Most outbreaks are caused by contaminated raw materials; however, outbreaks can also be caused by cross-contamination during processing. Bacteria which survive on surfaces such as knives and saws can be transferred to food.
Temperature and humidity
In order to assess the risks of cross-contamination, it's important to know a few things about how EHEC survives on surfaces. There are many factors that can affect the survival of bacteria on surfaces, but the most important ones in this context are temperature and humidity. Temperature and humidity in the production of meat products vary a lot depending on the stage in the process, the type of process and processing plant, and even in the course of the day and night in the same premises. Just after cleaning, humidity levels can be high, and under good drying conditions humidity will be low. The temperature during production and storage is often regulated and can vary from below freezing to a comfortable ambient temperature.
Die when desiccated
Karin Mo from the Norwegian University of Life Sciences (UMB) has recently completed her master's thesis at Nofima Mat. She has looked into how EHEC survive under various conditions on surfaces. Her results show that EHEC will die over time when they dry out on stainless steel, depending on the temperature and humidity. At 12 °C and 70% relative humidity (RH), about 99% of the bacteria will die within 24 hours, 99.99% within 7 days and 99.999% within 14 days. The study was carried out on 13 EHEC from different types of products and countries. There was little variation in survival between the different EHEC isolates, but EHEC is more resistant than non-pathogenic E. coli. The originators of the Norwegian outbreaks, E. coli O103 (cured sausage 2006) and E. coli O157 (current outbreak 2009), have similar survival abilities to other EHEC. Salmonella has been known to survive under dry conditions, and EHEC survives almost as well as Salmonella. However, EHEC has lower survival skills than many other types of bacteria found on surfaces in the meat industry.
Water better than blood
The study also included survival at different levels of humidity, with or without dirt, at various temperatures and on various materials. EHEC did not die under extremely humid conditions (95-100% RH). Surprisingly, EHEC had higher survival rates under dry conditions (40-70% RH) than under "semi-dry" conditions (85% RH). The reason for this is unknown, but it is possible that the bacteria have some sort of dormant phase at extremely dry conditions where little affects them.
EHEC will die more rapidly if they dry out in water compared to in substances such as blood. Proteins, sugar and salt increase survival rates. Thorough cleaning which leaves a minimum of surface meat and blood residue will therefore lead to quicker EHEC death. EHEC had higher survival rates at 12 °C than at 20 °C. No difference was found in survival rates between EHEC drying up on stainless steel and on plastic of the Ertacetal (POM) type.
The importance of the correct disinfectant
Disinfection further reduces the number of EHEC. Disinfectants have less of an effect on surface bacteria than on other bacteria, but the correct disinfectant should still achieve 99.99% bacterial death. Many disinfectants, however, have a limited effect on surface bacteria. It's important to be aware that the documentation presented in the marketing of disinfectants is in most cases based on testing with bacteria in solution and not surface bacteria.
In conclusion, thorough cleaning and disinfection will reduce the survival of surface EHEC. It is, however, important that surfaces dry after cleaning, so that the periods of very humid environments which give EHEC ideal survival conditions are kept as short as possible.
This research was financed by the Norwegian Research Council, the Foundation for Research Levy on Agricultural Products and the EU.