Escherichia coli is both a commensal and an opportunistic pathogen of the digestive tract of mammals. E. coli is also responsible for many extra-intestinal infections (ExPEC), such as those of the urinary tract (Pitout, 2012). Concomitantly, the spread of resistances to extended-spectrum cephalosporins (ESCs) used in human and veterinary medicine causes major therapeutic challenges worldwide (Naseer and Sundsfjord, 2011). Indeed, in E. coli, the vast dissemination of Extended-Spectrum β-Lactamases (ESBLs) is a concern, in particular due to the broad success of the CTX-M enzymes. ESBL genes are mostly plasmid-mediated and this obviously facilitates their dissemination amongst bacteria (Carattoli, 2008). In addition, plasmidic class C beta-lactamases (AmpCs), such as the CMY-2 enzyme, also confer resistance to those last-generations antibiotics and disseminate efficiently in Enterobacteriaceae. In certain countries, a significant prevalence of CMY-2 producers in animals has been reported (Mulvey et al., 2009).

Numerous Enterobacteriaceae naturally produce a chromosome-encoded AmpC cephalosporinase. In E. coli, this enzyme is usually expressed at very low levels, as a result of a weak promoter and a transcriptional attenuator preceeding the ampC gene. Nevertheless, constitutive over-expression of the chromosomal ampC gene was largely reported in clinical isolates. This was attributed to specific spontaneous mutations in the promoter (which creates close homologies with the E. coli perfect consensus sequence) or the attenuator (which destabilizes the mRNA hairpin structure) of the ampC gene. Consequently, both mechanisms confer resistance to narrow-spectrum cephalosporins. In human medicine, oxyminocephalosporins, such as cefepime and cefpirome, remain usually active against over-expressed AmpC-producing Enterobacteriaceae, thanks to their rapid penetration through the outer membrane and poor degradation by AmpC beta-lactamases.

More recently, nucleotides substitutions within the coding region of the ampC gene were shown to be responsible for an expanded hydrolysis spectrum of AmpC enzymes to oxyiminocephalosporins (Mammeri et al., 2004a, 2006). Amino acid deletions or insertions (Doi et al., 2004; Mammeri et al., 2007) of the cephalosporinase also broaden the hydrolysis spectrum. Those so-called Extended-Spectrum AmpC β-lactamases (ESACs) were not only reported in E. coli, but also in other Enterobacteriaceae, such as Enterobacter cloacae (Crichlow et al., 1999; Barnaud et al., 2001; Vakulenko and Golemi, 2002), Enterobacter aerogenes (Barnaud et al., 2004), Citrobacter freundii (Ahmed and Shimamoto, 2008) or Serratia marcescens (Matsumura et al., 1998; Raimondi et al., 2001; Mammeri et al., 2004b; Hidri et al., 2005), and even in Pseudomonas aeruginosa and Acinetobacter baumanii (Rodriguez-Martinez et al., 2009, 2010). ESACs were considered an emerging mechanism of resistance to beta-lactams and their prevalence was estimated around 0.2%, almost identical to that of plasmidic cephalosporinases (Mammeri et al., 2008). Among the modifications of the ampC gene in ESAC-producing E. coli, certain nucleotides substitutions were more frequently identified, such as the S287N replacement. Other replacements were found in the coding sequence of plasmid-borne AmpCs (Kim et al., 2006), which also contribute to the extension of their hydrolysis spectrum. Finally, ESAC E. coli producers were predominantly found to belong to the Clonal Complex (CC) 23 (Cremet et al., 2010), and this raises the question on the selection scheme of those isolates.

To date, ESACs were reported in human isolates only. The purpose of this study was thus to detect and characterize the very first ESAC producers in animals worldwide.

In this study, we report the first ESAC producers in animals worldwide. Contrary to humans (Barnaud et al., 2001; Mammeri et al., 2004b), ESACs were exclusively searched and found in the E. coli species. This results from the low proportion of animal infections due to other Gram negative bacteria expressing a chromosomal ampC gene, such as S. marcescens, E. cloacae, E. aerogenes, A. baumanii, or P. aeruginosa (Nordmann and Mammeri, 2007). Also, all ESAC-producing E. coli were from cattle as a result of our study design, and further investigations are surely needed to explore other animal sectors. The prevalence of ESAC-producing E. coli in cattle in France was investigated over a 6-year period of 2005–2010 and estimated at 0.37% (23/6158) in clinical isolates. Of note, this prevalence is similar to that found at a French hospital in the Paris suburb, where six ESAC producers were reported out of 2800 E. coli isolates (0.21%) (Mammeri et al., 2008). This prevalence is however four times higher than that found at another French hospital, where 41 AmpC-overproducing E. coli isolates accounted for 0.16% of all clinical E. coli isolates studied, of which only 11 produced ESACs (Cremet et al., 2010). However, in that study, inclusive criteria for ESAC producers included a MIC superior or equal to 64 mg/L of ceftazidime, which may have been a factor of underestimation.

