In the present case study, we investigated the causes of consistently elevated histamine content in raw
milk cheeses from a Swiss cheese dairy. Screenings of milk samples from 67 farmers revealed that 19.1%
of the raw milk samples were contaminated with histamine-forming bacteria. Lactobacillus parabuchneri
was detected in 97.4% of contaminated milk samples. The population density of L. parabuchneri was
almost always below the limit of quantification of the qPCR method used. Genotyping of 469 isolates
provided detailed insight into the diversity of L. parabuchneri in contaminated milks and cheeses. The
results of the trace-back study clearly demonstrated that several milk suppliers were responsible for the
histamine problem at the cheese dairy under study. Systematic controls of milking systems allowed
persistent contamination sources of L. parabuchneri at the farm level to be identified and eliminated and
thereby to reduce the percentage of contaminated milks to 8.5% within the study period.