A reader wrote “it seems to me that Clostridium XIVa is a very crucial bacterial group for improving health. Can you do a blog post on how to increase these bacteria? Thank you.”
For ulcerative colitis (UC), it appears to have benefit
- “we focused on Fusicatenibacter saccharivorans, which belongs to Clostridium subcluster XIVa and was successfully isolated and cultured in 2013…F. saccharivorans decreased in correlation to UC activity and suppresses intestinal inflammation. These results suggest that F. saccharivorans could lead to a novel UC treatment.” 
- Dysbiosis in the Gut Microbiota of Patients with Multiple Sclerosis, with a Striking Depletion of Species Belonging to Clostridia XIVa and IV Clusters .
- Dysbiosis of bifidobacteria and Clostridium cluster XIVa in the cystic fibrosis fecal microbiota . “members of Clostridium clusters XIVa (n=3) and IV (n=2), were significantly (p<0.05) underrepresented in samples of patients with CF.”
- “On the genus level an interesting link between reductions in the proportion of known butyrate producers belonging to Clostridium cluster XIVa, such as Roseburia and Ruminococcus, could be demonstrated in the elderly.” 
Scanning pubmed articles for what is known… often what we know comes from animal studies — which may or may not apply to humans.
- ” the addition of isoflavones, with enrichment of some bacterial gut members associated with the metabolism of phenolics and/or equol production, such as Collinsella, Faecalibacterium and members of the Clostridium clusters IV and XIVa.” 
- “Long-term calorie restriction induced significant shifts..enhancing the growth of beneficial microorganisms such as Bacteroides, Roseburia, Faecalibacterium and Clostridium XIVa. 
- “Vegetarian and fibre-rich diets have been shown to induce beneficial changes on gut microbiota in healthy people, with reduction of Bacteroides spp., Enterobacteriaceae, and Clostridium cluster XIVa bacteria.” 
- “Vegetarian diet…with lower ratio (%) of Clostridium cluster XIVa.” 
- “a strict vegetarian diet (SVD) for 1 month... an increase in commensal microbes such as Bacteroides fragilis and Clostridium species belonging to clusters XIVa and IV,” 
- “polyphenol-rich plant products… lower counts of Streptococcus spp. and Clostridium Cluster XIVa in the faecal microbiota (P < 0.05).” 
- “High Amylose starch…Clostridium clusters IV and XIVa were decreased (P < 0.01) in pigs” 
- Fasting has no impact .
- “Of particular interest, Clostridium cluster XIVa species were significantly reduced… by high-fat sucrose diet” 
- “bile acid therapy[tauroursodeoxycholic acid (TUDCA), or glycoursodeoxycholic acid (GUDCA)] normalized the colitis-associated increased ratio of Firmicutes to Bacteroidetes Interestingly, administration of bile acids prevented the loss of Clostridium cluster XIVa and increased the abundance of Akkermansia muciniphila, bacterial species known to be particularly decreased in IBD patients.” 
- “Increasing levels of the primary bile acid cholic acid (CA) causes a dramatic shift toward the Firmicutes, particularly Clostridium cluster XIVa” 
- “The alfalfa diet increased (P < 0.05) Clostridium cluster XIVa abundance,” 
- “We found that the concentration of the Clostridium coccoides group (Clostridium cluster XIVa), the Clostridium leptum subgroup (Clostridium cluster IV) and the Bacteroides fragilis group had significantly increased in partially hydrolysed guar gum-fed mice.” 
- “Xylo-oligosaccharides…significantly increased numbers of lactobacilli in the colon and Clostridium cluster XIVa in the ceca.” 
- “resistant starch, a fiber that was not completely fermented in the colon, whereas the other fibers induced similar responses on gene expression and microbiota. Consistent associations were revealed between fiber-induced enrichment of Clostridium cluster IV and XIVa representatives” 
- “a diet enriched in 5% acidic oligosaccharides derived from pectin (pAOS)…. stimulating the growth of species involved in immunity development, such as Bifidobacterium species, Sutturella wadsworthia, and Clostridium cluster XIVa organisms, and at the same time increased the production of butyrate and propionate.” 
- “Administration of either 0.33 or 3.3% apple pectin in the diet….became more prominent represented mainly Gram-positive anaerobic rods belonging to the phylum Firmicutes, and specific species belonging to the Clostridium Cluster XIVa.” 
- “Wheat dextrin mediated a significant increase in total bacteria in vessels simulating the transverse and distal colon, and a significant increase in key butyrate-producing bacteria Clostridium cluster XIVa and Roseburia genus in all vessels of the gut model.” 
- “Clostridium Cluster XIVa were higher (P < 0.001) in the casein-based diets” 
- “The faecal microbiota of the omnivorous group was enriched with Clostridium cluster XIVa bacteria, specifically Roseburia-E. rectale.” 
- “Especially the Clostridium cluster IV and subcluster XIVa were particularly increased in group B,[fed a probiotic mixture of: Lactobacillus delbrueckii subsp. bulgaricus, Bifidobacterium bifidum, Enterococcus faecium, Candida pintolopesii and Aspergillus oryzae 
- “pro-biotic yogurt containing Bifidobacterium animalis subsp. lactis LKM512… yogurt consumption, in particular, the bacterial species and phylotypes of Bifidobacterium, Clostridium cluster IV and subcluster XIVa were increased in number.” 
It appears that bifidobacterium probiotics will likely increase the level. Apple pectin becoming a regular diet additive is also suggested. Increasing/supplementing bile acids also appears to be well supported. Looking at Jarrow Bile Acid Factors ($20 for a month on Amazon), we see the acids cited in the above study.