9/18/2016 Injury Bacterial Injury following Mild Stress Non-selective medium Defined-inability of cells exposed to sublethal stress to grow on selective media,while lethality retaining culturability on non-selective media Selective 4 n山y medium Sub-lethal levels of heat,radiation,acid,or sanitizers may injure rather than kill cells Injured cells are less resistant to selective agents or have increased nutritional requirements (minutes) Recovery of injured cells may be needed before growth on selective media Injured bacteria are not detected on the selective media Cell Injury is a Threat to Food Safety How are Bacteria Injured? 1.If injured cells are classified as dead Heating,freezing,and detergents during heat resistance determination, Injury:damage membranes and leak then the heat process will be ineffective cytoplamic components 2.Injured cells that escape detection may Repair:membrane integrity is repair before food is eaten and cause reestablished illness ·Osmoprotectants 3.Selective agents may be common food ingredients like salt or organic acid that prevent injury prevent repair and lead to underestimation of the microbial levels9/18/2016 7 Injury • Defined – inability of cells exposed to sublethal stress to g , row on selective media, while retaining culturability on non-selective media • Sub-lethal levels of heat, radiation, acid, or sanitizers may injure rather than kill cells • Injured cells are less resistant to selective agents h d l or have increased nutritional requirements • Recovery of injured cells may be needed before growth on selective media 25 Bacterial Injury following Mild Stress Non-selective medium lethality Selective injury medium Injured bacteria are not detected on the selective media 26 Cell Injury is a Threat to Food Safety 1. If injured cells are classified as dead during heat resistance determination during heat resistance determination, then the heat process will be ineffective 2. Injured cells that escape detection may repair before food is eaten and cause illness 3. Selective agents may be common food ingredients like salt or organic acid that prevent repair and lead to underestimation of the microbial levels 27 How are Bacteria Injured? • Heating, freezing, and detergents - Injury: damage membranes and leak cytoplamic components - Repair: membrane integrity is reestablished • Osmoprotectants prevent injury 28