Food poisoning with a difference

The usual symptoms of food poisoning are vomiting, with or without diarrhoea, and sometimes fever.  Abdominal pain is also a frequent accompaniment.  Yersinia pseudotuberculosis doesn't fit this mould.

Since the beginning of September 2014, there has been a total of 124 confirmed cases and 18 suspected cases of Y. pseudotuberculosis in New Zealand.  I was asked by the Science Media Centre (NZ) to comment on this outbreak, and to provide a general overview of the bacterium, how it is transmitted in the food chain, and what the potential sources of the current outbreak.

Yersinia is a genus containing 11 species of Gram negative rod-shaped bacteria (the Gram stain is a relatively simple microscopy technique used by microbiologists to divide bacteria into convenient groups for study).  Three species of Yersinia cause illness in humans,  the most infamous member being Yersinia pestis, which caused the Black Death. 

The other two human pathogens are Yersinia enterocolitica and Yersinia pseudotuberculosis, the latter being much less commonly seen in infections.

The natural hosts of Y. pseudotuberculosis are rodents and sometimes other mammals.  Infection of humans normally occurs through consumption of foods contaminated with faeces or urine.  The outbreak currently occurring in New Zealand appears to be associated with bagged lettuce and raw carrots.  The full cause of the infections is still being investigated by Ministry of Primary Industry workers.

The symptoms of Y. pseudotuberculosis infection include fever and right-sided abdominal pain, which makes diagnosis difficult, since these symptoms are also seen in appendicitis.  The patient may have diarrhoea, but this is often absent.  Less frequently, long-lasting joint pain may occur.  Symptoms take 5 to 10 days to develop, so it is often difficult for the patient to remember what foods they had eaten that might have transmitted the infection.

Y. pseudotuberculosis is very well adapted to infecting humans.  The bacterial cells have a number of virulence factors, most encoded on a small transmissible piece of DNA called a plasmid.  The virulence factors enable the bacterial cells to adhere strongly to intestinal cells, and to evade the host’s normal defence mechanisms, while other proteins enable the bacteria to invade host cells.

The best way to avoid infection is probably to throw away bagged pre-prepared lettuce or shredded carrots that have been in your refrigerator for a while, and to prepare these items fresh as required.  Since Yersinia species are not heat resistant, they can be killed by normal cooking.  Be careful to maintain good kitchen hygiene, and avoid cross contamination of cooked foods from raw foods of any kind.

What is "sporulation"?

A friend asked me today "What is sporulation?"

There are essentially two genera of bacteria - Clostridium and Bacillus - that can produce very resistant stages called "spores" in response to unfavourable environmental conditions.  In contrast to the vegetative, or growing, cells, the spores are resistant to many environmental conditions, including heating and drying.  While the vegetative cells may be killed by heating to about 75C, the spores of C. botulinum can withstand boiling for more than four hours.

History gives us an example of how resistant spores can be:

In 1946, the British undertook some biological warfare experiments on Gruinard Island in the Inner Hebrides.  Small bombs containing anthrax spores (from Bacillus anthracis) were detonated near to groups of tethered sheep.  Within days, the animals were dying.  The government realised that the island would be hazardous to all mammals and quarantined the island indefinitely, occasionally testing the soil for viable anthrax spores.  On every testing occasion, the spores were shown to be viable.  This continued until a public campaign in 1981 forced the decontamination of the island.  This was achieved in 1986 by spraying 280 tonnes of formaldehyde solution onto the island and removing the most heavily contaminated topsoil.  The island was demonstrated to be safe and returned to its original owners.

It's important to note that soil normally contains some bacterial spores, including those of C. botulinum.  That means that raw vegetables and fruit carrying soil will often also be carrying spores.  Even milk drawn from healthy cows may initially be contaminated with soil organisms from their hides.  The normal pasteurisation process will kill the vegetative cells, but not the spores.