Detection and Prevalence of Listeria in U.S. Produce Packinghouses and Fresh-Cut Facilities

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Abstract
Listeria monocytogenes (LM) contamination of produce can often be traced back to the environment of packinghouses and fresh-cut facilities. As there is limited information on the detection, prevalence, and distribution of this pathogen in produce operations, environmental “routine sampling” plans for LM and other Listeria spp. were developed and implemented in three packinghouses and five fresh-cut facilities in the US. For “routine sampling”, a total of 2,014 sponge samples were collected over 6 - 8 separate samplings per operation, performed over one year; vector and pre-production samples (n=156) were also collected as needed to follow up on positive findings. In addition, a single “validation sampling” visit by an outside expert was used to evaluate the routine sampling. Among the 2,014 routine sponge samples collected, 35 and 30 were positive for LM and Listeria species other than LM (“LS”), respectively. LM prevalence varied from 0.8% to 5.8% for packinghouses and <0.4% to 1.6% for fresh-cut facilities. Among the 394 validation sponge samples, 23 and 13 were positive for LM and LS, respectively. Validation sampling found statistically significantly higher LM prevalence compared to routine sampling for 3/8 operations. For all samples collected, up to eight isolates per sample were characterized by sequencing of sigB, which allowed for classification into sigB allelic types. Among the 97 samples with more than one Listeria isolate characterized, 28 had more than one sigB allelic type present, including 18 sponges that were positive for LM and another Listeria spp and 13 sponges that were positive for more than one LM subtype. This indicates that collection of multiple isolates is necessary to capture Listeria diversity present in produce operations. Additionally, 17/77 sponges that were positive for LM were positive at only one enrichment time (i.e., 24 or 48h), indicating that LM testing after two different enrichment times provides enhanced sensitivity.

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