The relationship between gut and nasopharyngeal microbiome composition can predict the severity of COVID-19

Coronavirus disease 2019 (COVID-19) is a respiratory illness caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that displays great variability in clinical phenotype. Many factors have been described to be correlated with its severity, and microbiota could play a key role in the infection, progression, and outcome of the disease. SARS-CoV-2 infection has been associated with nasopharyngeal and gut dysbiosis and higher abundance of opportunistic pathogens. To identify new prognostic markers for the disease, a multicenter prospective observational cohort study was carried out in COVID-19 patients divided into three cohorts based on symptomatology: mild (n=24), moderate (n=51), and severe/critical (n=31). Faecal and nasopharyngeal samples were taken, and the microbiota was analyzed. Linear discriminant analysis identified M. salivarium, P. dentalis, and H. parainfluenzae as biomarkers of severe COVID-19 in nasopharyngeal microbiota, while P. bivia and P. timonensis were defined in faecal microbiota. Additionally, a connection between faecal and nasopharyngeal microbiota was identified, with a significant ratio between P. timonensis (faeces) and P. dentalis and M. salivarium (nasopharyngeal) abundances found in critically ill patients. This ratio could serve as a novel prognostic tool for identifying severe COVID-19 cases.


INTRODUCTION
Coronavirus disease 2019 (COVID-19) is a respiratory illness caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).The data reported in November 2023 revealed that over 700 million people have been infected with the virus [1].The appearance of mutations and variants of concern has caused several additional waves of infection and compromise the effectiveness of existing vaccines and antiviral drugs [2].
Moreover, SARS-CoV-2 infection has shown to cause long-term effects on human health, although the mechanisms are still poorly described [3].Even though most COVID-19 cases are mild, disease can be severe, resulting in hospitalisation, respiratory failure, or even death [1].Therefore, a remarkable feature of SARS-CoV-2 infection is the great variability in clinical phenotype among infected people.Many factors can correlate with COVID-19 disease severity, including age, gender, body mass index, previous comorbidities, immune responses, and genetics [4][5][6].Of note, and unfortunately, the determinants of infection outcome and the pathogenic mechanisms are not completely understood yet.SARS-CoV-2 primarily infects the respiratory tract by binding to angiotensinconverting enzyme 2 (ACE2) receptor [7].However, a growing body of evidence suggests that SARS-CoV-2 can also infect other organs since viral particles and nucleic acids have been found in different biological samples, like sputum, bronchoalveolar lavage fluid, faeces, blood, and urine [8,9].Thus, studies employing single-cell RNA sequencing have demonstrated that ACE2 is present in various organs and tissues, including the gastrointestinal tract, where ACE2 receptors have been reported to be highly expressed [10].This supports several lines of evidence suggesting a substantial involvement of the gastrointestinal tract in the pathogenesis of the disease, including the ability of SARS-CoV-2 to infect and replicate in intestinal enterocytes [11], or to increase expression of the viral entry receptor (ACE2 receptor) and several membranebound serine proteases (such as transmembrane protease serine 2 (TMPRSS2) and TMPRSS4) in intestinal epithelial cells [12].
In addition, reports have shown that SARS-CoV-2 infection can disrupt nasopharyngeal and intestinal microbiota composition and promote dysbiosis, reducing diversity and increasing the presence of opportunistic pathogens, including Staphylococcus, Corynebacterium and Acinetobacter bacteria [13,14], which can make patients more susceptible to secondary infections, rising morbidity and mortality [15].Besides local changes in the respiratory tract, alterations in the distal gut microbiota have also been observed in SARS-CoV-2 infection [16][17][18][19].Previous studies have evidenced a relevant connection between the microbiome in the nasopharynx and the gut, and preliminary data have suggested a bidirectional interaction that could play a role in the development of the immune response both in healthy and pathological conditions, including SARS-CoV-2 infection [20].In this sense, dysbiosis has been associated not only with the severity of the disease but also with the recovery processes [13,19]; however, little is understood of the link with the establishment of different symptomatic profiles in this condition, and to date, few studies have focused on the association between COVID-19 severity and the nasopharyngeal and faecal microbiota, being examined simultaneously.
Considering that the efficacy of the COVID-19 vaccines and antiviral drugs against SARS-CoV-2 is compromised with the emergence of mutations and new variants of the virus [2], new therapeutic approaches and prognostic tools are still necessary.Therefore, the characterization of the nasopharyngeal and intestinal microbiome will allow to identify predictive biomarkers for the diagnosis and prognosis of the disease, as well as possible therapeutic targets in the management of SARS-CoV-2.

