Abstract
This study aimed to analyze the Salmonella serovars, measure the minimum inhibitory concentration of antimicrobials, and examine the antimicrobial resistance genes of Salmonella isolated from 192 broiler flocks in Kagoshima Prefecture in Japan, from 2013 to 2016. We found that all Salmonella isolates belonged to three serovars: Salmonella Manhattan, S. Infantis, and S. Schwarzengrund. Among them, S. Schwarzengrund prevalence has recently increased annually making the main serovar. Most recovered isolates were highly resistant to streptomycin, sulfamethoxazole, and oxytetracycline. We saw the reduction of third-generation cephalosporin resistance and identified the reason of increased kanamycin resistance to be the increased number of S. Schwazengrund isolates. Among the kanamycin-resistant Salmonella isolates, aphA1 constituted the main resistance gene detected.
Keywords: antimicrobial susceptibility, broiler chicken, kanamycin resistance gene, Salmonella, Salmonella Schwarzengrund
Salmonellosis, one of the most important diseases in both humans and animals, has been described as the second most commonly caused foodborne bacterial disease worldwide [12]. Salmonella is one of the four key global causes of diarrheal diseases, with 2579 serovars identified till date [17]. Antimicrobial agents are widely used during poultry production for growth promotion, or treatment purposes [14]. Resistance to antimicrobial agents in bacteria is mediated by several mechanisms including changes in bacterial cell wall permeability, energy-dependent removal of antimicrobials via membrane-bound efflux pumps, modification of the site of drug action, and destruction or inactivation of the drug [3, 19]. Bacteria can acquire resistance genes through mobile elements, such as plasmids, which provide flexibility to the host bacteria and promote the spread and distribution of these genes across the diverse bacterial population [4].
Notably, we recently reported an increase in the prevalence of Salmonella in broiler chickens in Japan including the first report of Salmonella Schwarzengrund detection in 2012, which is the main serovar detected in Kagoshima Prefecture, Japan presently [9]. S. Schwarzengrund has been reported as an emerging pathogen in Asia, Denmark, the United States of America and Brazil [1, 2, 15]. In this study, we analyze the Salmonella serovars, measure the minimal inhibitory concentration (MIC) of antimicrobials, and examine the resistance genes in order to describe the recent fluctuations of antimicrobial susceptibility of Salmonella in broiler chickens and investigate its mechanism.
During 2013 to 2016, we analyzed 3069 cecal specimens from 192 broiler flocks (approximately 10,000 birds per flock) collected by the prefectural officials at an accredited poultry processing plant in Kagoshima Prefecture, Japan. Samples were delivered to the Laboratory of Veterinary Public Health, Kagoshima University, and cultured on the day of arrival. [8, 9]. The antimicrobial susceptibility of the Salmonella isolates was ascertained by the agar dilution method using Mueller Hinton agar (Oxoid Ltd., Basingstoke, UK) [20, 21, 25]. Two kanamycin resistance genes aphA1, and kn were detected by using PCR [7, 11, 13].
Salmonella prevalence in broiler chickens from 2013 to 2016 in Kagoshima Prefecture, Japan is shown in Table 1. Overall, the prevalence of Salmonella- positive flocks exhibited a dramatic increase during the last three years in the study period compared to that during the first year. In general, the incidence of Salmonella in the flocks was 78.6% (151/192; 48 flocks per year for four years) and the proportion of Salmonella-positive samples in the total number of samples from broiler chickens was 17.8% (546/3069). As shown in Table 1, Salmonella prevalence at both the flock and individual broiler chicken levels in the present study (78.6%) is much higher than that in our previous study (49.0%) [9]. Our report was similar to the Salmonella prevalence in Japan reported by Yamazaki et al. [24], and Sasaki et al. [18]. Alternatively, Salmonella prevalence was reported to vary considerably across different geographic regions worldwide. In Sweden, a study from 2007 to 2015 on housed broilers and laying hens reported that the percentage of Salmonella-positive broiler flocks was 2.0% [23]. A study in Egypt reported that 41.0% of tested broiler flocks were positive for Salmonella along with 1.09% of tested samples [10].
