According to a 1984 study, Salmonella survival in feed can last as long as 98 days, potentially longer. Since then, the literature on Salmonella in feed has only grown, supporting that Salmonella can be found everywhere and may infiltrate from anywhere. There are over 2500 different Salmonella serotypes, each possessing the ability to accommodate different environmental conditions.
No single intervention holds all the answers, and we turn to data and microbiological analysis to help determine the most impactful ways to gain the upper hand on controlling Salmonella in poultry. Posing a threat to food safety and consumer health, Salmonella in feed challenges producer reputation and brand value and serves as a potential liability. Consumers and regulatory body expectations demand effective intervention, and numerous starting points exist.
Feed as a Fomite
Aside from occurring naturally in the commensal microbial population in poultry, Salmonella has multiple entry points into live production, including pests, feces, water and feed and its ingredients. For example, Anitox’s laboratory database reports Salmonella contamination of 21%, 30% and 5% respectively in corn, soybean meal and wheat, which explains why studies have found a 27% incidence of mill ingredient receiving.
As one of the most significant inputs into live production, feed is central to nearly all operations, constantly provided to birds throughout the production cycle. Feed as a fomite facilitates consistent, repeated exposure of pathogens such as Salmonella to flocks. According to the literature, Salmonella contamination rates range from 1.1% to 41.7% in various finished animal feeds and 8.79% in mash feeds and 4.21% pelleted feeds. More recent trials have found 9.1% and 15% of sampled feed mills to be positive for Salmonella and 6.2% of the total feed samples collected to be Salmonella positive. The presence of Salmonella during times of bird stress increases the likelihood of colonization and shedding, repeatedly introducing Salmonella to live production.
Feed as a fomite for Salmonella has been historically underestimated. Salmonella in feed can be challenging to isolate, and when it is isolated, it isn’t always the serotypes that we find downstream in the production cycle. New technologies, such as CRISPR, could enable researchers to better understand and track serotypes through live production, from feed mill to processing. However, it’s essential to recognize that conventional methods have also made significant strides within the last decade.
Challenges to recovering Salmonella in feed
Salmonella in feed is difficult to recover and unfortunately in some cases a Salmonella negative result doesn’t mean the feed is Salmonella-free, it just means it wasn’t found. Factors impacting Salmonella recovery include:
Feed as a Matrix
Feed isn’t inherently homogenous – in fact its quite the opposite. As a mix of different ingredients feed consists of different particle sizes, comes in large quantities and its physical properties can vary by the sample. The use of different raw materials within a recipe contribute to variation observed in feed.
Salmonella in Feed
Subject to low moisture within the feed environment and exposed to high heat Salmonella in feed often becomes desiccated and injured.
Method Suitability for Feed
Salmonella in feed is sensitive to acid stress. Feed ingredients and other microbes residing in feed can lower media pH during vital method steps.
Animal feed is made in large batches and ingredient are received in large batches. The purpose of sampling is to take a small part of a bigger whole and use it as a representation. This is much harder to achieve than it sounds. Sampling is most representative when many aliquots from the bigger whole are taken and combined – creating a composite. All tested samples should be mixed and ground to a uniform particle size.
Feed Additive Inclusion
Certain diet components and feed pathogen control tools can mask the presence of Salmonella by inhibiting the methods ability to support Salmonella growth during pre-enrichment or by contributing to its injury. This can lead producers to underestimating the Salmonella transmission risk from feed to live production. When analyzing feed for Salmonella its important to understand how feed additives may impact the pH of media used in isolation methods.
Pathogens and other bacteria residing in feed can alter the pH during Salmonella recovery – again potentially masking its presence.
Method considerations for Salmonella in feed
Conventional Salmonella isolation methods were originally developed for human foods, which are very different from animal feed matrices. In general Salmonella isolation methods include several steps:
- Selective agars
- Colony selection
- Colony growth on non-selective agar
- Serotyping and molecular identification
Several of these steps have been optimized for feed over the years.
Salmonella recovery is dependent on its viability within a feed sample. Pre-enrichment requires the addition of feed to a non-selective liquid media. The goal is to get all the bacteria present in the feed to grow. Media commonly used in Salmonella isolation methods such as Lactose Broth and Buffered Peptone Water, are not always able to buffer the acid by-products produced by bacteria and feed fermentation, resulting in a pH dive during pre-enrichment. Successful pre-enrichment requires a media that can buffer against acid compounds as well as provide balance nutrients required for Salmonella growth. Studies have shown that a pH as low as 4 can injure or kill Salmonella. Acid injury often results in the inability of Salmonella to produce hydrogen sulfide – which impacts identification during subsequent methods steps.
After successful pre-enrichment a liquid aliquot from the sample should be added to a selective broth. Due to the amount of Salmonella serotypes and their abilities to adapt to certain environments it is best practice to use a couple different type of selective broths. The method used by the Anitox laboratory, uses two different selective broths, each incubated at different temperature – this helps cover the wide range of Salmonella capabilities and improves chances of isolation from feed.
Cultures grown from the selective enrichment broths can be streaked on to selective agar for colony identification. Again, using a couple different agars covers a wider range of biochemical properties inherent among the thousands of Salmonella serotypes.
Culture Growth on Non-Selective Agar
Once colonies have been grown on a selective agar and identified as potential Salmonella a few colonies should be selected and streaked onto a non-selective nutrient agar. Growth on a non-selective agar is designed to grow a health culture from a single colony – this growth can be serotyped for Salmonella confirmation and further identification.
While many conventional methods use serotype specific antigens for Salmonella identification, PCR technology can also be used. Technologies such as CRISPR, have granted scientists the ability to understand Salmonella types and populations within a single sample. Conventional isolation practices are highly dependent on human selection of colonies which can be biased towards certain serotypes that may grow bigger or healthier colonies.
As this review has made evident – Salmonella in feed can be difficult to find and without the proper methodology, negative results can’t always be trusted. Industry researchers have spent years troubleshooting conventional isolation methods to improve the accuracy of Salmonella recovery. However, finding Salmonella in feed isn’t solely dependent on the correct method, it also relies on proper sampling procedures, feed microbial load and ingredient composition and feed additive inclusion. Awareness of all these factors allows producers to improve their Salmonella monitoring efforts and intervention program efficacy. The last thing a producer wants to do is spend time and money on feed pathogen control, when the support to properly assess Salmonella in feed is not in place. Anitox is an expert when it comes to Salmonella in feed and provides technical laboratory support to ensure that feed pathogen control is a total system solution, not just a product to apply. Our experts provide sampling support as well as regular pathogen and product monitoring.
If you’re ready to implement feed pathogen control or if you want more information on recovering Salmonella in feed contact us.