Investigation of a staphylococcal food poisoning outbreak in Luxembourg using epidemiological case-control analysis, traditional typing and whole genome sequencing

In June 2014, a staphylococcal food poisoning outbreak occurred at an international equine sports event in Luxembourg requiring the hospitalization of 31 persons. We conducted a microbiological investigation of patients and buffet items, a case-control study and a carriage study of catering staff. Isolates of Staphylococcus aureus from patients, food and catering staff were characterized and compared by traditional typing methods and whole genome sequencing. Genotypically identical strains (sequence type ST8, spa-type t024, MLVA-type 4698, enterotoxin A FRI100) were isolated in ten patients, shiitake mushrooms, cured ham, and in three members of staff. The casecontrol study strongly suggested pasta salad with pesto as the vehicle of infection (p<0.001), but this food item could not be tested, because there were no leftovers. Additional enterotoxigenic strains genetically unrelated to the outbreak strain were found in four members of staff. Non-enteroxigenic strains with livestock associated sequence type ST398 were isolated from three food items and two members of staff. Not maintaining the cold chain after food preparation is likely to have been the main cause of the outbreak. Whole genome sequencing resulted in phylogenetic clustering which concurred with traditional typing while simultaneously characterizing virulence and resistance traits.