Table 1. Prevalence of Salmonella in broiler chickens during 2013–2016 in Kagoshima, Japan.
| Year | No. of flocks | No. of positive flocks (%) | No. of samples | No. of positive samples (%) |
|---|---|---|---|---|
| 2013 | 48 | 31 (64.6) | 767 | 82 (10.7) |
| 2014 | 48 | 41 (85.4) | 767 | 153 (19.9) |
| 2015 | 48 | 40 (83.3) | 768 | 157 (20.4) |
| 2016 | 48 | 39 (81.3) | 767 | 154 (20.1) |
| Total | 192 | 151 (78.6) | 3,069 | 546 (17.8) |
The Salmonella isolates from broiler chickens in Kagoshima Prefecture, Japan belongs to three serovars: Infantis, Manhattan, and Schwarzengrund across the four years of the present study, as also reported in our previous study [9], although the relative proportions differed as shown in Table 2. The largest differences were observed in Infantis and Schwarzengrund serovars. Across both studies, S. Infantis proportion exhibited a dramatic decrease. In contrast, S. Schwarzengrund and S. Manhattan percentage steadily increased from 2.1 and 40.3%, respectively, in 2009–2012 to 21.3 and 51.8%, respectively, in 2013–2016.
Table 2. Incidence of Salmonella serovars in broiler chickens in Kagoshima, Japan during two periods (2009–2012 and 2013–2016).
| Serovar | Survey period |
|
|---|---|---|
| 2009–2012a) (%) | 2013–2016b) (%) | |
| No. of S. Infantis | 140 (57.6) | 147 (26.9) |
| No. of S. Manhattan | 98 (40.3) | 283 (51.8) |
| No. of S. Schwarzengrund | 5 (2.1) | 116 (21.3) |
| Total | 243 | 546 |
a) Cited from our previous study [9]. b) This study.
In Japan, S. Schwarzengrund proportion of broiler chicken origin increased from 0% in 2000–2003 to 28.1% in 2005–2007 and was resistant to streptomycin, oxytetracyclin and kanamycin [2]; a high incidence of S. Schwarzengrund was also detected in Kyushu region, Japan with 123 positive samples from 184 Salmonella strains (66.8%) isolated from broiler chickens [24]. Moreover, a study conducted in Taiwan from 2004 to 2006 indicated S. Schwarzengrund contamination prevalence in raw chicken meat samples as 30.5% [6]. In our present study, the number of S. Schwarzengrund strains isolated increased dramatically from 5 to 116 (Table 2). Together, these studies support that S. Schwarzengrund has become one of the most prevalent serovars in broiler chickens in East Asia.
Table 3 describes that the proportion of Salmonella antimicrobial resistance slightly changed across the previous (2009–2012) [9] and current (2013–2016) study periods. Ampicillin, cefotaxime, and ceftiofur resistance was concurrently and markedly decreases. Conversely, kanamycin-resistant Salmonella proportion increased from 6.6% in 2009–2012 to 13.7% in 2013–3016. The majority of S. Schwarzengrund were sensitive to ampicillin, cefotaxime, cefoxitin, and ceftiofur (zero percent resistance).
Table 3. Antimicrobial susceptibility profiles from the current study and our previous study [9] of Salmonella isolates from broiler chickens in Kagoshima, Japan.
| Antimicrobial agent | MIC Break-point (µg/ml) | No. of resistant isolates (%) |
|
|---|---|---|---|
| Previous studya) |
Current studyb) |
||
| 2009–2012 | 2013–2016 | ||
| n=243a) | n=511*b) | ||
| AMP | ≥32 | 134 (55.1) | 148 (29.0) |
| CTX | ≥4 | 128 (52.7) | 132 (25.8) |
| CFX | ≥32 | 15 (6.2) | 42 (8.2) |
| CP | ≥32 | 0 (0.0) | 0 (0.0) |
| SM | ≥16 | 231 (95.1) | 484 (94.7) |
| SUL | ≥512 | 221 (91.0) | 463 (90.6) |
| OTC | ≥16 | 222 (91.4) | 451 (88.3) |
| KM | ≥64 | 16 (6.6) | 70 (13.7) |
| OFLX | ≥2 | 4 (1.6) | 3 (0.59) |
| CTF | ≥8 | 124 (51.0)b) | 112 (22.0) |
a) Cited from our previous study [9]. b) This study. *The number of strains (511) differs from the total given in Table 2 (546) because at the time of MIC testing, some strains were dried and not suitable for use. AMP, ampicillin; CTX, cefotaxime; CFX, cefoxitin; CP, chloramphenicol; SM, streptomycin; SUL, sulfamethoxazole; OTC, oxytetracycline; KM, kanamycin; OFLX, ofloxacin; CTF, ceftiofur.
