Interplay between host and Candida albicans during commensal gut colonization

Introduction

Mammals are naturally colonized with a diverse consortium of microbes composed of bacteria, fungi and other organisms. Although fungi make up a small percentage of the total microbial community, they have significant effects on the gastrointestinal ecosystem. For example, the presence of fungal species in the gut community affects the host immune system [1]. The bacterial community is affected by the presence of fungi and the response of the bacterial community to antibiotic stress is altered when a fungus is introduced. Additionally, fungi affect gut metabolite levels with impacts on the physiology of the host. In this communication, we will discuss some of the ways that the fungus Candida albicans interacts with its host, including the beneficial effects of C. albicans against disease due to Clostridioides difficile infection. We also consider the effects of C. albicans colonization on the gut–brain axis.

Gut colonizing fungal populations are the sources of organisms that cause invasive fungal infection

Many fungal pathogens such as the primary pathogens Coccidioides immitis, Histoplasma capsulatum, and Blastomyces dermatitidis and the opportunistic pathogens Cryptococcus neoformans or Aspergillus fumigatus are environmental. These organisms often initiate infection in the lungs after they are inhaled by the host.

On the contrary, C. albicans does not have a major environmental reservoir and can be acquired by human infants early in life [2], probably from caregivers. C. albicans is thought to colonize its host as a normal member of the human gut microbiome for long periods of time without causing disease. If the host becomes immunocompromised, colonizing C. albicans can disseminate from the gut and cause potentially life-threatening candidiasis [3]. Invasive candidiasis is usually caused by endogenous organisms [4].

Gene expression in colonizing C. albicans is regulated so that fungal behavior responds to and influences host immune status

During experimental murine colonization, gene expression in C. albicans cells was found to differ depending on the immune status of the host [5]. Expression of EFG1, encoding a key transcription factor that regulates many important activities of C. albicans, was higher during colonization of healthy mice (BALB/c) and lower during colonization of immunodeficient mice (nu/nu BALB/c). Under experimental conditions, immunodeficient mice showed no evidence of disease but the colonizing fungi responded to their immunodeficiency by expressing lower levels of EFG1.

The mechanism for coupling gene expression in colonizing fungi to the status of the host’s immune response rests on natural variability in EFG1 expression from cell to cell and the properties of low Efg1 activity cells [5,6]. These cells can be considered “testers” of the host’s immune status. Changes in EFG1 expression probably occur early during the transition of a host from an immunocompetent to immunosuppressed state and may herald the change in the fungal population from benign commensals to invasive pathogens.

Variation in expression of another C. albicans transcription factor, Ume6, during commensal colonization promotes the protective Th17 response of the host [7] providing additional evidence that variability in C. albicans gene expression impacts the interaction between C. albicans and the host. These examples illustrate the nuanced interactions between host and C. albicans that allow each to respond to changes in the other.

C. albicans gut colonization alters susceptibility to bacterial infection

Since the status of the host affects C. albicans gene expression and variation in gene expression alters interaction with the host, it seemed possible that C. albicans colonization could have far-reaching effects on host physiology. Here, we will discuss effects of colonization on host susceptibility to gastrointestinal infection and on the gut–brain axis.

Several studies show that C. albicans and bacteria affect each other’s virulence. For example, we analyzed the effect of pre-colonization with C. albicans on the susceptibility of mice to Clostridioides (formerly Clostridium) difficile infection and demonstrated a protective effect of pre-colonization against this important infection. C. difficile infection (CDI) is the most common nosocomial infection in the United States and is responsible for considerable morbidity and mortality. We asked whether the presence of C. albicans in the gut community would alter the course of CDI. Results showed that the presence of C. albicans was protective against lethal CDI in mice [8].

