Comparison of Antimicrobial Resistance in Escherichia coli Strains Isolated From Healthy Poultry and Swine Farm Workers Using Antibiotics in Korea

2.1. Collection of faecal samples and isolation of E coli from the samples

Faecal samples were collected from healthy persons, i.e., 30 from poultry farm workers and 31 from swine farm workers that often use antibiotics. For the comparison, 31 specimens were collected from healthy persons that work in the restaurants of the same regions (Table 1). The samples were placed in sterile plastic specimen tubes on ice, transported to our laboratory, and plated onto a MacConkey agar directly or, occasionally, after enrichment in trypticase soy broth (TSB) containing vancomycin (Sigma Chemical Co., St. Louis, MO, USA). Candidate colonies were then plated in trypticase soy agar medium and biochemically characterized using the API20E system (Biomerieux, Marcy l’Etoile , France). For individual samples, one or two E coli isolates were selected randomly for the purpose of determining susceptibility.

Table 1.

Collected faecal samples

2.2. Antimicrobial susceptibility testing

Susceptibility testing was conducted by disk diffusion according to the guidelines of the Clinical and Laboratory Standards Institute [8]. Antimicrobial susceptibility was determined by agar disk diffusion (Kirby-Bauer method) using Mueller–Hinton agar (Difco, MI, USA). The following antibiotics were tested: ampicillin (AM)/sulbactam (SAM), AM, tetracycline (TE), aztreonam (ATM), cefotetan (CTT), cefepime (FEP), cefoxitin (FOX), cefotaxime (CTX), tobramycin (NN), trimethoprim/sulfamethoxazole (SXT/TM), cephalothin (CF), imipenem (IPM), gentamicin (GM), amikacin (AN), piperacillin/tazobactam (TZP), and netilmicin (NET). E coli ATCC 25922 and E coli ATCC 35218 were used as quality controls.

2.3. Statistic analysis

The antimicrobial susceptibility data are expressed as percentages or frequency of the avian or human isolates. A one-way analysis of variance or x² statistics was used to estimate overall difference between the percentages or frequencies of resistance between avian and human E coli isolates. In all cases, p < 0.05 was regarded as statistically significant.

3.1. Collected faecal samples and isolated E coli strains from the samples

As shown in Table 1, many faecal samples were collected from people ages 30–50 in each group. In the workers that often use antibiotics, more samples were collected from men than women, while there were more samples of women than men in the restaurant workers. A total of 163 E coli isolates were obtained from the faecal samples, as shown in Materials and Methods section, of which 112 isolates were derived from livestock workers (44 isolates from poultry farm workers and 68 from swine farm workers) and 51 isolates from restaurant workers.

3.2. Antibiotic susceptibility of the isolates

Antibiotic resistance rates of each group were shown in Table 2. In the isolates of each group, high antimicrobial resistance to AM and TE was noted in the most tested isolates. In Enterobacteriaceae, resistance to AM is mainly due to ß -lactamases like bla (TEM-1) and bla (SHV-1) enzymes that hydrolytically cleave the ß-lactam ring. Plasmid-encoded derivatives of ß-lactamases that show an enhanced spectrum of catalytic activity have been known since the early 1980s [9]. In addition to the large number of extended-spectrum ß-lactamase (ESBL)-TEM and -SHV variants, other plasmidencoded ESBL such as bla (CTX-M) enzymes are now frequently reported [10]. The isolates showed higher resistance to cephalothin than other antibiotics among the cephems. Most of the E coli isolates were susceptible to CTX, FOX, and ATM. However, in the isolates of swine farmers, 3% showed resistance to cefotaxime, cefoxitin, and aztreonam. Our data showed that CTXM14 was detected in all cefotaxime resistant isolates (data not shown). TE is a commonly used first line

Table 2.

Antibiotic resistance rates of the isolates in each group

antibiotic for many domestic animals as a growth promoter or as an infection control agent and is often used before the antibiotic resistance profile of a pathogen has been determined [11-13]. Resistance to tetracycline is plasmid mediated, with a wide variety of genetic determinants. The presence and frequency of tetracycline resistance in E coli in this study agree with findings of other studies on antibiotic resistance in E coli [14,15]. The resistance to SXT/TM was also relatively higher than to other antibiotics. Among the aminoglycosides, the resistance to GM and NN occurred at higher frequencies in comparison with resistance to AN and NET. Aminoglycoside resistance in E coli most often occurs by aminoglycoside-modifying enzymes [16,17] encoded on transmissible plasmids [18]. Alternatively, no isolates showed resistance to FEP, CTT, IPM, AN, and NET.

There was a trend towards higher resistance frequency of the isolates of poultry and swine farm workers than restaurant workers, especially CF, GM, and NN (p < 0.05). Among the livestock workers, the resistance rates to CF of swine farm workers were significantly higher than poultry farm workers (p < 0.05). The findings that the highest prevalence of resistance occurred among swine and poultry farmer isolates suggest that antimicrobial use in these animals may be a factor in the emergence of antimicrobial resistance in the human faecal E coli isolates. In a study, faecal E coli isolates from swine showed higher levels of antibiotic resistance and multidrug resistance [19].

3.3. Multiresistance patterns

The percentage of multiple resistance patterns in E coli isolates of each group is given in Figure 1. Multidrug resistance was defined as resistance exhibited to two or more antimicrobials.

Figure 1.

Antibiotic multiresistance patterns of E coli strain in each group. DR = drug resistance.

Multidrug resistance was found in E coli from both groups, but was higher in frequency and proportion in livestock workers. A total of 78% (40 of 163 strains) of restaurant workers expressed resistance to two or more antimicrobials. Approximately 16% of the isolates showed resistance to four or more antibiotics. The resistance most frequently observed pattern in this group was resistance to AM/TE (41.2%) and AM/TE/SXT (41.2%). Among the isolates of livestock workers, 93% (63 of 68 strains) of swine farm workers and 91% (40 of 44 strains) of poultry farm workers exhibited resistance to two or more antimicrobials. The rates of antibiotic resistance to four or more antibiotics of livestock workers were higher than the rates of restaurant workers (66% of swine farm workers, 43% of poultry workers and 16% of restaurant workers). The resistance most frequently observed pattern was resistance to AM/TE (41.2%) in the poultry farm workers and AM//CF/GM/TE/SXT (41.2%) in the swine farm workers. The highest number of resistant antibiotics was nine (SAM/AM/CF/FOX/CTX/ NN/GM/TE/SXT) in an isolate of a swine farm worker.