Study patients characteristics
A total of 106 patients who had laboratory confirmation of SARS-CoV-2 infection were included in the present study.Of note, none of these patients reported another viral infection at the time of enrolment.Based on the clinical spectrum criteria reported in the COVID-19 treatment guidelines, patients were categorised into 3 groups: mild (24 patients), moderate illness that needed hospitalisation in Respiratory Unit (51 patients), and severe symptomatology and admitted in the ICU (31 patients) (Table 1).The age of the patients significantly increased with the severity of the symptoms.Patients included in the severe group were significantly older than those with mild or moderate symptoms (p < 0.05; ANOVA).Patient inclusion was carried out evenly in terms of gender as 52 women and 54 men were recruited; nevertheless, a gender-related impact on the clinical course of these patients was observed since males were predominantly in the group of patients with severe symptoms when compared with the mild illness group (p < 0.05; Fisher's test).Regarding the symptomatology, mild patients showed symptoms of a mild respiratory infection, including fever, cough and headache.33% of mild patients displayed dyspnoea and low oxygen saturation, 26% reported gastrointestinal complaints (including stomach ache, digestive discomfort and diarrhoea); and 4% reported high respiratory and heart rates.In contrast, moderate and severe patients showed higher frequencies of dyspnoea, low oxygen saturation and increased respiratory or heart rates (p < 0.05; Fisher's test).When the different comorbidities were considered, patients in the group with severe symptoms showed a higher percentage of cardiomyopathy compared to those from the mild or moderate cohorts (42% vs 3% and 6%, respectively; p < 0.05; Fisher's test).However, no significant differences were found in the prevalence of the other pathologies among groups.Regarding blood-based biomarkers, platelets (p < 0.05; ANOVA), D-dimer, ferritin and C reactive protein (p < 0.05; Kruskal-Wallis) correlated with the severity of the symptoms, being the severe group significantly different.

Bacterial composition differs between sample type and severity index in SARS-CoV-2 infected patients
Nasopharyngeal swabs and faeces were obtained from all the patients included in the study within seven days of symptom onset and used for characterization of the microbiota composition.In the nasopharyngeal microbiota, α-diversity was reduced in the moderate and severe groups compared to the mild group (p < 0.05 and p = 0.07 respectively; ANOVA) (Figure 1A).Conversely, when α-diversity was examined in stool samples, no significant modifications were observed among groups (Figure 1B).
On the other hand, ß-diversity analysis through the Bray-Curtis index revealed statistical differences between groups for both nasopharyngeal swabs and stools samples (p < 0.001; PERMANOVA) (Figure 1C and 1D).As a result, microbiota is grouped in three separate clusters depending on the severity of the symptoms, which indicates that microbiota differences are based on changes in bacterial composition.Similarly, the characterization of microbiota abundance composition revealed heterogeneity in the microbiota profile associated with severity and disease progression in these patients.Relative abundance levels for both phylum and genera were evaluated using the median abundance as a threshold.Specifically, genera with a median relative abundance greater than 0.5% were considered for detailed analysis.
At phylum level, in nasopharyngeal microbiota, the abundance of Bacillota was increased while the abundance of Bacteroidota and Actinobacteroidota was reduced in patients with severe symptomatology (Figure 2A).Conversely, the faecal microbiota of the three groups presented a more homogeneous distribution than the nasopharyngeal one, being the most abundant phyla Bacillota and Bacteroidota (Figure 2B).However, patients with severe symptoms, thus with a worse prognosis, showed a lower abundance of Bacteroidota (Figure 2B).Exact values of relative abundance for different phyla are shown in supplementary file 1.At genus level, the nasopharyngeal microbiome composition revealed that symptom severity was associated with a higher number of detected genera (Figure 2C).Of note, significant differences in genus abundance for each group of patients were found.Relative abundance levels for genera identified bacteria and exact p-values can be found in supplementary files 2 and 3.

