minute single-celled organisms which are present more or less everywhere. They resemble plants more than animals, but are usually considered as belonging to a third kingdom, Protista, in which they constitute the group Schizomycophyta. With a few exceptions, they do not feed by the ordinary plant process of photosynthesis but live, according to type, on an enormous range of substances including practically everything found in food, as well as in live animals or plants. They reproduce by splitting in half and some, in ideal conditions, can do this every twenty minutes, so they can spread very fast.
The effect of bacteria in foods ranges from highly desirable to harmful. The great majority are neither, though if they are allowed to grow unchecked in food they may spoil it with off flavours from their waste products, or by producing enzymes which cause softening or gases which cause bloating.
Of the really useful species the most familiar are the lactic acid-producing bacteria which cause fermentation in milk products of all kinds, in pickles and sauerkraut, in salami (see sausages of Italy), and in many of the soya bean products of the Orient. There are many varieties, each producing characteristic flavours and other effects. It is quite usual for types to work in sequence, one kind replacing another as the acidity increases and often living off the waste products of its predecessor. Typically, Leuconostoc spp in plants and Streptococcus spp in milk start a fermentation and are succeeded by Lactobacillus spp which produce and can tolerate high levels of lactic acid.
Other useful bacteria include acetic acid-producing types such as Acetobacter aceti, which turns alcoholic liquids to vinegar; propionic acid producers such as Propionibacterium shermanii, which creates the special flavour of Swiss cheese and also forms its ‘eyes’ by giving off carbon dioxide; Bacillus subtilis, which ferments certain vegetable products such as natto and arroz fermentado; and a motley crew of bacteria which co-operate with moulds and yeasts to develop the flavour of surface-ripened cheese: the most assertive of these is Bacterium linens, which gives Limburger cheese its pungent aroma.
It seems safe to assume that all useful bacterial fermentations were discovered by accident, the species involved being endemic in the relevant food or in the environment. It is, however, impossible to be sure that the right organisms are naturally present. Conditions can be adjusted in their favour, as when, in making yoghurt, the milk is kept above 32 °C (90 °F) so that Streptoccocus thermophilus and Lactobacillus bulgaricus, which thrive in such warmth, can outgrow and dominate rivals. In commercial food production it is usual to ensure the growth of the right bacteria by first killing off all those present, often by pasteurization, and then adding a pure culture of the desired type. After fermentation the food may be pasteurized again to stop further bacterial action.
Bacteria also have useful effects in living creatures. Nitrogen-fixing bacteria in the root nodules of leguminous plants (such as peas and clover) take in nitrogen direct from the air and pass it on to the plant, both feeding the plant and enriching the soil. Herbivorous animals could not, unaided, digest the cellulose in the plants which they eat. It is bacteria in their digestive tracts which break down the cellulose and change it to digestible sugars. Humans have gut bacteria as well, but these do not supply nourishment from cellulose. They do, however, break down certain plant substances: scientific opinion is still divided on how far, if at all, the process aids the nutrition of the host. One unwelcome by-product is wind.
Some cause disease. For example bacteria in unpasteurized milk have been known to infect those who drink it with polio, tuberculosis, typhoid, diphtheria, undulant fever, and foot and mouth disease. They also cause food poisoning. Fatal botulism is due to toxins created by Clostridium botulinum. Staphylococcus aureus, a species common on human skin, produces toxins which cause what used to be called ‘ptomaine’ poisoning; and Salmonella spp, abundant in many kinds of raw meat, especially chicken, are a common cause of less serious poisoning (see salmonella). Campylobacter jejuni, widespread in animals, causes diarrhoea and a typhoid-like illness. Other bacteria that have been causing concern since the late 20th century are Escherichia coli (E. coli), especially the strain O157:H7 which has been implicated in many outbreaks of food poisoning arising from eating pre-cooked meats and minced meats, for instance in hamburgers. The bacteria inhabit the intestines of many animals as a matter of course (they can be picked up from handling live farm animals, for instance during farm visits) and need exclusion from the food chain by sound slaughter practice and careful kitchen hygiene.
Harmful bacteria in food can be difficult to suppress. Many species can grow in a wide range of conditions and can survive, to resume growth when they have a chance, in a much wider one.
Aerobic bacteria are those which breathe air and stop breeding (but do not die) when air is excluded. Other, anaerobic, types breed only in airless conditions and stop (but again survive) when air is admitted. Certain types prefer particular temperatures. Thermophilic bacteria breed fastest between 42 °C (104 °F) and 75 °C (160 °F); mesophilic bacteria between 10 °C (50 °F) and 40 °C (104 °F); while psychrophilic bacteria, although they breed fastest between 150 °C (590 °F) and 20 °C (68 °F), can continue to breed right down to −5 °C (23 °F). One example of the way temperature favours different types of bacteria is the spoilage of unpasteurized milk. At room temperature, mesophilic bacteria turn milk sugar to lactic acid so that the milk goes sour. In a refrigerator these organisms are repressed; instead, psychrophilic bacteria attack the milk protein and turn the milk alkaline and smelly.
Only heating well above their preferred temperature, or antiseptic chemicals, can kill bacteria. Chilling, even freezing, or drying causes them to stop growing; but when warmth or moisture return they at once start again. Some bacteria which themselves are killed by heating form spores from which fresh bacteria can later develop; and these spores can survive very high temperatures. The most dangerous of these is Clostridium botulinum.
Another problem is that of bacterial toxins. Both C. botulinum and harmful Staphyloccocus spp poison not by their presence but by the toxins they produce. If they have been allowed to grow for long enough to produce an appreciable amount of toxin, the food is poisonous and remains so no matter how much it is heated.
All these harmful bacteria are common and we are constantly exposed to them; yet they seldom cause illness except in people with weak immune systems—the very young, the very old, and those who are already ill. A healthy person's immune system can kill any bacteria as long as these do not arrive in overwhelming numbers.
Alan Davidson was a distinguished author and publisher, and one of the world's best-known writers on fish and fish cookery. In 1975 he retired early from the diplomatic service—after serving in, among other places, Washington, Egypt, Tunisia, and Laos, where he was British Ambassador—to pursue a fruitful second career as a food historian and food writer extraordinaire. Among his popular books are Seafood of South-East Asia, North Atlantic Seafood, and Mediterranean Seafood. In 2003, shortly before his death, he was awarded the Erasmus Prize for his contribution to European culture.
