The Salmonella bacteria that are one of the most common causes of food poisoning have long posed a puzzle for microbiologists. You can swallow thousands of these bacteria on their own, au Naturel, without coming to any harm, but eating fewer than a hundred of the same bacteria in a piece of contaminated food can be enough to make you ill.

One answer to this long-standing microbiological conundrum is now provided in a report in Applied and Environmental Microbiology. The most immediate and greatest hazard food-poisoning bacteria face is the extremely acid environment of the human stomach. Except after a meal, when the stomach is full of food, the pH of the normal human stomach can be very low, around pH 2, which will kill Salmonella and many other bacteria that cause enteric diseases. Once past this barrier, however, the bacteria have quite a good chance of surviving to colonize the intestines.

So Scott Waterman of Imperial College, London, UK and P. Small of the National Institute of Allergy and Infectious Diseases at Hamilton, Montana, USA decided to see whether the food itself could be protecting these acid-sensitive bacteria. They inoculated strains of Salmonella and other acid-sensitive bacteria onto the surface of some solid foods, such as minced beef or egg-white, which were then suspended in an acid growth medium at pH ranging from 2.5 to 4 for several hours.

They found that bacteria that would normally be killed in these acid conditions survived on the food surface and would grow again when retrieved and restored to growth conditions that suit them better.

Waterman?s and Small?s experiments explain why Salmonella primarily cause food-borne infections, rather than being transmitted directly by person-to-person spread. In contrast, more acid-resistant microbes, such as the Shigella bacteria that cause dysentery, are commonly spread person-to-person or by contaminated water, as well as in contaminated food.

Their experiments also help explain why experiments in which human volunteers were given their dose of Salmonella in a small amount of liquid rather than in food seemed to show that very high numbers of bacteria were needed to cause illness.

The food may be protecting the bacteria in various ways. In bulk, food such as minced meat raises the pH (lowers the acidity) of any surrounding liquid, and this would allow acid-sensitive Salmonella to survive in a higher-pH (less acid) microenvironment created at the surface of the food particles. The pH of the stomach temporarily rises to around 6 as it fills with food immediately after a meal and this undoubtedly helps the bacteria to survive.

But there may be more to it than that. While minced meat and egg white provided protection, boiled rice did not. The common factor in meat and egg white is protein, as egg white is low in fat, while boiled rice is mainly carbohydrate. Naturally acid-resistant enteric bacteria such as Shigella and Escherichia coli strain O157 seem to require small quantities of extra amino acids to maintain their acid-resistance and the food protein might also be acting as a similar source of amino acids for Salmonella.