In vitro activities of antimicrobials against six important species of gram-negative bacteria isolated from raw milk samples in Korea.

Abstract Gram-negative bacteria (GNB) with multidrug resistance pose a serious threat to public health. They are environmental pathogens frequently isolated from raw milk and mastitis in dairy cattle. This study was to examine the in vitro antimicrobial activities against 225 isolates belonging to six important species of GNB from mastitic raw milk samples of dairy herds in the Republic of Korea: Acinetobacter baumannii (n = 17), Citrobacter freundii (n = 19), Enterobacter cloacae (n = 54), Klebsiella pneumoniae (n = 55), Pseudomonas aeruginosa (n = 45), and Serratia marcescens (n = 35). In general, amikacin, gentamicin, and piperacillin exhibited strong antimicrobial activities against all bacterial species tested, whereas rifampin, cephalothin, cefazolin, and ampicillin were ineffective against most of the bacterial species tested. Wide differences were observed in the patterns of resistance among the bacterial species; in particular, resistance to kanamycin, streptomycin, tetracycline, and chlorampheni...