We consider L. ruminis as a candidate probiotic, which we are also investigating as a potential responder for prebiotic/symbiotic supplementation in humans and animals. Several studies have identified L. ruminis in the gastrointestinal tract of humans [35–37]. L. ruminis was isolated from the bovine rumen , from the pig [4, 8], chickens , sheep , Svalbard reindeer , horses [41–43], cats [44, 45], dogs  and parrots . L. ruminis thus appears to be variably present in the microbiota of humans and many domesticated animals.
L. ruminis was previously described as a homofermentative bacterium, with the ability to ferment amygdalin, cellobiose, galactose, maltose, mannose, melibiose, raffinose, salicin, sorbitol and sucrose . In the current study, the nine strains of L. ruminis were unable to utilise sorbitol as a carbon source. L. ruminis has also been reported to have the ability to ferment D-ribose . However, we observed no growth for any of the nine L. ruminis strains when cultured in cfMRS supplemented with ribose. ATCC 27782 lacks a transaldolase gene (and the draft genome sequence suggests ATCC 25644 also lacks this gene), which would account for inability to utilise any of the pentose sugars tested. All of the L. ruminis strains tested (with the exception of ATCC 27782 which lacks a lacZ gene) had strong growth in lactose. This contrasts with a previous study, where moderate growth was recorded on lactose . It has also been reported that L. ruminis showed a strain dependent fermentation of starch , and very little growth was recorded for any of the strains tested here.
As a species, L. ruminis is generally able to ferment prebiotic compounds including FOS, GOS, lactulose, 1,3:1,4 β-D-Glucooligosaccharides, raffinose and stachyose. Only one strain, S36 was capable of (weakly) fermenting the prebiotic disaccharide palatinose. Palatinose is made by enzymatic rearrangement of the glycosidic linkages present in sucrose from an α-1,2-fructoside to an α-1,6-fructoside . This suggests that the catalytic enzymes involved in sucrose utilisation may no longer be able to degrade the α-1,6-fructoside linkage in this disaccharide. The majority of L. ruminis strains achieved higher cell densities when grown on the prebiotic carbohydrates raffinose, lactulose, FOS, GOS and stachyose than when grown in other mono- and disaccharide carbohydrates tested. This growth pattern may be attributed to a niche for L. ruminis in the lower gastrointestinal tract (GIT). Mono and disaccharides are often unable to resist the hydrolytic action of the upper GIT, unlike prebiotics, and would not therefore be as freely available as carbon sources for L. ruminis in the large intestine. Lactulose, a disaccharide derivative of lactose, has previously been shown to support high level growth of other lactobacilli namely L. rhamnosus, L. paracasei and L. salivarius. Lactulose also supported a high level of growth for the majority of L. ruminis strains. The β-galactosides lactulose and GOS are predicted to be transported and hydrolysed in ATCC 25644 by LacY and LacZ as part of the lactose operon. Two operons for β-galactoside utilisation were identified in the genome of ATCC 25644; however neither of these operons or any potential genetic determinants could be identified for lactose utilisation in ATCC 27782. The absence of a lactose operon in the genome may suggest an ecological niche adaptation by ATCC 27782 to an environment devoid of milk sugars.
β-glucooligosaccharides such as cellobiose are generally transported and hydrolysed using the cellobiose PTS and β-glucosidase enzymes. Both cellobiose and β-glucotriose B are 1,4-β-D-glucooligosaccharides with a similar structure which allows the transport and utilisation of these carbohydrates by the products of the cellobiose operon. The bovine L. ruminis isolates, ATCC 27780T, 27781 and 27782 were previously reported to utilise β-glucan hydrolysates as a carbohydrate source , and in that study, all bovine isolates utilised β-glucan hydrolysates of DP3, and only ATCC 27780T was unable to utilise DP4 oligosaccharide. ATCC 27781 was distinguished by being able to utilise the highest percentage of both DP3 and DP4 β glucan. We have shown that all the strains tested in this study were able to utilise the DP3 β-glucan hydrolysates to a moderate degree. The bovine isolate ATCC 27780T achieved the highest growth (data not shown) when utilizing β glucan hydrolysate, in contrast to a previous study which identified ATCC 27781 as having the highest percentage utilisation of β-glucan oligosaccharide .
In previous analysis of sixteen Lactobacillus species, only L. acidophilus L3, L. acidophilus 74-2 and L. casei CRL431 were able to utilise Raftilose P95, an oligofructose . In the current study, eight strains of L. ruminis were capable of utilizing Raftilose P95. In addition, L. ruminis was capable of moderate to strong fermentation of Raftilose Synergy 1, an oligofructose-enriched inulin. L. paracasei subsp. paracasei 8700:2 was previously shown to be the only strain, out of ten strains tested, that was capable of strong growth on Raftilose Synergy 1, while three other species were capable of moderate growth . Based on these comparisons, L. ruminis may have a growth advantage over other lactobacilli in the presence of fructooligosaccharides.
A novel β-fructofuranosidase was identified in the genome of L. ruminis ATCC 25644 that potentially hydrolyses the linkages present in chicory derived fructooligosaccharides. The cognate transporter OHS was identified only in the strains isolated from humans. Transport of FOS may be transported using the sucrose PTS transporter in the bovine strains ATCC 27780 and 27781. The human isolates of L. ruminis apparently use an OHS to transport FOS into the cell. Both sequenced strains likely use the ABC transport system to transport simple carbohydrates like maltose and glycerol. The most populated class of transporter identified was the phosphotransferase system transporter, with six such systems present. However, in L. ruminis many of the fermentable carbohydrates including α-galactosides and β-galactosides are predicted to be transported by GPH symporters. GPH transporters contain a C-terminal hydrophilic domain which interacts with the PTS system , which may thus be an important regulatory mechanism in L. ruminis.