ABSTRACT
AIMS: Mastitis causes economic losses and antimicrobials are frequently used for mastitis treatment. Antimicrobial resistance surveys are still rare in the ovine field and characterization of strains is important in order to acquire information about resistance and for optimization of therapy. METHODS AND RESULTS: Bacterial pathogens recovered in milk samples from mastitis-affected ewes were characterized for resistance to tetracyclines and aminoglycosides, members of which are frequently used antimicrobials in small ruminants. A total of 185 strains of staphylococci, streptococci, and enterococci, common mastitis pathogens, were tested for minimal inhibitory concentration (MIC) to tetracycline, doxycycline, minocycline, gentamicin, kanamycin, streptomycin, and for resistance genes by PCR. Effects of different tet genes arrangements on MICs were also investigated. Staphylococci expressed the lowest MIC for tetracycline and tet(K) was the most common gene recovered; tet(M) and tet(O) were also found. Gene content was shown to influence the tetracycline MIC values. Enterococci and streptococci showed higher MICs to tetracyclines and nonsusceptible strains always harboured at least one ribosomal protection gene (MIC above 8 µg ml(-1) ). Streptococci often harboured two or more tet determinants. As regards the resistance to aminoglycosides, staphylococci showed the lowest gentamicin and kanamycin median MIC along with streptomycin high level resistant (HLR) strains (MIC >1024 µg ml(-1) ) all harbouring str gene. The resistance determinant aac(6')-Ie-aph(2â³)-Ia was present in few strains. Streptococci were basically nonsusceptible to aminoglycosides but neither HLR isolates nor resistance genes were detected. Enterococci revealed the highest MICs for gentamicin; two str harbouring isolates were shown to be HLR to streptomycin. CONCLUSION: Evidence was obtained for the circulation of antimicrobial-resistant strains and genes in sheep dairy farming. SIGNIFICANCE AND IMPACT OF THE STUDY: Tetracycline MIC of 64 µg ml(-1) and high-level resistance were detected for streptomycin (MIC >1024 µg ml(-1) ), so that effectiveness of common treatments may be at risk.
Subject(s)
Aminoglycosides/pharmacology , Anti-Bacterial Agents/pharmacology , Enterococcus/drug effects , Staphylococcus/drug effects , Streptococcus/drug effects , Tetracyclines/pharmacology , Animals , Enterococcus/genetics , Enterococcus/isolation & purification , Italy , Mastitis/microbiology , Mastitis/veterinary , Microbial Sensitivity Tests , Milk/microbiology , Sheep , Sheep Diseases/microbiology , Sheep, Domestic , Staphylococcus/genetics , Staphylococcus/isolation & purification , Streptococcus/genetics , Streptococcus/isolation & purification , Tetracycline ResistanceABSTRACT
Different genetic markers were used to analyze 22 Mycobacterium bovis strains isolated from cattle in Sardinia and one human isolate. IS6110 DNA fingerprinting differentiated the strains into six patterns, whereas with enterobacterial repetitive consensus sequence primers produced seven clusters. PCR ribotyping followed by digestion with HaeIII and PvuII produced five and seven patterns, respectively. PCR with the (GTG)5 oligonucleotide primer showed the best discriminatory power, generating eight clusters among the strains analyzed.
