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This is an interesting and important question where there is no consensus. When I have been responsible for this situation in the laboratory we had several criteria that we used to determine if the growth in broth only was a likely contaminant or not. They were:
1. Is the organism recovered from the broth only a recognized frequent microbiological contaminant (i.e., coagulase-negative staphylococcus, corynebacterium)?
2. Does this organism grow well on the solid agar used for primary recovery?
3. Is the direct Gram stain negative?
If the answer is yes to all 3 questions then we would consider the organism growing in the broth only to be a contaminant and report the culture result as no growth.
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