5.1. Prebiotics, probiotics and synbiotics supplementation
The phenomenon of FGIDs is not an uncommon among the post-infection patients. The long-term impacts of cryptosporidiosis and giardiasis including IBS, cognitive deficiencies, chronic fatigues, and joint pains have been extensively highlighted [
53,
55]. The Rome Foundation, through its Rome IV working report, has indicated the significance of post-infection IBS (PI-IBS) [
56]. The report indicates a 4 - 36% occurrence of PI-IBS among enteritis patients and highlights a complete absence of pharmacologic strategy to treat PI-IBS.
In this context, probiotics have been investigated to address this problem. It has been suggested that in the cases of post infection diarrhea caused by parasites such as
Campylobacter,
Salmonella, Cryptosporidium, and
Giardia, the supplemented probiotics compete with the gut parasites for nutrition and resources, thereby increasing the anti-parasitic immunity within the gut [
57]. This in turn aided towards shortening the diarrheal period and reducing its severity. The predominant species used on commercial levels include
Lactobacillus,
Streptococcus,
Enterococcus,
Lactococcus,
Pediococcus,
Bacillus,
Escherichia, and sometimes
Saccharomyces [
58]. The American Gastroenterology Association (AGA), in its 2020 guideline, although indicated that there are knowledge gaps regarding the impact of probiotics on the post-infection conditions, conditionally recommended the use of probiotics during the antibiotic treatment. Although a reduction in diarrheal period by 21.91 – 28.9 hours was seen in paediatric acute infectious gastroenteritis, the experimental outcomes varied, resulting in AGA recommending against the probiotic use during infectious gastroenteritis [
59]. It is likely that this may be due to the symptom control and not targeting pathogenic mechanisms, approach of the current treatments [
60]. One of the recent studies exploring the IBS caused from cow milk allergy may shed some light into the change of approach. The study, based on brain-gut immunoendocrine microbiota axis, indicated that the use of extensively hydrolysed casein formula along the
Lactobacillus rhamnosus GG probiotic helped to decreased FGIDs in children [
61]. Similarly, the intake of low fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs) has shown to help alleviate the IBS [
62]. This is not surprising since these molecules are not digestible by the human gut, and act as good fibre sources.
Synbiotics and combination therapies have also been studied, and have indicated to have positive impacts on gut health. Complementary synbiotics are the mixture of prebiotic and probiotic components. In the inflammatory bowel disease (IBD) mouse model, recent study by Shinde, et al. [
63] showed that synbiotic additive, created through mixing of prebiotic such as green banana resistant starch, and probiotic such as
Bacillus coagulans, led to an increase on colonic SCFAs. This phenomenon was not observed with the addition of probiotic only supplementation, but was observed when prebiotic was added, indicating the important role of prebiotics in production of SCFAs. In addition to the gut modulation, synbiotics feeding has also showed to increase the levels of anti-inflammatory and chemopreventive metabolites such as 2-pentanone [
64], shown to inhibit prostaglandin and COX-2 protein expressions in colon cancer cells [
65]. Furthermore, the synbiotic therapy has shown to alleviate the chemotherapy effects, as recently reviewed by Singh, et al. [
66].
In contrast, synergistic synbiotics consist of adding a stimulated microbe, or enhancing the activity of delivered microbe through adding a specific substrate . A good example can be cited through the work of Boger, et al. [
67], who utilised short-chain inulin (sc-inulin) as a prebiotic and
Lactobacillus paracasei subsp. paracasei W20 as a Step 1 probiotic. The species was able to ferment the sc-inulin which, through a cross-feeding mechanism, enabled an increased fermentation by step 2 probiotic
Lactobacillus salivarius W57. Such synergistic synbiotics, although much difficult to obtain, have the ability to deliver significant impacts in selective, targeted manner [
68] such as to counter AMR and MDR. In the case of protozoal infection, early study on rats by Ribeiro, et al. [
69] showed that supplementation of synbiotic mixture of
Bifidobacterium animalis and Raftilose
® P95 fructooligosaccharides by its own, and as an addition to dexamethasone treatment.
Metronidazole (MTZ) is the most used drug to address giardiasis. However, emerging studies are showing an increasingly developing
Giardia resistance to MTZ [
70,
71]. In this context, nutritional sources have shown promise in mitigating the parasite removal. For example, the blueberry polyphenolic extract, under
in vitro conditions, has shown to inactivate > 90%
Giardia trophozoites at 167µg/mL, with respect to a 100% achieved by 67 µg/mL MTZ [
72]. The dichloromethane polyphenolic extracts of ginger and cinnamon, particularly at 20 mg dosage rate in albino rats, have shown to reduce
Giardia cyst count by 90.1% and 100% respectively, while reducing cyst count by 75.4% and 34.1%, respectively [
73]. In mouse model, BIOintestil
® (contains gingergrass (or palmarosa) essential oil, ginger powder and gingerol), when combined with MicrobiomeX
® (contains citrus extract flavonoids) at 100 mg/day dosage, eliminated 100% of
Giardia cysts in Swiss mouse model within five days, and was twice as effective as Albendazole and Metronidazole [
74].
