Your mouth, AKA the oral cavity, is actually the entry point into the digestive tract and harbors the oral microbiome – a highly diverse, complex, and fascinating community of microorganisms. The oral microbiota acts as a sort of gateway into the rest of the gastrointestinal tract microbiome, and is directly linked to the microbial community in the gut (as virtually everything that enters the gut passes through the oral cavity first) (1). Also, the oral microbiome is closely linked with the perception of taste and smell. Obviously this is because of the anatomical proximity of the mouth and nose, however the oral microbiota is also directly involved in both the breakdown (metabolising) and production (synthesising) of various substances that can activate sensory receptors, which in turn impacts taste and smell perception (2).
Scientifically speaking, the mouth is divided into separate regions, based on the various structures within each area. This means that within each region there are different bacteria that dominate, which was explored in detail in a 2023 study (3). For example, the surface of the tongue is one such environment, where bacteria must be able to deal with a constant flow of saliva, and the presence of tastebuds, and ongoing contact with virtually everything we consume – from our morning coffee to an after-dinner sweet treat. On the surface of the tongue, the dominant bacterial taxa include Veillonella spp., Rothia spp., Neisseriaceae spp., Actinomyces spp., and Streptococcus spp. Another area is the inside of the cheek – this is referred to as the buccal mucosa, and the predominant bacteria found in this area different to the species living on the tongue. Specifically, Streptococcus spp. and Pasteurellaceae spp. are dominant in the buccal mucosa.
Some of these bacterial genera may sound familiar as they often contain common opportunistic pathogens – for example, Streptococcus includes Streptococcus pyogenes, the infectious bacterium responsible for strep throat. Overall, it is clear that the mouth is a very unique part of the human body, and the microbes that live there must be able to endure a constantly changing environment.
In addition to the bacteria living in the mouth, other microorganisms, including fungi, can also be found here. Candida albicans is of the most commonly found fungi living in the mouth and it is also a leading cause of oral thrush, responsible for up to 80% of cases. Oral thrush, AKA oral candidiasis, can be caused by any Candida species, which includes C. albicans as well as Candida glabrata, Candida tropicalis, Candida kruesi, Candida guillermondii, Candida lusitaniae, Candida parapsilosis, Candida pseudotropicalis, and Candida stellatoidea – although these are much rarer in comparison to C. albicans (4).
C. albicans is in fact a normal member of the oral microbial community in up to 60% of adults, and although the majority of people will have some level of C. albicans living within their mouth, it typically should not cause any symptoms or discomfort. However, in scenarios where the normal defenses that keep C. albicans in a harmless commensal state become compromised, oral candidiasis can occur. This means that oral thrush is something that occurs as an unintended consequence of immunosuppression; for example, the use of an inhaler to manage asthma can be linked to oral thrush. This is because most asthma inhalers contain corticosteroids, which function to manage the symptoms of asthma by suppressing the local immune response. Disruptions to the oral microbiota is also involved in increased levels of Candida in the oral cavity, meaning it is therefore associated with oral thrush. In particular, one study reported that Campylobacter, Staphylococcus, and lactobacilli were detected at higher levels during oral candidiasis, while Neisseria, Prevotella, and Alloprevotella were found to be decreased (i.e. less abundant) (4) The same study also reported that there were differences following the restoration of the bacterial community post-antifungal treatment – meaning that patients were unable to get back to their original ‘baseline’. While this study was preliminary, the authors suggest that further long-term research may be useful to help uncover a potential role for using microbiome-mediated interventions (such as probiotics) in order to help re-establish a healthy bacterial and fungal community after antifungal treatment.
So how do you know if you have oral candidiasis? This disease can present in various ways – the most common manifestation is in the form of creamy white-coloured coating that forms on the tongue, mouth, and throat, however patches can also be red. Usually, scraping off this coating reveals inflamed red patches that can bleed. Other common signs and symptoms include (6)
Creamy white patches or spots on the tongue, inner cheeks
In severe cases these can appear sometimes on the roof of the mouth, along the gums and on the tonsils.
White patches that are slightly raised and have a cottage-cheese like texture
Redness, burning or soreness – especially if it feels severe enough that it creates difficulty eating, drinking, or swallowing
Bleeding when the patches or spots are scraped or rubbed
Redness and cracking at the corner of the mouth
A feeling of having ‘cotton mouth’ or ‘dry mouth’, even with good oral hygiene
An inability to taste
For individuals who wear dentures, pain, redness, and irritation underneath
Most importantly, oral candidiasis is a clinical diagnosis based on the signs and symptoms described above that must be confirmed by sampling the affected areas to prove that Candida spp. is the underlying cause. Once diagnosed, in most mild cases the use of topical antifungal therapy and improved oral hygiene habits are able to resolve oral thrush.
It is important to keep in mind that presence of Candida spp. is not a diagnostic measure, because as mentioned earlier, the fungus is a commensal that is commonly identified within the oral cavity. In other words, an important aspect to keep in mind is that Candida being present in your oral microbiome does NOT necessarily mean you have oral thrush (7)
However, there is an undeniable link between the bacterial and fungal community in the mouth, with probiotic interventions are being explored in the context of oral candidaisis and increasing evidence suggesting they may be useful especially if combined with antifungals (8).
Clearly, the complex and fascinating microbial community living in your mouth deserves just as much focus as our other microbiomes!
(1) https://doi.org/10.1038/s41579-024-01075-5.
(2) https://doi.org/10.1038/s41598-021-02558-8
(3) https://doi.org/10.1038/s41579-023-00963-6
(4) https://www.ncbi.nlm.nih.gov/books/NBK545282/
(5) https://doi.org/10.1007/s00784-026-06827-6
(6) https://www.mayoclinic.org/diseases-conditions/oral-thrush/symptoms-causes/syc-20353533
(7) https://doi.org/10.1080/19424396.2025.2552159