High resistance to tetracycline and ciprofloxacin in bacteria isolated from poultry farms in Ibadan, Nigeria

Interference of gastrointestinal barriers with antibiotic susceptibility of foodborne pathogens: an in vitro case study of ciprofloxacin and tetracycline against Salmonella enterica and Listeria monocytogenes

Minimum inhibitory concentrations (MIC) assays are often questioned for their representativeness. Especially when foodborne pathogens are tested, it is of crucial importance to also consider parameters of the human digestive system. Hence, the current study aimed to assess the inhibitory capacity of two antibiotics, ciprofloxacin and tetracycline, against Salmonella enterica and Listeria monocytogenes, under representative environmental conditions. More specifically, aspects of the harsh environment of the human gastrointestinal tract (GIT) were gradually added to the experimental conditions starting from simple aerobic lab conditions into an in vitro simulation of the GIT. In this way, the effects of parameters including the anoxic environment, physicochemical conditions of the GIT (low gastric pH, digestive enzymes, bile acids) and the gut microbiota were evaluated. The latter was simulated by including a representative consortium of selected gut bacteria species. In this study, the MIC of the two antibiotics against the relevant foodborne pathogens were established, under the previously mentioned environmental conditions. The results of S. enterica highlighted the importance of the anaerobic environment when conducting such studies, since the pathogen thrived under such conditions. Inclusion of physicochemical barriers led to exactly opposite results for S. enterica and L. monocytogenes since the former became more susceptible to ciprofloxacin while the latter showed lower susceptibility towards tetracycline. Finally, the inclusion of gut bacteria had a bactericidal effect against L. monocytogenes even in the absence of antibiotics, while gut bacteria protected S. enterica from the effect of ciprofloxacin.

Unveiling the landscape of resistance against high priority critically important antimicrobials in food-producing animals across Africa: A scoping review

The rapid population growth in Africa is associated with an increasing demand for livestock products which in turn can lead to antimicrobial use. Antimicrobial usage in animals contributes to the emergence and selection of resistant bacteria which constitutes a serious public health threat. This study aims to review and summarize the available information on highest priority critically important antimicrobials (HPCIAs) resistance in livestock production in Africa. This work will help to inform future policies for controlling antimicrobial resistance (AMR) in the food production chain. A scoping review was conducted according to the Cochrane handbook and following PRISMA 2020 guidelines for reporting. Primary research studies published after 1999 and reporting resistance of Escherichia coli, Enterococcus spp, Staphylococcus aureus, Salmonella spp, and Campylobacter spp to HPCIAs in poultry, cattle, pigs, goats, and sheep in Africa were searched in four databases. A total of 312 articles were included in the review. The majority of the studies (40.7) were conducted in North African countries. More than 49.0% of included studies involved poultry and 26.2% cattle. Cephalosporins and quinolones were the most studied antimicrobial classes. Of the bacteria investigated in the current review, E. coli (41.7%) and Salmonella spp (24.9%) represented the most commonly studied. High levels of resistance against erythromycin in E. coli were found in poultry (MR 96.1%, IQR 83.3-100.0%), cattle (MR 85.7%, IQR 69.2-100.0%), and pigs (MR 94.0%, IQR 86.2-94.0%). In sheep, a high level of resistance was observed in E. coli against nalidixic acid (MR 87.5%, IQR 81.3-93.8%). In goats, the low level of sensibility was noted in S. aureus against streptomycin (MR 86.8%, IQR 19.4-99.0%). The study provides valuable information on HPCIAs resistance in livestock production in Africa and highlights the need for further research and policies to address the public health risk of AMR. This will likely require an investment in diagnostic infrastructure across the continent. Awareness on the harmful impact of AMR in African countries is a requirement to produce more effective and sustainable measures to curb AMR.

Prevalence of Antimicrobial Resistance Among the Hydrogen Sulfide Producing Bacteria Isolated on XLD Agar from the Poultry Fecal Samples

