Methicillin-resistant Staphylococcus aureus (MRSA) is a drug-resistant pathogen causing subclinical and clinical bovine mastitis. This study examined the molecular properties, biofilm formation, virulence genes, and antimicrobial susceptibility of MRSA isolates from mastitis-infected dairy cow milk in South Korea. Whole-genome sequencing of an ST22-SCC mec IV MRSA strain positive for Panton–Valentine leukocidin (PVL) and toxic shock syndrome toxin-1 (TSST-1) was also performed. Of the 488 S . aureus isolates, 30 (6.1%) were identified as MRSA, harboring the mecA gene. MRSA exhibited the highest resistance to kanamycin (66.7%) among non-beta-lactam antibiotics. Multidrug resistance was observed in 83.3% of MRSA isolates. All MRSA strains had the capacity to form biofilm and harbored biofilm-related genes. The primary virulence genes included hla , hlb , lukED , seg , sei , sem , sen , and seo . The ST72-t324-SCC mec IV genotype was the most prevalent. Of note, three ST22-SCC mec IV isolates were positive for PVL and TSST-1. Our findings suggest that the majority of MRSA isolates from milk obtained from dairy cows with mastitis are multidrug-resistant, capable of forming robust biofilms, and harbor multiple virulence genes. The presence of PVL- and TSST-1-positive ST22-SCC mec IV isolates in mastitis-infected bovine milk highlights the need for persistent monitoring to mitigate possible public health risks. IMPORTANCE: This study reports on the presence and characteristics of methicillin-resistant Staphylococcus aureus strains in milk from mastitis-infected cows. To our knowledge, this is the first report of a Panton–Valentine leukocidin- and toxic shock syndrome toxin-1-positive ST22-SCC mec IV strain in South Korea.

Mycobacterium avium subsp. paratuberculosis (MAP) causes chronic infectious enteritis in various domestic and wild mammals and is widely distributed globally. Interspecies transmission has been frequently reported. We investigated the presence of MAP from December 2010 to March 2011 in blood and feces collected from 222 hunter-killed wild boars. We collected 197 serum and 180 fecal samples and examined them by culture, PCR, and enzyme-linked immunosorbent assay. We investigated the status of MAP infection and the MAP genotypes in the wild boar population of Korea by using IS900 PCR and IS1311-restriction endonuclease analysis typing. Of the 180 fecal samples cultured, MAP colonies were recovered from two. By PCR, 18 animals were positive for MAP and one serum sample had a strong humoral response to MAP. The PCR-positive DNA samples from the colonies and the feces samples were genotyped as "cattle type" and "bison type," which are major MAP genotypes infecting domestic species in Korea. Our study provides new information on mycobacterial infection among wild boars, and suggests that a more effective program should be developed to monitor mycobacterial infections in wild animal populations in Korea.

We demonstrate Mycobacterium bovis infection in wild boar (Sus scrofa) in South Korea. During 2012–15, we attempted to isolate M. bovis from 847 wild animals, mainly Korean water deer (Hydropotes inermis argyropus), raccoon dogs (Nyctereutes procyonoides), and wild boar, from 11 regions in South Korea. We isolated M. bovis from three of 118 wild boar (2.5%) captured in Gyeonggi Province, where bovine tuberculosis (bTB) outbreaks have also occurred in livestock. Spoligotypes and mycobacterial interspersed repetitive units–variable number tandem repeats types of these M. bovis isolates (SB0140 and SB1040, 4-2-3-3-7-5-5-4-4-3-4-3 and 5-2-3-3-7-5-5-4-3-10-5-2; MIRU4, MIRU16, MIRU27, MIRU31, ETR-A, ETR-B, ETR-C, QUB11b, QUB26, QUB3336, VNTR2401, and VNTR3171) have also been identified from farmed livestock such as cattle (Bos taurus coreanae), Formosan sika deer (Cervus nippon taiouanus), and American elk (Cervus canadensis) in the country. In South Korea, bTB appears to be endemic in livestock, and t...