As shown in Table 4, almost all Salmonella strains of the three serovars were sensitive to chloramphenicol and ofloxacin, whereas over 80% of each serovar exhibited resistance to streptomycin, sulfamethoxazole, and oxytetracycline. In our survey from 2009 to 2012, the increased proportion of the S. Manhattan serovar led to an annual increase in resistance to ampicillin, cefotaxime, and ceftiofur [9]. In the present study, although S. Manhattan percentage was 51.8% (Table 2) the resistance rate of all Salmonella serovars decreased compared to that from 2009 to 2012 [9]. This may be due to the decrease in the number of S. Infantis and increase in S. Schwarzengrund from 2013 to 2016, as all isolated S. Schwarzengrund (109 isolates) were sensitive to ampicillin, cefotaxime, and ceftiofur (Table 4). The β-lactam antimicrobial resistance rate of S. Manhattan was higher than those of S. Infantis and S. Schwarzengrund. In addition, considerable differences in kanamycin resistance were detected among the three serovars. While the majority of S. Manhattan was susceptible to kanamycin, S. Infantis exhibited a resistance rate at 10.8% and S. Schwarzengrund showed the maximum rate, with 47.7% resistance to kanamycin. The reduction of β-lactam resistance proportion in our study may be the same as reported by Mauro et al. [16], where the authors indicated that the off-label use of ceftiofur with Marek’s vaccine is associated with the short-term increase in ESBL-producing Escherichia coli in the gut of broiler chickens. In Japan, the same situation appeared following the cessation of ceftiofur use by the Japanese poultry industry [22].
Table 4. Comparison of antimicrobial resistance of Salmonella Schwarzengrund, S. Manhattan and S. Infantis during the 2013–2016 study period.
| Antimicrobial agent | No. of resistant isolates (%) |
||
|---|---|---|---|
| S. Schwarzengrund | S. Manhattan | S. Infantis | |
| n=109 | n=263 | n=139 | |
| AMP | 0 (0.0) | 119 (45.2) | 29 (20.9) |
| CTX | 0 (0.0) | 109 (41.4) | 23 (16.5) |
| CFX | 0 (0.0) | 27 (10.3) | 15 (10.8) |
| CP | 0 (0.0) | 0 (0.0) | 0 (0.0) |
| SM | 109 (100) | 257 (97.7) | 118 (84.9) |
| SUL | 102 (93.6) | 244 (92.8) | 117 (84.2) |
| OTC | 101 (92.7) | 238 (90.5) | 112 (80.6) |
| KM | 52 (47.7) | 3 (1.1) | 15 (10.8) |
| OFLX | 0 (0.0) | 0 (0.0) | 3 (2.2) |
| CTF | 0 (0.0) | 90 (34.2) | 22 (15.8) |
AMP, ampicillin; CTX, cefotaxime; CFX, cefoxitin; CP, chloramphenicol; SM, streptomycin; SUL, sulfamethoxazole; OTC, oxytetracycline; KM, kanamycin; OFLX, ofloxacin; CTF, ceftiofur.
Figure 1 shows a comparison of the specific antimicrobial resistance rates for S. Infantis (Fig. 1a) and S. Manhattan (Fig. 1b) between the current (2013–2016) and previous (2009–2012) [9] study periods. However, as only five strains of S. Schwarzengrund were isolated in the previous period [9], we did not perform the comparison for this serovar. S. Infantis proportion exhibiting antimicrobial resistance to ampicillin, cefotaxime, streptomycin, sulfamethoxazole, and oxytetracycline slightly decreased in the current study period compared to that in the previous study period (Fig. 1a). No change was observed in cefoxitin, chloramphenicol, and ofloxacin resistance, whereas kanamycin and ceftiofur resistance was slightly increased. In comparison, the resistance rate of S. Manhattan to streptomycin, sulfamethoxazole, oxytetracycline, chloramphenicol, kanamycin, and ofloxacin minimally fluctuated between the two periods. The percentage of resistance to three antimicrobials decreased in the present period: ampicillin (from 94.9% to 45.2%), cefotaxime (from 93.9% to 41.4%), and ceftiofur (from 74.5% to 30.0%). In contrast, cefoxitin-resistant S. Manhattan resistance increased from 0 to 10.3% between the previous and current study periods.
