Chickens can acquire bacteria at different stages, and bacterial diversity can occur due to production practices, diet, and environment. The changes in consumer trends have led to increased animal production, and chicken meat is one of the most consumed meats. To ensure high levels of production, antimicrobials have been used in livestock for therapeutic purposes, disease prevention, and growth promotion, contributing to the development of antimicrobial resistance across the resident microbiota. Enterococcus spp. and Escherichia coli are normal inhabitants of the gastrointestinal microbiota of chickens that can develop strains capable of causing a wide range of diseases, i.e., opportunistic pathogens. Enterococcus spp. isolated from broilers have shown resistance to at least seven classes of antibiotics, while E. coli have shown resistance to at least four.
Stępień-Pyśniak et al. (2021) carried out a study that included 35 Polish E. faecalis and 41 Danish E. faecalis strains collected during post-mortem examination from broiler chicks showing lesions characteristic of yolk sac infection. The most prevalent clonal lineage among the Polish isolates was ST59, followed by ST282 and ST16. Regarding the Danish isolates, the most prevalent clonal lineages were ST116 and ST16. Only two Danish isolates were identified as VRE, and one belonged to the ST387 clonal lineage, while the other belonged to ST838 [127]. A Brazilian study that analyzed 12 E. faecalis strains isolated from natural cases of vertebral osteomyelitis in broilers revealed that almost half of these belonged to ST49. In addition, ST202 was represented by one strain that was vancomycin-resistant [93]. A study performed with 45 E. faecalis strains isolated from the cloaca of healthy broilers in Saudi Arabia reported that most of those strains belonged to ST16, ST302, and ST179, respectively. Two isolates were VRE, and these also belonged to ST16 [128]. In China, 61 strains of E. faecalis isolated from the cecal tissue of broiler chickens with swollen cecal lesions belonged to 34 sequence types, and the most prevalent was ST631 [129]. Kim et al. (2018) studied the molecular characteristics of 85 E. faecalis strains isolated from chicken meat samples, and ST256 was observed in over 50% of the isolates [130]. E. faecalis strains isolated from retail chicken carcasses in the Emirate of Abu Dhabi were assigned to five different sequence types, and half of them belonged to the clonal lineage ST476 [101]. ST314, followed by ST16, were the most prevalent clonal lineages reported among broilers across Australia [60]. Overall, the most frequent and wide-ranging clonal lineage that has been identified among E. faecalis isolated from broilers or broiler meat since 2018 is ST16. This sequence type has already been identified in Poland, the Netherlands, Saudi Arabia, China, and Australia, and in both vancomycin-resistant and vancomycin-susceptible E. faecalis.
Leinweber et al. (2018) isolated three vancomycin-resistant E. faecium (VREfm) strains from Danish chicken meat, and all the strains belonged to ST32 [120]. VREfm strains were also isolated from cecal samples from healthy broilers in Sweden, but all of these belonged to ST310 [111]. In Turkey, a study that included vancomycin-susceptible E. faecium and VREfm isolated from broiler cloaca reported that ST1346 was the most prevalent clonal lineage among vancomycin-susceptible E. faecium, while all VREfm presented different and novel STs (ST1341, ST1342, ST1343, ST1244, and ST1345) [131]. A study that included 30 E. faecium strains isolated from the cloaca of healthy broilers in Saudi Arabia reported that most of those strains belonged to ST194, ST82, and ST157, respectively [128]. Kim et al. (2018) isolated one E. faecium strain from chicken meat samples that was revealed to belong to ST451 [130]. E. faecium isolated from retail chicken carcasses in Abu Dhabi Emirate has been assigned to four different sequence types: one known ST (ST195) and three novel STs (ST2236, ST2238, and ST2239) [101]. ST492, followed by ST195 and ST241, were the most prevalent clonal lineages reported among broilers across Australia [60]. Overall, E. faecium isolates from broilers or broiler meat since 2018 do not share many clonal lineages. However, ST194 and ST195 were already identified in two different sources (broilers and broiler meat) on at least two different continents.
Päivärinta et al. (2020) collected broiler cecal samples from a high-capacity slaughterhouse and from vacuum-packed raw broiler meat without marinade intended for consumer use, all from the same high-capacity slaughterhouse. In total, three ESBL-producing E. coli strains were isolated: two from the ceca that belonged to ST1594, and one from the meat that belonged to ST351 [175]. Retail chicken meat was also studied in Egypt, and ST1196 was the most prevalent sequence type among ESBL-producing E. coli, while ST156 and ST189 were identified among non-ESBL-producing E.coli [198]. Broilers infected with colibacillosis were studied in Norway, Croatia, Tunisia, and Pakistan [144][147][199][200]. In the Norwegian study, ST429 accounted for over 60% of the clonal lineages identified in E. coli isolates [144]. However, in Croatia, ST429 was reported at a much lower rate (0.65%). The most prevalent sequence types in Croatia were ST95 and ST117 [199]. ST117 was also predominant among the Pakistani E. coli isolates from broilers with colibacillosis [201]. The Tunisian study reported four different sequence types in ESBL-producing E. coli strains, with the majority belonging to ST4187 [147]. Two different Pakistani studies that included cecal and fecal samples from broilers reported ST131 between the most prevalent sequence types in ESBL-producing E. coli strains [202][203]. A study carried out by Aslantaş (2020) in Turkey detected 19 sequence types in 28 ESBL-producing E. coli isolates, and the most prevalent were ST114 and ST354 [200]. In Australia, ESBL-producing E. coli isolated from healthy broilers belonged to different clonal lineages, while E. coli from chickens with colibacillosis belonged mainly to ST354 [143]. Overall, according to the studies mentioned in Table 5, the most frequent and wide-ranging clonal lineage that was identified in both ESBL-producing and non-ESBL-producing E. coli isolated from broilers or broiler meat since 2020 was ST117.
Recently, broilers have increased significantly as a meat source, and the largest broiler meat producers worldwide include the United States, China, and Brazil, respectively. Within the European Union, Poland, Spain, Germany, France, and Italy present the higher gross domestic production of broilers [217]. Broiler meat produced by some of these countries is exported globally [23]. For example, Brazil is the world’s largest poultry exporter; about a third of Brazil’s chicken production is exported—4.6 million out of 14.3 million metric tons in 2020—to over 150 countries worldwide [218]. Therefore, ongoing surveillance systems for antimicrobial resistance in broiler production are mandatory to avoid the spread of antimicrobial resistance among broiler meat or other foods derived from these animals.
This entry is adapted from the peer-reviewed paper 10.3390/ani13081362