Pasteurization eliminates Listeria in milk but contamination can occur afterwards, said the analysis of the outbreak.
Inspection of the Crave Brothers site revealed issues during the cheese-making process, after the milk was pasteurized, which likely led to contamination.
The case of the outbreak which sickened six people last year is featured in the Morbidity and Mortality Weekly Report (MMWR) update.
Five cases were identified in four states (Minnesota, two cases; Illinois, Indiana, and Ohio, one each) by the end of June 2013, it said.
Median age of the patients was 58 years (range: 31–67 years), four patients were female, including one who was pregnant at the time of infection.
One death and one miscarriage were reported and all five were hospitalized.
All five patients had definitely or probably eaten one of three varieties of Crave Brothers soft-ripened cheese (Les Frères, Petit Frère, or Petit Frère with truffles).
Three patients had purchased it at three different restaurants, and two bought it at two different grocery stores.
The cheeses were shipped as intact wheels to the three restaurants and two grocery stores, where they had been cut and served or repackaged and sold to customers.
Crave Brothers halted production of Les Frères, Petit Frère, and Petit Frère with truffles at the start of July.
The firm issued a voluntary recall with a production date of July 1 2013, or earlier.
It voluntarily halted production of all cheese products manufactured at the facility on July 11.
After the recall, one other case was identified in Texas through whole genome sequencing, bringing the total case count to six.
Testing at the Minnesota Department of Agriculture identified the outbreak pattern of L. monocytogenes in two cheese wedges (Les Frères and Petit Frère with truffles) collected from two different grocery stores in Minnesota.
At the time of the outbreak George Crave, president, said: “We are cooperating with the regulatory agencies’ ongoing investigation of the cause of the potential health risks.”