With the rapid development of high-throughput sequencing technology and bioinformatics efforts, an increasing body of literature has demonstrated that intestinal microbiota has a profound impact on host health by regulating intestinal homeostasis. Although IBS is not directly life-threatening, it can have a serious impact on patients’ life quality and mental health(Gralnek et al., 2000). Besides, multiple recent studies have consistently revealed that the gut microbiota plays a substantial role in IBS(Simrén et al., 2013; Pittayanon et al., 2019). Thus, it is crucial to understand the progress and evolution about the role of intestinal microbiota in IBS. However, such information is very limited. In this study, the top 100 most influential documents in this field were selected for research data and bibliometric analysis was carried out to systematically explore the research output dynamics, hotspots, and frontiers of this field. It can help researchers accurately grasp basics, broaden the horizon, and learn status quo of current research efforts.
As can be seen from Fig. 2, these influential outputs characterized by international cooperation and witnessed a multi-national cooperation network with the United States and Ireland as the core. The United States and Ireland were the leading country with most top papers and have the most specialists (Supplementary Table 2). Additionally, more than half of the top 10 institutions were from these two countries (Supplementary Table 3). In addition, some European countries such as the Netherlands, Sweden, and Canada in North America have also made outstanding contributions in the field. In 2005, an article written by Finland researcher Malinen E revealed that there was extensive individual variation in the gastrointestinal (GI) microbiota between IBS group and control group devoid of GI symptoms, significant differences were found in Clostridium coccoides subgroup and Bifidobacterium catenulatum group among IBS patients and controls(Malinen et al., 2005). Besides, Kassinen A et al. characterized the differences in GI microbiota between patients with IBS and control subjects by extensive high-throughput 16S ribosomal RNA gene cloning and sequencing of clones in 2007 (Kassinen et al., 2007). These studies indicated that fecal microbiota is significantly altered in IBS. Although the molecular mechanisms underlying these alterations are poorly understood, targeting the composition and metabolic function of intestinal flora has become a new option for the treatment of IBS. In recent years, many countries have contributed substantially in this field. Studies have shown that probiotics can not only reduce the symptoms of patients suffering from IBS but also alleviated depression and increases quality of life in patients with IBS(Moayyedi et al., 2010; Pinto-Sanchez et al., 2017). The VOSviewer software was used to visualize the relationship between each institution in this study. As illustrated in Fig. 3, the National University Ireland University College Cork is the foremost research institution in this field, with the most top-notch papers and highest TC. Followed closely by McMaster University, University College Cork, University of California, Los Angeles, and University of Helsinki also with exceptional academic achievement. All of these organizations focus on researching alterations of intestinal bacteria in IBS and brain-gut-microbiome Axis(Kelly et al., 2015; Burokas et al., 2017; Foster et al., 2017; Martin et al., 2018).
The top-cited studies within the research field were more likely to be published in high impact journals such as Gastroenterology, Gut, American Journal of Gastroenterology, Biological Psychiatry, Nature Reviews Gastroenterology & Hepatology, Proceedings of The National Academy of Sciences of The United States of America, suggesting that these journals are prone to publishing research of gut microbiota in health and disease. Meanwhile, it can be seen that most of these papers are from journals with high impact factor value, indicating that the topic is considered of high value by academics worldwide. The results of bibliometric analysis also support the well-known convention that high-quality studies are often published in high-impact factor journals, which in turn maintains the high impact factor of these journals. Additionally, in addition to gastroenterology and microbiology, several studies published in these journals belonging to other fields such as psychiatry and physiology. These findings offer substantial support for the strong correlation between brain-gut axis and IBS. Numerous groundbreaking advances regarding gut microbiota have been made in these research directions. O'Mahony SM et al. designed a study to assess the effect of early life stress on the brain-gut axis, they found that early life stress could result in an altered brain-gut axis, which might be used for investigating potential mechanistic insights into stress-related disorders including depression and IBS(O’Mahony et al., 2009). Burokas A and colleagues reported that chronic prebiotic treatment exhibited both antidepressant and anxiolytic effects in mice, this therapeutic effect was primarily based on normalizing the intestinal flora and reducing chronic stress-induced elevations in corticosterone and proinflammatory cytokine levels (Burokas et al., 2017). A clinical trial performed in the England investigated the clinical efficacy of prebiotic trans-galactooligosaccharide (GOS) in IBS. Clinical study results showed that the prebiotic significantly improved anxiety scores in IBS patients and was effective in alleviating symptoms by stimulating gut bifidobacteria(Silk et al., 2009). Overall, there is growing appreciation for the influences and mechanisms of gut microbiota in brain-gut axis. According to the current study results, the main action mechanisms of probiotics, including promoting the colonization and normalization of perturbed intestinal microbial communities; competitive exclusion of pathogens; production of volatile fatty acids, which could interact with the brain-gut axis by regulation of endocrine and neurologic functions(Plaza-Diaz et al., 2019). Despite great progress has been made in recent decades, further research to elucidate the precise molecular mechanisms of probiotic actions is warranted. The major contributors (shown in Fig. 4), such as Cryan JF, Dinan TG, and Clarke G come from the same institution University College Cork. Their long-lasting efforts and closer collaboration have driven the development of this hotpot. Through a network of co-authors, the cooperative relationships among researchers are fragmented, indicating that most scholars prefer to cooperate with authors in a certain circle, especially from the same institution. As a result, it is necessary for research institutions and authors to strengthen cooperation with researchers in other countries and institutions.
