Salmonella enterica subsp. enterica serovar Heidelberg is most often associated with eggs and poultry (Hennessy et al., 2004; Chittick et al., 2006; Foley et al., 2008; Folster et al., 2012; Jackson et al., 2013). However, this scenario has changed since Salmonella Heidelberg has been associated with other sources from a broad host range, such as dairy calves (Centers for Disease Control and Prevention [CDC], 2017), swine (Cabral et al., 2017), wild birds (Liakopoulos et al., 2016a), environmental sources (Antony et al., 2018), human-derived clinical specimens (Abdullah et al., 2021), and outbreaks (Antony et al., 2018), which denotes their importance as a high-priority pathogen.

One of the most important risk factors surrounding foodborne illness is the international food trade that has been circumstantially accompanied by Salmonellae dispersal beyond borders. In this context, there is a global dispersal of multidrug-resistant lineages of serovar S. Heidelberg, reaching various countries in North America (Andrysiak et al., 2008; Centers for Disease Control and Prevention, 2014; Public Health Agency of Canada, 2014; Deblais et al., 2018; Cox et al., 2021), South America (Kipper et al., 2021), Europe (Liakopoulos et al., 2016b; Campos et al., 2018), and Asia (Wu et al., 2013). Therefore, the simultaneous increase and extended protraction of S. Heidelberg in many parts of the world have favored their genetic acquisition of virulence and antimicrobial resistance (AMR) genes through horizontal gene transfer (HGT), which has ultimately led to one of the most pressing global concerns.

Owing to their importance as a key poultry producer globally, Brazil quickly became the hotspot of S. Heidelberg and urgent actions were needed from the food safety authorities to mitigate this pathogen in order to reduce the economic losses in the poultry sector. In this context, most investigations to detect Salmonella enterica serovars in the poultry sector still had important methodological gaps, since the food industry focuses especially on Salmonella Typhimurium and Salmonella Enteritidis, which demonstrate the need for a combined approach between classical microbiology and high-resolution methods such as whole-genome sequencing (WGS) and clustered regularly interspaced short palindromic repeats (CRISPR) genotyping (Barrangou and Dudley, 2016; Thompson et al., 2018; Yousfi et al., 2020). Indeed, the use of these high-throughput sequencing analyses exemplifies a useful means, not only for identifying Salmonella serovars but also to trace back the origin of the contamination conferring a substantial aid in decision-making to the poultry sector. In this regard, we demonstrated the usefulness of WGS-based identification in our previous study for genotyping rare Salmonella enterica serovars isolated from food and related sources (Monte et al., 2021). This previous survey demonstrated that the CRISPR arrays were highly conserved, and this genomic inspection provides high-resolution genotyping of Salmonella serovars. Hence, we performed a genomic study by combining WGS and CRISPR genotyping to characterize S. Heidelberg isolates from different sources at broiler farms, slaughterhouses, transport, lairages, and retail markets in Brazil.

There has been a great interest in surveying the adaptation of Salmonella serovars to the poultry production chain because of their extensive persistence in the past, notably with S. Typhimurium and S. Enteritidis, which have caused significant economic losses to this sector. Furthermore, the prevalence of S. Heidelberg shown in this study is not the only issue, but the fact that highly drug-resistant and/or MDR isolates are being recovered in most steps of the poultry production chain, particularly in Brazil could be considered a public health threat, as there is a risk of it becoming globalized.

Based on AMR results, the β-lactam resistance was essentially driven by the presence of plasmid-mediated AmpC (pAmpC) bla_CMY–2 gene, given the isolates which did not carry this gene were susceptible to FOX, while QRDR such as gyrA and parC genes drove quinolone resistance (Table 1). Indeed, the presence of strains harboring bla_CMY–2 gene could have implications on a one health interface, since this plasmid is more likely to persist (Teunis et al., 2018). Besides that, all strains harbored chromosomal mutations in gyrA and parC genes promoting high-level resistance against quinolones that could have implications on human health as treatment options become limited. Disturbingly, this result corroborates the findings by van den Berg et al. (2019) that found 98.4% of the S. Heidelberg isolates resistant to fluoroquinolones. On the other hand, all S. Heidelberg strains from our collection, displayed susceptibility to azithromycin, which could be considered a promising agent against Salmonellae infections (Crump et al., 2015; Wen et al., 2017). Azithromycin has been used as an alternative treatment option for enteric fever even when the guidelines on susceptibility testing were not available. Like fluoroquinolones, azithromycin is an antimicrobial agent with efficient intracellular penetration (Crump et al., 2015; Wen et al., 2017).

