Staphylococcus aureus Growth and Thermostable Nuclease and Enterotoxin Production in Canned Salmon and Sardines

Abstract

Staphylococcus aureus growth, thermostable nuclease (TNase) and enterotoxin production in inoculated canned salmon incubated at 22 ± 1°C for 4 d were dependent on the size of inoculum, and on the amount of oxygen present in the headspace; under nitrogen with an inoculum of 7 cfu/can, 102–103 cfu/g, no TNase and traces of enterotoxins (A, B, C2) were observed; under oxygen with the same inoculum ≥109 cfu/g, ≥6.0 μg TNase and up to 5.2 μg total enterotoxins (A, B, and C2)/100 g of salmon were observed. Values were intermediate under atmospheric air. After 1 week, 2 months and 4–24 months of incubation of salmon under nitrogen, S. aureus cfus were 108, 106 and 104–105 per g; TNase ranged from trace amounts to 20 μg/100 g and total enterotoxins from <1.0 μg to 6.2 μg/100 g. In canned sardines stored from 1 d to 12 months at 22 ± 1°C, levels were 109 cfu/g and 3.7–3.9 μg total enterotoxins/100 g; after 1 week, counts declined to 105 cfu/g but total enterotoxins remained relatively stable in some cans with up to 6.2 μg/100 g of sardines after 12 months. TNase varied from <1.0 μg to 20 μg/100 g of salmon with 109 and 105 cfu/g, respectively. In sardines, similar variation in TNase was observed and there was no correlation between TNase, enterotoxins and cfu/g. After 2 d to 24 months, carbon dioxide, an acidic smell and unacceptable odors were detectable over the headspace of S. aureus contaminated salmon and sardines, but not all persons who sniffed the contaminated products could recognize off-odors that would warn them against consuming the food. To prevent canned foods from causing staphylococcal illness, the conditions allowing post-process contamination should be eliminated by the producer and distributor of the products.