Abstract:
Streptococcus thermophilus is a lactic acid bacterium used in the production of large types of
fermented dairy products such as yogurt and cheeses. According to the most recent
literature (Li et al., 2019; Sirichoat et al., 2020), this thermophilic species may be resistant
to antibiotics and cause potential transmission of antibiotic resistance to pathogenic
microorganisms of the gastrointestinal tract. A high risk of dissemination of resistance is also
promoted by the presence of mobile genetic elements (e.g. Plasmids), which are vectors
with high transfer capacity. The research papers of Tosi et al., (2006) and Mayo & Florez,
(2017) presented as part of this master’s work have reported in different strains of S.
thermophilus the phenomenon of resistance to antibiotics including: tetracycline,
erythromycin, streptomycin, clindamycin and neomycin. Tosi et al., (2006) evaluated the
antibiotic resistance of the studied S. .thermophilus by means of know methods namely:
micro-dilution, E-test and diffusion in agar by the disk method. The first two methods are
based on the determination of the minimum inhibitory concentration (MIC). The microdilution
method has been shown to be more reliable than the E-test method. However, the
third method, does not give the MICs, but is easier to apply and allows distinguishing
resistant isolates, by testing several antibiotics at the same time. Molecular related studies
are concerned with definition of the type and genetic localization of the resistance observed
in candidate strains for human and animal use, in order to assess their safety status. In this
regard, the work performed by Florez and Mayo, (2017) have led to the identification of
resistance genes encoded by nucleotide sequences (contigs) showing a large homology with
sequences derived from some conjugative plasmids (i.e. pRE25, pSM19035). Furthermore,
the ICEs (integrative and conjugative elements) some of which carry genes for resistance to
erythromycin identified in some isolates have functional modules involved in maintenance
and dissemination process. Lastly, under conditions similar to manufacture and storage of
yogurt-like fermented milks no transfer of resistance genes from S. thermophilus isolates to
Lactobacillus delbrueckii has been observed.