Similarly, the mouse model study has indicated that zinc supplementation and
Lactobacillus acidophilus + dill seed oil supplementation reduced the
Cryptosporidium oocysts by 98.3% and 95.8%, respectively, with respect to prescribed drug, nitazoxanide (91.6%) within eight days of treatment. Furthermore, these treatments were also found to significantly reduce the TNF-α levels in serum [
75].
Although supplementation of micronutrients such as zinc have been suggested to control parasitic activity in gut [
71], it has been shown that in the infant gut, supplementation of micronutrients such as iron and vitamins (A, C, D, folate), but without zinc, decrease the gut microbiome diversity and aid the growth of protozoal parasites such as
Entamoeba [
76]. Therefore, it is important to ascertain the impact of certain nutritional interventions.
5.2. Postbiotics and microbiome modulation to improve MDR resilience
Postbiotics, although have been traditionally remained neglected in modern medicine for FGID treatments, have started to gain relevance over last few years. The International Scientific Association for Probiotics and Prebiotics (ISAPP) defines postbiotics as “preparation of inanimate microorganisms and/or their components that confers a health benefit on the host” [
77]. They can range from sugar alcohols to amino acids, fatty acids, vitamins, and microbial peptides. A widely known example would be Vitamin K generated by
Escherichia coli in gut. Some of the recent additions to the postbiotics include SCFAs, D-amino acids, and small proteins/peptides.
SCFAs and D-amino acids, generated by gut microbes, have been shown to contribute towards the host immune response to infections [
63,
78,
79,
80]. In our mouse model study of cryptosporidiosis, the SCFA accumulation elevated in response to the infection. Particularly, significant acetate elevations were seen in duodenum and jejunum, while butyrate levels increased in caecum and colon [
19]. However, it appeared that these levels were much higher during giardiasis, particularly the propanoate and butyrate increase in colon [
81]. It has been shown that the supplementation of SCFAs such as acetate to the influenza infected mice, through the gut-lung axis, aided to improve the alveolar macrophage activity [
82]. On the other hand, propionate production by gut Bacteroides disrupted the intracellular homeostasis, inhibiting pathogenic
Salmonella enterica growth [
83]. In the aging mice,
Lactobacillus acidophilus DDS-1, when added as a probiotic, led to an increase of caecal butyrate levels, leading to downregulation of inflammatory cytokines [
84]. Similarly, the D-amino acid levels increased throughout the mouse gut during cryptosporidiosis [
19] and giardiasis [
81], but their levels were observed to be much higher in small intestine with respect to large intestine. Early assessment has shown that although D-amino acids were unable to prevent the
Staphylococcus aureus colonisation, they inhibited the biofilm assembly development under in vitro conditions [
85].
In addition to SCFAs and D-amino acids, small proteins such as bacteriocins/colicins, produced by gut Enterobacteriaceae, have shown to competitively inhibit the growth of pathogenic
Salmonella enterica [
86]. One of the very recent reviews by Upatissa and Mitchell [
87] has indicated the utilisation of these small proteins to control specific drug resistant pathogens. For example, microcin J25 has shown to inhibit more than 28 multi-antibiotic resistant
Salmonella enterica serovars [
88]. The protein also has shown effectiveness against some strains of multi-drug resistant
E. coli [
89]. The work of Upatissa and Mitchell [
87] provides a good insight into these proteins and their action mechanism. In the case of cryptosporidiosis, cathelicidin related antimicrobial peptide (CRAMP) has been indicated to significantly reduce the parasite burden. However, the indigenous CRAMP appeared to be downregulated during cryptosporidiosis. In such a case, oral feeding of 5 µg CRAMP has shown to aid the reduction of
Cryptosporidium sporozoites, but not the oocyst [
90]. Antimicrobial peptides, particularly from venomous insects such as bees, have shown inhibitory effects on protozoal parasites[
91,
92] and promise to be applied in protozoal infection treatment.
One of the components that may arguably be categorised as both prebiotics and postbiotics are enteric viruses, especially macrophages. In addition to chronic disorders such as colitis [
93], they have been proposed to be effective treatment for AMR and MDR pathogens [
94,
95,
96]. For example, the phages such as Bϕ-B1251 and PD-6A3 have shown to provide lytic activity against MDR resistant
Acinetobacter baumannii [
97],
E. coli and methicillin-resistant
Staphylococcus aureus [
98], respectively. However, due to their extreme specificity, more studies need to be undertaken to ascertain the impacts of bacteriophage treatment.
In addition, recent reviews of [
99] has also covered various new and emerging techniques of improving gut microbiome resilience, and microbiome resurrection post infection and in various other gut and extra-gut inflammations.