Submitted to: Journal of Food Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2014
Publication Date: N/A
Interpretive Summary: The food-grade lactic fermentation bacterium Streptococcus thermophilus is an essential starter culture in the industrial production of fermented dairy foods (cheeses, yogurt). The harmless bacteria are consumed in large quantities by humans as components of the finished food products. Since this beneficial species is closely related to pathogenic streptococci that may become resistant to antibiotics and transfer their resistance genes, contact of food-grade streptococci with clinical isolates of streptococci must be avoided. In research on the genetic development of S. thermophilus we discovered a variant of this bacterium that displayed resistance to antibiotics of the macrolide-lincosamide-streptogramin (MLS) group. Since several of these antibiotics have medical applications (erythromycin, tylosin), the origin of resistance in S. thermophilus was investigated in detail. Work established the absence of mobile genetic elements that may transfer resistance to the dairy fermentation bacterium. Analysis of the sites of protein synthesis (ribosomes) in the starter culture uncovered three types of spontaneous mutations that were responsible for resistance to MLS family of antibiotics. The research demonstrated for the first time that dairy streptococci may spontaneously develop resistance to antibiotics as the result of specific mutations that lead to alterations in the ribosomal architecture instead of developing resistance through the transfer of resistance genes from potentially pathogenic species of streptococci.
Technical Abstract: Resistance to macrolide-lincosamide-streptogramin (MLS) group antibiotics in the dairy bacterium Streptococcus thermophilus (ST) is documented but the mechanism of resistance has not been elucidated. MIC values for erythromycin (Erm), azithromycin (Azm), tylosin (Tyl), spiramycin (Spm), pristinamycin (Prm) and virginiamycin S (VirS) were determined by the disk diffusion method. PCR products were obtained with primer pairs for the L4, L22 and 23S rDNA (domain V) genes. The sequencing results ruled out mutations in the L4 and L22 ribosomal proteins and the presence of rRNA methylase, efflux, and inactivating genes. However, sequencing of domain V in each of the six ribosomal alleles detected by EcoRI/I-CeuI digestion in ST mutants identified three types of mutations that led to MLS resistance. Type A mutants, induced by Erm, had high resistance to 14- and 15-membered ring macrolides (Erm, Azm) and streptogramin B antibiotics (Prm, VirS), moderate resistance to 16-membered ring macrolides (Tyl, Spm), but remained susceptible to lincomycin. In Type B mutants, also induced by Erm, resistance was high to Erm, Tyl and Spm, and moderate to lincomycin but sensitivity was retained to Prm. Type C mutants, induced by Prm, showed high resistance to 16-membered ring macrolides but remained sensitive to Erm, Azm and lincomycin. The three identifiable resistance patterns were apparently due to point mutations in domain V of the 23S rRNA gene, resulting in three phenotypes among resistant S. thermophilus isolates. Type A phenotype mutants had a C2611G mutation in five of the six ribosomal alleles, Type B phenotypes had a A2058G mutation in five alleles, and Type C variants had a A2062C mutation in all six alleles. Resistance to MLS antibiotics in S. thermophilus was inducible by 14- and 15-membered ring macrolides and streptogramin B type antibiotics but not by 16-membered ring macrolides or lincosamides.