Efflux-mediated antibiotic resistance in Gram-positive bacteria

Efflux is a widely used mechanism of resistance to several toxic compounds, in both eukaryotic and prokaryotic cells, and it is a well-known mechanism of resistance to some antibiotics such as tetracyclines. Efflux-mediated resistance is being described increasingly for microorganisms in which, and antibiotics for which, it was previously unknown. In the 1990s efflux-mediated quinolone resistance was described in the important Gram-positive pathogens such as Staphylococcus aureus, Streptococcus pneumoniae, Streptococcus agalactiae and enterococci. Efflux-mediated macrolide resistance mechanisms were described during recent years in pneumococci, group A Streptococcus pyogenes and other Gram-positive cocci. In fact, efflux is the main mechanism of resistance to macrolides in Streptococcus pyogenes in many areas. Efflux has also been described as an important mechanism of resistance to tetracyclines in Gram-positive bacteria. In some cases, the increase in the mimimum inhibitory concentration derived from efflux is modest, and its impact on clinical resistance is unclear. In other cases, efflux can lead to clinical resistance by itself, and seems to have some influence on the rate of emergence of further mechanisms leading to higher level resistance.