Lettuce enjoy our bacteria

Allison Cartwright is the incoming web publication officer for our Early Career Scientists (ECS) group. As it’s summer, she’s been pondering salad and the microbes within. 

Coming from an environmental science background, I haven’t had much exposure to food microbiology. As the Society for Applied Microbiology’s Annual Applied Microbiology Conference in July is titled ‘New Insights into Food Safety’, I had to do some research of my own.

Involvement in an undergraduate project where the students were encouraged to design their own program to investigate differences in the microbial community on plants provided me with a perfect opportunity. I decided to carry out my own project comparing the microbes on loose and bagged lettuce leaves.


Tip of the iceberg

Once the microbes were suspended from the lettuce and put onto different agars, I impatiently waited for the growth. I didn’t expect to find high numbers of microbes, but was staggered to find that my agar plates were covered in organisms.

Fungi were rare but bacteria were common. Although, the loose lettuce leaves had on average 300 bacteria per mm2 and the bagged lettuce a higher growth of 1800 bacteria per mm2. I couldn’t help but wonder, was I safe eating lettuce?


Bag it up

Using selective agars, I identified the bacteria as actinomycetes; a bacteria group widely found environmentally. Most importantly, they do not contain microbes that are usually hazardous to health. Actinomycetes are even helpful to human health, as many antibiotic compounds have been isolated from this group and so they protect us from harmful bacteria.

Potentially, the humid environment created by the plastic caused an increase in bacteria on bagged lettuce, but some of these could be washed off with water before eating (but don’t forget, water also contains bacteria which will then be on the lettuce). The bagged lettuce also contained an expiry date. This has been carefully designed so that the bagged content will have bacteria counts below safe consumption levels.


Romaine or leaf?

On reflection, I am not afraid of consuming lettuce abundant in these microbes, but instead I wonder if they could be beneficial. If the bacteria from the lettuce can colonise my gut, potentially, they could release compounds protecting me from infection. Although food safety is often concerned with preventing harmful microbes, the microbes on the food could also be beneficial for good health.

Allison Cartwright completed her undergraduate degree in Marine Science before carrying out a MRes studying freshwater crayfish at Ulster University, Northern Ireland. She’s remained at Ulster University and is currently a third year PhD student. Alison’s project combines environmental science and microbiology to study freshwater sponges in Ireland.


Categories: Feature Articles

3 replies

  1. I studied nutrition and am Registered with the American Dietetic Association. I studied Food Born pathogens but never good bacteria. I think the bad bacteria is important because people get violently I’ll and die in some instances. It’s good that we get good bacteria from lettuce. I havent thought about it. Working in oncology in a hospital we take all fresh fruits and vegetables off patients when their white. Cells are low. The bad bacteria out ways the good then and with food poisoning.
    Thanks for this I would like to read more. It’s great to read things on the healthy side of things.
    Sue Breen

  2. I wonder how bacterial numbers would compare at a local farmers market, rather than a supermarket? Interesting read!

  3. Hi thanks for describing some interesting work. Because of my experiences in food chain microbiology I always re-wash salad stuffs that are sold pre-washed. This is because British industrial processes including potable water useage can be expensive & cost-cutting is sometimes achieved by re-cycling water used to wash salads; also I wonder about the organoleptic issue here. And as you write, although tap water is not sterile (and it shouldn’t be) I still prefer to re-wash ready washed salads. Now I’ll be shot down by the water conservationists!

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