How does salmonella travel to onions and then into humans?

Question

How does the salmonella get into onions?

Or if it only gets on the outside, how does it get into humans?

Answer 1

I'm guessing this is inspired by the recently reported outbreak in the US. From the media reports I've seen it's not clear if contamination was limited to biofilm or if it grew inside onions.

Generally speaking, salmonella contamination of vegetables often originates from contaminated manure used as fertilizer and can be absorbed through plant roots. Salmonella can definitely grow inside some plants thereafter (aka "internalization"), e.g. inside potatoes, watermelons, lettuce, oranges, or even tomatoes as shown by fluorescence microscopy and by laser scanning confocal microscopy. It can persist for weeks in such conditions, generally at room temperature.

More generally, Salmonella can form biofilm and persist on many more surfaces, sometimes with the cooperation of other microorganisms. It can even do that on the surface of tomatoes (which have some anti-microbial properties otherwise). Under some washing conditions, e.g. warm fruits washed with cold water, it can actually penetrate the skin of some plants as well, so ironically that may make the problem worse.

Reference for the above: https://www.tandfonline.com/doi/full/10.1080/01140671003767834

There is actually one (2015) study that found that Salmonella will readily grow inside onions at room temperature (0.3 log CFU/gram/hour), but will slowly degrade on the (outer, dry) skin thereof (0.3 log CFU/sample/day). Refrigeration inhibited growth inside onions.

There is also one (2019) study that discusses the issues with detecting Salmonella in spices (including onions), precisely because spices have some anti-bacterial properties retarding bacterial growth.