The distribution of the journals (Fig. 5A) demonstrates the studies of gut microbiota in IBS are increasingly reliant on molecular/biology/immunology field. As shown in Fig. 5B, some journals such as Gastroenterology and Gut have continued to remain influential in this research area. Interestingly, in addition to the field of digestive system, relevant studies were also published in the field of psychiatry. One possible explanation could be that a large proportion of patients with IBS have been observed to have high rates of anxiety and depression symptoms. The gut microbiota plays a crucial role in the communication between the intestine and central nervous system (CNS), also known as the microbiota-gut-brain axis, and is also an important signal for maintaining intestinal homeostasis(Liang et al., 2015; Foster et al., 2017). The wide application of probiotic for the treatment of IBS and depression suggested that targeting gut microbiota with probiotics may have a therapeutic potential for disease related to the brain-gut axis.
Through the analysis of high-frequency terms and phrases extracted from the titles and abstracts, the top 100 most-cited papers covered a wide range of topics regarding gut microbiota and IBS. As shown in Fig. 6, these terms or phrases including gut microbiota, irritable bowel syndrome, probiotics, microbiota, stress, depression, microbiome, anxiety, brain-gut axis, serotonin, inflammation, and inflammatory bowel disease appeared more frequently in the top 100 publications. From these terms, we briefly summarized the current attentions of this topic mainly focus on the clinical application of probiotics in IBS and depression. The next research hotspot in this field may revolve around the keyword brain-gut axis. Meanwhile, targeting gut microbiota in IBS and depression is gradually being valued in Fig. 7A. Keywords with higher burst strength may become the new turning point, which can lead us to find emerging hotspots and frontiers of the field. Figure 7B showed that earlier studies paid more attention on the links between gut microbiota and gastrointestinal symptom. However, more and more studies have increasingly focused on the role of intestinal flora in the brain-gut axis in the latest research. Actually, microbiota-gut-brain axis is one of the research frontiers. Beginning with the concept of the brain-gut axis, the interdependence between gastrointestinal tract and brain has been implicated in gut microbiome with increasing clinical applications. With the recent development of various techniques for microbiome analysis, there has been a rapid increase in the number of relevant preclinical and clinical studies on animals and human subjects. Various psychiatric symptoms including psychotic and depressive symptoms could alter the composition of the gut microbiome. Conversely, the resided microbiome influences psychological factors by means of their metabolites, either acting directly on the brain or through the synthesis of various neurotransmitters(Bravo et al., 2012; Dinan and Cryan, 2013; Kurokawa et al., 2018).
How peripheral gut microbes are associated with emotions and cognitive function is not yet clear. However, it is hypothesized that interactions occur through multiple signal pathways, such as microbial metabolites, immune system, and gut hormone signaling (Jang et al., 2020). Kelly JR et al. reported that fecal microbiota transplantation (FMT) from depressed patients to microbiota-depleted rats can lead to typical psychosocial and behavioral characteristics, including anhedonia and anxiety-like behaviors (Kelly et al., 2016). Therefore, highlighting the role of microbiomes has great potential for the treatment of psychological and gastrointestinal disorders in the future.
IBS usually co-morbid anxiety and depressive disorders and the latter characterized by elevated levels of inflammatory markers including interleukin IL-6, tumor necrosis factor alpha (TNFα) and C reactive protein (CRP)(Labus et al., 2004; Sperber and Dekel, 2010). Although series of studies proved that the gut microbiome played an important role in the treatment of IBS and depression via the microbiome-gut-brain axis. However, there are still some problems remain elusive, the exact role of the intestinal flora in the pathogenesis of depression and anxiety is unknown and the downstream physiological mechanisms by which gut microbes influence human behavior remain unclear. Due to the complexity of the intestinal flora itself, the mechanisms of its interaction with the body had not been fully elucidated, and the exact mechanism of participating in the metabolism of the body was still unclear, which needed to be investigated in further studies.