Animal rennets as sources of dairy lactic acid bacteria

Abstract

The microbial composition of artisan and industrial animal rennet pastes was studied by using both culture-dependent and -independent approaches. Pyrosequencing targeting the 16S rRNA gene allowed to identify 361 operational taxonomic units (OTUs) to the genus/species level. Among lactic acid bacteria (LAB), Streptococcus thermophilus and some lactobacilli, mainly Lactobacillus crispatus and Lactobacillus reuteri, were the most abundant species, with differences among the samples. Twelve groups of microorganisms were targeted by viable plate counts revealing a dominance of mesophilic cocci. All rennets were able to acidify ultrahigh-temperature-processed (UHT) milk as shown by pH and total titratable acidity (TTA). Presumptive LAB isolated at the highest dilutions of acidified milks were phenotypically characterized, grouped, differentiated at the strain level by randomly amplified polymorphic DNA (RAPD)-PCR analysis, and subjected to 16S rRNA gene sequencing. Only 18 strains were clearly identified at the species level, as Enterococcus casseliflavus, Enterococcus faecium, Enterococcus faecalis, Enterococcus lactis, Lactobacillus delbrueckii, and Streptococcus thermophilus, while the other strains, all belonging to the genus Enterococcus, could not be allotted into any previously described species. The phylogenetic analysis showed that these strains might represent different unknown species. All strains were evaluated for their dairy technological performances. All isolates produced diacetyl, and 10 of them produced a rapid pH drop in milk, but only 3 isolates were also autolytic. This work showed that animal rennet pastes can be sources of LAB, mainly enterococci, that might contribute to the microbial diversity associated with dairy productions.

FIG 1

Scheme of the analysis performed on rennet samples and LAB.

FIG 2

Relative abundances (%) of bacterial families identified by pyrosequencing in animal rennet pastes. Only families occurring at >1% abundance in at least one sample were included.

FIG 3

Distribution of bacterial genera and species as identified by pyrosequencing in animal rennet pastes. Only OTUs belonging to the lactic acid bacteria group and occurring at >0.1% abundance in at least one sample were included. The color scale indicates the relative abundance of each OTU within the samples.

FIG 4

Evolution of milk pH determined by the fastest acidifier rennet LAB. Bars represent standard deviations of the means. Tests were performed in duplicate. Vertical bars not visible are smaller than the symbol.

FIG 5

Autolysis of the fastest acidifier rennet LAB. Bars represent standard deviations of the means. Tests were performed in duplicate. Vertical bars not visible are smaller than the symbol.