Growth of Escherichia coli and Staphylococcus aureus in individual lasagne layers and evidence for migration of E. coli throughout the product

Growth of Escherichia coli and Staphylococcus aureus within individual layers of lasagne was studied after spiking of ∼105–106 CFU/g of each bacterial species into bolognese or béchamel sauces. Both E. coli and S. aureus grew by 3–4 log10 cycles in each meal component. In a second study, alternate layers within a composite lasagne meal were spiked with ∼105–106 CFU/g of E. coli and adjoining layers were monitored for possible bacterial migration. Spiked composite meals were subjected to either low temperature storage at 4 °C up to 8 h or to freeze chilling, which involved freezing at −18 °C for 24 h followed by thawing at 4 °C up to 40 h. Migration of E. coli from inoculated layers to the adjoining layers was indicated by a recovery of substantial populations following both storage treatments. Migration appeared to be more limited for meals which underwent freeze chill treatment. In contrast, migration was evident throughout all product layers in meals stored at 4 °C only. Migration of bacteria throughout a multi-layered food may arise from differing compositional or structural characteristics within the product or from differing storage treatments. Therefore as a result of bacterial migration, it would appear that microbiological safety of multi-layered products such as lasagne relies on ensuring safety of each individual layer.