Probiotics are living organisms that are beneficial to the host when administered in appropriate doses. They can be bacteria or fungi and exert their function either by direct action on the gut microbiota or by affecting the intestinal barrier and local and even systemic immune responses. Several studies have reported their benefit against bacterial, viral, and parasitic infections [1]. Probiotic bacteria include Lactobacillus spp. which show a high genotypic and phenotypic variability reflecting on their biological effects and interactions. Acid tolerance, oxygen tolerance and spore formation are characters that contribute to the survival of these microbes in challenging environmental condition such as gut acidity [2]. Among lactic acid producing bacteria, Lactobacillus acidophilus is a preferred probiotic agent due to its ability to survive the exposure to bile and acidic pH. In addition, it improves lactose metabolism, produces antibiotic products, and modulates the immune response of the host [3]. Probiotic therapy has therefore been described as a valuable bio-therapeutic adjuvant to the management of some parasitic diseases such as giardiasis, amoebiasis, cryptosporidiosis, and blastocystosis [4]. Blastocystis hominis (BH) infection stands out among other parasitic agents for its ambiguous pathogenic potential and clinical spectrum. It also displays an intricate and curious relation with gut microbes, reflecting both on local gastrointestinal and systemic symptomatology [5]. While some studies report a potential role for BH in intestinal dysbiosis and hence intestinal pathology, others suggest a supportive function of the colonization by BH to intestinal homeostasis and eubiosis. These conflicting studies could be attributed to the genetic diversity of BH subtypes or the influence of BH on the composition and density of both beneficial and harmful microbiota [6].

The link between BH and inflammatory GIT conditions like inflammatory bowel disease, ulcerative colitis, and Crohn's disease is a common research focus. Several mechanisms have been proposed to explain how BH contributes to GIT inflammation. These include modulating immune signaling pathways such as nuclear factor kappa beta (NF-κB) and MAPK pathways, as well as degrading IgA and blocking NO production. Additionally, BH promotes the secretion of IL-8, a key regulator of granulocyte movement, especially neutrophils [7]. Furthermore, some BH subtypes trigger the release of pro-inflammatory cytokines such as IL-6, IL-1 beta, and TNF-α [8].

Probiotic agents exert their anti-inflammatory effect by reversing several of the above-mentioned mechanisms. Several Lactobacillus strains are reported to induce the production of anti-inflammatory mediators and, in contrast, downregulate pro-inflammatory cytokines such as TNF-α, IL-6, and IL-1 beta. Probiotic bacteria also modulate immune signaling pathways to inhibit the expression of cyclooxygenase 2 and inducible nitric oxide synthase [9].

Inflammation of the GIT can be assessed by measuring fecal inflammation markers such as fecal calprotectin (FCP), lactoferrin, and fecal cytokines [10,11]. Fecal calprotectin (FCP) is a calcium and zinc-binding antimicrobial protein that makes up 30–60 % of neutrophilic cytosol protein content. Elevated FCP levels indicate neutrophil activation and an inflammatory response [12]. Fecal CP has been reported as an indicator of intestinal inflammation in BH infection, and its measurement can also help monitor treatment effectiveness [13,14]. Other markers of GIT inflammation during blastocystosis include pro-inflammatory cytokines such as IL-1 beta, IL-6, and TNF-α, which are increased during infection [15].

TNF-α plays discrepant roles in the GIT, where, on one hand, it is important in maintaining intestinal homeostasis, but on the other hand, it acts as an important mediator of chronic intestinal inflammatory conditions. It is secreted not only by intestinal immune cells but also by Paneth cells constitutively. Studies on mice secreting high amounts of TNF-α emphasize the interlacing correlation between TNF-α and gut microbiota in the development of intestinal inflammation, where these mice did not develop a local inflammatory response after eradication of the microbial colonization by antibiotic treatment [16]. Bacterial LPS also induces the secretion of TNF-α by immune effector cells, and TNF-α receptor mediates LPS-induced intestinal cell shedding [17].

In the current, the effect of probiotic therapy on blastocystosis was compared to conventional metronidazole therapy by estimating FCP and fecal TNF-α.