Fig. 1.
Change of antimicrobial resistance from 2009–2012 to 2013–2016 among (a) Salmonella Infantis and (b) S. Manhattan. AMP, ampicillin; CTX, cefotaxime; CFX, cefoxitin; CP, chloramphenicol; SM, streptomycin; SUL, sulfamethoxazole; OTC, oxytetracycline; KM, kanamycin; OFLX, ofloxacin; CTF, ceftiofur.
We further evaluated 68 kanamycin-resistant S. enterica isolates from Kagoshima Prefecture, Japan during the present study period (2013–2016) (13 S. Infantis, 3 S. Manhattan, and 52 S. Schwarzengrund) for kanamycin resistance genes (kn and aphA1) by PCR. None of the 68 isolates carried kn, whereas 65/68 (95.6%) carried aphA1 (Table 5). All the 13 S. Infantis isolates (MIC: 512 µg/ml) carried aphA1. Of the three S. Manhattan isolates, one (MIC: 512 µg/ml) carried aphA1 whereas two others (MIC: 256 and 128 µg/ml) did not. The 51 S. Schwarzengrund isolates with MIC of 512 µg/ml carried aphA1; that with MIC of 256 µg/ml did not.
Table 5. Distribution of the aphA1 kanamycin resistance gene from Salmonella serovars isolated from broiler chickens during the 2013–2016 study period.
| Serovar (no. of isolates) | MIC of kanamycin (µg/ml) |
No. of isolates tested | No. of isolates positive for aphA1 resistant gene (%) |
|---|---|---|---|
| S. Infantis (13) | 512 | 13 | 13 (100) |
| 256 | - | - | |
| 128 | - | - | |
| S. Manhattan (3) | 512 | 1 | 1 (100) |
| 256 | 1 | 0 (0.0) | |
| 128 | 1 | 0 (0.0) | |
| S. Schwazengrund (52) | 512 | 51 | 51 (100) |
| 256 | 1 | 0 (0.0) | |
| 128 | - | - | |
| Total | 68 | 65 (95.6) | |
MIC, minimal inhibitory concentration.
aph gene was found in almost kanamycin-resistant of S. enterica serovars isolated in some regions of the United States of America in 2005 [5]. A study in the United States of America and China [7] found aph in S. Enteritidis, S. Haardt, and an unidentified serovar from chicken meat and S. Derby from pork. In comparison, we found the aph gene (but not kn) in three serovars: S. Infantis, S. Manhattan, and S. Schwarzengrund. This may suggest that this gene commonly serves to provide kanamycin resistance in numerous Salmonella serovars.
Together, our findings revealed that there has been a recent increase in the population of the S. Schwarzengrund-strain, making it the main serovar of Salmonella isolated from broiler chickens in Kagoshima Prefecture in Japan, followed by S. Manhattan. In turn, the increase of S. Schwarzengrund, which exhibited a high level of kanamycin resistance, led to a decrease in the rate of antimicrobial resistance to ampicillin, cefotaxime, and ceftiofur among all Salmonella isolates and affected the increase in the percentage of kanamycin-resistant isolates. In addition, the resistance rate of S. Manhattan to β-lactams in this study slightly decreased compared to that in our previous study [9], which also affected the overall rate of resistance to β-lactams. Moreover, we demonstrated that aphA1 is the main antimicrobial resistance gene in Salmonella isolates. These changing profiles indicate the need for continual evaluation and research regarding the molecular characteristics of Salmonella in broiler chickens.
CONFLICT OF INTEREST
The authors declare that this research was conducted in the absence of any commercial or financial relationship that could be construed as a potential conflict of interest.
