Volume 20, Number 5—May 2014
Staphylococcus aureus Carrying mecC Gene in Animals and Urban Wastewater, Spain
To the Editor: A new methicillin resistance mechanism gene, a divergent mecA homologue named mecC (formerly mecALGA251), was recently described in Staphylococcus aureus (1). Methicillin-resistant S. aureus (MRSA) isolates carrying mecC have been recovered from humans, ruminants, pets, and other animals such as rats, seals, and guinea pigs (1–3). It has been suggested that mecC-carrying MRSA isolates might not be detected by using MRSA selective media (4). For mecC-carrying S. aureus isolates, cefoxitin MICs of 4–64 mg/L have been demonstrated (1–2,4), values that would normally include susceptible isolates, according to the epidemiologic cutoff value established by the European Committee on Antibiotic Susceptibility Testing (EUCAST; www.eucast.org). mecC-carrying S. aureus isolates have been classified as heteroresistant (5), and MICs can be affected by the drug-susceptibility testing method used (1,5).
These observations led us to retrospectively investigate the presence of mecC gene in a set of 361 mecA-negative S. aureus isolates collected during 2009–2012 (Table), independently of their susceptibility to cefoxitin. Isolates were recovered from healthy carriers in livestock (n = 39), from wild animals (n = 254), and from wastewater (effluents) from an urban sewage plant (n = 68). Specific amplification of the mecC gene was performed as described (6). The mecC-carrying S. aureus isolates were tested by broth microdilution using Microtiter EUST plates (Trek Diagnostic Systems, East Grinstead, UK) for susceptibility to benzylpenicillin, cefoxitin, chloramphenicol, ciprofloxacin, clindamycin, erythromycin, florfenicol, fusidic acid, gentamicin, kanamycin, linezolid, mupirocin, rifampin, sulfamethoxazole, streptomycin, quinupristin-dalfopristin, tetracycline, thiamulin, trimethoprim, and vancomycin. Additionally, susceptibility to oxacillin was determined by using microScan Gram Positive Combo panel 37 (Siemens, Erlangen, Germany). MICs were interpreted according to EUCAST epidemiologic cutoff values.
mecC was detected in a total of 4 isolates from wild boar (n = 1), fallow deer (n = 2), and urban wastewater (n = 1); these isolates represent 1% of the 361 tested isolates. The 3 isolates recovered from animals were susceptible to all antimicrobial drugs tested other than β-lactams and to oxacillin (MICs 0.5–1 mg/L) but were resistant to penicillin (MICs 0.5–2 mg/L). Two of the isolates were resistant to cefoxitin (MICs 8 and 16 mg/L) and the third was susceptible (MIC 4 mg/L). The wastewater isolate was resistant to penicillin (MIC 2 mg/L) and erythromycin (MIC 16 mg/L) and susceptible to all other antimicrobial drugs tested, including cefoxitin (MIC 4 mg/L) and oxacillin (MIC ≤0.25 mg/L).
Previous studies have described mecC-positive isolates as susceptible to all antimicrobial drugs tested except β-lactams (2,3), although sporadic resistance to fluoroquinolones has been found (4,7). We additionally found erythromycin resistance in 1 mecC-carrying S. aureus isolate. For the 4 mecC-carrying S. aureus isolates we detected, MICs of oxacillin were interpreted as susceptible, and 2 isolates were susceptible to cefoxitin according to EUCAST guidelines, findings that agree with previous reports (1–2,4). Thus, mecC presence is not always linked to resistance phenotypes for cefoxitin or oxacillin; such unclear findings could hinder the detection of mecC-carrying isolates.
We further characterized the 4 mecC-carrying S. aureus isolates by spa typing and detection of Panton-Valentin leukocidin (PVL) toxin genes (6,8). Multilocus sequence typing (MLST) was performed according to Enright et al. (9) by using self-designed primers arc (down 5′-CGATTTGTTGTTGATTAGGTTC-3′), tpi (up 5′-CATTAGCAGATTTAGGCGTTA-3′), and yqiL (down 5′-GATTGGYTCACCTTTRCGTTG-3′). All 4 isolates were PVL negative. The 3 animal isolates were assigned to a new spa type (t11212) and to clonal complex (CC) 425 and sequence type (ST) 425 (Table). ST425 has been previously associated with mecC-carrying S. aureus isolates in cattle and humans (1–2); the animals we sampled were from a game estate and may have had contact with cattle and with urban wastewater. The wastewater isolate was assigned to spa type t843 and to a new allelic profile, ST2676, in CC130 (Table). ST2676 represents a single-locus variant of ST130 carrying a different allele for the gene aroE. MRSA isolates of CC130 have been associated with humans and animals (1–4,6). This result indicates that mecC-carrying S. aureus isolates can be found in urban wastewater, which may act as an environmental reservoir, as has been demonstrated for mecA-carrying S. aureus (10).
In conclusion, we detected the methicillin resistance mechanism gene mecC in nonclinical S. aureus isolates from animals and urban wastewater in Spain. Although our data indicate that the frequency of this resistance mechanism is low, this gene appears to be expanding to new areas. Prospective studies should be performed to evaluate epidemiologic changes and to analyze the genetic lineages that carry this resistance mechanism.
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1Current affiliation: Innovación en Gestión y Conservación de Ungulados S.L., Cáceres, Spain.