Based on the above, we may say that the effort to control or reduce postoperative pain, by any means, has two simultaneous approaches: one relating to the pain of the surgical trauma on the abdominal wall which is purely of an inflammatory etiology, and two, the visceral pain, which is due to overdistension of the intestine before its mobilization from the paralytic ileus, resulting from anaesthesia.
4.1. Probiotics in relation to the inflammation-induced pain of surgical trauma
Surgical incisional trauma causes inflammatory reaction as part of the healing process. Inflammation seems to contribute to the sensitization of peripheral nociceptors, leading to hyperalgesia or allodynia, when noxious or non-noxious stimuli cause pain [
118]. Inflammation reduces the pain threshold of nociceptors and increases the individual pain response. Post-operative pain is exacerbated by the released pro-inflammatory cytokines, not only locally but in the spinal dorsal horn tissues, which are further involved in the pain mechanism, the magnitude of pain being proportionally dependent on the incision length and the surgical manipulations [
102,
119,
120]. The released chemo- and cytokines trigger the activation of the intracellular downstream signal pathways and, subsequently, to phosphorylation of receptors and ion channels, in primary sensory neurons. This whole process leads to neuronal hyperexcitability and peripheral sensitization [
15]. Furthermore, it is known that skin-deep incisions induce mechanical and heat hypersensitivity similar to incisions extended deeper than the skin; thus, cutaneous trauma drive the most of post-operative pain and, consequently, the most of the inflammatory process, or the opposite [
121,
122,
123].
Probiotic species seem capable to modulate the gut microbiota to prevent or alleviate the inflammatory pain, the beneficial effects being the consequence of their multifaceted action: to control pro-inflammatory signaling via increased expression of anti-inflammatory cytokines, or by directly limiting the bioavailability of specific pro-inflammatory cytokines [
112,
124]. The anti-inflammatory IL-10, binds to the IL-10 receptor, with the IL-10R1 being the high affinity sub-unit necessary for signal transduction, expressed by astrocytes, microglia, endothelial cells, and trafficking leukocytes [
125]. Its anti-inflammatory action is accelerated by blocking downstream signaling of TLR4, which normally promote a pro-inflammatory milieu, critical for the induction of neuropathic pain [
126]. Additionally, IL-10 signaling also induces the production of the suppressors of cytokine signaling 1 and 3, thereby further reducing pro-inflammatory cytokine production by targeting the p65 NF-κB subunit [
127]. This effect has also been confirmed in a clinical study on colorectal surgery patients treated with the four-probiotic regime
L. plantarum, L. acidophilus LA-5, B. lactis BB-12 and
S. boulardii [
115].
Diabetic rats subjected to a dorsal wound and topically treated with
Lactiplantibacillus plantarum presented a significantly increased expression of the anti-inflammatory mediators IL-10 and TGF1 to day 14, in parallel with a markedly decreased expression of the pro-inflammatory IL-1 and TNF from day 7 and thereafter, in comparison to control-treated rats. These findings were consistent to the speeding up of wound closure from day 3 and thereafter [
128].
Staphylococcus aureus-infected excisional wounds treated with
L. plantarum MTCC 2621 exhibited up-regulation of IL-10 in the later phase of the healing process in relation to controls, in parallel with earlier re-epithelization, accompanied by a reduction of leukocytes infiltration, and increased activity of fibroblasts and deposition of collagen [
129]. Other probiotic bacteria, such as
L. rhamnosus UBLR-58, L. acidophilus LA-5, L. fermentum SGL10, L. brevis GQ4237768, L. brevis SGL 12, L. paracasei SGL 04, and
B. longum UBBL-64 also exert anti-inflammatory action by means of similar mechanisms, but to a significantly lesser degree, in relation to
L. plantarum [
130,
131,
132].
TNF is considered of particular value as a marker of inflammation, due to its close relationship to the prostaglandin release implicated in pain, swelling and fever [
133]. In an ex-vivo human skin explant model, the administration of the postbiotic
B. longum Reuter lysate led to an improvement in parameters relating to inflammation, but mainly it significantly inhibited capsaicin-induced CGRP release by neurons. Similarly, the same regime in humans led to increased skin resistance to physical and chemical aggression in relation to control [
134].