C. albicans pre-colonization was associated with changes in the host GI tract. Stronger expression of the protective cytokine IL-17A was detected in C. albicans pre-colonized mice [8]. Additionally, changes in the gut metabolome associated with C. albicans colonization affected disease severity [9]. Specifically, elevated levels of unsaturated fatty acids were observed in C. albicans colonized mice and feeding mice without C. albicans a source of unsaturated fatty acids (olive oil) was protective against lethal CDI. Further, C. difficile cultured in laboratory media containing oleic acid (the major fatty acid constituent of olive oil) grew but with reduced expression of toxin genes. Also, C. difficile cells recovered from the cecal epithelium of olive oil-fed mice exhibited reduced toxin gene expression. The results thus show that C. albicans colonization changed the gut metabolite environment in a way that reduced C. difficile virulence.

C. albicans is not the only organism that is protective against C. difficile infection. Commensal bacteria such as Clostridium scindens [10], Lachnospiraceae [11], and Paraclostridium bifermentans [12] are also protective against murine CDI. The mechanisms involved in protection differ, but these results show that multiple commensal organisms can protect against this bacterial infection.

C. albicans shows interaction with other bacterial species. Pre-colonization with C. albicans reduced the virulence of Pseudomonas aeruginosa following oral inoculation of neutropenic mice [13]. In contrast, some combinations of C. albicans and bacteria have the opposite effect and increase virulence [14–16]. Additionally, some bacteria such as Enterococcus and Pseudomonas express antifungal activities [17,18]. These findings demonstrate that C. albicans–bacterial interactions affect virulence in multiple ways and are consequential for the host.

C. albicans colonization affects the gut–brain axis

Impacts of the gut microbiota on the brain through the gut–brain axis have recently become the subject of investigation. The gut–brain axis is defined as the bidirectional communication between the brain and the gastrointestinal tract through direct and indirect pathways including direct neuronal innervation of the intestines, metabolic pathways, and immune system modulation. All of these pathways can be impacted by gastrointestinal microbes or microbial products and can impact activities in the brain.

Gastrointestinal colonization by C. albicans resulted in altered behavior in mice [19]. Mice colonized with C. albicans had elevated afternoon corticosterone and an anxiety-like phenotype due to dysregulation of lipid metabolism and perturbation of the endocannabinoid system [19]. Endocannabinoids are signaling lipids that regulate synaptic transmission and modulate inflammation, neurological development, memory formation, anxiety, and depression [20]. Based on the functions of the endocannabinoid system, it is likely that C. albicans or other microbes that alter the endocannabinoid system would have other behavioral impacts. In fact, in a chronic unpredictable stress model, microbiota dysbiosis and endocannabinoid perturbations contributed to the development of depressive-like behavior [21].

Furthermore, C. albicans affects other pathways such as immune responses that could modulate neuroinflammation. Additionally, C. albicans impacts plasma levels of ketones such as β-hydroxybutyrate [22], a lipid molecule that has neuroprotective effects. β-hydroxybutyrate was reduced in mice colonized with microbiomes from humans with alcohol use disorder [23] and this reduction correlated with depression-like behavior and altered social behavior in these mice.

In summary, the gut–brain axis is extremely complex. Many different microorganisms are involved in its modulation through a number of pathways, leading to a variety of microbiota-responsive neuropsychological phenotypes.

Possible contributions of C. albicans and gut microbiota to the evolutionary fitness of the host

Why does C. albicans have such profound effects on its host? As discussed above, C. albicans is not the only organism to affect susceptibility to bacterial infection or signaling in the gut–brain axis. Other members of the gut microbiota have similar effects. These observations suggest that the impacts of microbiota on the host could represent an important way to generate diversity in a human population. Microbiota diversity could protect the population as a whole from extinction due to endemic or invasive pathogens that would otherwise cause widespread mortality in a genetically similar population. Additionally, variability in microbiota composition between humans could have very individual effects on each person’s gut–brain axis. As a result, each person’s response to a stressful condition could be slightly different. Diversity in responses to danger may promote survival of a group by increasing the chances that the group will find an optimal response. In small family groups where genetic diversity may not be high, microbiota diversity may provide a needed mechanism that allows the group to sample a wider variety of responses to danger. The microbiota–gut–brain axis thus provides survival benefits in a cruel world.

Acknowledgments

We thank past and present members of the group for their invaluable contributions.

