Investigation of the microbial contamination of food and the effects of storage, cooking and hand washing on microbial load
The results show that foods tested varied considerably both in the degree of microbial contamination and the types of bacteria present. Bile salts in the MacConkey agar allow selective growth of bacteria adapted to grow in the mammalian intestinal tract (Heritage et al. 1996).
The majority of these are Enterobacteriacae (enteric bacteria), including pathogens such as Escherichia coli (coliforms) and Salmonella spp., many of which are causal agents of human food poisoning (Murray et al. 2002). Bacterial colonies present on the nutrient agar are likely to be a mixture of non-pathogenic and spoilage bacteria, Staphlococcus spp. as well as pathogenic enteric species (Beuchat 2002). Analysis of microbial load on the MacConkey agar is therefore a more reliable indication of whether a food is contaminated with potentially pathogenic bacteria (although some non-enteric bacteria such as Staphlococcus aureus also cause food poisoning).
The majority of these bacteria are destroyed by adequate cooking (Murray et al. 2002). The finding that some foods, such as salad and cheese, had a high microbial load of enteric bacteria is worrying as these are intended to be eaten raw. The high level of bacterial growth on the nutrient agar is not necessarily as much of a health concern, as many of these bacteria are likely to be non-pathogenic spoilage bacteria commonly found on vegetables and fruit, such as species of Pseudomonas, Corynebacterium and Streptomyces (Adams & Moss 2000). These will not usually cause disease but may decrease the storage life of the food.
Other studies have also found salad to be contaminated with pathogenic microbes, such as Salmonella, Listeria and E.coli (Sagoo et al. 2003, Viswanathan & Kaur 2001) and 19 cases of S. newport were traced to eating contaminated salad.
The finding of high levels of enteric and other microbes on cheese was not as predictable, as the cheese-making process usually decreases microbial load as cheese has a lower water content and pH than many other dairy products (Adams & Moss 2000). The high load could have resulted from contamination at the cheese making plant or subsequent handling in the kitchen/laboratory.
The high level of enteric bacteria in flour was also not predictable. The dry nature of flour usually limits bacterial growth and moulds are usually more of a problem for grain products, rather than bacteria (Adams & Moss 2000). Again, this may have resulted from cross-contamination of the flour in the kitchen/lab following handling of highly contaminated foods, such as meat, or during the laboratory procedures. All of the food types should be retested with newly purchased items to check these results, with strict isolation of each food and handling hygiene protocols.
The enteric bacteria isolated on the selective agar could have arisen from a number of sources of contamination throughout the food production process (Hobbs & Roberts 1993). Meat, poultry and eggs are frequently contaminated with bacteria such as coliforms and Salmonella spp. on the farm or abattoir. Inadequate storage or subsequent cooking then allows the bacteria to multiply and is more likely to induce disease when eaten. Enteric microbes can also be spread from person to person, via the faecal-oral route, due to inadequate hygiene following contact with faeces. Handling raw food then cooked food and inadequate cooling and reheating of cooked food are also common causes of high bacterial growth on food. Contaminated water, flies and pets may also spread bacteria.
The presence of such pathogenic bacteria in food is a health risk, and although it increases the risk of becoming ill, there are other factors involved. One of these is the amount of bacteria on the food and the infective dose for each bacteria. Bacterial strains such as E. coli 0157: H7, which causes food poisoning that can progress to serious complications such as kidney failure and death, has an infective dose of around 100 bacteria. Species of Salmonella require at least 100,000 bacteria to be ingested before food poisoning symptoms are seen (Murray et al. 2002), so very low numbers of viable organisms are unlikely to cause disease.
Immunocompromised people, such as babies, the elderly and those with immunodeficiencies are at increased risk of becoming ill following ingestion of smaller doses of bacteria. There is also variety in the susceptibility of healthy individuals to disease, which was highlighted by Parry and colleagues (2005) who found similar levels of Salmonella contamination on dish cloths and refrigerator swabs in the kitchens of people who had suffered Salmonella food poisoning and controls that had not.
Although most people are able to fight off infection from very low doses of ingested bacteria, the aim of food producers and handlers should be to eliminate bacterial contamination to reduce the risk of disease. This can be achieved through the HACCP system (Hazard Analysis of Critical Control Points) at the farm and industrial stages of food production (Hobbs & Roberts 1993) and through suitable storage, cooking and handling in the domestic kitchen.
The results from our study of meat storage show that freezing had the greatest effect in reducing viable enteric bacteria, but still did not completely eliminate them, which is consistent with other findings (Bolton et al. 2001). Refrigeration resulted in a lower enteric microbial load, which although still significantly better than storage at room temperature is still likely to pose a risk to health. The microbial load of the meat prior to storage was not recorded, so it cannot be determined if there had been any growth whilst in the refrigerator or if it had remained constant, which would be in keeping with previous findings of E. coli growth between 6 and 45oC (Tamplin et al. 2004), but not at the normal fridge temperature of 4oC (Berry & Koohmarie 2001). Many people do not know the actual temperature of their kitchen fridge, which means that they could be storing food above the 6oC at which coliform growth occurs (Kennedy et al. 2005), thereby increasing the risk of illness.
Adequate cooking is commonly stated as a sure way to kill enteric bacteria in contaminated food (Hobbs & Roberts 1993), but the results of our study question this. Although all the beefburger cooking regimes reduced the microbial load significantly, viable organisms remained even after 14 minutes grilling on each side. Although another study also found that grilling beefburgers on one side with infrequent turning did not eliminate all E. coli bacteria (Rhee et al. 2003), the experiment should be repeated to ensure there was no cross-contamination during the laboratory procedures.
Another important method of decreasing the risk of human illness due to ingestion of contaminated food is to improve hygiene whilst handling food. Hand washing after handling raw meat is a simple, quick procedure and the results show that it reduces the microbial load on the hands. It did not, however, eliminate bacteria, which could therefore still be transferred to subsequent foods handled. As well as bacteria from any raw foods handled, there are likely to be other bacteria on the hands which can lead to food-borne illness, such as Staphlococci spp., enteric bacteria following defaecation and soil microbes such as Clostridrium perfringens. Contamination from hands could also be spread to other areas of the kitchen, such as chopping boards, which then act as sources of contamination themselves (Wachtel et al. 2003), or may spread directly to the mouth.
Haas and colleagues (2005) used quantitative microbial risk assessment to determine the benefits of hand washing in the kitchen and found that washing with normal, non-germicidal soap reduced the microbial load, which agrees with this study, and is therefore beneficial, if not ideal. The most effective way to reduce the bacterial load further was to use alcohol based topical products.
Tags: bacteria, contamination, cooking, HACCP, kitchen, pathogens, salmonella, washing
