Abstract
BackgroundLactobacillus rhamnosus is a non-starter lactic acid bacterium that plays a significant role during cheese ripening, leading to the formation of flavor. In long-ripened cheeses it persists throughout the whole time of ripening due to its capacity to adapt to changing environmental conditions. The versatile adaptability of L. rhamnosus to different ecosystems has been associated with the capacity to use non-conventional energy sources, regulating different metabolic pathways. However, the molecular mechanisms allowing the growth of L. rhamnosus in the cheese dairy environment are still poorly understood. The aim of the present study was to identify genes potentially contributing to the growth ability of L. rhamnosus PR1019 in cheese-like medium (CB) using a transcriptomic approach, based on cDNA-amplified fragment length polymorphism (cDNA-AFLP) and quantitative real-time reverse transcription-PCR (qPCR).ResultsUsing three primer combinations, a total of 89 and 98 transcript-derived fragments were obtained for L. rhamnosus PR1019 grown in commercial MRS medium and CB, respectively. The cDNA-AFLP results were validated on selected regulated genes by qPCR. In order to investigate the main adaptations to growth in a cheese-mimicking system, we focused on 20 transcripts over-expressed in CB with respect to MRS. It is worth noting the presence of transcripts involved in the degradation of pyruvate and ribose. Pyruvate is a intracellular metabolite that can be produced through different metabolic routes starting from the carbon sources present in cheese, and can be released in the cheese matrix with the starter lysis. Similarly the ribonucleosides released with starter lysis could deliver ribose that represents a fermentable carbohydrate in environments, such as cheese, where free carbohydrates are lacking.Both pyruvate degradation and ribose catabolism induce a metabolite flux toward acetate, coupled with ATP production via acetate kinase. Taking into account these considerations, we suggest that the energy produced through these pathways may concur to explain the great ability of L. rhamnosus PR1019 to grow on CB.ConclusionsBy a transcriptomic approach we identified a set of genes involved in alternative metabolic pathways in L. rhamnosus that could be responsible for L. rhamnosus growth in cheese during ripening.
Highlights
Lactobacillus rhamnosus is a non-starter lactic acid bacterium that plays a significant role during cheese ripening, leading to the formation of flavor
In order to investigate the main adaptations of L. rhamnosus to the Parmigiano Reggiano (PR) cheese environment, we focused on Transcript-derived fragments (TDFs) over-expressed in Cheese Broth (CB)
Identification of TDFs over-expressed in CB Twenty TDFs strongly over-expressed by L. rhamnosus in CB compared to MRS were extracted from gel and used as templates for re-amplification by PCR
Summary
Lactobacillus rhamnosus is a non-starter lactic acid bacterium that plays a significant role during cheese ripening, leading to the formation of flavor. In long-ripened cheeses it persists throughout the whole time of ripening due to its capacity to adapt to changing environmental conditions. L. rhamnosus has been shown to become dominant within NSLAB population in several cheeses, including Parmigiano Reggiano (PR) [8,9]. It persists throughout the whole time of PR cheese ripening (1 to 20 months) and this implies its capacity to adapt to changing environmental conditions [10]. Different L. rhamnosus strains have been detected in relation to specific steps of cheese ripening, suggesting that these strains may have specific metabolic activities, which could account for the adaptation to the changing microenvironment of cheese ripening [11]
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