Fecal microbes, short chain fatty acids, and colorectal cancer across racial/ethnic groups

Abstract

Aim: To investigate differences in microbes and short chain fatty acid (SCFA) levels in stool samples from Hispanic and non-Hispanic African American, American Indian, and White participants.

Methods: Stool samples from twenty participants were subjected to analysis for relative levels of viable bacteria and for SCFA levels. Additionally, the samples were subjected to 16S rRNA gene pyrosequencing for identification of bacteria present in the stool. We used a metagenome functional prediction technique to analyze genome copy numbers and estimate the abundance of butyrate kinase in all samples.

Results: We found that African Americans had significantly lower levels of acetate, butyrate, and total SCFAs than all other racial/ethnic groups. We also found that participant microbial profiles differed by racial/ethnic group. African Americans had significantly more Firmicutes than Whites, with enriched Ruminococcaceae. The Firmicutes/Bacteroidetes ratio was also significantly higher for African Americans than for Whites (P = 0.049). We found Clostridium levels to be significantly and inversely related to total SCFA levels (P = 0.019) and we found Bacteroides to be positively associated (P = 0.027) and Clostridium to be negatively associated (P = 0.012) with levels of butyrate. We also identified a correlation between copy number for a butyrate kinase predicted from 16S rRNA gene abundance and levels of butyrate in stool.

Conclusion: The identified differences in gut flora and SCFA levels may relate to colorectal cancer mortality differentials and may be useful as targets for future clinical and behavioral interventions.

Keywords: Butyrate; Colorectal cancer; Microbiota; Racial/ethnic disparities; Short chain fatty acids.

Figure 1

Levels of individual and total short chain fatty acids by race/ethnicity. All values are in mg/mL of stool. A: The levels of short chain fatty acids (SCFAs) in stool across all four racial/ethnic groups; B: The levels of SCFAs for African Americans vs those of all other races/ethnicities. ¹Statistically significant difference by independent samples t test, two sided, equal variances not assumed. Acetate: P = 0.045; Butyrate: P = 0.043; Total SCFAs: P = 0.039.

Figure 2

Stool pH by race/ethnicity. A: Stool pH across all four racial/ethnic groups; B: Stool pH for African Americans vs those of all other races/ethnicities.

Figure 3

Human gut microbiota analysis from different racial/ethnic groups. The 16S rRNA gene (v1-v3 regions) was sequenced from fecal samples using 454 Jr. sequencing and analyzed using QIIME platform. A: The phylum distribution among different groups was generated by comparing with the GreenGene (gg_otus-4feb2011) data base; B: The ratio of Firmicutes and Bacteroidetes is represented among different racial/ethnic groups. Statistics were performed using unpaired two-tailed t test using Graphpad Prism 4.0 software (^aP < 0.05 vs control).

Figure 4

PICRUSt analysis of 16S rRNA and correlating with experimental butyrate levels. A: Spearman Correlation analysis suggest moderately strong correlation (r = 0.52) prediction of K00929 pathway (butanoate pathway) vs experimental butyrate levels with significant P value (P = 0.023); B: Box plot indicating relative abundance of KO K00929 across ethnic groups (* = mean; + = points beyond 25^th-75^th percentile).