Poultry products remain as one of the most popular and extensively consumed foods in the world and the introduction of hydrogen sulfide (H2S) producing antibiotic resistant bacterial species into it is an emerging challenge. The current study has been designed to analyze the distribution of antibiotic resistance among the H2S producing bacteria isolated from the fecal samples of chickens from different poultry farms. Here, twenty bacterial isolates were selected based on their ability to produce H2S on XLD agar, and the16S rDNA sequencing was carried out for their molecular identification. The results showed the isolates as belong to Salmonella spp. and Citrobacter spp. and in the antibiotic susceptibility test (AST), three of the Salmonella strains were found to be resistant to antibiotics such as tetracycline, doxycycline, nalidixic acid, and amikacin. Also, fourteen Citrobacter strains showed resistance towards azithromycin, and furthermore, eleven of them were also resistant to streptomycin. Resistance towards tetracycline was observed among five of the Citrobacter strains, and seven were resistant to doxycycline. Further molecular screening by the PCR has showed three of the Salmonella strains along with eight Citrobacter isolates to have tetA gene along with four of the Citrobacter strains to have co-harbored blaTEM gene. The results on biofilm formation have also demonstrated three Salmonella strains along with nine Citrobacter strains to have the ability to form moderate biofilm. The study thus describes the occurrence of H2S producing multidrug-resistant bacteria in poultry feces, which might contribute towards the dissemination of antibiotic resistance genes to other microorganisms including human pathogens with likely risk to treat disease conditions.

Occurrence profiling, risk assessment, and correlations of antimicrobials in surface water and groundwater systems in Southwest Nigeria

The presence of antimicrobials in water has grown into a major global health concern. This study thus focused on the presence, ecological implications, and potential health risks associated with nine antimicrobials: five antibiotics (ampicillin, chloramphenicol, ciprofloxacin, metronidazole, and tetracycline) and four parabens (methylparaben, ethylparaben, propylparaben, and butylparaben) in surface water and groundwater samples collected from three Southwestern States in Nigeria (Osun, Oyo, and Lagos States). These antimicrobials were widely detected across the three States with ciprofloxacin being the most dominant having maximum average concentrations of 189 μg L-1 and 319 μg L-1 in surface water and groundwater respectively. The range of average concentrations of antibiotics in surface water are 47.3-235 μg L-1 (Osun), 27.9-166 μg L-1 (Oyo) and 52.1-159 μg L-1 (Lagos). For groundwater, it is 35.3-180 μg L-1 (Osun), 26.5-181 μg L-1 (Oyo) and 32.3-319 μg L-1 (Lagos). The average concentrations of all parabens were 32.4-153 μg L-1, 53.4-80.1 μg L-1, and 83.2-132 μg L-1 for surface water and 46.7-55.7 μg L-1, 53-117 μg L-1, and 62.4-118 μg L-1 for groundwater in Osun, Oyo, and Lagos States respectively. Methylparaben was most frequently detected paraben with average concentrations of 153 μg L-1 and 117 μg L-1 in surface water and groundwater respectively. The measured environmental concentrations of these antimicrobials pose a significant ecological risk while those of ciprofloxacin and ampicillin pose a high health risk to all population groups studied. The average concentrations of antibiotics investigated in this study exceeded their threshold values for Predicted No-Effect Concentrations (PNEC) associated with resistance selection, except for tetracycline.

Genetic characterization of Escherichia coli and Klebsiella spp. from humans and poultry in Nigeria

The emergence of antibiotic resistance in livestock, especially food-producing animals, is of major public health importance as a result of the possibility of these bacteria entering the food chain. In this study, the genetic characteristics of antibiotic-resistant Escherichia coli and Klebsiella spp. isolates from humans and poultry in Edo state, Nigeria, were investigated. In April 2017, 45 Klebsiella spp. and 46 E. coli isolates were obtained from urine, clinical wounds, nasal and chicken faecal samples. Isolates were recovered and identified as previously described. Species identification was achieved by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry and ribosomal multilocus sequence typing. Antimicrobial susceptibility testing was carried out using the Kirby-Bauer method for 12 antibiotics. A double disc synergy test was used to screen for extended-spectrum beta-lactamse (ESBL) production. Whole genome sequencing was performed for strain characterization of the isolates. Thirteen Klebsiella spp. isolates yielded positive results by the ESBL phenotypic test and harboured ESBL genes. Of the 46 E. coli isolates, 21 human and 13 poultry isolates were resistant to at least one of the tested antibiotics. Four human E. coli isolates harboured ESBL genes and revealed positive results when applying ESBL double disc synergy tests. ESBL genes in the Klebsiella spp. and E. coli isolates include bla CTX-M-15 and bla SHV-28. Whole genome-based core gene multilocus sequence typing of the Klebsiella spp. and E. coli isolates revealed a close relatedness among the isolates. An integrated 'One Health' surveillance system is required to monitor transmission of antimicrobial resistance in Nigeria.

