Antibiotics loose their effect over time if they kept at +4 °C and/or subjected to multiple freeze and thaw cycles. They should either be added to the transport medium when the samples are ready to be collected or added during preparation of the media which should then be frozen at -20 °C and thawed only when needed.
If attempts to isolate virus are to be made, transport media containing specimens should not be stored at -20°C but at -70°C or in a liquid nitrogen storage unit. Storage at -20 °C is adequate if the specimen is to be used for PCR tests only.