The total AMR load also included encoding resistance genes for aminoglycoside [aac(3)-VIa, aph(3′)-Ic, aadA1, aadA2], chloramphenicol (cmlA1), macrolides [Inu(G)], trimethoprim (dfrA12), and ammonium quaternary compounds (qacL) (Figure 1). Interestingly, the unique strain (SSc139) that carried the qacL gene was isolated from the retail market, which makes large use of such compounds to disinfect surfaces. Moreover, this strain nested in the same cluster with five strains isolated from broiler farms and slaughterhouse sources, in different states such as São Paulo, Santa Catarina, and Parana, denoting the successful establishment of this lineage (ST15) in the Southern and South regions of Brazil. In this regard, we visualized the co-occurrence of AMR genes in all S. Heidelberg strains.

The presence of AMR genes and transmissible plasmids demonstrated little variation across the strains. The broad distribution and abundance of S. Heidelberg in broiler farms, slaughterhouses, transport, lairages, and retail markets suggest the high adaptability of this serovar in the poultry production chain in Brazil. Similarly, a study by Edirmanasinghe et al. (2017) examining FOX-resistant S. Heidelberg strains isolated from human, abattoir poultry, and retail poultry sources in Canada revealed a potential common source among strains, which suggest the simultaneous dispersal of S. Heidelberg strains carrying CMY-2 gene in several sources and different geographical locations. In convergence with our results, another study revealed a high occurrence of S. Heidelberg in imported poultry meat in the Netherlands containing bla_CMY–2 gene (van den Berg et al., 2019). Although we could not find colistin-resistant strains, it is worthwhile to note the presence of the IncX4 plasmid since it is a promiscuous plasmid with a high capacity of self-transmissibility that is commonly associated with the presence of plasmid-mediated colistin-resistance (mcr-1) in Brazil (Moreno et al., 2019).

Consistent with observations obtained in our study, recent surveys strongly support that S. Heidelberg may originate from a common ancestor, which circulates and persists in the Brazilian poultry production chain since at least 2004 (Kipper et al., 2021). More importantly, these strains are also nested with strains isolated from several sources and countries around the world supporting the hypotheses of intercontinental spread, which demonstrate that probably the common ancestor underwent diversification through genetic changes over time (Supplementary Material).

Protracted dissemination of S. Heidelberg via poultry might be a risk for a globalized food trade era. The introduction and clonal expansion of Salmonella strains across borders remain challenging due to the difficulties of identifying the origins of contamination. In light of this, the continued need for combined approaches between classical microbiology and high-resolution methods such as WGS and CRISPR genotyping truly illustrate to us what is hidden in plain sight.

For the purpose of discussion, other studies provide compelling validation data to support the usefulness of high-resolution methods for genotyping rare Salmonella enterica serovars (Monte et al., 2021) and/or to resolve S. Heidelberg isolates involved in foodborne outbreaks (Vincent et al., 2018). Of the latter, while assessing the CRISPR array of 145 S. Heidelberg isolates, Vincent et al. (2018) found 15 different CRISPR profiles endorsing our results.

This study further illustrates the potential of CRISPR for the tracking of variable genotypes in diverse Salmonella strains, as previously determined (DiMarzio et al., 2013; Shariat et al., 2013a,b, 2015; Monte et al., 2021), with noteworthy methodological convenience. Indeed, CRISPR-based analyses have proven relevant for subtyping of Salmonella enterica serovars Typhimurium and Heidelberg strains involved in outbreaks (Shariat et al., 2013b) and occasionally associated with antibiotic resistance (DiMarzio et al., 2013). Our findings underscore the potential role of S. Heidelberg as a key pathogen in the poultry production chain, particularly in Brazil.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/Supplementary Material.

DM, RB, MN, PF-C, and ML designed the study. RB, PF-C, NL, and ML supervised the work. DM, MN, HB, SK, NL, PF-C, RB, and ML participated, coordinated, and analyzed the data. DM, MN, and RB wrote the original draft. All authors approved the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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