Lactobacillus paracasei MSMC39 or placebo given as mouth wash in 30 patients who had had an impacted mandibular third molar removed, was found to significantly reduce the TNF level in gingival crevicular fluid, in relation to placebo. Pain, swelling and trismus, evaluated in a visual analog scale score, were also reduced, but not to a statistically significant degree [
135]. Thirty-eight patients subjected to third molar surgery were randomized to
Levilactobacillus brevis CECT7480 and
Lactoplantibacillus plantarum CECT7481 or control regime for one week. Probiotics were found to significantly reduce pain and eating difficulty scores on the 5th post-operative day and thereafter. The infection rate and swelling values showed no significant difference between probiotics and placebo at any time point [
136]. [
Table 1].
A total of 283 patients with a single rib fracture were allocated to either
Lactobacillus casei Shirota in skimmed milk or placebo daily for a month following the fracture. Pain relief was assessed during activities specifically to elicit pain, while sleep quality and maximal inspiration lung volumes were also examined. Probiotic treatment was found to be more effective in relieving pain during deep breathing, coughing and turning over, while patients also had increased inspiration volumes - sleep quality being unaffected [
137]. Contrary to these positive findings,
Lactobacillus reuteri LR06 or
Bifidobacterium longum BL5b, given in the drinking water in a rat model of inflammatory pain by means of injection of complete Freund's adjuvant into the left hind paw, were found to have no antinociceptive effect. This was assessed by means of mechanical and thermal hyperalgesia, while Iba1 was used to verify the microglial inflammatory activation in relation to placebo [
138].
There is also evidence that probiotic beneficial bacteria may affect inflammatory cytokine levels by acting on the NF-κB and mitogen-activated protein kinases [MAPK] signaling pathways [
139].
L. rhamnosus GG ATCC 53103 was found to modulate signal transduction pathways by triggering MAPKs, while
L. acidophilus LA-5 were found showing higher values in all MAPKs analyzed [
140,
141].
Finally, besides anti-inflammatory cytokines, microbiota also produce neuro-transmitters, which is able to alter pain signaling.
Lactobacillus spp., B. dentium and
Bifidobacterium spp. produce GABA, the most important inhibitory neuro-transmitter in the brain, through enzymatic decarboxylation of glutamate. GABA binds to its receptors on the surface of DRG neurons, resulting to their depolarization, thus inhibiting the nociceptive transmission [
142].
4.2. Probiotics in relation to gut distension-induced visceral pain
Visceral or splanchnic pain results from the activation of nociceptors of the thoracic, pelvic, or abdominal organs. Splanchnic organs are highly sensitive to distension, ischemia and inflammation, but are relatively insensitive to other stimuli that normally evoke pain, such as cutting or burning [
16,
38]. Thus, the colorectal distention model – although scheduled for inflammatory bowel disease or irritable bowel syndrome simulation – could be considered a model characterized by visceral pain associated with altered transit time, as occurs in the post-operative situation of paralytic ileus, before the gut starts to move again.
One of the basic findings correlating probiotic strains with pain relief is that documented by Rousseaux et al. They initially hypothesize that some probiotics may stimulate the expression of receptors on epithelial cells that locally control the transmission of nociceptive stimuli to the gut nervous system. After testing different probiotic bacteria, they found that both
L. acidophillus NCFM and
L. salivarius Ls-33 induced a sustained increase in opioid receptor μ- [OPRM1] mRNA expression in human HT-29 epithelial cells, but only the former, either live or heat-killed, was able to also induce significant cannabinoid receptor [CNR2] mRNA expression. Then, they conducted the same experiment in mice and rats, orally administering live
L. acidophillus NCFM strain for 15 days, and found the same receptors expressed in 25% to 60% of epithelial cells as against 0% to 20% in placebo treated animals. Additionally, in treated rats the mean colorectal distention required to induce pain was 20% more than the 50±2mm Hg of controls [
143].
Similar results were obtained in humans. Twenty women experiencing mild to moderate abdominal pain were enrolled to receive either
L. acidophilus NCFM or this in combination with
Bifidobacterium lactis Bi-07 for 21 days.