Citation: Day AW, Kumamoto CA (2023) Interplay between host and Candida albicans during commensal gut colonization. PLoS Pathog 19(9): e1011607. https://doi.org/10.1371/journal.ppat.1011607

References

1. d’Enfert C, Kaune AK, Alaban LR, Chakraborty S, Cole N, Delavy M, et al. The impact of the Fungus-Host-Microbiota interplay upon Candida albicans infections: current knowledge and new perspectives. FEMS Microbiol Rev. 2021;45(3). Epub 2020/11/25. pmid:33232448; PubMed Central PMCID: PMC8100220.

2. Russell C, Lay KM. Natural history of Candida species and yeasts in the oral cavities of infants. Arch Oral Biol. 1973;18(8):957–62. Epub 1973/08/01. pmid:4581575.

3. Sprague JL, Kasper L, Hube B. From intestinal colonization to systemic infections: Candida albicans translocation and dissemination. Gut Microbes. 2022;14(1):2154548. Epub 2022/12/13. pmid:36503341; PubMed Central PMCID: PMC9746630.

4. Zhai B, Ola M, Rolling T, Tosini NL, Joshowitz S, Littmann ER, et al. High-resolution mycobiota analysis reveals dynamic intestinal translocation preceding invasive candidiasis. Nat Med. 2020;26(1):59–64. Epub 2020/01/08. pmid:31907459; PubMed Central PMCID: PMC7005909.

5. Pierce JV, Kumamoto CA. Variation in Candida albicans EFG1 expression enables host-dependent changes in colonizing fungal populations. mBio. 2012;3(4):e00117–12. Epub 2012/07/26. pmid:22829676; PubMed Central PMCID: PMC3413400.

6. Pierce JV, Dignard D, Whiteway M, Kumamoto CA. Normal adaptation of Candida albicans to the murine gastrointestinal tract requires Efg1p-dependent regulation of metabolic and host defense genes. Eukaryot Cell. 2013;12(1):37–49. Epub 2012/11/06. pmid:23125349; PubMed Central PMCID: PMC3535844.

7. Shao TY, Kakade P, Witchley JN, Frazer C, Murray KL, Ene IV, et al. Candida albicans oscillating UME6 expression during intestinal colonization primes systemic Th17 protective immunity. Cell Rep. 2022;39(7):110837. Epub 2022/05/19. pmid:35584674; PubMed Central PMCID: PMC9196946.

8. Markey L, Shaban L, Green ER, Lemon KP, Mecsas J, Kumamoto CA. Pre-colonization with the commensal fungus Candida albicans reduces murine susceptibility to Clostridium difficile infection. Gut Microbes. 2018;9(6):497–509. Epub 2018/04/19. pmid:29667487; PubMed Central PMCID: PMC6287688.

9. Romo JA, Markey L, Kumamoto CA. Lipid Species in the GI Tract are Increased by the Commensal Fungus Candida albicans and Decrease the Virulence of Clostridioides difficile. J Fungi (Basel). 2020;6(3). Epub 2020/07/09. pmid:32635220; PubMed Central PMCID: PMC7557729.

10. Buffie CG, Bucci V, Stein RR, McKenney PT, Ling L, Gobourne A, et al. Precision microbiome reconstitution restores bile acid mediated resistance to Clostridium difficile. Nature. 2015;517(7533):205–8. Epub 2014/10/23. pmid:25337874; PubMed Central PMCID: PMC4354891.

11. Reeves AE, Koenigsknecht MJ, Bergin IL, Young VB. Suppression of Clostridium difficile in the gastrointestinal tracts of germfree mice inoculated with a murine isolate from the family Lachnospiraceae. Infect Immun. 2012;80(11):3786–94. Epub 2012/08/15. pmid:22890996; PubMed Central PMCID: PMC3486043.

12. Girinathan BP, DiBenedetto N, Worley JN, Peltier J, Arrieta-Ortiz ML, Immanuel SRC, et al. In vivo commensal control of Clostridioides difficile virulence. Cell Host Microbe. 2021;29(11):1693–708 e7. Epub 2021/10/13. pmid:34637781; PubMed Central PMCID: PMC8651146.

