Adaptation or resistance responses of microorganisms to stresses in the food processing environment

Foodborne bacteria are exposed to a variety of stresses in the environment. Oftentimes, they are able to tolerate such stresses, survive and/or grow in food and cause spoilage as well as illness. If the stress is mild, it causes injury to the bacteria and if it is severe, it causes inactivation. Injured bacteria in food are of concern, since they can revive when favorable conditions are encountered, as well as multiply and grow in food. Such mild stresses are very often encountered by the bacteria in food as well as in the food processing environment. For instance, with the present day consumer demand for fresh-like, preservative-free food products with good nutritional quality, minimal processing is done in which mild treatments are given to the food product. The bacteria once exposed to a mild stress are able to tolerate further severe stresses. This ability of the bacteria is called stress adaptive response (SAR) or stress hardening, which enables the bacterium to resist further homologous as well as heterologous stresses (Yousef, 2000). In the food processing environment, several treatments are given to the food to preserve its quality as well as shelf-life. The environment and the equipment used for processing in a plant handling wet processes are regularly or periodically cleaned to keep them, as well as the processed food, contamination free. Under such conditions bacteria are exposed to a variety of chemicals, sanitizers, heavy metal ions, antibiotics, etc. If these treatments are not severe enough, the bacteria survive and are able to adapt to even harsher treatments. These bacteria can form microcolonies on the equipment surfaces or other areas of the plant which, in course of time, form biofilms. Also, there are certain areas either in the equipment or other places in a plant that are inaccessible or hard to reach for cleaning, and the bacteria escape treatment. These areas are also most vulnerable for biofilm formation. Once biofilms have been formed it becomes very difficult to eradicate them. For instance, Listeria monocytogenes is a foodborne pathogen well known for its presence in processing plants (Smoot and Pierson, 1998; Cox et al., 1989) and for the formation of biofilms, due to which the food industry has incurred heavy losses especially in dairy (Mafu et al., 1990) and processed ready-to-eat meat products. This bacterium is easily disseminated by aerosols and contaminated food products in the processing plants (Cox, 1996) and can survive in aerosols (Spurlock and Zottola, 1991). When this bacterium forms biofilms it has an enhanced resistance to sanitizers (Frank and Koffi, 1990). Listeria monocyotgenes was isolated from domestic, retail and industrial refrigerators in Greece (Sergelidis et al., 1997). Pathogens such as Listeria, Staphylococcus aureus and Salmonella were isolated from a poultry abattoir in South Africa (Geornaras et al., 1997) and Salmonella from healthy swine and from abattoirs in Brazil (Lázaro et al., 1997). In a poultry slaughtering facility the main airborne contaminants (bioaerosols) were bacteria and the highest count was found in the shackling area and decreased toward the packaging area (Lutgring et al., 1997). The microbial load of the floor after cleaning in different types of food processing facilities was assessed and it was determined that a milk site had the lowest load followed by the pastry site where the load decreased initially after cleaning and then increased, while a meat site had the highest load with a rapid increase (Mettler and Carpentier, 1998). Salmonella serovars were isolated from a citrus processing plant both inside the premise (in the juice as well as surface of the fruits) and outside the premise, from amphibians captured outside the plant (Parish, 1998). In plants handling wet processes, enough moisture and other favorable conditions to promote microbial growth are found. In plants where processing takes place under dry conditions, bacteria are able to enter the plant through air, raw ingredients, worker traffic and other means. They are exposed to dehydration and are able to adapt to such conditions, survive and contaminate food products. For instance, Salmonella has been a problem due to its contamination from the plant environment in dry dairy products, grain products, chocolate products and others, causing several recalls (Gabis and Faust, 1988). Aerobic spore-formers such as Bacillus species are the predominant microflora in the food packaging material such as paper and board and these bacteria were found to produce enzymes which degrade papermaking chemicals and were resistant to industrial biocides (Väisänen et al., 1989). In dry processing plants, bacteria may encounter areas where there is lack of nutrients. In such conditions bacteria are exposed to starvation stress (Figure 5.1). Some other stresses that a bacterium may encounter in a processing environment during exposure to chemicals, sanitizers, or otherwise, include oxidative stress, osmotic stress, acidic/alkaline stress, etc. (Figure 5.1). According to Bower and Daeschel (1999), the resistance responses of bacteria in food environments are conferred by various factors including innate structures such as impermeable outer membrane of bacterial cell, mechanisms for antibiotic inactivation, and biofilm formation on food processing surfaces as an adaptive response to prevent the hazardous effects of cleaners and sanitizers. Antibiotic resistant bacteria present in food animals, exhibiting cross resistance to biocides, entering a food processing plant may pose a contamination risk to the processed product. Antibiotic residues present in meat and milk have contributed to the development of resistant bacteria (Brady and Katz, 1992; Brady et al., 1993). In this chapter, various aspects dealing with the adaptation as well as resistance responses of bacteria to various stresses in the food processing environment such as chemicals, sanitizers, metal ions, starvation and antibiotics will be discussed.