Profiling of antimicrobial resistance and plasmid replicon types in β-lactamase producing Escherichia coli isolated from Korean beef cattle.

In this study, 78 isolates of Escherichia coli isolated from Korean beef cattle farms were investigated for the production of extended-spectrum β-lactamase (ESBL) and/or AmpC β-lactamase. In the disc diffusion test with ampicillin, amoxicillin, cephalothin, ceftiofur, cefotaxime, ceftazidime, and cefoxitin, 38.5% of the isolates showed resistance to all of ampicillin, amoxicillin, and cephalothin. The double disc synergy method revealed that none of the isolates produced ESBL or AmpC β-lactamases. DNA sequencing showed that all isolates encoded genes for TEM-1-type β-lactamase. Moreover, 78.2% of the isolates transferred the TEM-1-type β-lactamase gene via conjugation. In plasmid replicon typing of all donors, IncFIB and IncFIA were identified in 71.4% and 41.0% of plasmids, respectively. In transconjugants, IncFIB and IncFIA were the most frequent types detected (61.5% and 41.0%, respectively). Overall, the present study indicates that selection pressures of antimicrobials on β-lactamases in beef cattle may be low relative to other livestock animals in Korea. Moreover, to reduce selection pressure and dissemination of β-lactamase, the long-term surveillance of antimicrobial use in domestic beef cattle should be established.

Introduction

The prevalence of β-lactam-resistant Enterobacteriaceae has increased consistently over the past few decades. Escherichia (E.) coli producing plasmid-mediated AmpC β-lactamases and/or extended-spectrum β-lactamases (ESBLs) has been of particular concern because of their implications in human and food animal health [19]. These strains encode β-lactamases that mediate resistance to β-lactam antimicrobials included penicillins and extended-spectrum cephalosporins such as 3rd and 4th generation cephalosporins [4]. Genes encoding β-lactamases are located on mobile genetic elements, mostly plasmids, which can transfer resistance genes horizontally to non-resistant isolates. Thus, these elements are believed to be responsible for the acquisition and dissemination of β-lactam antimicrobial resistance in the bacterial population.

The incidence of resistance to extended-spectrum β-lactam antimicrobials has increased in Korea [317]. Most studies that have been performed to date have focused on the characterization of β-lactamases in human clinical isolates [1314212330]. However, there is little information available regarding the prevalence and characteristics of plasmid-mediated AmpC β-lactamases and ESBLs among E. coli isolates in the Korean veterinary industry [18273132]. Furthermore, β-lactamases-producing E. coli isolated from beef cattle have rarely been reported in Korea.

Enteric bacteria, especially E. coli, derived from livestock animals are potentially infectious pathogens and reservoirs for β-lactamase genes; accordingly, investigations of these microorganisms are necessary for public health. In view of the risk of spreading ESBL and AmpC β-lactamase resistance determinants among E. coli isolates, it is important to elucidate the mechanism by which resistance is transferred between isolates. Thus, in the present study, we investigated antimicrobial resistance profiles and plasmid replicon types of ampicillin (AMP)-resistant E. coli isolates recovered from the feces of beef cattle with the goal of investigating the transfer of β-lactamase genes and antimicrobial resistance to non-resistant E. coli.

Materials and Methods

Bacterial strains

A total of 290 E. coli strains were isolated from feces collected from clinically healthy beef cattle during 2011-2012 [29]. Briefly, E. coli isolates of this study were isolated from 830 fecal samples collected from healthy beef cattle on eight farms from six different provinces in South Korea. The fecal samples were collected from rectum and pats of cattle and plated onto MacConkey agar (Becton, Dickinson and Company, USA) for selection, then incubated at 37℃ for 18 h. From each sample, three to five colonies suspected of being E. coli were sub-cultured onto blood agar plates. Isolates were confirmed as E. coli by a standard biochemical test and by the Vitek2 system (bioMérieux, France).

