Do Bacteria Smell?

So, supermarket chiefs eat food past its Use By date and use the ‘smell test’ to identify if the food is safe to eat. Words fail me. Except “Dickheads”, possibly.

This isn’t about the issue of food waste (with which it’s being conflated), it’s all about Listeria, you silly, silly twats. Cases of Listeriosis run at about 200 annually in the UK, about a third of people who get it die from it and it costs us about £245m per year. The problem with Listeria is that unlike other foodborne bugs it can still grow at fridge temperatures – albeit slowly – and this is one of the main reasons for having a Use By date on chilled food. This is also the principal reason it is a legal requirement that commercial fridges are kept between 0ºC and 5ºC. (See below for explanation of Best Before vs Use By dates).

According to the article:

Listeria Stats
Listeriosis UK Click to embiggen

“Dalton Philips, CEO of Morrisons said when at home he subjected food to what he described as a ‘smell test’… Speaking to The Times, Mr Dalton said: “Meat especially… I would smell it. Our date codes in the UK are pretty strict. But good, aged beef is nicer. I am always going over the date on my yoghurts. A sell-by date for lots of cheese is ridiculous – they get better with age. The rule is, smell it.”

How anyone running a supermarket chain could go on the record with such dumbarse advice staggers me.

The HPA runs a campaign every year highlighting the issue of – principally – pensioners buying food at its Use By date that has been reduced in price then keeping it for a few days. This is why Listeriosis is far more prevalent in the elderly than the pregnant – the group usually thought of as being at the most risk.

Smell the Bugs!

Aside from his cluelessness about basic food hygiene (always a good look in a food business CEO, I think) Mr Phillips’s ‘smell test’ for foodborne bugs is Utter Utter Bollocks (µ²B) too. By and large those of Our Microbial Overlords that can kill us via what we eat are odourless – so even if Mr Phillips’s olfactory skills are positively bloodhound-like in their acuity I am truly grateful I am never likely to be asked to his house to dine.

But I’ve heard many a food hygiene expert say “bacteria don’t smell”. I never correct them just in case it encourages the idea that Mr Phillips’s ‘smell test’ might be of use but actually, that’s Utter Utter Bollocks (µ²B) as well. The anaerobes (those what don’t need or can’t tolerate oxygen) can produce some foul odours because many metabolise sulphur. But plenty of bacteria have a very distinct whiff. For example:

  • The Streptococcus milleri group – viridans strep that can cause abscesses – produce large amounts of diacetyl which has a distinctive, caramel odour. Mmmm! Buttered popcorn abscess!
  • Pseudomonas aeruginosa smells of grapes. Some say tortillas, I say artificial grape flavouring.
  • Clostridium difficile smells of elephant dung – and dogs have been trained to detect it. Some say it smells of horse dung; it’s similar but the smell is definitely elephants, trust me.
  • Staphyloccocus aureus: sort of a skin smell combined with a bread smell. Like a skin sandwich. Sort of.
  • Eikenella corrodens smells of bleach.
  • Giant African pouched rats have been trained to smell tuberculosis in sputum samples as a diagnostic test.
  • Streptomyces produce geosmin which is a major component of petrichor – that very distinctive smell you get from warm, dry soil immediately after a rainstorm.

Underarm and foot odours are a result of several bacterial species acting on sweat – and we have two different types of sweat gland: our 3,000,000 eccrine glands produce sweat principally composed of water and salts to cool our bodies. Our 2,000 apocrine glands are principally found in hair follicles and when we’re stressed produce a sweat rich with proteins, fats and carbs. This is a great food source for Micrococcus luteus, Bacillus subtilis and Staphylococcus epidermidis which contribute to foot odour, plus S. epidermidis, Corynebacterium xerosis and Propionibacterium acnes which are the main culprits for underarm odour production – coincidentally some of the same species are used in the production of many strong cheeses!

Oh, and not only can Our Microbial Overlords talk to each other, they can smell things themselves too!

Footnote: Best Before vs Use By

A Best Before date means ‘I’m safe to eat after that date, I just might not taste as good. Or taste of the tin. But the only way I can kill you is to put lots of me on a pallet and have someone drop it on you’. 

Use By dates are different; after that date microbiological safety cannot be assured, even if the product has been refrigerated. It means ‘eat me and you may well end up rough as a badger’s arse. Or dead. Your call...’

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