Comparative Analysis of Chloramphenicol-Resistant Enterococcus faecalis Isolated from Dairy Companies in Korea

Although chloramphenicol is currently banned from use in livestock, other phenicols, such as florfenicol and thiamphenicol, have been used for the treatment of bacterial infections in domestic cattle in Korea. This study compares the characteristics of chloramphenicol-resistant Enterococcus faecalis isolated from the bulk tank milk of four major dairy companies in Korea. Although the distribution of multidrug resistance patterns showed no significant differences between the four companies, 85 chloramphenicol-resistant Enterococcus faecalis isolates showed a significantly high number of resistances against five or six antimicrobial classes (37.6%, respectively) (p < 0.05). When analyzing the distribution of phenicol resistance genes, 31 (36.5%) isolates only carried the catA gene, and two (6.3%) isolates from company A only carried the cfr gene. No isolates carried the catB or fexA genes. Regarding the distribution of other resistance genes, both the tetL and tetM (45.9%), ermB (82.4%), and both aac(6 '')-Ie-aph(2 '')-la and ant(6 ')-Ia genes (30.6%) showed a high prevalence, and the optrA and poxtA genes were observed separately, each in only two (2.4%) isolates. Our results confirm that the dissemination of chloramphenicol-resistant Enterococcus faecalis and some antimicrobial resistance genes show significant differences between dairy companies. Therefore, our results support that each dairy company should undertake effective surveillance programs to better understand and minimize the emergence of resistance on a multidisciplinary level.