Aumento de resistencia de Streptococcus agalactiae vaginal-anal en el tercer trimestre de gestación a eritromicina y clindamicina al cabo de una década de tamizaje universal Fernando Abarzúa C., Alejandra Arias E., Patricia García C., Constanza Ralph T., Jaime Cerda L., Ingrid Riedel K. y Cynthia Gárate O.Streptococcus agalactiae increase in resistance to erythromycin and clindamycin in vaginal-anal colonization in third quarter of pregnancy in one decade of universal screening Introduction: Streptococcus agalactiae (GBS) is the main causative agent of early perinatal sepsis. The acquisition of prevention policies has led to frequent use of intrapartum antibiotics. Surveillance of antimicrobial resistance is indispensable for defi ning drugs of choice and alternatives for such prophylaxis. Objectives: To determine the evolution of antimicrobial resistance of GBS from maternal colonization to drugs used in the prevention of neonatal sepsis, between 2002 and 2008. Methods: We studied 100 GBS positive vaginal and anal samples from pregnant women. Disc diffussion susceptibility method was performed for penicillin, ampicillin, cefazolin, erythromycin and clindamycin according to the Clinical and Laboratory Standards Institute (CLSI). Results: We analyzed the susceptibility of 99 strains. Seventeen were resistant to erythromycin (17.1%) and 13 were resistant to clindamycin (13.1%). Thirteen of the 17 strains resistant to erythromycin had the MLS phenotype (resistance to erythromycin and clindamycin) and 4 had the M phenotype (resistance to erythromycin only). Within the MLS phenotype, resistance was constitutive in 9 strains, and induced in 4 strains (positive D test). Compared with 2002 there was a signifi cant increase in resistance to clindamycin (from 3.27% to 13.1% p < 0.002) and erythromycin (1.09% to 17% p < 0.001). 100% GSB remained sensitive to penicillin and ampicillin. Conclusions: GBS remains highly susceptible to drugs of choice for prevention of perinatal sepsis. There is a signifi cant increase in antimicrobial resistance to clindamycin and erythromycin. Therefore, it is necessary to request susceptibility testing in GBS from third trimester of pregnancy screening in patients allergic to penicillin.