L. acidophilus alone, but not with
Bifidobacterium, induced colonic opioid receptor μ- [MOR] mRNA and protein expression. In contrast, cannabinoid receptor [CNR2] mRNA expression was decreased, as seen in colonic biopsies obtained before and at the end of treatment. Both treatment groups trended towards improvement in symptoms, but not significantly [
144].
In a colorectal distention model, the commercially available fermented dairy product containing
Bifidobacterium lactis CNCM I-2494 and
Lactcoccus lactis CNCM I-1631 and two common yogurt starters,
Streptococcus thermophilus and
Lactobacillus bulgaricus, was found to reduce stress-induced visceral hypersensitivity and pain. Of interest, the anti-nociceptive effect of fermented dairy products was dose-dependent and generally more effective than
B. lactis CNCM I-2494 strain alone, at the same dose, findings supporting a synergistic interplay. In detail, the anti-nociceptive action of
B. lactis CNCM I-2494, when given as a single dose per day, was clear at a dose of 10
10 CFU while, when in co-administration with the fermented dairy product at the lower dose of 10
8 CFU [
145].
In a model of colonic visceral hypersensitivity induced by infusion of 2,4,6-trinitro-benzene-sulfonic acid into the proximal colon, the 8-day probiotic
Lactobacillus rhamnosus Lcr35 treatment produces an anti-hypersensitivity activity. As this model is known to involved an increase in IL-13 secretion, it may be that the action of probiotic treatment involves the regulation of the local IL-13/Th17 immune activation [
146].
Colonic visceral hypersensitivity of inflammatory origin, occurring in rats after intracolonic instillation of zymosan, was found to be significantly attenuated by
Lactobacillus rhamnosus GG ATCC53103, the viscero-motor response for grading colorectal distension being determined by measurement of the electro-myographic activity of the abdominal external oblique muscles. The levels of neuro-transmitters serotonin, noradrenaline, and dopamine and biogenic amines quantified in the frontal cortex, subcortex, brain stem, and cerebellum were also found significantly altered in
L. rhamnosus-treated rats, suggesting that they could be involved in pain modulation [
147,
148].
Rats exposed to maternal separation, from weaning onwards, were given drinking water with or without supplementation of
Lactobacillus rhamnosus GG soluble mediators. Maternal separation followed by restraint stress in adulthood led to increased splanchnic sensitivity and corticosterone plasma levels, as well as to alterations in β-diversity and abundance of specific bacteria, including
parabacteroides - all these effects being ameliorated through
L. rhamnosus GG soluble mediator supplementation [
149].
Intrarectally given butyrate enemas effected reduction in pain perception and discomfort, when given to healthy volunteers [
150]. Based on this knowledge,
Roseburia hominis, a species of
Lachnospiraceae family, well known to consume lactate and acetate to synthesize butyrate via different pathways, was administered orally in rats subjected to water avoidance stress [
151]. Rats subjected to stress but having received placebo treatment exhibited a reduced representation of the pathways involved in the metabolism of butyrate and a reduced abundance of several operational taxonomic units associated with butyrate-producing bacteria, such as
Lachnospiraceae. However,
Roseburia hominis treatment led to alleviation of visceral pain perception and hypersensitivity, as well as to an increase in cecal butyrate concentration [
152].
On the other hand, in a rat model of colonic hypersensitivity elicited by butyrate enemas, the hypersensitivity improved after administration of
Lactobacillus acidophilus NCFM strain, since treatment increases the colorectal distention threshold by 44% compared to untreated rats. It is very important to underline the fact that the anti-nociceptive effect of
L. acidophillus was similar to that elicited by the subcutaneous injection of 1mg/Kg body weight morphine, and it enhanced by 65% the suboptimal analgesic effects of 0.1mg/Kg morphine. Furthermore,
L. acidophillus-induced analgesia was significantly inhibited by an intraperitoneally given dose of the cannabinoid receptor 2 [CB2] selective antagonist AM-630, but not by the opioid receptor antagonist naloxone methiodide, thus given indirect evidence for the physiological role of CB2 in the regulation of splanchnic pain [
143].
