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Spectrum of activity and MICs
A range of bacterial agents may cause mastitis in cattle. The most
significant include Streptococcus uberis, Streptococcus dysgalactiae
and agalactiae, Staphylococcus aureus, Escherichia coli, Klebsiella
sp., pseudomonas aeruginosa and Actinomyces (Arcanobacterium) pyogenes8.
In addition, coagulase negative staphylococcus (CNS), Corynebacterium
bovis, some moulds and yeasts are frequently isolated. Recent work
suggests that Corynebacterium bovis may be a commensal organism
essential for a healthy bacterial quilibrium in the udder of normal cows9.
Penethamate hydriodide is a time dependent bacteriocidal antibiotic
with a spectrum of activity primarily against non-penicillinase
producing gram+ve cocci. Penethamate is particularly active against
streptococci such as Streptococcus uberis, Streptococcus dysgalactiae
and Streptococcus agalactiae and penicillin sensitive staphylococci such
as Staphylococcus aureus. The Minimal Inhibitory Concentration (MIC) of
penethamate (after hydrolysis to penicillin G) is recorded in the
literature as follows. (table 1)
The data presented shows that penicillin sensitive staphylococci and
streptococci generally have MIC 90 values between (0.04-0.25μg/ml), and
streptococci consistently below 0.07μg/ml. These data have been
collected in a range of European countries and with a value
consistently below 0.1μg/ml this shows a strong sensitivity of these
organisms to penicillin. It is still generally assumed in the UK and
Ireland that all isolates of Streptococcus uberis are sensitive to
penicillin.
Following doses of 15 and 7.5mg/kg, the Cmax of penethamate in the milk
rises to approximately 0.6μg/ml and 0.2μg/ml respectively4.This
represents between approximately 3 and 10 times the maximum MIC quoted
in the literature. The progress of degradation of penicillin in the
udder following administration of penethamate shows that levels above
the MIC are maintained for almost 24 hours following each dose4.Therefore
the recommended course of 4 days treatment maintains an antibacterial
effect against Streptococcus uberis for full 96 hours.
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