Reasons for Negative Culture Results
Originally published in the NMC Newsletter "Udder Topics", December 1989

Reports indicate that 25-40 percent of all clinical samples are negative on routine culturing. Reasons include:

Numbers of certain organisms, such as mycoplasma, Staphylococcus aureus, and coliforms, can vary greatly in infected quarters, and may occasionally be less than the minimum detection limit of the assay. The minimum detection limit when plating 0.01 ml of milk is about 100 colony forming units per ml.

The organism may no longer be present and the clinical signs are due to by-products such as endotoxins.

Somatic cells may have phagocytized the organisms.

Antibiotics may have killed or suppressed organism numbers to unrecoverable levels.

Storage may have reduced numbers of viable organisms to undetectable levels.

The organism may require cultural conditions other than those used for isolation (i.e. reduced temperature, prolonged incubation, special media, anaerobic conditions, etc.).

Source: National Mastitis Council Publication "Laboratory and Field Handbook on Bovine Mastitis" (1987)

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