Presence/absence testing in foods: In qualitative assays, “examination of the presence/absence of organisms” most correctly refers to which workflow?

Introduction / Context:
Presence/absence tests are qualitative. They aim to detect very low levels of target pathogens or indicators by first enriching them to detectable levels, then isolating on selective/differential media for confirmation.

Given Data / Assumptions:

• Food matrices often contain low pathogen numbers and high background flora.
• Enrichment increases target concentration.
• Selective media suppress competitors and reveal characteristic colonies.

Concept / Approach:
A typical workflow is sample homogenization → non-selective or selective enrichment → selective plating → confirmation (biochemical/serological/molecular). This yields a qualitative result (Detected/Not detected) per sample size, aiding release decisions and corrective actions.

Step-by-Step Solution:
State the objective: detection at low levels, not quantitation. Justify enrichment to resuscitate injured cells and amplify targets. Apply selective plating to differentiate targets from background. Select the option that captures enrichment followed by selective isolation.

Verification / Alternative check:
Standards for Salmonella, Listeria, and E. coli O157 routinely include primary/secondary enrichments prior to selective plating and confirmatory tests.

Why Other Options Are Wrong:
Non-selective-only plates permit overgrowth; enrichment without isolation is incomplete; “none” and direct PCR omit the culture-based definition asked here.

Common Pitfalls:
Skipping selective plating after enrichment can yield ambiguous flora and false negatives/positives.

Final Answer:
Incubating a food suspension in an enrichment medium followed by inoculation onto a suitable selective medium.