Antibiotic resistance of lactic acid bacteria and bifidobacteria from dairy and human sources

Abstract

General Introduction In the fight against bacterial infections, the discovery and use of antibiotics is one of the most important achievements of medicine in the 20th century. However, the initial success has been tarnished over time due to the appearance of antibiotic resistances and their spread among pathogens, which hinders the treatment of bacterial infections. The antibiotic resistance is a natural phenomenon linked to evolutionary processes of adaptation of the microbes to their environment. Adaptation might be caused by the selective pressure imposed by the presence of antibiotics in many ecosystems, including human and animal clinical settings, but also in agriculture, animal husbandry and aquaculture. The adverse impact of resistances on the human health increases with the coding of the resistances in mobile genetic elements (plasmids, integrons, transposons, etc.), which promote their horizontal spread to susceptible species within the shared environments.

Lactic acid bacteria (LAB) and bifidobacteria Different LAB and bifidobacteria species are applied as technological agents in food fermentations and used as probiotics for the improvement of human and animal health. Although phylogenetically unrelated, the presence of LAB and bifidobacteria in the same ecosystems and their use for the same purposes has reinforced the study of these bacteria as a single group.

Antibiotic resistance in LAB and bifidobacteria As commensal and beneficial bacteria, antibiotic resistance in LAB and bifidobacteria does not in itself pose a risk, since they do not cause diseases. It is therefore not surprising that research on antibiotic resistance has initially focused on pathogens of clinical relevance. However, the potential exists for commensal populations to become reservoirs of resistance determinants and vehicles for gene transfer from the environment to pathogens. The reservoirs constitute a set of resistance genes (¿pool¿) shared between pathogenic and non-pathogenic organisms. Reducing or eliminating this pool of resistances would also reduce its transmission. The food chain has been identified as a key element in the transmission of resistances due to the high cell densities reached at certain manufacturing time points and the stress conditions in which microorganisms are found. Transfer can occur during food processing and after consumption during intestinal transit. Our group has been involved for the last decades in determining the antibiotic susceptibility/resistance profiles of LAB and bifidobacteria species and strains isolated from cheese and other dairy products, as well as from the human gastrointestinal and genitourinary tracts. The genetic basis of the identified resistances has been characterized by molecular methods, including genome sequencing and analysis. Making use of techniques such as DGGE, qPCR, microarray hybridization and functional metagenomics, we have studied the composition and evolution of the resistant population in cheese. Finally, in vitro and in vivo conjugation systems have been applied to measure the actual risk of transfer of the resistances.