Antibiotic use in infants within the first year of life is associated with the appearance of antibiotic-resistant genes in their feces

Background

Antibiotic resistance, an increasing challenge, is not only a national threat but also a global threat. Carriage of resistance genes is not limited to adults alone, various microbiota niches present in the body system of children have been found to harbor bacteria carrying resistant genes, especially, their gut microbiota. This study aims to identify selected antibiotic-resistant genes from the fecal samples of infants and the association of antibiotics use with the occurrence of resistant genes in the infant's gut.

Methods

A total number of 172 metagenomic DNA samples previously extracted from stool samples of 28 Nigerian babies longitudinally within their first year of life were screened for the presence of ESBL genes (blaSHV, blaTEM, and blaCTX-M), PMQR genes (qnrA, qnrB, qnrS, qepA), ribosomal protection protein tetracycline resistance gene, (RPP) β-lactamase (blaZ), macrolide (ermA, ermB, mefA/E), aminoglycoside modifying enzymes gent^R (aac(6')/aph(2″)) and dfrA genes by PCR. Nineteen (19) of the 28 babies used antibiotics during the study. The association between antibiotic use by the babies within the first year of life and occurrence of resistant genes were analyzed by Spearman rank correlation.

Results

One hundred and twenty-two (122) samples (71%) out of the 172 isolates had antibiotic-resistance genes. PMQR genes were absent in all the samples. Three isolates had blaTEM gene, nine isolates had blaSHV gene, six isolates had blaCTX-M gene and 19 isolates had dfrA gene, 31 samples had tet gene, 29 samples had mef gene, 27 samples had ermB gene, four samples had ermA gene, 13 samples had blaZ gene and 16 samples had aac gene. The babies whose samples had resistant genes used antibiotics in the same months the samples were collected. Interestingly, the 11 babies whose samples had the dfrA gene all used antibiotics in the same months their samples were collected but none of them used trimethoprim/sulfamethoxazole antibiotic. The overall correlation matrix of the babies showed a strong association between antibiotic use (AU) and antibiotic use presence of resistance genes (AUPRG) with a coefficient of 0.89. Antibiotic-resistant genes are present in the gut of infants and their occurrence is strongly connected with antibiotic use by infants.

Antibiotic resistance genes, mobile elements, virulence genes, and phages in cultivated ESBL-producing Escherichia coli of poultry origin in Kwara State, North Central Nigeria

The paucity of information on the genomic diversity of drug-resistant bacteria in most food-producing animals, including poultry in Nigeria, has led to poor hazard characterization and the lack of critical control points to safeguard public health. Hence, this study used whole genome sequencing (WGS) to assess the presence and the diversity of antibiotic resistance genes, mobile genetic elements, virulence genes, and phages in Extended Spectrum Beta Lactamase producing Escherichia coli (ESBL - E. coli) isolates obtained from poultry via the EURL guideline of 2017 in Ilorin, Nigeria. The prevalence of ESBL - E. coli in poultry was 10.5 % (n = 37/354). The phenotypic antibiotic susceptibility testing showed that all the ESBL- E. coli isolates were multi-drug resistant (MDR). The in-silico analysis of the WGS raw-read data from 11 purposively selected isolates showed that the isolates had a wide array of ARGs that conferred resistance to beta-lactam antibiotics, and 8 other classes of antibiotics (fluoroquinolones, foliate pathway antagonists, aminoglycoside, phenicol, tetracycline, epoxide, macrolides, and rifamycin). All the ARGs were in the bacterial chromosome except in two isolates where plasmid-mediated quinolone resistance (PMQR) was detected. Two isolates carried the gyrAp.S83L mutation which confers resistance to certain fluoroquinolones. The mobilome consisted of several Col-plasmids and the predominant IncF plasmids belonged to the IncF64:A-:B27 sequence type. The virulome consisted of genes that function as adhesins, iron acquisition genes, toxins, and protectins. Intact phages were found in 8 of the 11 isolates and the phageome consisted of representatives of four families of viruses: Myoviridae (62.5 %, n = 5/8), Siphoviridae (37.5 %, n = 3/8), Inoviridae (12.5 %, n = 1), and Podoviridae (12.5 %, n = 1/8). ESBL - E. coli isolates harboured 1-5 intact phages and no ARGs were identified on any of the phages. Although five of the isolates belonged to phylogroup A, the isolates were diverse as they belonged to different serotype and sequence types. Our findings demonstrate the high genomic diversity of ESBL - E. coli of poultry origin in Ilorin, Nigeria. These diverse isolates harbor clinically relevant ARGs, mobile elements, virulence genes, and phages that may have detrimental zoonotic potentials on human health.

