The risk to import ESBL-producing Enterobacteriaceae and Staphylococcus aureus through chicken meat trade in Gabon

The global trade volume is steadily increasing and the exchange of various goods may contribute to economic wealth and development. However, global travel and trade bear the risk of transmitting pathogens through travelers or goods. While measles, syphilis, tuberculosis or smallpox have spread in the age of exploration and colonization, nowadays, new pathogens are emerging [1].

For instance, enterohemorrhagic Escherichia coli O104:H4, which caused an outbreak in central Europe in 2011, was most likely imported through seeds from Egypt [2]. Similarly, international travel, transfer of patients and medical tourists facilitate the import of various pathogens such as extended spectrum beta-lactamase (ESBL)- and carbapenemase-producing Enterobacteriaceae or Panton-Valentine leukocidin (PVL) positive Staphylococcus aureus [3]–[5].

The frequent use of antibiotics - not only as therapeutics but also as growth promoters - has contributed to the emergence of antibiotic resistance in animal husbandry [6]. Hence, in European countries, industrially raised poultry and chicken can be contaminated with ESBL-producing E. coli (ESBL E. coli, up to 58–93.3% of chicken) and methicillin-resistant S. aureus (MRSA, 0.7-37.2% of chicken) [7]–[12]. One study showed that contamination of food items with multidrug-resistant bacteria might be an important vehicle for the spread of antibiotic resistance [13].

While current literature mostly indicates an import of antimicrobial resistant pathogens from developing countries to the industrialized world, there is a potential transmission pathway for such bacteria, which is diametrically opposed: Many industrialized countries do not only produce poultry meat for their domestic market but also for export to low income or developing countries. The leading exporters of poultry meat in 2007 were the USA, Brazil and the Netherlands. Main import markets were sub-Saharan Africa and Asia (http://kids.fao.org/glipha).

Our hypothesis was that the import of meat from industrialized countries contributes to the emergence of multidrug resistance among humans in developing countries such as Gabon, Central Africa [14]. We therefore analyzed the contamination of chicken meat with ESBL-producing Enterobacteriacae (ESBL Enterobacteriaceae) and S. aureus in Gabon and compared genotypes of these pathogens with isolates from human carriage and infection.

We assessed the import and potential spread of ESBL E. coli and S. aureus through chicken meat in Gabon, Central Africa. Main findings were a high proportion of imported chicken meat in our sample collection and a high prevalence of ESBL E. coli. Beta-lactamases associated with chicken meat were not found in ESBL E. coli from carriage and infection in humans. Chicken-related S. aureus genotypes were found in human isolates from carriage and infection.

The high proportion of imported chicken products in Gabon is not surprising because the country has an excess of import of poultry meat. The import of poultry products increased in Gabon from 11,149 Mt (1997) to 43,875 Mt (2007), the ratio of exports to imports in 2007 was 0.0 (Germany: 0.8, http://kids.fao.org/glipha).

No ESBL Enterobacteriaceae other than ESBL E. coli were detected which is in line with recent reports where the proportion of ESBL E. coli among all ESBL-producers from chicken was 93.8% (Germany), 76.8% (the Netherlands) and 58-67% (Spain) [8],[11],[15]. The prevalence of ESBL E. coli in chicken meat was lower in chicken products from the US compared to Spain which could be explained by a generally lower ESBL E. coli contamination rate in chicken from the US [11]. This lower contamination rate might be achieved by a “pathogen reduction treatment” with chemical compounds (e. g. chlorinated water) in the US which is prohibited in the European Union [16].

The predominance of the phylogenetic groups A and B1 in poultry products was expected as these groups are mostly found in the environment in contrast to B2 which is associated with mammals [17].

The ESBL encoding genes found in chicken meat in Gabon mainly belonged to CTX-M 1 and CTX-M 14 (Table 3). The predominance of CTX-M 14 over CTX-M 1 reflects the proportion of CTX-M subtypes in Spain [7]. Our data do not provide any evidence for a transmission of ESBL E. coli from poultry to humans as CTX-M 1 and CTX-M 14 from chicken meat were neither detected in ESBL E. coli from carriage nor infection in humans (Table 3). This is in contrast to Europe and the Americas, where CTX-M 1 was the most common beta-lactamase in ESBL E. coli from humans [18].

It is unclear why the same ESBL subtype (CTX-M 1) occur in poultry related and human isolates in Europe but not in Gabon [18]. One might argue that the prevalence and concentration of ESBL E. coli in chicken meat in Gabon is not yet high enough to be transmitted and to establish a sustainable colonization in humans [19]. It is also possible, that our samples size was too small to detect CTX-M subtypes with lower prevalence such as CTX-M 1 which accounts for 7% of all ESBL/AmpC types in humans in Europe [18].

Chicken meat can be contaminated not only with ESBL Enterobacteriaceae but also with S. aureus and MRSA, in particular. S. aureus research in livestock animals has been focused on MRSA. The prevalence of MRSA in poultry products is up to 37.2% in Germany [10],[20], but varies significantly within and between the different countries in Europe (0% in Switzerland, 6.9% in the Netherlands) [10],[12]. This variation in MRSA colonization might be the reason why we did not detect any MRSA among the S. aureus isolates in our study. Nevertheless, genotyping of methicillin susceptible S. aureus (MSSA) revealed that some S. aureus from poultry meat and humans in Gabon share the same spa type (t002, ST5). On the one hand, it was postulated that a human-to-poultry transmission of S. aureus ST5 isolates occurred in the 1980s and that this clonal lineage now spreads in chicken around the world (except for Australia, where stringent controls of imported poultry products are applied) [21]. On the other hand, there might be a risk of transmission of isolates belonging to t002 back to humans [22].

Although our study provides important information about the spread of pathogens through the trade with chicken products, some limitations need to be addressed. First, the prevalence of S. aureus in chicken meat in Gabon might have been underestimated as we did not use S. aureus selective enrichment broths and chromogenic agars. This practice was chosen because our objective was to use preferentially methods which can be easily applied in rather basally equipped laboratories. Second, few months after the start of the study we realized the high burden of ESBL-producing Enterobacteriaceae in humans Gabon [14]. From that point, we decided to use chromogenic media for the screening of ESBL E. coli in chicken meat. Therefore only a proportion of samples were consecutively screened for ESBL-producing Enterobacteriaceae (60 of 151, Table 1). Third, more comprehensive genotyping approaches (e.g. sequencing of plasmids) would have been preferred to detect more reliably potential sources of transmission. Fourth, our results might not be representative for the whole country due to the small sample size. Fifth, due to the cross sectional study design we cannot draw any conclusions regarding the direction of transmission between chicken meat and humans, in case there is any. Sixth, the chromogenic agar that we used might suppress the growth of Enterobacteriaceae carrying the AmpC resistance type. Therefore, the prevalence of CMY-2 refers to ESBL-producers only.

In conclusion, there is a risk to import ESBL E. coli to Gabon but molecular differences between isolates from humans and chicken meat argue against a further dissemination. No MRSA isolate was imported through poultry meat.

FS, ASA and AM were involved in the conception, design and analysis of the study. Microbiological analyses were done by FS, LF, RK, and AM. FS, MPG, KB, SI, PGK, GP and AM drafted the manuscript or revised critically the results. All authors read and approved the final manuscript.