If a SCIGIENE Salmonella plate (or any selective media for Salmonella) initially showed no growth after 24 hours of incubation but later developed pink colonies, several factors could be involved:
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Delayed Growth: Some Salmonella strains may have slower growth rates, which can lead to delayed colony formation. It's possible that they could start growing after the initial 24-hour check, especially if the temperature or conditions in the incubator fluctuate.
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Contamination: Contaminants from the environment or handling can lead to unexpected colony growth. If pink colonies appeared, they might not be Salmonella but rather a different organism that thrives under those conditions.
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pH Indicator Reaction: If the media contains a pH indicator that turns pink in the presence of a byproduct from bacterial metabolism, the colonies may indicate the presence of certain bacteria that can grow under the same conditions but are not Salmonella.
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Nutrient Shift: There may have been a nutrient shift in the medium after prolonged incubation, leading to changes in the metabolic activity of certain opportunistic flora, causing them to start growing and possibly turning pink.
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Rehydration Effect: If the plate was dehydrated or not stored properly, the rehydration effect when removed from the incubator could allow for growth to begin after the initial inspection.
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Incorrect Interpretation: If there were visible colonies that were mistaken for contamination or failed to be seen during the initial observation, they may have been small or not yet visibly clear during the first inspection.
In all cases, additional confirmatory tests (such as biochemical tests or molecular methods) should be performed to properly identify the organism causing the pink colonies.