Livestock have been repeatedly implicated as the source of antimicrobial resistance, but new research from Scotland shows that, at least in the case of Salmonella Typhimurium DT104, resistance profiles are often showing up in humans before animals.

In the paper from the University of Glasgow, An ecological approach to assessing the epidemiology of antimicrobial resistance in animal and human populations,” by Mather et al, published in the Proceedings of the Royal Society B, the researchers looked at Salmonella Typhimurium DT104 in animals and humans in Scotland. They demonstrated that, while ecologically connected, at the level of resistance phenotype, animals and humans have distinguishable microbial communities, differing in linkage and diversity.

The paper says while transmission is likely to occur in both directions, animal-to-human and human-to-animal, the critical issue is the proportion of transmission that occurs in each direction. Examination of temporal and phenotypic similarity suggests that a plausible and justifiable conclusion based on these data is that the sympatric animal population is unlikely to be the major source of resistance diversity for human DT104. 

Three conclusions

The research reached three conclusions:

  1. While a number of the DT104 phenotypes are common to both animal and human populations, there are a significant number of phenotypes that are unique to each population. Research indicates that there are distinguishable microbial communities, with significantly fewer shared profiles and significantly more human-only profiles than would be expected if the animal and human microbial communities were completely shared.
  2. For resistance to individual antimicrobials, there is no clear pattern, but the majority of resistance profiles that are common to both human and animal populations were identified first in humans.
  3. The origins of resistance profiles can be estimated. When a putative source population can be identified for each profile, and for each isolate, significantly more of these putative precursors were identified in the human population.

Big picture impact
What does this research mean in the bigger picture? Guy Loneragan, BVSc, PhD, veterinary epidemiologist at Texas Tech University, summarizes the major points from this research and puts it in context of the issue of antimicrobial resistance.

  • The inference is squarely focused on Scotland and the authors readily acknowledge that.
  • They are not making claims about the origin of Salmonella DT104 but are discussing the likely/probable/possible origin and diversity of resistance phenotypes within Salmonella DT104.  That is, it is important to note that they talk to resistance emergence in Salmonella DT104 and not emergence of Salmonella DT104.
  • Basically, there is some overlap between cattle and people but there is greater phenotypic diversity in humans than cattle. It also appears that in addition to the greater diversity of phenotypes, many of the shared phenotypes appeared (or were detected first) in the Scottish human population. One might conclude, therefore, that the origin of some of the phenotypes is likely human rather than cattle/animals. 
  • This certainly does not preclude animal-to-human spread and we know that foodborne outbreaks happen and animal contact is a risk factor for salmonellosis.
  • It is possible that both animals and people had a common/shared source of Salmonella DT104 such as imported foods, migratory birds, etc. If so, it appears this common source might have seeded humans with a greater diversity of phenotypes than animals or after a common source, the bacterium evolved with greater diversity in humans than animals.
  • I think the paper does highlight that Salmonella Typhimurium DT104 might be considered a ‘quasi-clone’ that is evolving both independently and dependently in humans and cattle. That is, there are macro (i.e., Scotland) and micro-ecological niches (i.e., animals versus humans) that influence the bacterium’s evolution.

“This highlights the zoonotic nature of Salmonella Typhimurium DT104 in that it is associated with a bidirectional movement of the organism,” explains Loneragan. “We tend to focus on movement from animals to people but the other direction clearly happens and may be precedent in many instances.”

Read the research report here. Read an article about how this research fits into the larger issue of antimicrobial resistance here.