13. Lopez-Medina E, Fan D, Coughlin LA, Ho EX, Lamont IL, Reimmann C, et al. Candida albicans Inhibits Pseudomonas aeruginosa Virulence through Suppression of Pyochelin and Pyoverdine Biosynthesis. PLoS Pathog. 2015;11(8):e1005129. Epub 2015/08/28. pmid:26313907; PubMed Central PMCID: PMC4552174.

14. Todd OA, Fidel PL Jr., Harro JM, Hilliard JJ, Tkaczyk C, Sellman BR, et al. Candida albicans Augments Staphylococcus aureus Virulence by Engaging the Staphylococcal agr Quorum Sensing System. mBio. 2019;10(3). Epub 2019/06/06. pmid:31164467; PubMed Central PMCID: PMC6550526.

15. Bergeron AC, Seman BG, Hammond JH, Archambault LS, Hogan DA, Wheeler RT. Candida albicans and Pseudomonas aeruginosa Interact To Enhance Virulence of Mucosal Infection in Transparent Zebrafish. Infect Immun. 2017;85(11). Epub 2017/08/30. pmid:28847848; PubMed Central PMCID: PMC5649025.

16. Shing SR, Ramos AR, Patras KA, Riestra AM, McCabe S, Nizet V, et al. The Fungal Pathogen Candida albicans Promotes Bladder Colonization of Group B Streptococcus. Front Cell Infect Microbiol. 2019;9:437. Epub 2020/01/31. pmid:31998657; PubMed Central PMCID: PMC6966239.

17. Cruz MR, Cristy S, Guha S, De Cesare GB, Evdokimova E, Sanchez H, et al. Structural and functional analysis of EntV reveals a 12 amino acid fragment protective against fungal infections. Nat Commun. 2022;13(1):6047. Epub 2022/10/14. pmid:36229448; PubMed Central PMCID: PMC9562342 peptides described in this work. The other authors declare no competing interests.

18. Morales DK, Jacobs NJ, Rajamani S, Krishnamurthy M, Cubillos-Ruiz JR, Hogan DA. Antifungal mechanisms by which a novel Pseudomonas aeruginosa phenazine toxin kills Candida albicans in biofilms. Mol Microbiol. 2010;78(6):1379–92. Epub 2010/12/15. pmid:21143312; PubMed Central PMCID: PMC3828654.

19. Markey L, Hooper A, Melon LC, Baglot S, Hill MN, Maguire J, et al. Colonization with the commensal fungus Candida albicans perturbs the gut-brain axis through dysregulation of endocannabinoid signaling. Psychoneuroendocrinology. 2020;121:104808. Epub 2020/08/03. pmid:32739746; PubMed Central PMCID: PMC7572798.

20. Osafo N, Yeboah OK, Antwi AO. Endocannabinoid system and its modulation of brain, gut, joint and skin inflammation. Mol Biol Rep. 2021;48(4):3665–80. Epub 2021/04/29. pmid:33909195.

21. Chevalier G, Siopi E, Guenin-Mace L, Pascal M, Laval T, Rifflet A, et al. Effect of gut microbiota on depressive-like behaviors in mice is mediated by the endocannabinoid system. Nat Commun. 2020;11(1):6363. Epub 2020/12/15. pmid:33311466; PubMed Central PMCID: PMC7732982.

22. Peroumal D, Sahu SR, Kumari P, Utkalaja BG, Acharya N. Commensal Fungus Candida albicans Maintains a Long-Term Mutualistic Relationship with the Host To Modulate Gut Microbiota and Metabolism. Microbiol Spectr. 2022;10(5):e0246222. Epub 2022/09/23. pmid:36135388; PubMed Central PMCID: PMC9603587.

23. Leclercq S, Le Roy T, Furgiuele S, Coste V, Bindels LB, Leyrolle Q, et al. Gut Microbiota-Induced Changes in beta-Hydroxybutyrate Metabolism Are Linked to Altered Sociability and Depression in Alcohol Use Disorder. Cell Rep. 2020;33(2):108238. Epub 2020/10/15. pmid:33053357.