Abstract
<p>Resistance to macrolide-lincosamide-streptogramin (MLS) group antibiotics in the dairy bacterium <em>Streptococcus thermophilus</em> (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 (Vir) 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 <em>Eco</em>RI/I-<em>Ceu</em>I digestion in ST mutants identified three types of mutations that led to MLS resistance. Type<strong> </strong>A mutants, induced by Erm, had high resistance to 14- and 15-membered ring macrolides (Erm, Azm) and streptogramin B antibiotics (Prm, Vir), 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 <em>S. thermophilus</em> 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 <em>S. thermophilus</em> was inducible by 14- and 15-membered ring macrolides and streptogramin B type antibiotics but not by 16-membered ring macrolides or lincosamides.</p>
Highlights
Streptococcus thermophilus is a widely used food-grade lactic acid bacterium responsible for carrying out essential biocatalytic functions in the industrial production of yogurt and other fermented dairy foods, including Italian and Swiss style cheeses
Since dairy fermentations provide a suitable environment for the growth of pathogenic bacteria, there has been a persistent concern about the possible interchange of antibiotic resistance genes between S. thermophilus and pathogens that may proliferate in the human gastrointestinal tract
Based on the results of these bioassays, the Erm resistant mutants were grouped in 2 categories: Type A mutants were highly resistant to Erm, Prm and VirS, with increased resistance to the 16-membered lactone macrolides Spm and Tyl but remained sensitive to Cli and Lin
Summary
Streptococcus thermophilus is a widely used food-grade lactic acid bacterium responsible for carrying out essential biocatalytic functions in the industrial production of yogurt and other fermented dairy foods, including Italian and Swiss style cheeses. Vol 4, No 1; 2015 resistance to antibiotics of the macrolide-lincosamide-streptogramin (MLS) group in this important industrial species has not been studied at the molecular level. In clinical isolates of streptococci, resistance to macrolide antibiotics is orchestrated predominantly by two well-known resistance mechanisms, target site modification and macrolide-specific efflux. An acquired ermB gene methylates A2058 in 23S rRNA and blocks the binding of macrolides by causing conformational changes in the 50S ribosomal subunit, resulting in cross-resistance to 14-(erythromycin), 15-(azithromycin), and 16-membered (tylosin) macrolides. The mechanism of resistance to MLS antibiotics was elucidated in mutants of S. thermophilus ST113 displaying different phenotypes that were isolated following exposure to macrolide antibiotics
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