Antimicrobial resistance, virulence associated genes and phylogenetic background versus plasmid replicon types: the possible associations in avian pathogenic Escherichia coli (APEC)

BACKGROUND

Antimicrobial resistance (AMR) in bacterial isolates from food producing animals not only challenges the preventive and therapeutic strategies in veterinary medicine, but also threatens public health. Genetic elements placed on both chromosome and plasmids could be involved in AMR. In the present study, the associations of genomic backbone and plasmids with AMR were evaluated. We also provided some primary evidences that which genetic lineages potentially host certain groups of plasmids.

RESULTS

In the current study, 72 avian pathogenic Escherichia coli (APEC) strains were examined. Isolates resistant to tetracycline and trimethoprim-sulfamethoxazole (87.5%; each), and harboring bla_TEM (61.1%) were dominant. Moreover, phylogroup D was the most prevalent phylogroup in total (23.6%), and among multidrug-resistant (MDR) isolates (14/63). The most prevalent Inc-types were also defined as follows: IncP (65.2%), IncI1 (58.3%), and IncF group (54.1%). Significant associations among phylogroups and AMR were observed such as group C to neomycin (p = 0.002), gentamicin (p = 0.017) and florfenicol (p = 0.036). Furthermore, group D was associated with bla_CTX. In terms of associations among Inc-types and AMR, resistance to aminoglycoside antibiotics was considerably linked with IncP (p = 0.012), IncI1 (p = 0.038) and IncA/C (p = 0.005). The bla_TEM and bla_CTX genes presence were connected with IncI1 (p = 0.003) and IncFIC (p = 0.013), respectively. It was also shown that members of the D phylogroup frequently occured in replicon types FIC (8/20), P (13/47), I1 (13/42), HI2 (5/14) and L/M (3/3).

CONCLUSIONS

Accorging to the results, it seems that group D strains have a great potential to host a variety of plasmids (Inc-types) carrying different AMR genes. Thus, based on the results of the current study, phyogroup D could be a potential challenge in dealing with AMR in poultry. There were more strong correlations among Inc-types and AMR compared to phylotypes and AMR. It is suggested that in epidemiological studies on AMR both genomic backbone and major plasmid types should be investigated.

Knowledge, Attitudes, and Practices of Community Pharmacy Professionals on Poultry Antibiotic Dispensing, Use, and Bacterial Antimicrobial Resistance in Zambia: Implications on Antibiotic Stewardship and WHO AWaRe Classification of Antibiotics

Globally, the inappropriate dispensing and use of antibiotics in animals has contributed to the development of bacterial antimicrobial resistance (AMR). In Zambia, there is insufficient information among community pharmacy professionals on antibiotic use (ABU) and AMR in food-producing animals. This study assessed community pharmacy professionals’ knowledge, attitudes, and practices regarding poultry antibiotic dispensing, use, and bacterial AMR in the Lusaka district of Zambia. A cross-sectional study was conducted among 178 community pharmacy professionals between February and April 2022 using a semi-structured questionnaire. Data were analyzed using Stata version 17. Of the total participants (n = 178), 51.1% (n = 91) were pharmacists. The most dispensed antibiotic was oxytetracycline, a Watch antibiotic, mainly without prescriptions. Good knowledge of ABU and AMR was associated with work experience for more than one year (p = 0.016), while good practices were associated with male gender (p = 0.039) and work experience of more than one year (p = 0.011). The study found moderate knowledge, positive attitudes, and moderate practices of pharmacy professionals on poultry ABU and AMR. There was high dispensing of poultry antibiotics without prescriptions, which calls for strict implementation of antimicrobial stewardship and surveillance programs in poultry production in Zambia to reduce AMR.

