifidobacterium when administered to IL-10 knockout mice could alleviate severe colitis. The same study showed that the introduction of live and non-viable bacteria subcutaneously could produce the same benefits. Lammers et al. showed IL-10 production is enhanced by Bifidobacterium genomic DNA thereby demonstrating the immunomodulatory effects of bacteria DNA. Petrof et al. proposed that the probiotic growth conditioned media possess some of the beneficial activities that are exerted by the live cells. This may suggest that the cells release certain useful factors in the media. A thorough knowledge of these excreted extra-cellular by-products can make the use of probiotics in a desired condition more efficient.
Pros and cons of using dead probiotic cells:
There are several added advantage of using dead probiotic cells in the modification and reduction of diseased condition of the host. Chances are that the live probiotics can anytime cause some patho-physiological conditions of their own. Also, the immuno-compromised patients can be at risk if they are ingested with live cells and for them the non-viable species are always a safer option. There would also be no viability concern with them as they could easily tolerate the extreme conditions in the stomach and intestine. So, the concern of reduction in the number of viable cells as also the shelf-life in the probiotic products could be done away with. Therefore, the thrust may be on the production of immunologically active forms of probiotic drugs and food products irrespective of their microbiological viability.
However, notwithstanding the various usefulness that the non-viable probiotics has on offer, utmost care should be taken to cross-check any side effects that these dead cells or their cellular components might cause to the recipients before they can be put to practical use.
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