In this study, we identified several compositional and functional alterations of the gut metagenome that may be related to symptomatic atherosclerosis. The differences in the metagenome between patients and controls did not seem to be related to smoking, diabetes or body mass index (Supplementary Figs S14–S19
), but these factors and the potentially modifying effects of different types of medication and diet that may be different between patients and controls require further investigation. Interestingly, we observed enrichment of patients within the Ruminococcus
The metagenomes of patients were enriched in genes associated with peptidoglycan biosynthesis, which suggests that increased peptidoglycan production by the gut metagenome may contribute to symptomatic atherosclerosis by priming the innate immune system and enhancing neutrophil function. Indeed, inflammation has been identified as an important contributor to the pathogenesis of atherosclerosis20
. The increased abundance of genes in this pathway cannot be explained solely by a general increase in Gram-positive bacteria because both Gram-positive and Gram-negative bacteria have peptidoglycan and even more, abundant Gram-positive groups of bacteria such as Eubacterium
were enriched in controls.
Our finding of enriched levels of phytoene dehydrogenase in the metagenomes of healthy controls and its association with elevated levels of β-carotene in the serum may indicate that the possible production of this anti-oxidant by the gut microbiota may have a positive health benefit. Lycopene and β-carotene adipose levels are associated with a reduced risk of CVD in epidemiological studies21
, but several large randomized, placebo-controlled studies with durations up to 12 years have failed to show that supplementation of pure β-carotene reduces CVD risk23
. However, lycopene has been related to intima-media thickness of the common carotid artery25
and suggested to have a role in the early stage and prevention of atherosclerosis26
. A previous study encompassing >500 participants failed to observe an association between lycopene intake and plasma lycopene levels27
, indicating that other mechanisms might be more important in determining plasma levels than oral intake of lycopene. Together with evidence that bacterial species from the human gut can synthesize carotenoids28
, we propose that our findings of increased prevalence of phytoene dehydrogenase, and increased levels of β-carotene in plasma of control subjects represent an important step towards elucidating the importance of carotenoids in the development of atherosclerosis. It is worth noting that peptidoglycan and phytoene dehydrogenase genes were not linked to obesity as there was no significant difference in abundance of these genes between lean and overweight/obese subjects in our study (Supplementary Fig. S14
), or in the meta-analysis of an independent study13
(Supplementary Fig. S17
In conclusion, here we observed associations between enterotypes, genera and species and symptomatic atherosclerosis at the taxonomical level. Within the metagenome, genes in the peptidoglycan pathway were enriched in patients, whereas genes involved in synthesis of anti-inflammatory molecules (for example, butyrate) and antioxidants were enriched in controls, suggesting that the metagenome may contribute to the development of symptomatic atherosclerosis by acting as a regulator of host inflammatory pathways. Even though our study cannot provide evidence for direct causal effects, these findings indicate that the gut metagenome may have a role in the development of symptomatic atherosclerosis.