To investigate occurrence of acquired antimicrobial resistance in udder pathogens MICs in Staphylococcus aureus (n=211),
coagulase-negative staphylococci (CNS) (n=56), Streptococcus uberis (n=113), Streptococcus dysgalactiae (n=152), Streptococcus agalactiae (n=6), Escherichia coli (n=163), and Klebsiella spp. (n=42) were determined using microdilution. Isolates were from a nation wide survey employing strict inclusion criteria. Presence of acquired resistance was evaluated by species-specific epidemiological cut-off values issued by EUCAST.
Penicillin or methicillin resistance in staphylococci were however evaluated by
beta-lactamase production or presence of the
mecA gene, respectively. Staphylococci were mostly susceptible to antimicrobials tested but 7.1% of S. aureus and 12.5% of CNS were resistant to
penicillin by
beta-lactamase production. Methicillin resistance was not found in S. aureus. All Streptococcus dysgalactiae and S. agalactiae were susceptible to
penicillin. Bimodal MIC distributions for
tetracycline in S dysgalactiae and S. uberis indicate acquired resistance in some isolates. Among E. coli 12.3% of isolates were resistant to one or more antimicrobials. Resistance to
streptomycin (11.0%), sulphametoxazole (8.6%),
ampicillin (7.4%), or
tetracycline (4.9%) were the most common traits. Klebsiella spp. were resistant to
ampicillin and some isolates also to
tetracycline (7.1%) or sulphonamide (9.5%). The study shows that in Sweden bacteria associated with acute clinical
mastitis for the most part are susceptible to antimicrobials used in
therapy but resistance to
penicillin in S. aureus is not uncommon.
Penicillin is recommended for treatment of
mastitis caused by gram-positive pathogens and regular monitoring of
beta-lactamase production in S. aureus is therefore recommended in herds with udder health problems.