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Minimum Inhibitory Concentrations and Disk Diffusion Zone Diameter for Selected Antibiotics Against Streptococci Isolated from Bovine Intramammary Infections1

  • W.E. Owens
    Affiliations
    Hill Farm Research Station, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Route 1, Box 10, Homer 71040
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  • J.L. Watts
    Affiliations
    Hill Farm Research Station, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Route 1, Box 10, Homer 71040
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  • B.B. Greene
    Affiliations
    Hill Farm Research Station, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Route 1, Box 10, Homer 71040
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  • C.H. Ray
    Affiliations
    Hill Farm Research Station, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Route 1, Box 10, Homer 71040
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  • Author Footnotes
    1 Approved for publication by the Director of he Louisiana Agricultural Experiment Station as Manuscript Number 89-8@3352.
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      Abstract

      The antimicrobial susceptibility to 11 antibiotics was determined of 358 streptococcal isolates representing six species was determined. All isolates were from milk samples obtained from clinical or subclinical cases of bovine mastitis. Agar disk diffusion zone diameters and minimum inhibitory concentrations were obtained for all isolates. Of the antibiotics tested that are available for mastitis therapy, cephalothin, penicillin, ampicillin, novobiocin, and erythromycin demonstrated acceptable activity against nonenterococcal streptococci, but only penicillin and ampicillin demonstrated moderate activity against Enterococcus faecalis. Enterococcus faecalis demonstrated substantial resistance to cephalothin, erythromycin, gentamicin, streptomycin, and tetracycline, while the nonenterococcal streptococci demonstrated substantial resistance to gentamicin, streptomycin, and tetracycline. Streptococcus uberis demonstrated greater antibiotic resistance than did Streptococcus dysgalactiae, while Aerococcus viridans demonstrated the greatest overall resistance of the nonenterococcal species.

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