Antimicrobial susceptibility test

For selection of β-lactam-resistant E. coli, all isolates were screened by plating on MacConkey agar plates containing AMP (16 µg/mL) because the minimum inhibitory concentration (MIC) value of AMP for E. coli was above or at the breakpoint (≥ 32 µg/mL) for AMP resistance [5]. Overall, a total of 78 E. coli isolates were selected for characterization of β-lactamases in this study. All 78 E. coli isolates were tested using antimicrobial-containing discs according to the Clinical and Laboratory Standards Institute (CLSI) guidelines [5]. The following antibiotics were tested: AMP, 10 µg; amoxicillin (AMX), 20 µg/10 µg; cephalothin (CF), 30 µg; ceftiofur (EFT), 30 µg; cefoxitin (FOX), 30 µg; cefotaxime (CTX), 30 µg; and ceftazidime (CAZ), 30 µg (Oxoid, UK). The MICs of the isolates were also determined by the micro-broth dilution method using the same antibiotics. The MIC test was conducted according to the recommendations of the CLSI [5]. The breakpoint of EFT (MIC ≥ 8 µg/mL) was used based on the results of a previous study [8], because the CLSI guidelines do not include a MIC breakpoint of EFT for E. coli of bovine origin. E. coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853 were used as quality control organisms in the antimicrobial susceptibility tests and ESBL and/or AmpC β-lactamases in the phenotypic screening test.

Screening and phenotypic identification of ESBLs and AmpC β-lactamases

A double disc diffusion method was performed with CTX (30 µg)/CTX-clavulanate (30 µg/10 µg; Becton, Dickinson and Company) and CAZ (30 µg)/CAZ-clavulanate (30 µg/10 µg; Becton, Dickinson and Company) to detect ESBL production according to CLSI guidelines [5]. Similarly, plasmid-mediated AmpC β-lactamase production was screened by the cefoxitin-cloxacillin double disc synergy method using FOX (30 µg)/FOX-cloxacillin (30 µg/200 µg; Himedia, India), as described in a previous study [33].

Detection of β-lactamase-encoding genes

PCR amplification of genes of the ESBL (blaTEM, blaSHV, blaOXA, and blaCTX-M) and plasmid-mediated AmpC was carried out as previously described [292527]. The primers used to detect β-lactamases in this study are shown in Table 1. The DNA templates used in this study were prepared by the boiling method. In all PCR amplifications, distilled water was used as a negative control. A positive control organism was not used in this assay as all DNA products were sequenced by a dye-termination sequencing system using an automatic sequencer (Macrogen, Korea). Homologous sequence searches were performed against the GenBank database using the BLAST tool of the National Center for Biotechnology Information (NCBI, USA) website.

Table 1

Primers for the detection of β-lactamase genes used in this study

Conjugation assay

To determine the transferability of the β-lactamase-encoding genes, a conjugation assay was conducted. A mixed broth culture mating method in a previous study was applied with sodium azide-resistant E. coli J53AzR as a recipient strain, with some modifications [27]. Single colonies of donor and recipient isolates were incubated in tryptic soy broth (TSB; Becton, Dickinson and Company) and grown at 37℃ for 20 h. The donor and recipient strains were grown in TSB for 8 hrs, after which the cultures were mixed at a ratio of 1: 2 and incubated at 37℃ for 20 h. Transconjugants were selected on Mueller-Hinton agar (Becton, Dickinson and Company) supplemented with AMP (100 µg/mL) and sodium azide (200 µg/mL). The conjugation frequency of each isolate was calculated as the number of CFU transconjugants per CFU donor. In addition, transfer of the genes was confirmed by PCR amplification of specific genes in the transconjugants.

Typing of plasmid replicons

For typing plasmid replicons, PCR was performed using DNA extracted from all donor and transconjugant strains. The primers used in this study targeted 18 different replicons (Table 2), as described previously [16].

