Transmission and persistence of methicillin-resistant Staphylococcus aureus in milk, environment, and workers in dairy cattle farms.

The aim of this study was to determine the presence and persistence of methicillin-resistant Staphylococcus aureus (MRSA) in milk, farm environment, and farmers on 22 dairy cattle farms in Korea during 2008-2009. Genetic relatedness among the MRSA isolates was also investigated. Of 1146 samples examined, 35 of 559 (6.3%) quarter milk samples from 371 cows, four of 86 (4.7%) hand and nose samples from 43 farmers, and 6 of 501 (1.2%) farm environment samples were MRSA positive. Except for three isolates, all MRSA were classified into ST72-spa t324-SCCmec IV with PVL negative, the most predominant clonal type among community-associated MRSA in South Korea. All 35 MRSA-positive milk samples from 19 cows were obtained from a single farm (Farm G) out of 22 (4.5%) farms tested. The farm G was revisited 1 year later and milk samples were collected for examination of MRSA again. Two of six previous MRSA-positive cattle that had been kept on the farm still harbored MRSA genetically identical to MRSA strains, which were isolated from the same farm a year ago. The results of this study provide the evidence of transmission of MRSA among cattle, farm environment, and farmers and also long-term persistence of MRSA in animals.

Macrolide resistance mechanisms and virulence factors in erythromycin-resistant Campylobacter species isolated from chicken and swine feces and carcasses.

Resistance to antimicrobials was measured in 73 isolates of Campylobacter jejuni (C. jejuni) and 121 isolates of Campylobacter coli (C. coli) from chicken and swine feces and carcasses in Korea. Both bacterial species showed the highest resistance to (fluoro) quinolones (ciprofloxacin and nalidixic acid) out of the nine antimicrobials tested. Erythromycin resistance was much higher in C. coli (19.0%, 23/121) than in C. jejuni (6.8%, 5/73). The mutation in the 23S rRNA gene was primarily responsible for macrolide resistance in Campylobacter isolates. Several amino acid substitutions in the L4 and L22 ribosomal proteins may play a role in the mechanism of resistance, but the role requires further evaluation. A total of eight virulence genes were detected in 28 erythromycin-resistant Campylobacter isolates. All C. jejuni isolates carried more than four such genes, while C. coli isolates carried fewer than three such genes. The high rate of resistance highlights the need to employ more prudent use of critically important antimicrobials, such as fluoroquinolones and macrolides, in swine and poultry production, and to more carefully monitor antimicrobial resistance in Campylobacter isolates in food animals.

Prevalence and characterization of apramycin-resistant Salmonella enterica serotype Typhimurium isolated from healthy and diseased pigs in Korea during 1998 through 2009.

Apramycin resistance was observed in 22.8% (81 of 355) of Salmonella Typhimurium isolates collected from pigs from 1998 through 2009 in Korea. All apramycin-resistant Salmonella Typhimurium isolates also were cross-resistant to gentamicin and tobramycin. Among the seven types of aminoglycoside resistance genes tested, only four types were detected in the apramycin-resistant Salmonella Typhimurium isolates: aac (3)-IV, aac (3)-II, aac (3)-III, and ant (2'')-I. Although the aac (3)-IV gene was found in all apramycin-resistant Salmonella Typhimurium isolates, aac (3)-II, aac (3)-III, and ant (2'')-I genes were detected in five (6.2%), two (2.5%), and three (3.7%) isolates, respectively. The apramycin-resistant isolates comprised six phage types, of which PT193 (16 of 81 isolates, 19.8%) was most commonly observed. To our knowledge, this is the first report describing characteristics of apramycin-resistant Salmonella Typhimurium isolates in Korea. Further study is warranted to determine whether apramycin use in animals results in cross-resistance to gentamicin, which may affect public health when gentamicin is required for disease treatment in humans.

The first detection of methicillin-resistant Staphylococcus aureus ST398 in pigs in Korea.

In order to determine the presence of methicillin-resistant Staphylococcus aureus (MRSA) in pigs in Korea, a total of 657 nasal swabs were collected from pigs on 66 different pig farms nationwide during February 2008-May 2009. The prevalences of MRSA positive samples in pigs and farms were 3.2% (21/657) and 22.7% (15/66), respectively. Two different types were found among the 21 MRSA isolates: 17 strains of livestock-associated type (LA; ST398 or ST541/spa t034) and 4 strains of human-associated type (HA; ST72/spa t664 or t2461). The most prevalent type of MRSA strain was ST398/t034 (12/21, 57%), followed by ST541/t034 (5/21, 23.8%). The rest of the isolates were ST72/t664 (n=2) and ST72/t2461 (n=2), respectively. Our data provide evidence for the existence of not only LA types (ST398 and ST541) but also HA type (ST72) MRSA in pigs in Korea. This survey provides the first evidence of LA type MRSA in animals in Korea. In addition, the presence of human MRSA clones in pigs observed in this study suggests an additional reservoir for human MRSA infection, and vice versa.

Detection and characterization of aparmycin-resistant Escherichia coli from humans in Korea.

To investigate apramycin resistance in humans in Korea, a total of 138 human Escherichia coli strains confirmed as gentamicin-resistant were collected from Korean Culture Collection Antimicrobial-Resistant Microbes. Apramycin resistance (minimum inhibitory concentrations ≥1,024 µg/ml) was observed in 16 (11.6%) of the 138 gentamicin-resistant E. coli (GREC) strains. Among the seven different kinds of aminoglycoside resistance genes tested, only four kinds were detected in the apramycin-resistant GREC strains: aac (3)-II, aac (3)-III, aac (3)-IV, and armA. The aac (3)-IV gene was found in all apramycin-resistant GREC strains, whereas aac(3)-II, aac(3)-III, and armA genes were detected in 8 (50.0%), 6 (37.5%), and 1 (6.3%) GREC strains resistant to apramycin, respectively. Of 16 apramycin-resistant GREC strains, transfer of apramycin resistance was observed in seven (43.8%), and co-transfer of resistance to other antimicrobials along with apramycin resistance was also found in four strains (25.0%) by broth mating. The results of this study suggest that more prudential use of apramycin in animals is needed.