If the world was enriched and homogenized, we would actually have a very good idea of what the microbiological community within looks like. Fortunately, the world is much more complex than the miniature environments we culture in the lab, and high throughput sequencing (HTS) is allowing us to fully appreciate micro-biodiversity. As new information becomes available, many of our models for microbial communities continue to be challenged by the actual composition of species in natural environments.
In the world of food safety, we rely on these models to set policy on a regulatory level, and to set critical limits down at the production level. Which tests we run on what products depend directly on what organisms (that cause food borne illness or spoilage) are supposed to be found on that type of food. The authors of this study that came out in PLOS ONE this February examined the microbiome associated with poultry products from farm to fork (meaning from clucking chicken to packaged poultry product) using HTS rather than culture/enrichment methods. The results indicate that there is an unappreciated amount of diversity between different stages of the poultry production process, and that we may not acknowledge the presence of some organisms as much as we should.
In the study, samples were taken from multiple steps in the poultry production process: wet and dry litter, fecal samples, fluid from carcasses collected during the cooling process following slaughter, and fluid from raw retail poultry products (legs, wings, and breasts). Other than the retail portion, all of the samples collected were from the same batch of birds from start to finish. The available RNA from viable cells in each sample was amplified and identified as belonging to specific species using a combination of Illumina sequencing and database referencing (blastn and usearch).
From this pile of data, lists of organisms were compiled to compare the ecosystem profile for each point in production.
The authors were very surprised by the amount of diversity between the two litter samples (wet and dry) and the fecal sample. They expected to see very similar profiles, as all of the predicted microbes in those groups would be inoculated from contact with fecal material (young chicks have no inherited microflora, and are coprophagous); however, all of the groups’ microbial communities had very little in common. As shown above, of the hundreds of unique species identified, only 52 were actually found at every stage from farm to fork.
In evaluating food safety, several results are of concern. The first was that the authors found significant amounts of Shigella spp., which have traditionally not been associated with poultry products and may not be a part of many sanitation programs. The second is that in one of their dry litter samples, the authors found a large amount of C. jejuni. It’s presence was interesting as previous studies have found it difficult to cultivate C. jejuni onto dry litter, suggesting that it will not grow in that environment. This discovery further shows that our attempts to cultivate bacteria are not indicative of their behavior in “the wild”. There may be nutrient gradients or a symbiont in play that allows C. jejuni to grow; therefore the possible contamination of dry litter has to be acknowledged in that facility’s Campylobacter monitoring program.
The last point of interest I’ll discuss here is the large amount of unique species that were found in samples following slaughter. This suggests that these species did not come from the farm, but rather were introduced during slaughter and processing. Interestingly, among Campylobacter spp., there was little to no abundance of C. jejuni in the samples, but differing amounts of other Campylobacter spp. This is revealing, as we have been predisposed to expect C. jejuni to be present due to our use of selective media.
Let’s fully appreciate the amount of diversity found within the processing facility, the authors collected two post-processing samples labeled carcass rinse and carcass weep. The rinse was composed of fluid shaken off of the carcass following its removal from the chlorinated chill tanks, and the weep was the drippings from the same carcass 48 hours later. 2/3 of the unique species found the weep samples were not found in the rinse. The authors interpret this as being due to the fact that the sterilization of carcasses is not the goal of poultry processing, and provide the example that viable Salmonella can be recovered from carcasses even after they are sent through the standard antimicrobial processes. The goal is to reduce enumeration, not sterilization.
Finally, in examining the retail samples, we get what we expect. Similar organisms as the weep, with some new faces, presumably because they persisted through processing at undetectable levels, and slowly grew as the product was stored in refrigeration.
The authors conclude by examining some potential symbionts that would allow C. jejuni to persist, but ultimately say that due to the high number of environments C. jejuni can occupy, attempting to exclude it in a universal way will not be very effective.
So all in all, a thorough example of the misdirection we receive from culture bias, and a startling look at how, given enough incubation time, properly processed meat can still support a huge amount of microbial diversity, including many food borne pathogens.
Appreciate this diversity, and make sure you cook your chicken to temperature.
Oakley BB, Morales CA, Line J, Berrang ME, Meinersmann RJ, Tillman GE, Wise MG, Siragusa GR, Hiett KL, & Seal BS (2013). The Poultry-Associated Microbiome: Network Analysis and Farm-to-Fork Characterizations. PloS one, 8 (2) PMID: 23468931