Genetic Characterization of Antibiotic Resistant Enterobacteriaceae Isolates From Bovine Animals and the Environment in Nigeria

There is a link between antibiotic resistance in humans, livestock and the environment. This study was carried out to characterize antibiotic resistant bovine and environmental Enterobacteriaceae isolates from Edo state, Nigeria. A total of 109 consecutive isolates of Enterobacteriaceae were isolated from March–May 2015 from 150 fecal samples of healthy bovine animals from three farms at slaughter in Edo state Nigeria. Similarly, 43 Enterobacteriaceae isolates were also obtained from a total of 100 environmental samples from different sources. Isolates were recovered and identified from samples using standard microbiological techniques. Recovered isolates were pre-identified by the Microbact Gram-Negative identification system and confirmed with Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry and ribosomal multilocus sequence typing (rMLST). Antibiotic susceptibility testing was carried out by Kirby-Bauer method for 14 antibiotics. Whole genome sequencing (WGS) was carried out for isolate characterization and identification of resistance determinants. Out of 109 animal and 43 environmental Enterobacteriaceae isolates, 18 (17%) and 8 (19%) isolates based on selection criteria showed antibiotic resistance and were further investigated by whole genome sequencing (WGS). Resistance genes were detected in all (100%) of the resistant bovine and environmental Enterobacteriaceae isolates. The resistance determinants included β-lactamase genes, aminoglycoside modifying enzymes, qnr genes, sulfonamide, tetracycline and trimethoprim resistance genes, respectively. Out of the 18 and 8 resistant animal and environmental isolates 3 (17%) and 2 (25%) were multidrug resistant (MDR) and had resistance determinants which included efflux genes, regulatory systems modulating antibiotic efflux and antibiotic target alteration genes. Our study shows the dissemination of antibiotic resistance especially MDR strains among Nigerian bovine and environmental Enterobacteriaceae isolates. The presence of these resistant strains in animals and the environment constitute a serious health concern indicated by the difficult treatment options of the infections caused by these organisms. To the best of our knowledge we report the first detailed genomic characterization of antibiotic resistance in bovine and environmental Enterobacteriaceae isolates for Nigeria.

Prevalence and Antibiotic Resistance in Campylobacter spp. Isolated from Humans and Food-Producing Animals in West Africa: A Systematic Review and Meta-Analysis

Campylobacter species are one of the leading causes of gastroenteritis in humans. This review reports on the prevalence and antibiotic resistance data of Campylobacter spp. isolated from humans and food-producing animals in West Africa. A systematic search was carried out in five databases for original articles published between January 2000 and July 2021. Among 791 studies found, 38 original articles from seven (41%) out of the 17 countries in West Africa met the inclusion criteria. For studies conducted in food-producing animals, the overall pooled prevalence of Campylobacter spp. was 34% (95% CI: 25–45). The MDR prevalence was 59% (95% CI: 29–84) and half (50%, 13/26) of the animal studies had samples collected from the market. The human studies recorded a lower pooled prevalence of Campylobacter spp. (10%, 95% CI: 6–17), but a considerably higher rate of MDR prevalence (91%; 95% CI: 67–98). The majority (85%, 11/13) of the human studies took place in a hospital. Campylobacter jejuni and Campylobacter coli were the most common species isolated from both animals and humans. Our findings suggest that Campylobacter spp. is highly prevalent in West Africa. Therefore, improved farm hygiene and ‘One Health’ surveillance systems are needed to reduce transmission.

Drug- and Multidrug-Resistance Pattern of Enterobacteriaceae Isolated from Droppings of Healthy Chickens on a Poultry Farm in Southwest Ethiopia

Background

In Ethiopia, the precise attribution of animals and their food products as the sources of resistant strains and the consequences of it on human health have not yet been seriously evaluated. Therefore, the aim of this study was to assess the drug- and multidrug-resistance pattern of Enterobacteriaceae isolated from chicken droppings at Jimma University poultry farm, College of Agriculture and Veterinary Medicine, southwest of Ethiopia.

Methods

A cross-sectional descriptive study was conducted from April, 2018 to June, 2018. A total of 140 fresh chicken dropping samples were collected and transported to Jimma University Medical Microbiology Laboratory for analysis. All samples were inoculated on MacConkey agar and xylose lysine deoxycholate agar. Gram stain and relevant biochemical tests were done for identification of isolates. Antimicrobial susceptibilities were tested by the Kirby–Bauer disk diffusion method.

Results

Out of 140 chicken dropping samples, 61 (43.6%) showed bacterial growth. Of these, E. coli accounts for 39.0% followed by K. pneumoniae (22.0%), P. mirabilis (19.3%), and Salmonella species (17.7%). With regard to antibiotic resistance pattern, E. coli demonstrated a high rate of resistance against ampicillin (91.7%), tetracycline (75.0%), and trimethoprim-sulfamethoxazole (70.8%). K. pneumoniae showed a high resistance rate against ampicillin, trimethoprim-sulfamethoxazole, and tetracycline, with a resistance rate ranging from 76.9% to 85.6%. P. mirabilis and Salmonella spp. also showed high resistance against ampicillin, tetracycline, and trimethoprim-sulfamethoxazole with a resistance rate ranging from 72.7% to 83.3%. All isolates relatively showed lower resistance rates with a range of 20.8% to 41.7% against third-generation cephalosporins (ceftazidime and ceftriaxone), ciprofloxacin, and gentamicin. Totally, 32 (52.5%) of the isolates showed MDR against three or more antibiotics.

