Flavor is an important sensory property of fermented food products, including cheese, and largely results from the production of aroma compounds by microorganisms. Propionibacterium freudenreichii is the most widely used species of dairy propionibacteria; it has been implicated in the production of a wide variety of aroma compounds through multiple metabolic pathways and is associated with the flavor of Swiss cheese. However, the ability of other dairy propionibacteria to produce aroma compounds has not been characterized. This study sought to elucidate the effect of interspecies and intraspecies diversity of dairy propionibacteria on the production of aroma compounds in a cheese context. A total of 76 strains of Propionibacterium freudenreichii, Propionibacterium jensenii, Propionibacterium thoenii, and Propionibacterium acidipropionici were grown for 15days in pure culture in a rich medium derived from cheese curd. In addition, one strain each of two phylogenetically related non-dairy propionibacteria, Propionibacterium cyclohexanicum and Propionibacterium microaerophilum were included. Aroma compounds were analyzed using headspace trap-gas chromatography-mass spectrometry (GC–MS). An analysis of variance performed on GC–MS data showed that the abundance of 36 out of the 45 aroma compounds detected showed significant differences between the cultures. A principal component analysis (PCA) was performed for these 36 compounds. The first two axes of the PCA, accounting for 60% of the variability between cultures, separated P. freudenreichii strains from P. acidipropionici strains and also differentiated P. freudenreichii strains from each other. P. freudenreichii strains were associated with greater concentrations of a variety of compounds, including free fatty acids from lipolysis, ethyl esters derived from these acids, and branched-chain acids and alcohols from amino acid catabolism. P. acidipropionici strains produced less of these compounds but more sulfur-containing compounds from methionine catabolism. Meanwhile, branched-chain aldehydes and benzaldehyde were positively associated with certain strains of P. jensenii and P. thoenii. Moreover, the production of compounds with a common origin was correlated. Compound abundance varied significantly by strain, with fold changes between strains of the same species as high as in the order of 500 for a single compound. This suggests that the diversity of dairy propionibacteria can be exploited to modulate the flavor of mild cheeses.

In addition to their use in cheese technology, dairy propionibacteria have been identified as potential probiotics. However, to have a probiotic effect, propionibacteria have to survive and to remain metabolically active in the digestive tract. The aim of the present study was to investigate the survival and metabolic activity of Propionibacterium freudenreichii within the gastrointestinal tract of human microbiota-associated rats, and its influence on intestinal microbiota composition and metabolism. Twenty-five dairy Propionibacterium strains were screened for their tolerance towards digestive stresses and their ability to produce propionate in a medium mimicking the content of the human colon. Three strains were selected and a daily dose of 2 x 10(10) colony-forming units was fed to groups of human microbiota-associated rats for 20 d before microbiological, biochemical and molecular investigations being carried out. These strains all reached 8-log values per g faeces, showing their ability to survive in the gastrointestinal tract. Transcriptional activity within the intestine was demonstrated by the presence of P. freudenreichii-specific transcarboxylase mRNA. The probiotic efficacy of propionibacteria was yet species- and strain-dependent. Indeed, two of the strains, namely TL133 and TL1348, altered the faecal microbiota composition, TL133 also increasing the caecal concentration of acetate, propionate and butyrate, while the third strain, TL3, did not have similar effects. Such alterations may have an impact on gut health and will thus be taken into consideration for further in vivo investigations on probiotic potentialities of P. freudenreichii.

