Characterization of resistance to tetracyclines and aminoglycosides of sheep mastitis pathogens: study of the effect of gene content on resistance.
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(2″)-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.
['Lollai|S A|SA|http://orcid.org/0000-0001-6037-6614', 'Ziccheddu|M|M|', 'Duprè|I|I|', 'Piras|D|D|']
D000617:Aminoglycosides D000818:Animals D000900:Anti-Bacterial Agents D016983:Enterococcus D007558:Italy D008413:Mastitis D008826:Microbial Sensitivity Tests D008892:Milk D012756:Sheep D012757:Sheep Diseases D034561:Sheep, Domestic D013210:Staphylococcus D013291:Streptococcus D013753:Tetracycline Resistance D013754:Tetracyclines
resistance genes antimicrobials polymerase chain reaction veterinary