The replacement of glucose by sorbitol in MRS agar should
inhibit the development of the lactic acid bacteria, since these
microorganisms cannot ferment sorbitol, such as S. thermophilus
strains (Hardie & Whiley, 2006). However, this behavior was not
verified in this study, since pure cultures of Lactococcus lactis, L.
bulgaricus and S. thermophilus have developed in the culture medium
containing sorbitol at 37 C (L. lactis) or 45 C (yoghurt cultures).
Possibly the unusual growth of these strains in the presence
of sorbitol could be explained by culture rotation, which is a
technique performed by several industrial manufacturers of cultures
in order to control bacteriophages. The inhibition of Lactococcus
lactis in MRSS agar incubated at 45 C suggests that the
incubation temperature was responsible for the lack of development
in this medium as Lactococcus lactis does not grow at 45 C
(Nomura, Kobayashi, Narita, Kimoto-Nira, & Okamoto, 2006;
Teuber & Geis, 2006). The incubation temperature was also
responsible for the inhibition of L. acidophilus in M17 agar at 30 C
from cheeses (Table 4), since this microorganism has the optimum
growth temperature between 35 and 40 C (Hammes & Hertel,
2006; IDF, 1995). The selectivity of the M17 agar is also related to
the inhibitory effect of sodium glycerophosphate on different lactobacilli
strains (Shankar & Davies, 1977). This inhibitory effect was
confirmed by the lower counts of pure cultures of L. acidophilus in
M17 incubated at 37 C (Table 4) or 45 C (Table 3), and L. bulgaricus
in M17 incubated at 45 C (Table 3).
The replacement of glucose by sorbitol in MRS agar shouldinhibit the development of the lactic acid bacteria, since thesemicroorganisms cannot ferment sorbitol, such as S. thermophilusstrains (Hardie & Whiley, 2006). However, this behavior was notverified in this study, since pure cultures of Lactococcus lactis, L.bulgaricus and S. thermophilus have developed in the culture mediumcontaining sorbitol at 37 C (L. lactis) or 45 C (yoghurt cultures).Possibly the unusual growth of these strains in the presenceof sorbitol could be explained by culture rotation, which is atechnique performed by several industrial manufacturers of culturesin order to control bacteriophages. The inhibition of Lactococcuslactis in MRSS agar incubated at 45 C suggests that theincubation temperature was responsible for the lack of developmentin this medium as Lactococcus lactis does not grow at 45 C(Nomura, Kobayashi, Narita, Kimoto-Nira, & Okamoto, 2006;Teuber & Geis, 2006). The incubation temperature was alsoresponsible for the inhibition of L. acidophilus in M17 agar at 30 Cfrom cheeses (Table 4), since this microorganism has the optimumgrowth temperature between 35 and 40 C (Hammes & Hertel,2006; IDF, 1995). The selectivity of the M17 agar is also related tothe inhibitory effect of sodium glycerophosphate on different lactobacillistrains (Shankar & Davies, 1977). This inhibitory effect wasconfirmed by the lower counts of pure cultures of L. acidophilus inM17 incubated at 37 C (Table 4) or 45 C (Table 3), and L. bulgaricusin M17 incubated at 45 C (Table 3).
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