The microbiology laboratory at Southampton General Hospital is part of the Public Health Laboratory Service, and operates PHLS standard operating procedures. Currently the laboratory routinely examines all faecal specimens for Salmonella species, Shigella species, Campylobacter species, E coli O157 and Cryptosporidium .

Beyond that, what we look for depends on what you tell us. Specimens from patients where foreign travel is indicated are cultured for Vibrio species and are referred for parasite examination. If food poisoning is mentioned on the request form, the specimen may be cultured for Staphylococcus aureus, Bacillus species and Clostridium species, depending on the nature of the suspect food. Where an outbreak (not usually sporadic cases) of non-bacterial gastroenteritis is suspected, specimens are referred to the Enteric Viruses Reference Laboratory in London. Unusual pathogens such as Yersinia enterocolitica may be looked for when we are informed of special circumstances which include mesenteric lymphadenitis, terminal ileitis, reactive arthritis or "pseudo-appendicitis" – pigs are frequently identified as the possible source of this infection.

What we obviously need to know about a specimen is:

The best results are obtained when an appropriate, labelled, well-taken specimen in the proper container is promptly delivered to the laboratory with relevant clinical information provided on the request form. Please do not send specimens in non-sterile containers, and obtain them before antibiotics are started. (It’s difficult to grow something that’s already dead – Ed)

Unfortunately in most cases there is no simple test that will instantly reveal what the patient is suffering from. Bacteria will take at least overnight to grow to detectable amounts, and may take forty-eight hours or longer to be isolated and identified. But reports are printed and dispatched every working day, including Saturdays, although we do not fax copies of results. Results of urgent tests, and results that may have an immediate impact on patient management, will be telephoned.

For unusual enquiries on faecal specimen results, please ‘phone 023 8079 4803. Andy Tuck, Head MLSO

So – someone comes to you and says they’ve been poisoned by food. What happens next? This is a very topical subject, with the Food Standards Agency currently beginning its publicity drive.

First, was it food that caused the poisoning? (And here I’m using "food" very loosely, including water, confectionary, even Big Macs.) Not all that causes D&V is food poisoning – what has this year gone under the title of Winter Vomiting Virus (Norwalk-like Virus or NLV) is usually spread by aerosols, rather than food. A pointer towards an illness being caused by food is a number of people who ate the same food becoming ill at a very similar time afterwards.

Secondly, which food caused the poisoning? You will know that illness from eating food can be caused by bacteria multiplying in the gut, as do Campylobacter Salmonella, Shigella, and E. coli. These are by a long margin the commonest causes. They take time to multiply – at least 12 hours, and more typically 24 to 48 hours. So if you go down with Campylobacter enteritis at 3 o’clock in the morning, the one thing that didn’t cause it was the curry from the take-away the night before. But occasionally food poisoning can be caused by bacteria multiplying in the food before eating – sometimes before cooking – and producing a toxin which then causes the symptoms. Staph. aureus, Bacillus cereus , Clostridium perfringens and Cl botulinum can all do this. Onset is then much quicker, and the picture is of a number of people rapidly becoming (sometimes seriously) ill in a short time.

Bear in mind that glibly to tell a patient because they have D&V and have been eating, that the two are necessarily linked, can trigger a chain of complaints and recriminations that can end in court. Post hoc non est propter hoc. Most people who get Campylobacter infection have succeeded in giving it to themselves, often by preparing raw chicken for the family. The chicken is well cooked afterwards, but in the preparation the bugs got onto fingers, work surfaces, chopping boards, taps … (and a mouth – Ed).

So you’ve borne in mind those two caveats, and you still think that your patient may have food poisoning. Then please notify the CCDC. That will initiate action to alert local Environmental Health Officers (EHOs) who will be able to investigate a possible source, and if necessary take action to control it. But if you notify, then please also sample. Conducting a food poisoning investigation without knowing the causative organism is like hunting the snark without knowing what a snark looks like. Ask the patient for a faecal sample, and on the request record:

1. Suspected food poisoning.
2. Suspected food.
3. Length of time between food and onset of symptoms.

Much food poisoning is not food poisoning – but you’re entitled to notify on suspicion. And most food poisoning is not caused by retail outlets – but when it is, we need to know fast, and take action guided by your initial discovery.

Andrew Rivett, SCMO (CDC)