Differences in bacteria composition could be used as biomarkers to predict disease severity and outcome in SARS-CoV-2 infection.
We investigated whether some specific taxa could be associated with the severity of the symptoms.ASVs were evaluated to determine core taxa along with the specific bacteria of each group of patients and samples (Figure 3A,B).Core taxa were identified using a detection threshold of 0.1 and a prevalence threshold of 0.1.In nasopharyngeal swabs, Venn diagram revealed that the three cohorts shared 51 core taxa.60 specific bacteria were identified in patients with mild symptoms while 32 were observed in patients with moderate symptoms and 8 in patients with severe symptoms.When stools were considered, the three cohorts shared 159 core taxa.27 were specific of mild patients, another 33 of patients with moderate symptoms and 27 of the severe patients (For more details see supplementary file 4).and Prevotella copri as predictors of moderate status; and Escherichia, Enterococcus durans, Alistipes onderdonkii, Prevotella timonensis and Prevotella bivia as markers of severe condition.Unlike the Venn diagram, the presence of a species in the LEfSe analysis does not imply its absence in the other groups, but rather indicates significant differences in abundance.Hence, detailed abundance of these bacteria can be found in supplementary files 5 and 6.
To further assess the role of these potential biomarkers in the prediction of COVID-19 severity, a correlation analysis was performed between these species and symptomatology.In summary, biomarkers identified in nasopharyngeal swabs in patients with severe condition, M. salivarium and Leptotrichia, showed a positive correlation with D dimer and cardiomyopathy, respectively (R > 0.3).In addition, the other two biomarkers related to the highest severity, H. parainfluenzae and P. dentalis, also showed a positive association with CRP, D dimer and cardiomyopathy, even though it was not statistically significant (R = 0.2).More precisely, M. salivarium and Leptotrichia along with P. dentalis showed a significantly negative correlation towards lymphocytes count (p<0.05;R<-0.25) (Figure 4A).In the case of stool samples, biomarkers for mild symptomatology (B.coprocola, R. bicirculans and S. stercoricanis) presented a negative correlation with CRP levels and respiratory rate (p<0.05;R < -0.25).In contrast, the severe biomarkers identified, presented important correlations towards dyspnoea, cardiomyopathy or respiratory rate.Concretely, P. bivia and P.
timonensis, revealed a positive correlation with D dimer, ferritin levels and respiratory rate, as well as a negative correlation with lymphocyte count (R > 0.6) (Figure 4B).
Additionally, these two species showed a significant positive correlation against CRP levels and a negative correlation towards lymphocyte count (p<0.05).Exact correlation and p-values are shown in supplementary file 7.

Identification of a novel microbiome-based COVID-19 prognosis approach.
Considering the possible identified biomarkers and their relationship towards clinical variables, we proposed a new approach to predict disease severity in patients suffering SARS-CoV-2 infection by evaluating correlation among specific bacteria abundance from nasopharyngeal and stool samples The analysis revealed no important associations between nasopharyngeal and faecal microbiota in mild and moderate patients (Figure 5A,B).Nonetheless, in patients with severe symptoms the Spearman's rho coefficient showed a significant positive correlation between P. timonensis towards P. dentalis and M. salivarium (Figure 5C).Consequently, the ratios P. timonensis / M. salivarium and P. timonensis / P. dentalis were calculated to determine whether they could be used as a predictor of COVID-19 severity.Both seemed to be significantly increased in patients with severe symptoms compared to those with mild or moderate symptomatology (p < 0.05; Kruskal-Walli) (Figure 5D,E).As a result, the ratio between the abundance of these bacteria could serve as reliable predictors of severity of COVID-19.