Propionibacteria are cheese starters, also studied for their probiotic potential. An interesting feature is the ability of some strains to synthesize nitric oxide (NO) from nitrate reduction. NO production is hypothesized to be the mechanism by which some probiotics improve the mucosal barrier in rats. We therefore investigated the ability of two Propionibacterium acidipropionici strains (TL15 P TL223), which differ in their in vitro production of NO, to improve colitis in rats. Three groups of rats in which colitis was induced by trinitrobenzene sulfonic acid (TNBS), received intracolonic infusions of 9 g.L-1 NaCl (n=6), or 0.9x10(10) CFU.d(-1) of TL15 (n=7), or 1.4x10(10) CFU.d(-1) of TL223 (n=7), for 7 days. Following treatment, animal weight, food consumption, inflammatory score and myeloperoxydase (MPO) activity, together with changes in intestinal bacteria and SCFA concentrations, were investigated. As expected, both TL223 and TL15 supplements induced detectable counts of P. acidipropionici in colonic contents of rats (7.67 +/- 0.67 and 7.43 +/- 0.50 log eq. CFU.g(-1), respectively, as determined by real-time PCR). Food consumption and body weight of rats receiving propionibacteria were overall higher than those of rats with NaCl (P=0.007 and P=0.004). Moreover, both propionibacteria infusions similarly and dramatically reduced the gross score for inflammation (P=0.004) as compared with NaCl (0.6 +/- 0.3 and 0.7 +/- 0.5 vs. 4.8 +/- 1.5). A similar effect was observed for MPO activity (P=0.056), which reached 2.50 +/- 1.37 U.g(-1) of mucosal tissue for NaCl vs. 0.05 +/- 0.02 and 0.26 +/- 0.18 U.g(-1) for TL223 and TL15, respectively. This study shows for the first time the potential benefit of propionibacteria in colonic mucosa healing, but the mechanism involved needs to be elucidated.

Propionibacteria, which are largely used as Swiss-type cheese starters, also constitute promising human probiotics because of their beneficial effects on intestinal bifidobacteria and bacterial enzyme activities. To be efficient, they must survive acid stress imposed within the stomach and the surfactant effects of bile salts. We thus investigated the ability of Propionibacterium freudenreichii (strain Propiofidus® SI41) to adapt to, and survive these stresses. In an in vivo study, 7 volunteers received Propiofidus® SI41 in 3 different modes. The probiotic was supplied during 3 periods (i) in classical capsules at a low dose (5 × 109 CFU·d-1), (ii) in the same capsules at a higher dose (5 × 1010 CFU·d-1), (iii) in acid-resistant capsules at the low dose. In the first case, propionibacteria were recovered above 105 CFU·g-1 in only 8 out of 14 faecal samples. In the two following cases, they were enumerated in all faecal samples between 105 and 107 CFU·g-1. The recovery of live propionibacteria in faecal samples seemed to be related to a higher content in short chain fatty acids. This study, which clearly showed the role of gastric protection in the survival of propionibacteria, led us to investigate stress adaptation in this strain. Acid tolerance response was evidenced: transient exposure to pH 5 afforded protection towards acid challenge at pH 2. Moreover, we showed that pre-exposure to a moderate bile salts pre-treatment at 0.2 g·L-1 sharply increased its survival against a subsequent lethal challenge (1 g·L-1). Important changes in cellular protein synthesis were observed during adaptation using two-dimensional electrophoresis. Global protein synthesis decreased during adaptation treatments. In contrast, specific proteins were overexpressed during acid adaptation and others during bile salts adaptation. Among the up-regulated polypeptides, a carboxyl carrier protein (BCCP) and the recombinase RecR were identified during both stress responses while the universal chaperonins GroEL and GroES, as well as the replicase RepB, were only up-regulated during acid adaptation. These results bring new insights on the tolerance of P. freudenreichii to acid and bile salts, which should be taken into consideration for the development of probiotic preparations.

The influence of dietary fiber on xenobî- otic-metabolizing enzymes (XME) was assessed using germ-free rats fed inulin and other sources of fiber (wheat bran, carrot, cocoa and oat). The consumption of cocoa fiber greatly modified the hepatic cytochrome P450 isoenzymatic profile, causing a strong enhance ment of 1A2 and 2B1/B2 forms, concomitant with a significant decrease of the constitutive form 2C11, compared with all of the other types of fiber. Moreover, rats fed the cocoa fiber diet had a higher specific activ ity of hepatic UDP-glucuronosyl transferase than their carrot fiber- and wheat bran-fed counterparts. Intesti nal UDP-glucuronosyl transferase was unaffected by the type of ingested fiber. Diet composition also did not alter the specific activity of glutathione-S-trans- ferase in the liver, small intestine, or colon. Using ear lier results obtained in heteroxenic rats, we show that intestinal microflora plays a key role in some of the effects of fiber on XME, although this is not a neces sary prerequisite for all of the liver alterations. J. Nutr. 126: 403-409, 1996.