Table 2

Primers for analysis of plasmid replicon types used in this study

F, forward; R, reserve.

Results

Antimicrobial resistance

Resistance to AMP and AMX was observed in all isolates, and 30 isolates (38.5%) were resistant to CF. None of the isolates showed resistance to any of the extended-spectrum β-lactams used in the test (EFT, CAZ, CTX, and FOX) (Table 3). The MIC values of the different β-lactams tested for the 78 E. coli isolates are shown in Table 3. All isolates were highly resistant to AMP (MIC > 1024 µg/mL) and AMX (MIC > 1024 µg/ml). Cephalothin resistance (MIC ≥ 32 µg/mL) was detected in 32 isolates (41.0%). None of the isolates was resistant to EFT (MIC ≤ 4 µg/mL), CAZ (MIC ≤ 8 µg/mL), CTX (MIC ≤ 2 µg/mL), or FOX (MIC ≤ 8 µg/mL) (Table 3). However, intermediate resistance to EFT (MIC = 4 µg/mL), CAZ (MIC = 8 µg/mL), and CTX (MIC = 2 µg/mL) was detected in 39.7%, 17.9%, and 46.2% of the isolates, respectively. The resistance patterns of the isolates were [AMP-AMX] (61.5%) and [AMP-AMX-CF] (38.5%).

Table 3

Antimicrobial susceptibility of 78 Escherichia coli isolates to β-lactam antimicrobial agents

MIC, minimum inhibitory concentration; R, resistant; I, intermediate; S, susceptible.

Screening of ESBL and AmpC β-lactamase production

None of the isolates were positive for ESBL or AmpC β-lactamase production. In the MIC test, none of the isolates were resistant to CTX, CAZ, or FOX, even though 36 (46.2%), 14 (17.9%), and 4 (5.1%) of the E. coli isolates showed intermediate MIC values against CTX (MIC, 2 µg/mL), CAZ (MIC, 8 µg/mL), and FOX (MIC, 8 µg/mL), respectively (Table 3).

Molecular characterization of β-lactamase-encoding genes

All 78 E. coli isolates harbored a TEM-type gene. None of the genes encoding the ESBLs (blaSHV, blaOXA, and blaCTX-M) or pAmpC β-lactamases were found in any of the isolates. Sequence analysis identified TEM-1-type β-lacatamase in all isolates.

Transferability of β-lactamase resistance and plasmid replicon analysis

Plasmid replicon typing and conjugal transferability of plasmids revealed that the blaTEM-1, gene for β-lactamase resistance was transferred in 59 (75.6%) of the isolates (Table 4). The transfer frequency of the isolates ranged from 1.29 × 10-6 to 9.22 × 10-4. Plasmid replicon typing of the transconjugants was performed to identify the transfer of plasmids in E. coli carrying the TEM-1 gene. The prevalence of the plasmid replicon type of the donor isolates was as follows: IncFIB (71.8%); IncFIA (41.0%); IncP (34.6%); Frep (29.5%); IncY (29.5%); IncI1 (28.2%); IncN (15.4%); IncB/O (10.3%) and IncHI1 (1.3%). Among the 10 plasmids detected from the isolates, the main plasmid for the horizontal dissemination of blaTEM-1 in E. coli isolated from beef cattle was the IncFIB (Table 4). Plasmid replicon typing revealed that all donor isolates exhibited 32 different replicon combinations. The most frequent combination was [FIA-FIB-Y], which was detected in eight isolates (Table 4). For transconjugants, a total of five classes of replicon were detected. IncFIB and IncFIA were the most frequently detected replicons, being found either alone or in combination at ratios of 61.5% and 41.0%, respectively. The prevalence of the remaining plasmid replicons of transconjugants was as follow: IncI1 (17.9%); Frep (16.7%) and IncB/O (5.1%). PCR revealed that all 59 transconjugants harbored TEM-1-type β-lacatamase transferred from the donors.

