From the NMC Newsletter "Udder Topics", June 2005

Most mastitis-causing organisms survive refrigeration for several days or freezing for several weeks. Isolation of most staphylococci and streptococci is not affected greatly when samples are stored at 4 dgrees C for 1 week or frozen for up to 6 weeks. Nocardia spp. are an exception to this general rule, as storage of samples for only a few hours or freezing can reduce the likelihood of isolating these organisms. Freezing samples may reduce the sensitivity for isolation of Escherichia coli.

Source: NMC publication: Microbiological Procedures for the Diagnosis of Bovine Udder Infection and Determination of Milk Quality, pg. 2 (4th Edition, 2004).