Here, ESAC producers were mostly detected in diarrheic calves, which are also known as the main reservoir of ESBL genes in cattle (Madec et al., 2008, 2012; Valat et al., 2012). This may reflect the overall high antibiotic pressure with beta-lactams in infections of the enteric tract of young animals. However, a subset of 607 E. coli isolates from healthy cattle was also investigated, which revealed an even higher ESAC prevalence compared to diseased ones (5/607, 0.82%). This mirrors the situation in humans where ESAC producers were most likely found in E. coli belonging to the less pathogenic phylogroups A or B1 (Mammeri et al., 2006; Corvec et al., 2007). Moreover, the cattle ESAC-producing E. coli isolates predominantly belonged to the CC23, similarly to what has been reported in humans (Cremet et al., 2010). Further studies would be needed to understand why this evolution of the chromosomal ampC gene of E. coli seems restricted to the same genetic lineages in humans and animals despite different antimicrobial pressures and a rather host-specific distribution of the E. coli clones.

This study also provides an overview of 23 ampC promoter/attenuator sequences in cefoxitin-resistant E. coli from cattle. Contrary to humans, reports on mutations in the ampC promoter/attenuator regions of E. coli from animal sources are scarce (Brinas et al., 2002; Guillouzouic et al., 2009). The polymorphism at positions −88, −82, −42, −18, −1, +58 found in strong ampC promoters (Olsson et al., 1983; Guillouzouic et al., 2009) was highly conserved in our collection. In particular, this includes the −42 mutation shown to increase ampC gene expression (Nelson and Elisha, 1999; Caroff et al., 2000), and the association of the −42 and −18 changes, which creates two displaced new −35 and −10 boxes with an optimal 17-pb spacer region. The −18 mutation without the −42 mutation was rare (one isolate only) but had already been reported, in particular in E. coli isolates which do not over-express their chromosomal AmpC (Brinas et al., 2005). Other reported changes were not detected here, such as mutations −32, −11, or +6 (Caroff et al., 2000; Corvec et al., 2002; Mulvey et al., 2005; Tracz et al., 2007). However, these mutations might be more frequent in E. coli isolates of B2 or D phylogroups, which are not well represented here (Mammeri et al., 2008). The deletion of the entire attenuator was not observed either (Tracz et al., 2005).

In this study, all relevant amino acid replacements found in the ampC gene (namely S287N, S287C, A292V, and H296P) were single substitutions, confirming that cumulating substitutions in the catalytic site is not a preferential evolutionary scheme for AmpCs, contrary to class A beta-lactamases, as previously demonstrated (Le Turnier et al., 2009). A new A215V replacement was reported here in four E. coli isolates, either as a single or an associated mutation. This mutation, which occurs close to the DAEX motif in the Ω loop, has obviously little if no impact on the cefepime susceptibility. Indeed, in the corresponding native isolates, reduced susceptibility to cefepime may result from decreased permeability and/or oxacillinase production. Also, in the recombinant clones, the wild-type ampC gene of the cefepime susceptible control conferred the same MIC to cefepime than the A215V replacement. The main structural alteration found was the S287N replacement, which occurred in the H-9 helix close to the R2 loop. This mirrors the situation in humans where the S287N substitution has been shown to widely contribute to the ESAC phenotype (Mammeri et al., 2006). Similarly, other substitutions were also found, such as the S287C mutation, or both the H296P or the A292V mutations inside the R2 loop. Comparison of MICs of cefepime also confirmed that the S287N substitution has greater impact on resistance than the others (Mammeri et al., 2006).

Altogether this study is, to our best knowledge, the very first report of ESACs in animals. In humans, those enzymes have likely evolved from wild-type AmpCs by modifications in the vicinity of the R1 or R2 active sites. While the ESACs described here strongly resemble the human ones in both their genetic backgrounds and the responsible mutations, the driving forces for the selection of ESACs in animals are still unknown. In humans, the ESAC phenotype frequently occurs after cefepime therapy. On the contrary, different ESCs are used in animals, whose possible impact needs to be considered. In particular, ceftiofur is an ESC which is widely used in animals, and elevated MICs values to ceftiofur were found here in the recombinant E. coli clones. Hence, the use of veterinary cephalosporins in the selection of ESAC producers in animals might be a plausible hypothesis. Of note, most human and animal ESAC E. coli reported so far belong to the CC23—a well-adapted E. coli clone in the two hosts -, so that the transfer of ESAC producers from humans to animals may have contributed as well. Alternately, this may result from a favorable genetic background of the ampC genes in E. coli isolates of phylogroup A, irrespective of their human or animal origin (Mammeri AAC 2009). Even though data in humans report similar ranges of ESACs vs. plasmidic AmpCs (Mammeri et al., 2008), the true prevalence of ESACs in animals should be further investigated since this emerging mechanism surely contributes to the global burden of resistance to ESCs in animals.