DISCUSSION
Recent findings have evidenced the prominent role of the microbiome in several diseases, including those caused by viruses, proving that commensal microbiota could either enhance or hinder viral infections [30,31].A large body of evidence indicates that respiratory virus infections can induce a microbial imbalance in the airways, increasing the risk of secondary bacterial infections and worsening the prognosis [32].
Besides, alterations in the gut microbiota have also been noted during respiratory virus infections, likely influenced by the "gut-lung axis" [33].In this regard, different studies have tried to establish relationships between microbiota composition and SARS-CoV-2 infection [34,35] but just a few have explored microbiota compositional variations among patients with different symptomatology [36].However, only a few studies have explored the nasopharyngeal-faecal axis as a potential biomarker of severity in patients infected with SARS-CoV-2.Hence, the present study provides some biomarkers located in nasopharyngeal and gut microbiota that could help to predict COVID-19 severity.
In this context, 106 patients with SARS-CoV-2 infection were included in this study and classified into 3 severity groups according to their symptoms.To ensure results were derived only from SARS-CoV-2 infection, included patients were not positive for other viral infections.Considering clinical parameters, the current study confirmed that age and gender are positively associated with more severe symptoms like dyspnoea, increased heart and respiratory rates together with lower oxygen saturation.Results that are in line with other COVID-19 studies conducted globally, which have shown that men and women are disproportionately affected.Data revealed that males suffered from more severe disease than females, including higher ICU admission rates, dyspnoea, increased heart rate, being all clinical signs for higher severity of the COVID-19 disease [37,38].Regarding some biochemical parameters those groups of patients with moderate or severe symptoms showed significantly increased levels of D-dimer, ferritin and CRP, which are typically described as biomarkers for COVID-19 severity [39][40][41].
Interestingly, previous studies have reported that alterations in the gut microbiota composition in COVID-19 patients could be also associated with disease severity, measured with the elevated concentration of the blood markers described above.[42][43][44].In this sense, we have confirmed these results in our study as specific bacteria from severe condition positively correlates with the levels of ferritin, CRP, D-dimer or lymphocytes.Consequently, the microbiota could be key in the clinical phenotype of COVID-19 patients, although its specific contribution to the progression of the infection and a poor prognosis needs to be better understood.
The results of alpha diversity revealed only substantial changes in richness in the nasopharyngeal microbiome associated with moderate and severe symptoms.Although most of the studies have proposed that SARS-CoV-2 infection is associated with lower microbial diversity in nasopharyngeal samples [13,45,46], others did not find differences in alpha diversity composition among groups with different symptomatology [47,48].This could be explained by the diverse methods of sample collection, and also the SARS-CoV-2 variant and the treatment with antibiotics [49].
Furthermore, in the present study, beta diversity analysis showed that in both types of samples, every group of patients presented distinctly differentiated clusters.As previously reported, COVID-19 disease severity is more dependent on the presence or absence of certain bacteria rather than alterations in bacterial diversity and richness [ 50].This argument was supported by the characterization of bacterial microbiota composition at phyla and genera levels.In nasopharyngeal swabs, the abundance of Bacteroidota and Actinobacteriota was reduced compared to mild patients, which goes in accordance with previous reports where these bacteria have been previously linked to a better prognosis of SARS-CoV-2 infection, since can exert beneficial effects by preventing respiratory diseases, including COVID-19 [51][52][53][54][55].Moreover, the abundance of Bacillota and Pesudomonadota was increased in patients with severe symptomatology, thus supporting previous studies in which higher counts of Bacillota (Staphylococcus sp. and Streptococcus sp.) and Pesudomonadota (Pseudomonas sp.) were associated with moderate and severe symptoms of COVID-19 [56].
Regarding genera composition, Alistipes and Muribaculaceae were highly abundant in patients with mild symptoms.These bacteria have been well characterised in gut microbiota, but little is known about their presence in nasopharyngeal microbiota.
Different experimental studies in mice have suggested their role in viral infections.
Thus, Muribaculaceae was found in the lung microbiota in SARS-CoV-2 infected mice that were treated with a selective inhibitor of the main protease (M pro ) [57].In the case of Alistipes, in a study conducted in children infected with respiratory syncytial virus (RVS), its abundance was reduced in the nasopharyngeal microbiota in comparison with non infected subjects [58].Overall, these genera could be associated with a protective role against viral infection, and their higher presence in nasopharyngeal samples from mild COVID-19 patients could prevent the progression to severe disease.
