Antibiotic resistance and genetic diversity in Staphylococcus aureus from slaughter pigs in Switzerland

Nasal carriage of Staphylococcus aureus was evaluated in pigs at slaughterhouse. The nasal cavities of 304 pigs from 54 herds were screened. Eighty-nine percent of the farms harbored pigs that were colonized with S. aureus. Among them, no MRSA were found, indicating a low prevalence. However, pigs were found to harbor S. aureus, which displayed resistance to penicillin (blaZ) (62.5%), tetracycline [tet(M)] (33.3%), streptomycin (strpS194) (27%), clindamycin [erm(B)] (4.1%), erythromycin [erm(B)] (4.1%), kanamycin (4.1%), chloramphenicol (catpC194) (2%) and gentamicin [aac(6')-Ie-aph(2')-Ia] (2%). The S. aureus isolates mainly belong to Ridom spa type t034 (31.3%), t208 (14.6%) and t899 (12.5%). These pig-associated spa types have not yet been detected in hospitalized human patients in Switzerland. Surveillance programs are now necessary at both inland and import levels to rapidly detect and suppress the emergence of MRSA in pigs in Switzerland.

Molecular epidemiology and genetic linkage of macrolide and aminoglycoside resistance in Staphylococcus intermedius of canine origin

A collection of 77 Staphylococcus intermedius isolates from dogs and cats in Switzerland was examined for resistance to erythromycin. Resistance profiles for 14 additional antibiotics were compared between erythromycin-resistant and susceptible isolates. A resistance prevalence of 27% for erythromycin was observed in the population under study. Complete correlation between resistance to erythromycin, and to spiramycin, streptomycin, and neomycin was observed. The erythromycin-resistant isolates all had a reduced susceptibility to clindamycin when compared to the erythromycin-susceptible isolates. Both constitutive and inducible resistance phenotypes were observed for clindamycin. Ribotyping showed that macrolide-aminoglycoside resistance was randomly distributed among unrelated strains. This suggests that this particular resistance profile is not related to a single bacterial clone but to the horizontal transfer of resistance gene clusters in S. intermedius populations. The erythromycin-resistant isolates were all carrying erm(B), but not erm(A), erm(C), or msr(A). The erm(B) gene was physically linked to Tn5405-like elements known as resistance determinants for streptomycin, streptothricin, neomycin and kanamycin. Analysis of the region flanking erm(B) showed the presence of two different groups of erm(B)-Tn5405-like elements in the S. intermedius population examined and of elements found in Gram-positive species other than staphylococci. This strongly suggests that erm(B) or the whole erm(B)-Tn5405-like elements in S. intermedius originate from other bacterial species, possibly from enterococci.

Genetic characterization of antimicrobial resistance in coagulase-negative staphylococci from bovine mastitis milk

Coagulase-negative staphylococci (CNS; n=417) were isolated from bovine milk and identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Nineteen different species were identified, and Staphylococcus xylosus, Staphylococcus chromogenes, Staphylococcus haemolyticus, and Staphylococcus sciuri were the most prevalent species. Resistance to oxacillin (47.0% of the isolates), fusidic acid (33.8%), tiamulin (31.9%), penicillin (23.3%), tetracycline (15.8%), streptomycin (9.6%), erythromycin (7.0%), sulfonamides (5%), trimethoprim (4.3%), clindamycin (3.4%), kanamycin (2.4%), and gentamicin (2.4%) was detected. Resistance to oxacillin was attributed to the mecA gene in 9.7% of the oxacillin-resistant isolates. The remaining oxacillin-resistant CNS did not contain the mecC gene or mecA1 promoter mutations. The mecA gene was detected in Staphylococcus fleurettii, Staphylococcus epidermidis, Staph. haemolyticus, and Staph. xylosus. Resistance to tetracycline was attributed to the presence of tet(K) and tet(L), penicillin resistance to blaZ, streptomycin resistance to str and ant(6)-Ia, and erythromycin resistance to erm(C), erm(B), and msr. Resistance to tiamulin and fusidic acid could not be attributed to an acquired resistance gene. In total, 15.1% of the CNS isolates were multidrug resistant (i.e., resistant to 2 or more antimicrobials). The remaining CNS isolates were susceptible to antimicrobials commonly used in mastitis treatment. Methicillin-resistant CNS isolates were diverse, as determined by mecA gene sequence analysis, staphylococcal cassette chromosome mec typing, and pulsed-field gel electrophoresis. Arginine catabolic mobile element types 1 and 3 were detected in both methicillin-resistant and methicillin-susceptible Staph. epidermidis and were associated with sequence types ST59 and ST111. Because this study revealed the presence of multidrug-resistant CNS in a heterogeneous CNS population, we recommend antibiogram analysis of CNS in persistent infections before treatment with antimicrobials.

Antibiotic resistance genes in coagulase-negative staphylococci isolated from food

Coagulase-negative staphylococci were isolated from different raw milk cheeses and raw meat products and screened for their antibiotic resistances. They were identified as Staphylococcus xylosus, S. lentus, S. caprae, S. epidemidis and S. haemolyticus. The most frequent resistances found were those to chloramphenicol, tetracycline, erythromycin and lincomycin. They have been characterized on the molecular level. The chloramphenicol resistance genes were localized in several S. xylosus and S. caprae on plasmids with sizes ranging from 3.8-kb to 4.3-kb and were identified as chloramphenicol acetyltransferase (cat). All the tetracycline resistant strains were identified as S. xylosus and harboured a 4.4-kb plasmid carrying the tetracycline efflux resistance gene (tetK). The two erythromycin/lincomycin resistant S. caprae and S. epidermidis strains did not hybridize with the MLSB resistance genes ermAM, ermA, ermB and ermC. Three erythromycin resistant Staphylococcus sp. strains harboured an erythromycin efflux resistance gene (msr) localized twice on a 18-kb plasmid and once on the chromosome. A S. haemolyticus strain showing resistance to both lincomycin and clindamycin harboured a linA gene-carrying 2.2-kb plasmid. Further resistances to gentamicin, penicillin and kanamycin were less frequently observed and yet not characterized on a molecular level.