Last year it was the bagged spinach. This year it’s tomatoes. Just when you think a nice cool salad would be perfect on a hot, muggy day — BAM! Salmonella strikes again.

Here’s what’s going on this time:

US Salmonella outbreak explained
New Scientist

This week, the world’s biggest restaurant chain, McDonalds, stopped topping their Big Macs, Quarter Pounders and sandwiches with sliced tomatoes due to concern over a Salmonella outbreak that has affected at least 145 people, resulting in 23 hospitalisations.

Wal-Mart and some US grocery stores have shelved several varieties of the fruits, while federal investigators determine where the tainted tomatoes came from.

Here’s our guide to why Salmonella outbreaks seem to be increasing in frequency and why cleaner vegetables might mean more outbreaks.

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How common are Salmonella outbreaks on vegetable and fruit produce?

A recent census of produce outbreaks between 1996 and 2007 counted no fewer than 33 epidemics from Salmonella-contaminated fruits and vegetables. In five of them, tomatoes were the culprit. Cantaloupe melons and sprouted seeds, such as clover and alfalfa, were also common victims. Animal pathogens tend to infect only a limited range of plants.

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Shouldn’t consumers and regulators demand cleaner produce?

Not necessarily, Warriner says. More thorough washing will also kill off the harmless bacteria that coat tomatoes and other produce. These bacteria compete against pathogens like Salmonella.

Warriner’s lab found that tomatoes coated with a harmless form of a bug called Enterobacter were less likely to test positive for Salmonella. “If we make them too clean then it’s going to be a bigger problem. Salmonella seems to like it when there’s no competitor,” he says.

(shaking my head) We were just watching news of a tomato processing facility where they spray the tomatoes with bleach water. We thought that was a good thing.

I guess not.