Except for the treatment of streptococcus agalactiae, there is no evidence that antibiotic use is cost-effective in treating mastitis during lactation. Infections which do not originally respond to antibiotic therapy are almost invariably non-responsive to additional therapy regardless of the antibiotic used or the organism's sensitivity pattern in vitro. Therapy results are poorly correlated to Ln vitro sensitivity. Rapid growth on enriched media does not reflect growth in the normal in vivo environment. Rapidly multiplying bacteria may show larger zones of inhibition for the antibiotics indicating a greater sensitivity pattern than that achieved in the gland.
Most antibiotics destroy organisms as they replicate.
When the gland, leukocytes, or other enzyme activity affects bacterial growth this can affect the ability of the antibiotic to kill the microorganisms. Variability in survival of therapy is due to several factors including susceptibility, penicillin resistance, media interaction, and leukocyte activity. Milk components such as casein and other lipoproteins and lipids can inhibit antibiotic activity. For example, casein will link to tetracycline due to calcium binding and inhibit its activity on microorganisms. Antibiotic resistance or resistance development.