Five people fell ill from two states: Illinois (4) and Michigan (1) between June and August and were hospitalised.
Wholesome Soy Products issued a recalled in August due to possible Listeria monocytogenes contamination after the Food and Drug Administration (FDA) isolated the pathogen from samples.
Two of the five people interviewed reported eating bean sprouts in the month before becoming ill.
WGS findings
Whole genome sequences of the Listeria strains isolated from sprouts produced by Wholesome Soy Products and environmental isolates from the production facility were found to be highly related to sequences of strains isolated from the ill people.
The FDA isolated Listeria monocytogenes from mung bean sprouts and sprout irrigation water samples during a routine assignment on August 13 at the company.
Based on this, the agency held an inspection from August 12 to September 3 and isolated Listeria monocytogenes from 25 environmental swabs.
Wholesome Soy Products ceased production of sprouts on August 28 and resumed on September 15 after the pathogen was not identified in finished product.
FDA site inspection
From October 7 to 31, FDA re-inspected the facility and identified it in nine environmental swabs.
FDA also found 12 inspectional observations, citing the firm for numerous unsanitary conditions and poor equipment maintenance.
Nine of these observations had persisted from the previous inspection.
On October 14, Wholesome Soy Products stopped production of all products except mung bean and soy bean sprouts.
FDA is ensuring they do not produce sprouts until it has adequate assurances that the strain of Listeria monocytogenes is sufficiently controlled.
The agency performed pulsed-field gel electrophoresis (PFGE) and whole genome sequencing (WGS) on isolates from mung bean sprouts and environmental samples from the firm to characterize Listeria isolates.
Compared with PFGE, WGS provides a clearer distinction of genetic differences among Listeria isolates (strains that are highly related by WGS are more likely to have a common source), said CDC.