REFERENCES
- 1.Aarestrup F. M., Hendriksen R. S., Lockett J., Gay K., Teates K., McDermott P. F., White D. G., Hasman H., Sørensen G., Bangtrakulnonth A., Pornreongwong S., Pulsrikarn C., Angulo F. J., Gerner-Smidt P.2007. International spread of multidrug-resistant Salmonella Schwarzengrund in food products. Emerg. Infect. Dis. 13: 726–731. doi: 10.3201/eid1305.061489 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Asai T., Murakami K., Ozawa M., Koike R., Ishikawa H.2009. Relationships between multidrug-resistant Salmonella enterica Serovar Schwarzengrund and both broiler chickens and retail chicken meats in Japan. Jpn. J. Infect. Dis. 62: 198–200. [PubMed] [Google Scholar]
- 3.Barbosa T. M., Levy S. B.2000. The impact of antibiotic use on resistance development and persistence. Drug Resist. Updat. 3: 303–311. doi: 10.1054/drup.2000.0167 [DOI] [PubMed] [Google Scholar]
- 4.Blair J. M., Webber M. A., Baylay A. J., Ogbolu D. O., Piddock L. J.2015. Molecular mechanisms of antibiotic resistance. Nat. Rev. Microbiol. 13: 42–51. doi: 10.1038/nrmicro3380 [DOI] [PubMed] [Google Scholar]
- 5.Chen C.Y., Lindsey R. L., Strobaugh T. P., Jr., Frye J. G., Meinersmann R. J.2010. Prevalence of ColE1-like plasmids and kanamycin resistance genes in Salmonella enterica serovars. Appl. Environ. Microbiol. 76: 6707–6714. doi: 10.1128/AEM.00692-10 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Chen M. H., Wang S. W., Hwang W. Z., Tsai S. J., Hsih Y. C., Chiou C. S., Tsen H. Y.2010. Contamination of Salmonella Schwarzengrund cells in chicken meat from traditional marketplaces in Taiwan and comparison of their antibiograms with those of the human isolates. Poult. Sci. 89: 359–365. doi: 10.3382/ps.2009-00001 [DOI] [PubMed] [Google Scholar]
- 7.Chen S., Zhao S., White D. G., Schroeder C. M., Lu R., Yang H., McDermott P. F., Ayers S., Meng J.2004. Characterization of multiple-antimicrobial-resistant salmonella serovars isolated from retail meats. Appl. Environ. Microbiol. 70: 1–7. doi: 10.1128/AEM.70.1.1-7.2004 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8.Chuma T., Miyasako D., Dahshan H., Takayama T., Nakamoto Y., Shahada F., Akiba M., Okamoto K.2013. Chronological change of resistance to β-lactams in Salmonella enterica serovar Infantis isolated from broilers in Japan. Front. Microbiol. 4: 113. doi: 10.3389/fmicb.2013.00113 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Duc V. M., Nakamoto Y., Fujiwara A., Toyofuku H., Obi T., Chuma T.2019. Prevalence of Salmonella in broiler chickens in Kagoshima, Japan in 2009 to 2012 and the relationship between serovars changing and antimicrobial resistance. BMC Vet. Res. 15: 108. doi: 10.1186/s12917-019-1836-6 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.El-Sharkawy H., Tahoun A., El-Gohary A. E. A., El-Abasy M., El-Khayat F., Gillespie T., Kitade Y., Hafez H. M., Neubauer H., El-Adawy H.2017. Epidemiological, molecular characterization and antibiotic resistance of Salmonella enterica serovars isolated from chicken farms in Egypt. Gut Pathog. 9: 8. doi: 10.1186/s13099-017-0157-1 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11.Frana T. S., Carlson S. A., Griffith R. W.2001. Relative distribution and conservation of genes encoding aminoglycoside-modifying enzymes in Salmonella enterica serotype typhimurium phage type DT104. Appl. Environ. Microbiol. 67: 445–448. doi: 10.1128/AEM.67.1.445-448.2001 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12.Gast R. K.2008. Paratyphoid infections. pp. 636–665. In: Disease of Poultry, 12th ed. (Saif, Y. M., Fadly, A. M., Glisson, J. R., McDougald, L. R., Nolan, L. K. and Swayne, D. E. eds.), Blackwell Publishing, Ames. [Google Scholar]