The combination of
Bifidobacterium longum and
Lactobacillus helveticus, as pretreatment, in mice subjected to water avoidance stress, significantly reduced the chronic stress-induced visceral hypersensitivity, in comparison to a single one. Furthermore, the combination of
L. helveticus and
B. longum was superior in regulating glucocorticoid negative feedback on the HPA axis [
153].
The same authors, having previously reported that
Lactobacillus farciminis suppresses stress-induced hypersensitivity in response to colorectal distension, evaluated whether this anti-nociceptive result is related with changes in neuronal activation at spinal and supraspinal sites induced by
L. farciminis strain. The neuronal activation was assessed by means of Fos protein expression, which is a marker of neuronal activation, rapidly expressed in neurons of the central nervous system in response to somato-cutaneous or splanchnic sensory stimuli. They then assessed whether restrain stress-induced splanchnic hyperalgesia in rats modifies Fos protein expression provoked by colorectal distention and whether this expression can be modulated by
L. farciminis. After colorectal distention or restrain stress, Fos expression was found increased in the sacral spinal cord, in the nucleus tractus solitarius, the hypothalamus paraventricular nucleus, and in the medial nucleus of the amygdala. When both stimuli were applied, Fos was overexpressed in the sacral spinal cord section, in the paraventricular nucleus and the medial nucleus of the amygdala, but not in nucleus tractus solitarius.
L. farciminis pretreatment largely reduced the Fos positive cells in all these areas, suggesting that its anti-nociceptive effect is the result of the reduction in the activation of sensory neurons in the spinal and supraspinal level due to stress [
154,
155].
The implication of probiota in the modulation of the gut–brain axis and the HPA axis was further documented after an oral administration of
L. plantarum PS128 in rats for a 14-d period.
L. plantarum PS128 was found to inhibit the 5-hydroxytryptophan-induced visceral hypersensitivity during colorectal distention, the effect followed by decreased serum corticosterone, decreased neurotransmitter protein [substance P, CGRP, BDNF, and NGF] in the spinal cord, and increase of the glucocorticoid receptor and decrease of mineralocorticoid receptor in the amygdala. These findings suggest that
L. plantarum PS128 decreased splanchnic hypersensitivity through modulating the gut–brain axis and the HPA axis [
156].
Faecalibacterium prausnitzii is an extremely oxygen-sensitive commensal butyrate-producer bacterium, populating the most anaerobic parts of GI tract of mammals [
157]. In a neonatal-maternal-separation induced stress model, a decrease in Faecalibacterium prausnitzii has been confirmed. This is also evidence in several intestinal disorders, including that of colon anastomosis leakage, after colon surgery, the
F. prausnitzii being generally recognized as a biomarker of intestinal health [
157,
158,
159]. In two experimental models of chronic [neonatal-maternal-separation] and acute stress [partial restraint stress] colorectal distension was applied, for colonic hypersensitivity induction; then the
F. prausnitzii A2-165 strain, or its supernatant were tested [
157]. Both demonstrated anti-inflammatory properties, as also seen by others; however, only the
F. prausnitzii decreased colonic sensitivity and exhibited curative anti-nociceptive properties in response to a colorectal distension [
157,
160,
161]. The authors suggest that the
F. prausnitzii-induced anti-nociceptive properties are possibly correlated to intestinal epithelial barrier enhancement, since others support the option that it can modulate tight junctions in animal models of low grade or of acute inflammation [
157,
162,
163].
