Impact of spiking techniques on the survival of Staphylococcus aureus in artificially contaminated condiments

• Reduction rates after spiking depend on inoculation techniques and matrices.
• Spiking techniques affect S. aureus survival during long-term storage.
• At least two inoculation techniques should be considered for tenacity studies.

Tenacity studies are of relevance for microbial food safety risk assessment. Investigations on the survival of microorganisms in food matrices often require an artificial contamination step prior to microbial testing. Artificial contamination of low-moisture foods, like dried culinary herbs and spices, is challenging. In order to investigate the effect of several spiking techniques on the survival of Staphylococcus (S.) aureus in condiments, dried and ground paprika, pepper, and oregano were spiked with S. aureus using four different techniques: (1) aqueous bacteria suspensions were air-dried directly on the condiments; (2) aqueous bacteria suspensions were air-dried on sand, and then added to the condiments; (3) aqueous bacteria suspensions, or (4) bacteria suspended in liquid nutrient media with lyoprotectant were freeze-dried, crushed to powder, and added to the condiments. For each technique-condiment combination, and point in time, three distinct spiked samples (in three flasks) were prepared and stored in a dark environment at 24 ± 2 °C for up to 25 weeks. Cell counts directly after spiking and over storage time were analyzed. For the latter approach, mathematic models were used to fit the survival curve of each technique-condiment combination and to estimate their D-values. Results indicated that the reduction of the initial S. aureus cell counts depended on the applied spiking techniques and the used matrix. For instance, the initial cell counts in oregano were reduced by 0.8 ± 0.1 log10 cfu/g with technique 2 and by 6.1 ± 0.5 log10 cfu/g with technique 1. Technique 1 at the same time reduced the initial cell counts in paprika by only 0.4 ± 0.1 log10 cfu/g. The calculated D-values ranged from 1.3 ± 0.1 days (technique 3) to 120.1 ± 33.4 days (technique 4). The lowest as well as the highest D-value were observed for paprika samples, which emphasized the effect of the spiking technique. Thus, the behavior of bacteria can be strongly influenced by the selection of the spiking technique used for tenacity studies. Therefore, at least two spiking techniques should be considered, if the survival of microorganisms in low moisture food has to be investigated. In favor of high cell counts after drying, techniques 2 and 4 should be considered, as they performed best in our study.