Background/Objectives: Antimicrobial resistance (AMR) threatens global public health. This systematic review and meta-analysis, as part of the “ENVIRE” project (interventions to control the dynamics of antimicrobial resistance from chickens through the environment), assesses the prevalence of phenotypic and genotypic resistance, including extended-spectrum beta-lactamases (ESBLs), AmpC beta-lactamases, carbapenemases, colistin, and fluoroquinolone resistance, in broiler chickens and their environment. Methods: The analysis covers the years 2002–2022, focusing on Escherichia (E.) coli, Klebsiella spp., Enterobacter spp., and Citrobacter spp. in fecal, meat, environmental, and other-than-feces samples from observational studies published in PubMed and Web of Science. Quality assessment was performed using the Alberta Heritage Foundation criteria. Results: Data from 170 studies, conducted in Europe, North Africa, and North America, were included. The most frequently studied resistance was to beta-lactam, with focus on ESBL-producing and AmpC beta-lactamase isolates. The pooled prevalence of ESBL-resistant E. coli observed in meat samples at 41% and in fecal samples at 38% demonstrated significant heterogeneity between the studies. The negative binomial regression analysis of prevalence data revealed significantly higher ESBL-producing E. coli rates in European meat samples compared to North African samples. Conclusions: This systematic review revealed substantial variation in prevalence and emphasizes the need for standardized surveillance systems and robust study designs.

Antimicrobial-resistant bacteria originating from broilers pose an ongoing challenge as they can spread in the environment and food chain. One approach to lower colonization is to administer live bacteria to broilers. A live Escherichia coli (E. coli) vaccine, consisting of a single E. coli strain used as a competitive exclusion (CE) approach, was evaluated to decrease the colonization with ESBL-/pAmpC- producing E. coli. 168 ESBL-/pAmpC- negative Ranger Gold broilers were divided into six groups (3 x n = 46 experimental groups; 3 x n = 10 control groups for quality assurance). The E. coli vaccine was administered on day one through coarse spray or by drinking water on day five. Experimental groups were orally co-colonized with 10² cfu of one ESBL- (ST410, bla _CTX−M−15) and one pAmpC- producing E. coli (ST10, bla _CMY−2/mcr-1) on day three. Colonization status was monitored throughout the trial and quantified at the end of the study (day 49). A transient reduction in ESBL-producing E. coli colonization (p < 0.001) was observed following coarse spray administration. However, this decrease was not sustained over time. It can be concluded that a single E. coli strain originating from a live vaccine cannot decrease colonization of broilers with ESBL-/pAmpC- E. coli throughout a fattening period.

Background: Plasmid-mediated conjugation is a major form of horizontal gene transfer (HGT), facilitating dissemination of antimicrobial resistance (AMR) and the emergence of multi-drug-resistant (MDR) strains. In poultry, Escherichia coli producing extended-spectrum β-lactamases (ESBL) and plasmid-mediated AmpC β-lactamase (pAmpC) enzymes are common and contribute to antibiotic resistance. Additionally, plasmid-mediated colistin resistance gene mcr-1 in poultry requires attention, as it is a last-resort antibiotic in human medicine. Although plasmid-mediated conjugation is known to play a role in spreading antimicrobial resistance, its specific impact on resistance transmission within the broiler microbiota is still not well understood. We assessed conjugation dynamics of the mcr-1 gene from a pAmpC- E. coli to an ESBL- E. coli observed in an in vivo broiler chicken trial and compared them to conjugation frequencies under different in vitro conditions (LB broth, intestinal chicken cells in DMEM/F-12 medium, and DMEM/F-12 medium alone), with two initial bacterial loads. Results: From in vivo trial, among 138 broiler chickens sampled after a 49-day fattening period, transconjugants were detected in the cecal content of 35 broilers. The median conjugation frequency observed was of -5.02 log ₁₀ transconjugants/donor. Median conjugation frequencies across all in vitro conditions varied by less than one log unit (between − 6.8 and − 6.0 log ₁₀ transconjugants/donor), and no significant differences in conjugation efficiency were observed between initial bacterial concentrations. Conclusions: We confirmed bacterial conjugation between pAmpC-producing E. coli carrying the mcr-1 gene and ESBL-producing E. coli both in vitro and in vivo. The similar conjugation efficiencies observed across different in vitro methods suggest that experimental conditions have minimal influence under controlled settings. In contrast, the in vivo results underscore the significance of the host’s physiological environment in HGT. The presence of transconjugants after a 49-day fattening period indicates that intestinal bacteria function as reservoirs for resistance plasmids and could facilitate their spread throughout the broiler production chain. However, limitations like the possibility of plasmid transfer to other bacteria, unknown persistence of the plasmid in the gut, and potential modulations of transfer efficiency under antibiotic selection must be considered when interpreting the results.