The emergence of third-generation cephalosporin-resistant bacteria poses a significant human health hazard worldwide. We aimed to investigate the prevalence of extended-spectrum β-lactamase (ESBL)/AmpC β-lactamase-producing third-generation cephalosporin-resistant Escherichia coli in ceftiofur-use and non-use pig farms. A total of 585 E. coli isolates were collected from pig feces ( n = 354) and farm environments ( n = 231) across various farms between 2022 and 2023 in South Korea. Among them, 61% of E. coli from pigs and 22.9% from the environment were resistant to ceftiofur, with significantly higher rates in ceftiofur-use farms. The most common ESBL gene detected was bla _CTX-M-55 (59.9%), followed by bla _CTX-M-15 (12.3%) and bla _CTX-M-14 (7.4%). Resistance to non-beta-lactam antimicrobials varied, with bla _CTX-M-55 -carrying E. coli showing high resistance. Gene transfer via conjugation was observed, particularly for bla _CTX-M-1 (85.7%) and bla _CTX-M-55 (46.6%). Of note, seven isolates co-harboring mcr-1 and bla _CTX-M-55 were found in pigs. Replicon type IncFIB (85.3%) was mainly found in bla _CTX-M-55 -carrying plasmids and IncI1 (91.7%) in bla _CTX-M-1 . The most frequently observed sequence types were ST457, ST410, ST101, ST75, and ST10, which comprised 38.4% of the isolates. The transposable elements IS 26 -IS Ecp1-bla _CTX-M-55 - orf477 (76%), IS Ecp1-bla _CTX-M-55 - orf477 (22.7%), bla _CTX-M-65 -IS 903 (50%), and bla _CTX-M-14 -IS 903 (100%) were predominantly detected. This study implies that ceftiofur use, clonal, and horizontal transfer might cause a high prevalence of ESBLs in pig farms. Thus, these findings emphasize the importance of prudent antimicrobial use and biosecurity measures to combat antimicrobial resistance. IMPORTANCE: Antimicrobial resistance in bacteria is a growing concern for both humans and animals. Pigs and their farm environment can carry and spread antimicrobial-resistant bacteria. Moreover, these resistant bacteria can be transmitted to humans and cause complications in treating infections. In this investigation, we present evidence for the widespread occurrence of third-generation cephalosporin-resistant extended-spectrum β-lactamase (ESBL)/AmpC β-lactamase (AmpC) genes in Escherichia coli isolates in ceftiofur-use pigs, which were clonally and horizontally transferred and detected significantly in ceftiofur non-use pigs and their farm environment. Furthermore, we observed different transposable elements, sequence types, and replicon types of E. coli that potentially participate in disseminating antimicrobial resistance. Our study will contribute to a better understanding of the emergence and dissemination of ceftiofur-resistant E. coli and develop a strategy for preventing its spreading to humans and other animals.

Klebsiella pneumoniae , a zoonotic agent, is a causative pathogen of bovine mastitis. Despite its clinical relevance in dairy farms, studies on K. pneumoniae in bovine mastitis remain limited. Additionally, studies on K. pneumoniae ’s genetic diversity and virulence characteristics in South Korea remain limited. Therefore, in this study, we aimed to elucidate the genetic diversity, antimicrobial resistance, virulence genes, and biofilm-forming capacity of 29 K . pneumoniae strains isolated from bovine mastitis milk samples in South Korea between 2017 and 2023. Multilocus sequence typing revealed 23 sequence types, four of which were novel, indicating substantial genetic heterogeneity and the absence of a dominant clonal lineage. Excluding intrinsic resistance, the highest resistance rates were observed for tetracycline (34.5%) and sulfisoxazole (31.0%), whereas resistance to the other antibiotics tested ranged from 0% to 20.7%. In addition, multidrug resistance (MDR) was noted in 20.7% of isolates. Virulence gene analysis revealed that most isolates carried the ureA , uge , wabG , and fimH genes, whereas allS , rmpA , iucB , and iroNB were not detected. Two isolates exhibited a hypermucoviscous phenotype, and one belonged to the capsular serotype K2. All isolates demonstrated biofilm-forming ability, with moderate-to-strong production observed in over 89.0% of cases, indicating potential for persistence and treatment challenges. In conclusion, K. pneumoniae isolates from mastitis milk carried multiple virulence genes and showed MDR as well as robust biofilm formation. Therefore, continued surveillance and further characterization of K. pneumoniae are needed to support mastitis control and protect public health. IMPORTANCE: Klebsiella pneumoniae is an emerging environmental pathogen associated with clinical mastitis in dairy cows, raising concerns regarding antimicrobial resistance and public health. To the best of our knowledge, this study provides the first comprehensive characterization of K. pneumoniae isolates from mastitis milk in South Korea, including analyses of genetic diversity, antimicrobial resistance, virulence factors, and biofilm formation. The findings advance our current understanding of K. pneumoniae associated with bovine mastitis and highlight the need for continued surveillance that will contribute to mastitis control efforts and safeguard public health.