Comparison of cultural media for the enumeration of sourdough lactic acid bacteria

An accurate enumeration of lactobacilli in sourdough is sometimes difficult to achieve because of bacterial cell stress due to acidity or the presence of fastidious species, such as Lactobacillus sanfranciscensis, as happens for sweet baked products in which this species predominates. Fifty seven sourdough samples of Panettone, Pandoro, and Cornetto brioche from six different industrial bakeries were analysed using three different cultural media, and a comparison of the results was made. The highest lactobacilli counts (a significant P < 0.001) were found with modified SanFrancisco medium, with a mean log count of 8.409 CFU/g, followed by the SDB medium with 7.079 CFU/g. The MRS medium proved unsuitable as no colonies were detected in 28% of the cases, not even at low dilutions (log count < 3 CFU/g), the other samples had a mean log count of 5.426 CFU/g. The 16S rDNA sequencing of some isolates, collected from the highest dilutions, identified and confirmed the prevalence of Lactobacillus sanfranciscensis in this kind of sourdough. Furthermore, the mean counts in the “mother” samples (8.611 log CFU/g) were significantly different (P < 0.001) from those of the “refreshment” samples (7.948 log CFU/g), whereas there was very little difference (P = 0.118) in the mean counts of the samples from the different bakeries.