Table 4

Profile of plasmid replicon typing and transferability of 78 Escherichia coli isolates

Discussion

In the present study, we conducted phenotypic and genotypic characterization of β-lactamase of E. coli strains isolated from Korean beef cattle farms from 2011 to 2012. None of the E. coli isolates were found to produce ESBL and/or AmpC β-lactamase.

High MIC values for AMP in E. coli isolated from calves with diarrhea and dairy cattle were reported in previous studies [2028]. In the present study, the extremely high resistance to AMP (MIC > 1024 µg/mL resistance, 100%) and AMX (MIC > 1024 µg/mL resistance, 100%) of these E. coli isolates might have been caused by selection pressures from their excessive use in beef cattle farms over the last decade [1]. Additionally, the use of β-lactam antimicrobials, such as penicillins and cephems, has increased gradually [1]. In addition, the antimicrobial resistance to CF of the E. coli isolates used in this study was high, with 32 (41.0%) isolates showing resistance to CF (MIC ≥ 32 µg/mL), and this resistance was much higher than that of E. coli (1.0%) in a previous national report [1]. A considerable number of isolates exhibited intermediate resistance to CTX (n = 36), EFT (n = 31), and CAZ (n = 14), although none of the isolates in this study were identified as resistant to these compounds (Table 3). E. coli isolates showing intermediate resistance to these compounds may acquire resistance to β-lactams by selection pressure if they are exposed to continuous use of antimicrobials.

In this study, no ESBL- and/or AmpC β-lactamase-producing E. coli isolates were detected, which is consistent with the results of a previous study showing a low prevalence (< 2%) of β-lactamase-producing E. coli isolates [183132]. Although recent reports indicated that there are various types of ESBL- and AmpC β-lactamase-producing Enterobacteriaceae [1112142124], only TEM-1-type β-lactamase was detected in the present study. These findings suggest that less third- and fourth-generation cephalosporins might be used in the production of Korean beef cattle than in the human population and production of other livestock. In the present study, PCR and sequencing results revealed that all AMP-resistant isolates were only associated with TEM-1-type β-lactamase, which is known to be widely distributed in Korea [2227]. These results are in agreement with those of a previous study, which showed that most of the AMP-resistant E. coli harbored the TEM-1 β-lactamase gene as the only plasmid-mediated β-lactamase [6].

Continuous selective pressure exerted by β-lactams is an important reason for occurrence of ESBL- and AmpC β-lactamase determinants [10]. Similarly, genetically non-resistant strains might be able to acquire resistance plasmids, either randomly or specifically, due to constant antimicrobial use, leading to widespread occurrence of resistance plasmids [26]. Replicon typing of the transconjugant of E. coli isolates revealed that the IncFIA and IncFIB plasmids, which are commonly found in the fecal flora of humans and animals, were most frequently detected [7]. We found that strains that carried F plasmid (IncFIB, IncFIA and Frep) and I1 either alone or combination had transferred the TEM-1-type β-lactamase. These results suggest that blaTEM-1 gene, a primitive type of β-lactamase encoding gene, is harbored by these kind of plasmids and associated with old type β-lactams such as AMP and AMX [15] Two isolates that carried IncB/O did not transfer TEM-1-type β-lactamase to the recipients.

When compared to other veterinary studies, our results are unusual as no resistance to cephems was found and only one kind of β-lactamase was detected. These results suggest that the present selection pressure of antimicrobial use on β-lactamases in beef cattle may be relatively low in comparison to other livestock in Korea. However, increased exposure to antimicrobials could increase selection pressure for β-lactamases, which presents a critical risk to human and animal health. Thus, the use of β-lactam antimicrobials such as extended-spectrum cephalosporin should be restricted. In addition, monitoring the use of antimicrobials and assessment of antimicrobial resistance mechanisms in the bacteria of beef cattle could reduce selection pressure and may help enhance treatment for both humans and animals.