- 13.Gebreyes W. A., Thakur S.2005. Multidrug-resistant Salmonella enterica serovar Muenchen from pigs and humans and potential interserovar transfer of antimicrobial resistance. Antimicrob. Agents Chemother. 49: 503–511. doi: 10.1128/AAC.49.2.503-511.2005 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.Gyles C. L.2008. Antimicrobial resistance in selected bacteria from poultry. Anim. Health Res. Rev. 9: 149–158. doi: 10.1017/S1466252308001552 [DOI] [PubMed] [Google Scholar]
- 15.Moreno L. Z., Gomes V. T. M., Moreira J., de Oliveira C. H., Peres B. P., Silva A. P. S., Thakur S., La Ragione R. M., Moreno A. M.2019. First report of mcr-1-harboring Salmonella enterica serovar Schwarzengrund isolated from poultry meat in Brazil. Diagn. Microbiol. Infect. Dis. 93: 376–379. doi: 10.1016/j.diagmicrobio.2018.10.016 [DOI] [PubMed] [Google Scholar]
- 16.Saraiva Mauro M. S., Moreira Filho Alexandre L. B. Moreira Filho, Freitas Neto Oliveiro C. , Silva Núbia M. V., Givisiez Patrícia E. N., Gebreyes Wondwossen A., Oliveira Celso J. B.2018. Off-label use of ceftiofur in one-day chicks triggers a short-term increase of ESBL-producing E.coli in the gut. PLoS One. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17.Patrick A. D. Grimont and Francois-Xavier Weill. 2007. Antigenic formulae of the Salmonella serovars 9th ed. WHO Collaborating Centre for Reference and Research on Salmonella. Institute Pasteur, Paris. [Google Scholar]
- 18.Sasaki Y., Ikeda A., Ishikawa K., Murakami M., Kusukawa M., Asai T., Yamada Y.2012. Prevalence and antimicrobial susceptibility of Salmonella in Japanese broiler flocks. Epidemiol. Infect. 140: 2074–2081. doi: 10.1017/S0950268812000039 [DOI] [PubMed] [Google Scholar]
- 19.Schwarz S., Chaslus-Dancla E.2001. Use of antimicrobials in veterinary medicine and mechanisms of resistance. Vet. Res. 32: 201–225. doi: 10.1051/vetres:2001120 [DOI] [PubMed] [Google Scholar]
- 20.Shahada F., Amamoto A., Chuma T., Shirai A., Okamoto K.2007. Antimicrobial susceptibility phenotypes, resistance determinants and DNA fingerprints of Salmonella enterica serotype Typhimurium isolated from bovine in Southern Japan. Int. J. Antimicrob. Agents 30: 150–156. doi: 10.1016/j.ijantimicag.2007.03.017 [DOI] [PubMed] [Google Scholar]
- 21.Shahada F., Sugiyama H., Chuma T., Sueyoshi M., Okamoto K.2010. Genetic analysis of multi-drug resistance and the clonal dissemination of β-lactam resistance in Salmonella Infantis isolated from broilers. Vet. Microbiol. 140: 136–141. doi: 10.1016/j.vetmic.2009.07.007 [DOI] [PubMed] [Google Scholar]
- 22.Shigemura H., Matsui M., Sekizuka T., Onozuka D., Noda T., Yamashita A., Kuroda M., Suzuki S., Kimura H., Fujimoto S., Oishi K., Sera N., Inoshima Y., Murakami K.2018. Decrease in the prevalence of extended-spectrum cephalosporin-resistant Salmonella following cessation of ceftiofur use by the Japanese poultry industry. Int. J. Food Microbiol. 274: 45–51. doi: 10.1016/j.ijfoodmicro.2018.03.011 [DOI] [PubMed] [Google Scholar]
- 23.Wierup M., Wahlström H., Lahti E., Eriksson H., Jansson D. S., Odelros Å., Ernholm L.2017. Occurrence of Salmonella spp.: a comparison between indoor and outdoor housing of broilers and laying hens. Acta Vet. Scand. 59: 13. doi: 10.1186/s13028-017-0281-4 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 24.Yamazaki W., Uemura R., Sekiguchi S., Dong J. B., Watanabe S., Kirino Y., Mekata H., Nonaka N., Norimine J., Sueyoshi M., Goto Y., Horii Y., Kurogi M., Yoshino S., Misawa N.2016. Campylobacter and Salmonella are prevalent in broiler farms in Kyushu, Japan: results of a 2-year distribution and circulation dynamics audit. J. Appl. Microbiol. 120: 1711–1722. doi: 10.1111/jam.13141 [DOI] [PubMed] [Google Scholar]
- 25.Antibiotics Tested by NARMS (National Antimicrobial Resistance Monitoring System for Enteric Bacteria). https://www.cdc.gov/narms/antibiotics-tested.html [accessed on October 23, 2019].