Sir, Methicillin-resistant Staphylococcus aureus (MRSA) are increasingly associated with non-clinical settings or with companion and food-producing animals. This has raised questions about staphylococcal host specificity and the potential transmissibility from animals to humans or vice versa. Several clonal complexes (CCs) were reported as specifically deriving from animals (CC151 in cows) or humans (CC15, CC25 and CC45), whereas others are not restricted to particular hosts. In particular, the ST398 clone, originally described in pigs, has since been identified in humans, horses, cattle and poultry, and has become a public health concern, especially for persons in contact with animals. Conversely, coagulase-negative staphylococci (CoNS) are also responsible for severe infections in both animals and humans, and constitute a less-documented reservoir of mecA-carrying cassettes. In this context, our objective was to document the presence of MRSA and methicillin-resistant CoNS (MRCoNS) in veal calves at the slaughterhouse, and to characterize the isolates to gain insight into the potential risks for human health. Between April and June 2009, nasal swabs of 123 veal calves coming from 14 different feeding farms were sampled at the slaughterhouse in Lyon, France. Fifty-three (43.1%) methicillinresistant staphylococci, including 8 MRSA (6.5%) and 45 MRCoNS (36.6%), were isolated on selective plates after a pre-enrichment step. MRSA were sampled from eight different calves coming from four geographically unrelated farms. All belonged to the t899 spa-type ST398 clone, as confirmed by PCR, displayed an SCCmec type IV cassette and the agr1 allele, and shared identical resistance and genetic profiles by microarray (S. aureus genotyping; Identibac, Alere) and Cfr9I PFGE (see Figure S1, available as Supplementary data at JAC Online). Since ST398 can rapidly spread at slaughterhouses, a unique clone might have disseminated within this environment or during transport to it—possibly originating from the only calf fed on a pig–veal mixed farm. Nevertheless, the presence of a specific clone in the four farms cannot be excluded. Antimicrobial susceptibility tests (disc diffusion using clinical breakpoints recommended by the Antimicrobial Committee of the French Society of Microbiology, www.sfm-microbiologie.org) revealed phenotypic resistances to penicillin, tetracycline and tobramycin, as confirmed by the detection of blaZ, tet(M) and aadD genes on the microarray. This multidrug pattern was completed by the presence of the vga(A) and dfrA genes, and by the phenotypic detection of additional resistances to fusidic acid, ciprofloxacin, lincomycin and macrolides. Interestingly, while devoid of toxin genes, all isolates carried immune evasion cluster (IEC) type B genes (sak, chp and scn, but not sea), which are known to play an important role in human colonization but are usually not found in ST398 or animal staphylococci. The link with human adaptation remains to be elucidated, especially since MRSA or methicillin-sensitive S. aureus (MSSA) ST398 have rarely caused hospitalizations in France until now, even in the main pig-farming area (F. Laurent, National Reference Centre for Staphylococci, personal communication). Yet, severe infections have been reported in The Netherlands, and the acquisition of IEC genes in ST398 should be surveyed, especially regarding the transmission capacities and resistance patterns of this clone. In parallel, MRCoNS were identified by matrix-assisted laser desorption ionization–time-of-flight mass spectrometry (Bruker Daltonics, Bremen, Germany) as Staphylococcus haemolyticus (n1⁄41) and Staphylococcus sciuri (n1⁄444), and confirmed by sequencing of the tuf gene. In addition to cefoxitin and penicillin resistance, susceptibility tests revealed resistances to antibiotics of human and veterinary interest, including fusidic acid, tetracycline, macrolides or aminoglycosides (Table 1), but not to the molecules prescribed in human medicine (vancomycin, teicoplanin, pristinamycin, linezolid, tigecycline and rifampicin). Results revealed a wide variety of 24 different susceptibility patterns. Overall, the number of associated resistances was considerably higher than what has recently been reported in human S. sciuri isolates, but the potential interspecific transmission of these resistance determinants remains to be investigated. The SCCmec cassette was nontypeable for 16 isolates (mecA, mecI and IS431, but no ccr gene) and 28 S. sciuri presented an SCCmec type III element (mecA, mecI and ccrAB3). Since all MRSA presented a type IV cassette (mecA, ccrAB2 and IS1272), there was no indication for a direct genetic exchange between S. sciuri and S. aureus, as recently reported for human staphylococci. Lastly, three veal calves coming from different farms carried both an MRCoNS and an MRSA, but, in each case, both bacteria shared no common resistance pattern or SCCmec cassette.
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Jul 1, 2011
M Schim Van Der Loeff + 4
Open Access
