Abstract
BackgroundThis study was conducted to analyze the genetic characteristics of 41 β-lactam-resistant Escherichia coli isolates, which are one of the common causes of environmental mastitis, isolated from the bulk tank milk of 290 dairy farms in five factories operated by three dairy companies in Korea.ResultsAnalysis of the phenotypic and genotypic characteristics of β-lactam-resistant E. coli isolates revealed differences between factories even within the same company. Isolates from factory A1 and C1 showed high resistance to cephalothin (76.9 and 100%, respectively), which is a first-generation cephalosporins, whereas resistance to tetracycline was showed by only the isolates from factories B1 (60.0%), C2 (66.7%), and C3 (100%). Although all the 41 β-lactam-resistant E. coli isolates were positive for blaOXA-1, blaTEM-1 was highly prevalent in isolates from factories C2 (100%) and C3 (100%). Among 17 isolates resistant to both β-lactams and aminoglycosides, the most common multilocus sequence type was ST399 (13isolates, 76.5%). Furthermore, 2 (11.8%) and 12 (70.6%) isolates belonged to the phylogenetic groups B2 and D, respectively, which are invasive strains that cause intestinal infections, respectively. The predominant serogroup was O15 (70.6%), which is a globally distributed extraintestinal pathogen. Interestingly, one isolate from factory A1 belonged to O157 and carried six virulence genes, simultaneously.ConclusionsAlthough E. coli isolates were isolated from bulk tank milk, and not the clinical mastitis samples, the presence of the phylogenetic groups B2 and D, and the serogroups O15 and O157, which harbor antimicrobial resistance genes and virulence factors, can pose a threat to public health.
Highlights
This study was conducted to analyze the genetic characteristics of 41 β-lactam-resistant Escherichia coli isolates, which are one of the common causes of environmental mastitis, isolated from the bulk tank milk of 290 dairy farms in five factories operated by three dairy companies in Korea
Antimicrobial susceptibility testing According to the Clinical and Laboratory Standards Institute guidelines (CLSI, 2019) [18]. 41 β-lactamresistant E. coli were examined for antimicrobial susceptibility using antimicrobial disc (BD biosciences, San Jose, CA, USA) as follows: ampicillin (AM, 10 μg), amoxicillin-clavulanate (AMC, 20 μg), chloramphenicol (C, 30 μg), ceftazidime (CAZ, 30 μg), cefadroxil (30 μg), cephalothin (CF, 30 μg), ciprofloxacin (5 μg), colistin (CL, 10 μg), cefotaxime (CTX, 30 μg), cefuroxime (CXM, 30 μg), cefazoline (30 μg), cefepime (FEP, 30 μg), cefoxitin (30 μg), cefpirome (30 μg), gentamicin (GM, 10 μg), imipenem (10 μg), nalidixic acid (30 μg), trimethoprim/sulfamethoxazole (SXT, 1.25 μg), and tetracycline (TE, 30 μg)
Isolates from factories A1 and C1 showed the high resistance to CF (76.9 and 100%, respectively), the first-generation cephalosporin, but each of the six isolates from factories C2 and C3 of the same company as C1 showed no resistance to CF
Summary
This study was conducted to analyze the genetic characteristics of 41 β-lactam-resistant Escherichia coli isolates, which are one of the common causes of environmental mastitis, isolated from the bulk tank milk of 290 dairy farms in five factories operated by three dairy companies in Korea. Beta-lactam antibiotics kill bacteria by inhibiting penicillin-binding proteins (PBPs) essential for the cross-linking process during cell wall biosynthesis [1, 2]. In Korea, β-lactam antibiotics are widely used for treating bacterial infections in food-producing livestock [4, 5]. Resistance to β-lactam antibiotics in a variety of pathogens from livestock has been continuously reported in recent years, and these bacteria are considered as the primary reservoir of zoonotic pathogens [6, 7]. Aminoglycosides, which are an important class of antibiotics used frequently, are primarily used in combination with β-lactams to treat severe infections caused by gramnegative bacteria [9]. The co-occurrence of β-lactamase and AME genes in gram-negative bacteria has been continuously reported worldwide [11, 12]
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