|MadSci Network: Microbiology|
Let's begin by defining the question: "What kinds of bacteria grow on fruit?" "Bacteria" are one-celled life forms that do not have their genetic material in a separate nucleus ("prokaryotes"); the individual cells can be seen only with a microscope. "Grow," in this case, refers to multiplication: bacteria don't get large; rather, when one cell reaches a certain size, it divides into two, then the two become four, the four become eight, etc. ("multiplication"). "Fruit" is the part of a plant that produces seeds, although some varieties of fruits have been bred to be seedless; we eat many types of fruits, but there are many other types of fruits that are not eaten by people. "On" could mean just on the outside of the fruit, but I don't think that's what you have in mind.
Some fruits grow on trees, others on low-lying plants, and still others in contact with the ground. Tree fruits include the apple family, the orange family, the peach family, nuts, and others. Fruits on low-lying plants include grapes, tomatoes, and many more. Fruits that grow in contact with the ground include strawberries, melons, peanuts, and others. Air carries bacteria on dust particles and in rain, even onto fruits on trees. Irrigation adds bacteria; the kind of bacteria present depends on where the water came from. Fruits in contact with the ground get bacteria from the soil. Tree nuts contact the ground when they are shaken from the tree during harvesting. Birds, squirrels, insects, and other animals are natural sources of bacteria. Finally, when fruits are harvested, they may get bacteria from humans. Most bacteria are harmless, but some can make us sick.
Surfaces of many fruits are well designed to keep bacteria out. Melons usually have a tough rind, and citrus fruits have a thick, leathery skin. The skins of apples, peaches, and tomatoes are more delicate and bruise easily. The outsides of strawberries and grapes are very delicate, but even a nut shell is not completely bacteria-proof. Openings such as the stem scar of a tomato can let bacteria inside the fruit. So, we have two different situations -- bacteria on the outside of the fruit and those that get inside, where the environment is often juicy.
Bacteria need water to multiply, as well as certain nutrients and favorable conditions. Bacteria on the rind of a watermelon or the skin of an orange will not have the water and nutrients they need, so they can't grow but may "hang out" there for a long time. If the rind or skin gets broken, bacteria get inside the fruit, where there is plenty of water. Most of the nutrients (sugars, proteins, etc.) that bacteria need are likely to be present; however, the acidity inside an orange will prevent most kinds of bacteria from multiplying, whereas the inside of a melon is not so acid. Acidity can be measured with laboratory instruments, but most of us recognize acidity in foods by a sour taste; sourness may be masked by other flavors. Another important factor is temperature: once a melon is sliced, the slices need to be carefully refrigerated to prevent bacteria (especially disease bacteria) from growing. Outbreaks of human illness from melons that have been pre-sliced and not properly refrigerated are fairly common -- the bacteria involved are often Salmonella, Listeria, or E. coli O157:H7. These bacteria were on the outside of the melon and rode in on the slicing knife.
Acid fruits, such as oranges, lemons, and most types of tomatoes, prevent multiplication of most kinds of bacteria that may get in. However, acid fruits can still spoil because molds and yeasts are able to multiply in the presence of the acidity. In fact, molds use the fruit's acid as if it were sugar and gradually make the juice of the fruit less acid, which might even permit some bacteria to multiply. Growth of molds and yeasts may be limited by lack of oxygen; but they can grow in the refrigerator, although more slowly than at room temperature. This is why fruits get moldy in the refrigerator, if they stay there too long, especially if they have been bruised or peeled.
The obvious way to limit all this is to keep bacteria, molds, and yeasts off the surfaces of fruits. This is not easy to do, because of all of the sources of contamination that I mentioned earlier. Alternately, fruit surfaces can be washed, and perhaps disinfected, before they are peeled, cut, and served. Washing and disinfection may remove or kill some of the surface bacteria, etc. However, fruits with rough surfaces, such as cantaloupe, are very difficult to wash effectively; and disinfection is usually effective only after efficient washing. Some other fruits, such as strawberries and raspberries, should only be washed just before they are served, because washing breaks down any barrier that they had to keep bacteria out, and they cannot be effectively disinfected. Once these kinds of berries are washed, they may spoil within a day.
I have given you only a very broad view of the situation with fruits that are common in the United States. Warmer parts of the world produce many kinds of fruit that are seldom seen here, but the general principles are the same. When bacteria are present only on the outside of a fruit's skin, they are unlikely to grow but may survive for a long time. Bacteria that get into the juicy part of a fruit will have all the water and nutrients they need for multiplication, but may still be limited by acidity or low temperature. Keeping bacteria off the surfaces of fruits, or killing the bacteria by disinfection, are good ideas, but not always possible. Most of us like to eat many kinds of fruit, but we need to treat them with respect.
Dean O. Cliver
Possible further reading:
M. P. Doyle, L. R. Beuchat, and T. J. Montville (eds.), 1997. Food Microbiology: Fundamentals and Frontiers. American Society for Microbiology, Washington, D.C. [There is now a third edition of this, but I donít have the bibliographic details.]
Try the links in the MadSci Library for more information on Microbiology.