While events that conspired to sweep petfood into the Food Safety Modernization Act of 2011 were not exclusively pathogen related, the net result has been an almost singular focus on eliminating Salmonella in petfood. Before 2007, the only area in the news that seemed to be affected by Salmonella was poultry products (i.e., chicken and eggs). Now there is a near-constant stream of recalls from Salmonella in petfood and treats. Clearly, Salmonella is the one ingredient no one wants in their pet's food, treat, or chew toy. But can it truly be eliminated?
Annually, Salmonella leads to some 16 million cases of typhoid fever, 1.3 billion cases of gastroenteritis and 3 million deaths worldwide. It is universally dispersed in nature with animals (including humans) serving as the primary reservoir. It is harbored in the gastrointestinal tract and transmitted from one host to the next by fecal shedding along with anything this might come into contact with (such as water supplies). Transmission can be from one animal to the next, including from animal to man and back; the so-called "zoonotic" transmission.
Of the 2,463 Salmonella serovars, food poisonings are typically associated with the enteric species such as S. enteritidis, S. typhimurium and S. heidelberg. In the intestine of the host, Salmonella adhere to the endothelial cells with fimbriae and invade the cell by bacterial-mediated endocytosis. Once fully inside the intestinal cell, chloride ions are released into the lumen of the bowel contributing to a shift in osmotic balance (diarrhea) along with a cascade of immune and inflammatory responses.
Salmonella can multiply under various environmental conditions and are capable of living for long periods outside the host. They proliferate because they can grow rapidly on a wide array of substrates. For instance, visible colonies can occur within 24 hours at 37ºC. Their growth can be arrested (but not necessarily killed) below temperatures of 5-6ºC or in excess of 45ºC and will not survive baking and general wet cooking temperatures that exceed 75-80ºC.
Salmonella thrive at a pH around neutral with the ideal pH of 6.6 to 8.2. Conditions outside pH 4 and 9 can be bactericidal. They can use amino acids, and more elemental sources of nitrogen (e.g., nitrate, nitrite and NH3) as substrates for growth. Salmonella cannot tolerate high salt concentrations (>9%) and they grow best in a wet environment with water activity greater than 0.94. All conditions that pretty much describe the gastrointestinal tract of most animals.
Salmonella can survive in the environment for a long time and can be transmitted by various vectors including insects, rodents and birds. The organism can be picked up by farm livestock that then harbor it in their gastrointestinal tract before passing it along through milk, eggs and meat. The other challenge with Salmonella in modern food processing facilities is its assimilation into biofilms. They use "quorum sensing," a type of cell-to-cell chemical communication much like hormonal queues in animals. Salmonella are known to exhibit this sort of signaling when turning on "auto inducers" such as the LuxS/AI-2 gene system. This plays a role in microbial survival and promoting virulence of the Salmonella organism.
So, what can we do  to destroy Salmonella and make ingredients and petfoods safer? Prevention is at the top of the list. Simple procedural changes can be introduced to the operation which include controlling dust, managing equipment and personnel flow by compartmentalizing facilities and functions, controlling pests that can transfer bacteria, developing rigorous sanitation programs, attending to inbound and outbound transport vehicles, reducing multiplication sites within the facility, and using appropriate kill steps (Jones, 2011).
The standard is a thermal processing (cooking) step where temperatures exceed 75-80ºC. For some ingredients and processes this may not always be practical. In these cases, technologies do exist to control and eliminate Salmonella; they include high pressure processing, pulsed electric field processing, ionizing radiation, ozone processing, ultra-violet light, ultra-sound, electrolyzed oxidized water, high pressure CO2 processing and bacteriophage treatment (Mukhopadhyay and Ramaswamy, 2012).
Monitoring food and facilities for Salmonella is important, too. The standard method of analysis is the Bacteriological Analytical Manual (BAM; Chapter 5) procedure specified by the Food and Drug Administration. This multi-step procedure involves enrichment steps and incubation in selective media. The procedure is tedious, time consuming (three to five days) and requires very specialized training and skills. It is not conducive to just-in-time commerce or untrained receiving personnel.
There are several "kit" methods that have been developed to speed up the process, but these still require at least 24 hours of incubation. They rely on immunological techniques such as the "ELISA" or enzyme-linked immunosorbent assay. This is similar in principle to the kits currently in use for mycotoxin detection. The other technology relies on genetic testing procedures using "PCR" or polymerase chain reaction to determine genetic tags on the incubated organisms. There are really no true "rapid" methods to identify presence or absence of Salmonella in ingredients, equipment or finished products.
With this arsenal  of challenges, it is no wonder petfood manufacturers are wringing their hands regarding the "zero-tolerance" mandate resulting from the Food Safety Modernization Act. While improvements have been made in incidence of Salmonella-positive ingredients and finished petfoods (Li et al., 2012), there is still a long way to go before the matter is resolved.
Threats still exist from cross contamination, growing antibiotic resistance of Salmonella species and gene-jumping of virulence factors from one microorganism to another. We have to realize that these microorganisms are very resourceful and will probably challenge our every effort at eradication. However, nothing ventured, nothing gained. With continued vigilance and industry-wide adoption of best practices, the petfood industry will continue to deliver safe and wholesome foods to pets and their human family members.
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