Zone 1 Listeria Positive: Closing the Finding on a Single Retest
An auditor is conducting a three-day audit at a ready-to-eat salad processor that chops about 12,000 kg of romaine per day on two lines. Over the past 90 days the plant produced 48 lots. Its environmental monitoring program samples 20 Zone 1 food-contact sites weekly. One swab from a conveyor at Line 2 was positive for Listeria monocytogenes. The plant re-cleaned the conveyor and re-swabbed the same site 24 hours later; that result was negative. Sanitation records show quaternary ammonium at 200 ppm with a 15-minute contact time. Two sanitation employees describe the pre-shift cleaning differently: one says the conveyor was disassembled, the other says only the exposed surface was wiped. The corrective action record says 'retrain sanitation team' and attaches a sign-in sheet. The auditor has two days left and must decide whether the finding can be closed.
1. Is the single negative retest enough to close the positive Zone 1 Listeria finding? Set out the evidence you would require and explain your reasoning.
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No. A negative result from the same site 24 hours after a targeted re-clean is weak evidence because the cleaning was performed specifically for that sample and the sample is a single point in time. A Zone 1 Listeria monocytogenes positive means a food-contact surface may have transferred the pathogen to product. To close the finding I would require:
- a root-cause investigation covering conveyor design, disassembly, welds, cracks, biofilm, maintenance work, and cleaning chemistry and contact time;
- verification that the SSOP actually requires disassembly and that cleaning was performed as written;
- at least three consecutive rounds of environmental sampling after cleaning and during production, covering the original site plus adjacent framework, dead spots, drains, and squeegees;
- a review of 90-day Zone 1 and Zone 2 trends, not just the one positive;
- a documented risk assessment and disposition decision for the 48 lots produced in the 90-day window;
- evidence that corrective action effectiveness was independently verified.
The retrain sign-in sheet does not establish root cause. The finding stays open until the plant shows the contamination was eliminated and the control is effective, not merely that one swab turned negative.
2. How would you resolve the conflicting accounts about disassembly, and what interview and sampling follow-up would you perform?
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I would separate the two employees, ask open questions about what they actually did, and then observe a cleaning cycle. I would compare their accounts with the SSOP, disassembly checklist, maintenance records, cleaning logs, and training records. Conflicting accounts are a signal that the procedure is ambiguous or not verified, not just a personnel issue. I would interview the supervisor about who verifies cleaning and how. For sampling, I would collect aseptic swabs from 10-15 sites across both lines, including the original conveyor, frame joints, welds, drain pans, squeegees, and a control site. I would sample pre-operation and again mid-production, keep pathogen samples separate rather than compositing, and use an accredited laboratory. I would also review ATP results for the same sites, but ATP is only a hygiene indicator. Until the sampling and records show consistent control, I would not accept the cleaning step as verified.
3. Write the audit conclusion and follow-up requirement, including a proposed classification and the objective evidence you would cite.
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Classification: major nonconformity. Objective evidence: a Zone 1 food-contact swab positive for Listeria monocytogenes; a single negative retest at the same site; no root-cause analysis; a corrective action limited to retraining with a sign-in sheet; and conflicting accounts of disassembly. The risk is direct product contamination from a food-contact surface. The report should state the requirement, the evidence, the risk, and the open status. Follow-up should require a written root-cause analysis, a revised SSOP with disassembly and verification, intensified environmental monitoring for at least four weeks, finished-product risk assessment and disposition, and independent verification by QA. I would schedule a follow-up audit in 30 days and require objective evidence such as three consecutive negative sampling rounds and a completed corrective action record. The finding should not be closed on the retest alone.