Welcome to the new “Making the Most of Micro” series by world-renown Microbiologist and Food Safety Practitioner, Professor Chris Griffith
(Technical Director of Von Holy Consulting). Happy reading our first article: The Rise and Rise of Cross Contamination…


The Rise and Rise of Cross Contamination: The X Contamination Files

by Prof Chris Griffith

In many countries when an outbreak of food poisoning occurs environmental health officials will investigate and try to determine the cause of the problem. Although reported differently in different countries one trend to emerge in a rumber of countries collecting data is the increasing importance of cross contamination. Defined as ;“ the transfer of microorganisms or contaminants from one surface or food (usually raw) to another surface or food either directly or indirectly through hands or equipment "

Simply put it describes the ways in which microorganisms can spread within a food handling/processing environment and is an increasingly reported risk factor in outbreaks of food poisoning ( see Box 1 and 2 for examples). In the 1970s in the UK it was typically reported in only 5-7% of outbreaks by the mid 2000s this had increased to 38 %. However even this is likely to be an underestimate given the way that outbreaks are investigated. People are much more likely to remember if food was undercooked than if contaminated equipment came into contact with ready to eat food.

Box 1. Salmonella Cross Contamination

Salmonella enteritidis infections associated with a contaminated immersion blender, New Hampshire, 2009

This outbreak of Salmonella enteritidis was caused by a Salmonella-contaminated immersion blender. A malfunctioning blender shaft seal is suspected to have resulted in contamination of the blender and subsequently products prepared using the blender.







Box 2. Listeria Cross Contamination

Listeria monocytogenes detected in 70 (3.5%) environmental swabs and 16 (7.4%) product from Swiss sandwich plant. Of the 86 isolates 93% were serotype 1/2a with 6 genetic profiles. 78% belonged to one genotype found on slicers, conveyors, tables, bread feeding machine, salmon and egg sandwiches. These strains persisted for more than 9 months on slicers and conveyors. Revision of cleaning programmes solved the problem. Emphasizes importance of environmental monitoring to identify potential contamination problems and as early warning.






There are several reasons for the rise in cross contamination although one of the most important is likely to be due to the changing nature of the pathogens contributing to the present patterns of food borne disease. Many of the pathogens of current concern have a low infectious dose i.e. few need to be consumed to make someone ill (especially if they are a higher risk consumer). This is also reflected in the types of foods increasingly reported in outbreaks. No longer are these the traditional meat, dairy, eggs and poultry but now include breakfast cereals, peanut butter, nuts, chocolate and with seemingly greater frequency “leafy greens” or fresh fruit and vegetables. This makes cross contamination important throughout the food chain but especially in businesses were it may be harder to control, including small businesses and food service. Factors important in cross contamination include the design, construction, work flow and cleaning of premises and equipment along with aspects of personal hygiene, employee health and hand hygiene. Many of these will be discussed in general as well as in more detail when we examine specific outbreaks to try and identify what key lessons can be learned. However a careful reading of the definition will illustrate that the dangers of cross contamination do not lie just solely with microorganisms. Problems associated with allergen contamination of foods are not far from international headlines and preventing transfer of allergens between foods and surfaces can be time consuming for some businesses depending on the products they produce.

Table 3. Look Local (http://looklocal.co.za , 20 April 2011)










To control cross contamination effectively requires a knowledge of the underlying causes of it and the efficacy of different control measures. Of particular importance here is cleaning and disinfection. This is often allocated a low status by industry with “cleaning” perceived as a menial task however, its importance needs to be recognised and it requires the correct allocation of resources including not just money but also time. We will look at how cleaning should be managed next month.


Can you think of any similarities between cross-contamination in food safety and the TV programme the X files?

See next month for the answer !

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