In contrast, the increased presence of other genera, such as Corynebacterium, Acinetobacter, Staphylococcus and Veillonella, was associated with the severity of SARS-CoV-2 infection.This is supported by previous studies in which these genera were associated with both disease severity and systemic inflammation [59].In addition, higher abundance of Enterococcus was observed in severe patients, thus confirming other studies in critically ill patients [60].
On the contrary, findings at phylum level in stool samples did not reveal notable modifications among patient groups.However, in mild ill patients, different genera from Clostridia class (Clostridia, Coprococcus, Dorea, Lachnospiraceae, Roseburia and Ruminococcus), and Barnesiella were identified as highly abundant.While the class Clostridia was associated with a reduced production of proinflammatory cytokines in COVID-19 patients and in those who recovered from the infection [61,62], Barnesiella prevents colonisation by antibiotic-resistant bacteria such as Enterococcus, which has been reported as a frequent cause of systemic infection in critically ill COVID-19 patients [63,64].Thus, the reduction of gut abundance of Barnesiella and Clostridia members could be associated with more severe symptoms in COVID-19 patients.
Moreover, the specific genera detected in the severe illness group, Lachnocostridum, Anaerococcus and Peptoniphilus, have been previously recognised as opportunistic pathogens, which could induce gut inflammation and contribute to a poor prognosis [65].
Considering the variations observed in both nasopharyngeal and gut microbiota composition and their miscellaneous effects, the identification of specific bacteria could be used as a biomarker to predict COVID-19 severity.Accordingly, the present study has identified unique ASVs for the different severities of COVID-19.For nasopharyngeal samples, species belonging to the genus Lactobacillus (L.fermentum or L. reuteri) or Prevotella (P.pallens, P. ori and P. shahii) have been pinpointed.
Interestingly, mild patients have also shown a higher content of Lactobacillus, which could also contribute to the host defence against viral infection at early stages, as reported in asymptomatic COVID-19 patients [66].In the case of Prevotella sp., its role in COVID-19 infection has not been clearly elucidated, but previously published microbiome analysis has revealed that its abundance was higher in mild patients [67].
Nevertheless others have suggested that it could be a biomarker of critical phenotype in COVID-19 patients [68,69].It is interesting to highlight that Anaerococcus prevotii was one of the species exclusively found in stools in mild patients.This microorganism has already been linked to lower inflammation in COVID-19 patients [70].Conversely, Coprobacillus cateniformis was only found in severe patients, and could be involved in the development of a worse condition in these patients through ACE2 upregulation [17].
Even though these bacteria were found to be unique for each group, LEfSe was performed to obtain specific severity prognostic biomarkers depending on the abundance threshold.Of note, the use of these bacteria as biomarkers of severity in SARS-CoV-2 infection is further supported by the fact that these species exhibited positive correlations with critical clinical variables mentioned above.Specifically, possible severe biomarkers from nasopharyngeal samples such as M. salivarium, H. parainfluenzae and P. dentalis showed a negative correlation towards lymphocytes count.This also happens for stool severe biomarkers P. bivia and P. timonensis.As it is well established that microbiota can modify blood cells count [81], it would not be a surprise that these bacteria could promote COVID-19 severity by lowering lymphocyte count.In addition, P. bivia and P.
timonensis showed positive correlation with ferritin, CRP and D-dimer levels, as well as cardiomyopathy and respiratory rates.Several studies have revealed both the relationship between CRP levels, gut microbiota and COVID-19 severity [82], as well as the positive correlation of specific bacteria with D-dimer, CRP and the levels of proinflammatory mediators in plasma [83].
Finally, although the composition of the nasal and faecal microbiota in COVID-19 patients has been previously studied [32], the interaction between these microbiotas has not been extensively evaluated.In this study, we analyzed the relationship between specific bacteria from nasopharyngeal and stool samples, confirming a correlation between the abundances of species in these different sample types.Concretely, a strong positive correlation between P. timonensis (in stool samples) towards P. dentalis and M. salivarium (in nasopharyngeal samples) was found in severe patients.To maximise the potential use of these biomarkers, the ratio of the abundance of these species was also significantly increased within the highest severity of this condition.As a result, the ratio proposed in this study could be used as a novel predictor to identify critically ill COVID-19 patients as the ratio Bacillota and Bacteroidetes has been used as a marker of dysbiosis [84].In this case, the ratio P. timonensis/P.dentalis and P. timonensis /M.
Salivarium could be a prognostic tool for severe SARS-CoV-2, and its increase could be associated with a higher risk COVID-19 severity development.

