Although conventional culture-based methods did not approve the existence of a human blood microbiome, development of NGS allowed a complete study of microbial diversity at various niches including blood samples taken from healthy volunteers.
There are some studies about gut microbiota composition in healthy Iranian subjects (Marvasti et al. 2020; Heidarian et al. 2019), but as far as we know, their blood microbiome has not been examined so far. In the current study, we performed 16S rRNA gene NGS sequencing of DNA from the blood of healthy women in Iran and illustrated the bacterial composition of healthy blood subjects.
Similar healthy blood microbiome compositions reported in some studies worldwide imply the presence of a predominant microbiome profile in the bloodstream (Amar et al. 2013; Païssé et al. 2016; Lelouvier et al. 2016), and eliminate the hypothesis of contamination during the collection of blood samples. Here are the key findings of these researches. Amar et al. (2011) stated that Proteobacteria represented 90% of the overall microbiota in human blood samples in France. Shah et al. (2019) compared the blood microbiome of 20 healthy controls and 20 patients with CKD and showed that Proteobacteria (54%), Bacteroidetes (21%), Actinobacteria (18%), Firmicutes (4%) mostly consist of the blood microbiome of healthy controls in the USA. Païssé et al. (2016) illustrated that blood fractions contain bacterial DNA mainly from the Proteobacteria phylum (more than 80%), followed by Actinobacteria (6.7–10%), Firmicutes, and Bacteroidetes. According to Rajendhran et al. (2013), Proteobacteria and Firmicutes were the two major phyla with relative abundances in the blood microbiota of healthy human samples in India. Among non-cultured blood samples examined by Panaiotov et al. (2018), bacterial phyla of Proteobacteria (93%), Actinobacteria (2%), Planctomycetes (2%), and Firmicutes (2%) predominated in blood samples of healthy individuals from Bulgaria. Whittle et al., (2019) detected 16S rDNA of Proteobacteria (74.9%), Firmicutes (19.5%), Bacteroidetes (0.05%), and Actinobacteria (0.01%) in the blood of the healthy population of the United Kingdom. We confirmed the previous studies and observed the presence of Proteobacteria (83%), Firmicutes (8%), Bacteroidetes (6%), and Actinobacteria (2%) phyla in healthy subjects in the Iranian population (Fig. 1). The difference in the values reported in various studies may be attributed to a difference in geographic factors, such as local cuisines and the ethnicity of healthy humans studied.
Based on the sequencing data, Betaprotebacteria (47%), Gammaproteobacteria (29%), Bacteroidia (8%), Clostridia (7%), Alphaproteobacteria (4%), and Bacilli (3%) consist the predominant classes on average, while Païsséet al. (2016) stated that Alphaproteobacteria (34–54%), Betaprotebacteria 17.35–21.9%), Gammaproteobacteria (10–28%), Actinobacteria (6–10%), Bacilli (2.68–3.74%) are the most predominant classes of bacteria in healthy blood.
According to the results, a diverse bacterial family was found in the whole blood of the healthy subjects that the most abundant of them belong to Burkholderiaceae (69%) and Enterobacteriaceae (13%) families on average.
The relative abundance of a few genera reported in this study turned out to be contrasting to some previous studies on healthy individuals from other geographical areas. This may attribute to the lifestyle of the host, and behavioral patterns of the healthy subjects, or the relatively limited sample size of this study. According to Qian et al. (2018), and Whittle et al. (2019), the major bacterial DNA genera found in the blood samples of healthy controls are Limnobacter and Achromobacter, respectively. Interestingly, Iranian women appear to have a high blood microbiota diversity, with dominant bacterial genera such as Ralstonia, Rhizobium, Ignatzchineria, Lactococcus, Stenotrophomonas, and Ideonella detected in the blood of healthy individuals. This is the first report of Ideonella presence in the blood of healthy individuals.
In the current study, Ralstonia by more than 1300 OTU counts is the most predominant genus that closely resembles the oral microbiomes. It showed that damage caused by various daily activities including tooth brushing, potentially as a result of damage caused by leakage across the mucosal surfaces, chewing, and flossing could lead to the translocation of bacteria from the oral cavity into the bloodstream (Parahitiyawa et al. 2010; Horliana et al. 2014). Widmer et al., (2018) showed that pulp spaces of pristine healthy teeth contain detectable bacterial DNA including Ralstonia, Actinetobacter, and Staphylococcus (43–78% of the total community). Whittle (2019) suggests that the blood microbiome community is perhaps more likely to result from the translocation of organisms from the oral cavity and skin, than from organisms that colonize the gastrointestinal tract.
On the other hand, the airborne soil bacteria are continuously inhaled, being constantly devastated in the surface mucosa of the respiratory system, then the released rDNA may enter the bloodstream directly or carry by phagocytes. This may also explain why some of our blood-associated rDNA sequences differed from those found in studies reported previously from geographically different laboratories (Nikkari et al. 2001; McLaughlin et al. 2002; Brecher and Hay 2005). According to the results, Rhizobium and Stenotrophomonas are the most abundant soil-derived bacteria in the Iranian blood microbiome of a healthy human, whereas Li et al., (2018) stated that some soil-derived bacteria were dominated the blood microbiota in 17 healthy individuals from China that included Stenotrophomonas, Pseudomonas, and Corynebacterium. Païssé et al. (2016) detected the presence of Stenotrophomonas in RBCs of healthy donors assessed by 16S targeted metagenomic sequencing, too. Besides, Shah et al. (2019) showed the presence of Stenotrophomonas in a healthy blood microbiome.
Some variables such as dietary habits, age, and the use of probiotics and antibiotics influence the composition of an individual’s microbiota (Azad et al. 2016). As a result, the relative percentage of each taxon varies between individuals. Further study of individuals representing different cultural traditions and ethnic origins will allow us to observe greater diversity in the microbiome composition according to individual provenance (Davenport et al. 2017).
Identification of the blood microbiome provides novel insights into characteristics and diagnostics of transfusion-transmitted bacterial infection through blood transfusion. Further study is required to associate the possible impact of the blood microbiome on health and disease.