One of the fundamental receptors responsible for pain perception in the intestine is the transient receptor potential vanilloid 1 [TRPV1], which is a member of the vanilloid receptor family [
164,
165,
166]. It is expressed in spinal and vagal primary afferent neurons and activated by capsaicin, noxious heat, acidosis, depolarization and endovanilloids [
164,
166,
167]. In a distension-dependent gut pain model the effectiveness of
Lactobacillus reuteri DSM 17938 was tested. In a dose-depended manner,
L. reuteri DSM 17938 was found to reduce the jejunal spinal nerve firing evoked by distension; 80% of this reduction being inhibited by the TRPV1 channel antagonist, which mediates nociceptive signals [
168]. In order to further confirm the effect of
L. reuteri on TRPV1, the authors used a murine jejunal mesenteric nerve bundles model in which capsaicin was applied on serosa; capsaicin induces intracellular calcium in dorsal root ganglion [DRG] neurons but
L. reuteri DSM 17938 inhibits it, as assessed by means of Ca2+ fluorescence intensity [
168]. Furthermore, the
L. reuteri DSM anti-nociceptive effects, when given by gavage, were tested on gastric distension in rats:
L. reuteri DSM as pre-treatment was found to inhibit bradycardia induction after the painful gastric distension [
168]. Finally, it has recently found that both isoflurane inhalation and ketamine intravenously affect acetylcholine-activated TRPC4 channels, which considerably inhibit the muscarinic cation current in ileal myocytes, even when G proteins are activated directly by intracellular GTPγS, i.e., by-passing muscarinic receptors, thus seriously implicated in anaesthesia-induced postoperative ileus [
169,
170].
The same strain of
Lactobacillus reuteri DSM 17938, given orally in a dose of 10
8 CFU for 21 days, was reported to be effective in improving symptoms of infantile colic and thus reduce the crying time [
171]. Previous research had revealed that the gut bacteria in colicky infants varies in relation to healthy ones, exhibiting a reduction in bacterial richness and specifically in
Lactobacillus and
Bifidobacterium genera, while Gram-negative bacteria are increased [
172,
173]. Newer research also reveals that 30 days
Lactobacillus reuteri DSM 17938 treatment significantly alters the mRNA levels of the transcription factors retinoid-related orphan receptor-γ [RORγ] and forkhead box P3 [FOXP3] in the peripheral blood, which modulate T-cell responses to gut microbes: it increases the FOXP3 concentration, thus resulting in decreased RORγ/ FOXP3 ratio. In parallel, it increases the percentage of
Lactobacillus in feces and decreases calprotectin [
174].
Trichinella spiralis infection-induced muscle hypercontractility in mice was attenuated after
Lactobacillus paracasei, but not after
Lactobacillus johnsonii, Bifidobacterium lactis, or
Bifidobacterium longum treatment from days 10 to 21, post infection. This finding was related to a decrease in the T-helper 2 response triggered by
T. spiralis and in transforming growth factor-1, cyclooxygenase-2, and prostaglandin E2 levels in muscles. Based on this experiment, the authors conclude that muscle hypercontractility attenuation is a strain-dependent effect, which follows both immunologic response to infection and a direct effect of
L. paracasei on muscles [
175]. Similarly, acute ex-vivo exposure of colonic mucosa to
Lactobacillus rhamnosus GG resulted in a significant [almost 70%] impairment in smooth muscle cell contraction, the effect being attributed to the reduced contractile response to Acetylcholine [
176].
Using the same as previous colorectal distension-dependent gut pain model the effectiveness of
Lactobacillus rhamnosus JB-1 was also tested; although its visceral anti-nociceptive activity was confirmed, the nociceptive signals were not found mediated by the specific TRPV1 channel antagonist [
168]. It had been previously found in rats that
L. rhamnosus JB-1, as well as
L. reuteri ATCC23172, later recognized as being
L. rhamnosus, and
L. plantarum NCIMB 826 [WT] inhibited pain perception after even the maximum colorectal distension pressure of 80mmHg, by altering signaling in DRG fibers [
177,
178]. Similarly,
L. reuteri treatment was found to inhibit the mechano-sensitive response to gastric distension, but not the
L. plantarum NCIMB 826 treatment [
178].
The probiotics
Bifidobacterium infantis 35624, Lactobacillus salivarius UCC4331 or
Bifidobacterium breve UCC2003 were given by gavage for a 14-day period, to two strains of visceral normo-sensitive and visceral hyper-sensitive rats, exposed to a novel stress and then to colorectal distension. Their nociceptive responses were analyzed by recording visceral pain behavior.
B. infantis 35624 only was found to reduce the total number of pain behaviors in the open field assessment, while significantly increasing the threshold pressure of the first pain behavior [
179]. There is also evidence in irritable bowel syndrome patients, where it was found to alleviates symptoms of pain/discomfort, bloating/distention, and bowel movement difficulty [
180,
181]. [
Table 2]