Conclusion

Antibiotic resistant isolates against commonly prescribed single and multiple drugs were common. This highlights that chickens in the farm may serve as the reservoirs of antibacterial resistant bacteria that might infect humans through the food chain. Therefore, emphasis on the usage of antibiotics in chicken farms has to be considered.

Whole genome sequencing of extended-spectrum β-lactamase genes in Enterobacteriaceae isolates from Nigeria

Extended Spectrum β-lactamase (ESBL)-producing Enterobacteriaceae are of major concern as they are implicated in multidrug resistant nosocomial infections. They are listed on a recently published global priority list of antibiotic-resistant bacteria by the World Health Organization which raises concern in both healthcare and community settings. This study aimed at determining the frequency of ESBL genes in multidrug resistant human clinical Enterobacteriaceae isolates from Edo state Nigeria and to characterize the resistance mechanisms using whole genome sequencing. A total of 217 consecutive clinical isolates of Enterobacteriaceae, selection based on inclusion criteria, were collected from March-May 2015 from three medical microbiology laboratories of hospitals in Edo state Nigeria. All isolates were analyzed using matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry. Antibiotic susceptibility testing was performed by Kirby-Bauer method and minimum inhibitory concentration (MIC) determination by E-test method. Double disc synergy test was used to screen for the production of ESBL. Whole genome sequencing (WGS) was performed for isolate characterization and identification of resistance determinants. Out of 217 consecutive clinical Enterobacteriaceae isolates, 148 (68.2%) were multi-drug resistant. Of these multi-drug resistant isolates, 60 (40.5%) were positive for the ESBL phenotypic test and carried ESBL genes. CTX-M-15 was the predominant ESBL found, among 93.3% (n = 56/60). Thirty-two plasmid incompatibility groups and 28 known and two new sequence types were identified among the ESBL isolates. The high occurrence of CTX-M-15 with associated resistant determinants in multidrug resistant Enterobacteriaceae harboring different plasmid incompatibility groups and sequence types calls for the need of continuous monitoring of this resistance threat to reduce its public health impact. To our knowledge, this study presents the first genomic characterization of ESBL production mediated by blaCTX-M-15 in human clinical isolates of Enterobacter hormaechei, Citrobacter werkmanii and Atlantibacter hermannii from Nigeria.

The dynamic of antibiotic resistance in commensal Escherichia coli throughout the growing period in broiler chickens: fast-growing vs. slow-growing breeds

Antimicrobial resistance (AMR) is an important threat to public health worldwide. Furthermore, different studies have demonstrated a close association between antibiotic use in animal production and AMR in humans. It is well known that it is necessary to reduce antibiotic administration in farms by finding effective alternative treatments, using more resistant breeds and improving animal welfare. However, to be able to assess the alternatives proposed, it is essential to study the epidemiology of AMR under production conditions. Hence, the aim of this study was to investigate the AMR dynamic in 2 genetic poultry breeds during the growing period. The study was performed in 2 experimental poultry houses to simulate real production conditions, and no antibiotics were administered during the growing period. In addition, 2 poultry breeds were used, fast-growing and slow-growing. To evaluate AMR evolution, Escherichia coli was selected as indicator bacterium. To this end, animals from each experimental group were sampled at different times: on day of arrival, at mid-period, and at slaughter day. In the laboratory, cecal content was removed and inoculated in selective media. Then, biochemical tests were performed to confirm E. coli. Finally, antibiotic susceptibility was assessed according to Decision 2013/653. At the onset of the cycle, significant differences were observed between breeds, as the E. coli strains isolated from fast-growing 1-day-old-chicks showed higher AMR rates. However, at the end of the period, no significant differences were found between breeds and their presence of resistant bacteria (above 95%). Therefore, although no antibiotics were administered during the growing period, a high level of AMR at slaughter day was demonstrated. Further studies are necessary to determine the main risk factors that increase the level of AMR throughout the productive cycle in broiler chickens. In conclusion, it is important to highlight that although it is crucial to control both antibiotic use and animal welfare during the growing period, measures should be taken at all levels of the production chain.