Figure 1 .
Figure 1.Nasopharyngeal and gut microbiota composition is modified depending on the severity of COVID-19 symptoms.(A) Alpha diversity index for nasopharyngeal swab samples microbiota.(B) Alpha diversity index for stool samples microbiota.(C) PCoA for Bray-Curtis index of nasopharyngeal swab microbiota.(D) PCoA for Bray-Curtis index of stool samples microbiota.Values are represented as mean ± SD.Differences were displayed as: a p<0.05Severe sv.Mild; b p<0.05 Moderate vs. Mild; c p<0.05 Severe vs. Moderate.PERMANOVA test was employed to determine Bray-Curtis significance differences.

Figure 2 .
Figure 2. Microbiota composition of nasopharyngeal and stool samples at phylum level is slightly modified by COVID-19 symptoms severity.In contrast, at genus level, severity increases the total amount of detected bacteria in nasopharyngeal swabs while in stool samples it is promoting a reduction.(A) Representation of the relative abundance of the main phyla in nasopharyngeal swab samples.(B) Representation of the relative abundance of the main phyla in stool samples.(C) Relative abundance of the principal genera detected in nasopharyngeal swab samples.(D) Relative abundance of the principal genera detected in nasopharyngeal swab samples.For C and D only those ASVs with a median higher to 0.5 were chosen.

Figure 3 .
Figure 3. Differential analysis expression of microbiota composition from nasopharyngeal and stool samples revealed the presence of specific bacteria related to COVID-19 severity index.(A) Venn diagram showing ASVs distribution in nasopharyngeal swab samples.(B) Venn diagram showing ASVs distribution in stool samples.(C) LEfSe plot of taxonomic biomarkers present in nasopharyngeal swab samples (p value = 0.01 and LDA value = 4).(D) LEfSe plot of taxonomic biomarkers present in stool samples (p value = 0.01 and LDA value =4).Venn diagrams were acquired with the following parameters: detection level = 0.01 and prevalence level = 0.01.Besides, the linear discriminant analysis (LEfSe) was performed to identify differential microorganisms for each group of patients (Figure 3C,D).In nasopharyngeal samples from mild patients, Burkholderia sp., Paraburkholderia sp. and Massilia sp. were determined; in moderate patients, Pseudomonas veronii, Stenotrophomonas rhizophila and Azotobacter chroococcum; and in severe patients, Mycoplasma salivarium, Prevotella dentalis, Leptotrichia and Haemophilus parainfluenzae.On the other hand, in stool samples, Bacteroides coprocola, Veillonella sp., Ruminococcus bicirculans and

Figure 4 .
Figure 4. Whereas mild biomarkers showed negative correlations towards clinical variables, severe biomarkers presented positive correlations.(A) Correlation plot of nasopharyngeal swab biomarkers and clinical variables.(B) Correlation plot of stool samples biomarkers and clinical variables.RR: respiratory rate; HR: heart rate; GI: gastrointestinal alterations.Correlation was calculated by taking into account abundance levels of each bacteria against the clinical variable of severe ill patients.Red asterisks stand for p-values < 0.05 and correlations greater than or equal to 0.3 or less than or equal to -0.25.

Figure 5 .
Figure 5.The existence of a relationship between the abundance of nasopharyngeal severe biomarkers and stool severe biomarkers allows the employment of an abundance ratio between them as a new tool for predicting COVID-19 severity.(A) Correlation plot among biomarkers found in nasopharyngeal swab and stool samples in mild condition.(B) Correlation plot among biomarkers found in nasopharyngeal swab and stool samples in moderate condition.(C) Correlation plot among biomarkers found in nasopharyngeal swab and stool samples in severe condition.(D) Ratio of the abundance between P. timonensis (stool) and M. salivarium biomarkers.(E) Ratio of the abundance between P. timonensis (stool) and P.dentalis (nasopharyngeal swab) biomarkers.a p<0.05Severe sv.Mild; b p<0.05 Moderate vs. Mild.

Table 1 . Clinical data description of enrolled patients. Differences
were displayed as: a p<0.05Severe sv.Mild; b p<0.05 Moderate vs. Mild; c p<0.05 Severe vs. Moderate.ANOVA or Kruskal were employed for numerical variables and Fisher's test for categorical variables.
[17]50]][80]or mild patients, Burkholderia and Paraburkholderia were identified in nasopharyngeal swabs and B. coprocola and R. bicirculans in stool samples.Although the information regarding the first two species in humans is limited, a few studies have reported their presence in the commensal human microbiota[71,72].speciesrecognisedasbiomarkers.Both P. bivia and P. timonensis have been defined as unique microorganisms in COVID-19 patients microbiota[73,77], whilst a higher abundance of M. salivarium, H. parainfluenzae and E. durans were related to poor prognosis[78][79][80].When considering A. onderdonkii, it is a short-chain fatty acidproducing bacteria that has been reported to be inversely associated with COVID-19 severity[17,50]; however, there are conflicting evidences regarding its pathogenicity that indicate that A. onderdonkii may have protective effects against some diseases, including COVID-19[17].Nonetheless, considering that Zuo T et al. employed a different methodology to analyse microbiota composition from stool samples, A. onderdonkii could be considered as a biomarker of severe condition in SARS-CoV-2 infected subjects.