21.6: Microbial Hazards in Foods

Each year, millions of Americans are afflicted with food-borne illnesses, most of which are caused by microbes (e.g., bacteria, viruses). For some people, the result is a day or two of stomach cramps, diarrhea and other familiar intestinal symptoms. For others, particularly infants and older adults, the result can be deadly.

Most of these cases go unreported. Most people made ill by food think they have “a bug,” “a virus,” “intestinal flu,” or “indigestion.” Usually the discomfort goes away and the misery is forgotten; but the more seriously afflicted don’t forget so easily. The vast majority of such uncomfortable, wasted days are needless. So let’s see what we can do to protect ourselves.

We might begin with one simple but useful fact. By and large, those food-borne infections which can cause us trouble can’t be seen, smelled, or tasted in food.

Protecting ourselves requires more intellectual than sensory vigilance, because such infections are carried on foods that our senses tell us are perfectly good. While we can’t defend ourselves against all food-borne infections with personal care and hygiene, we can reduce our risk considerably with a little knowledge and effort.

How Food Becomes Infected

Most of us think of microbes in a negative way, and imagine we’re getting rid of them with soap and water and smelly solutions. But microbes are more plentiful and pervasive and tougher than most of us suspect. Some exist 18 miles above earth at the edge of space; others are found at the bottom of the sea, in bubbling hot springs, in polar ice or in powerful acids.

We’re covered with microbes, inside and out. All of life and food, all of air and soil are replete with them. Despite their tiny size, microbes as a group vastly outweigh man. One authority has calculated that the microbes of our planet outweigh all animal life (on land and sea) 25-fold.

So we can forget about trying to get rid of microbes. They are survival experts. Leave one bacterium alive and it becomes two, in a matter of minutes to a few hours at most. The consequence of this rapid doubling and redoubling is breathtaking. It’s been said that if one cell of E. coli had enough food, in three days it could yield a mass of bacteria greater than the earth. And in a matter of minutes, it would double again.

The key phrase here, of course, is “had enough food.” What serves as food for microbes is astonishingly varied. Most can live on our food; some do so by living in our digestive tract. And some of these give us back such things as vitamins, or join in performing essential functions for us. Microbes are an essential part of life processes on earth.

Our first effort is to keep our food from becoming a growth medium for harmful microbes, and to avoid eating large groups of them—or large amounts of the toxins some produce. The chief ways we can do this are by:

• Not inoculating our food with such microbes
• Rejecting food which has been handled in ways that encourage microbial growth
• Keeping our food in environments which are typically too hot or cold for microbial growth

Because microbes are everywhere, they enter food by endless routes. In a few cases we can see them (as in the case of molds), or smell the evidence (as in sour milk). So of course we shouldn’t place a new loaf of bread against a moldy one, or allow drops of soured milk to get into fresh milk. But in most cases, we can’t perceive microbes. We must understand where they are likely to be. That is why you shouldn’t mix a meat loaf with your bare hands when you have a cut finger—there may be staphylococci in the cut.

Some Kinds of Harmful Microbes

According to the Center of Disease Control and Prevention (cdc.gov), Norovirus is the leading cause of disease outbreaks from contaminated food in the United States. Most outbreaks occur in central food-service settings where food is prepared and served to large groups, e.g., nursing homes, cruise ships. Noravirus is extremely infectious—it only takes a small amount to make people sick. It’s transmitted mostly by contamination with norovirus-infected stool or vomit. Food can also be infected at its source, e.g., fruits and vegetables in the field. Outbreaks have also occurred from shellfish harvested from contaminated waters and eaten raw.

---

The term “microbes” generally includes organisms that can be seen with a microscope, so includes viruses. A virus, however, can only reproduce inside a cell, unlike a bacterium which is itself a single cell.

---

Staphylococci (“staph”) is a particularly interesting cause of food poisoning because many of us carry staph—on our skin, in nasal passages, and in wounds and skin eruptions. We can transmit the microbe to food by direct handling, or through the air by talking, sneezing, and coughing. It isn’t so much what the bacteria themselves will do to us. But as they eat and grow, they produce an intestinal toxin. Since there’s so much staph around, don’t leave prepared food at a temperature which isn’t hot or cold. Heat or cold slows microbial growth; moderate temperatures encourage it (see Fig. 21-2).

We don’t want staph in our food. They are easily killed by the heat of cooking, but the toxin isn’t alive in the first place and is highly heat-resistant. It remains intact and can make us sick.

Salmonella bacteria is another common cause of food-borne illness. Ingesting salmonella from egg or chicken can result in the digestive upheavals of salmonellosis. On the other hand, botulinum organisms aren’t the direct cause of the nerve-deadening effects of botulism; what poisons is the toxin they produce.

Again, these organisms are all around us. Bread molds are everywhere. Botulinum is common in soil. So when we undertake the preservation of our food, we must process so as to defend our food and ourselves against a vast microbial world. Some microbes that cause food-borne illnesses are listed in Table 21-2.

Keeping Microbes in Check

Whether or not disease-causing microbes in our food present a hazard is a matter of amount. It’s generally only when these microbes are allowed to flourish that there are enough of them or their toxins to make us sick. One exception is noravirus—just a few of them can make us sick.

Microbes can be killed outright by heat or irradiation, or they can be kept from flourishing by an inhospitable environment. Bread can be frozen, apricots dried, beef heavily salted, cucumbers pickled in vinegar, strawberries heavily sugared to make jam. Some methods for keeping microbes in check are:

• Handle foods properly, under sanitary conditions
• Cook foods thoroughly
• Refrigerate foods promptly
• Avoid unpasteurized milk and cheese
• Properly process home-canned foods

The heat doesn’t always have to be so high as to kill all the microbes. For example, the heat used to pasteurize milk kills the disease-causing ones, but isn’t high enough to kill those that cause spoilage. Much higher heat can be used to kill all the microbes (i.e., sterilize the milk). An “ultra-high” heat treatment is used for “fresh” milk that can be stored for months at room temperature.

---

A few seconds at very high heat sterilizes the milk without destroying much of the fresh flavor.

---

Irradiation of food can be used to kill microbes, and also for other purposes. The allowable uses for food irradiation in this country include killing pests in spices and tea, use on certain crops (to replace the post-harvest use of pesticides), killing Trichinella worms sometimes found in pork, inhibiting the post-harvest sprouting of potatoes and onions, and delaying ripening or spoiling of some fruits. Irradiating a food does not make it radioactive, as some consumers think.

E. coli O157:H7

E. coli (Escherichia coli) is a common microbe normally present in the intestine and feces of all vertebrates, including humans. Most strains are quite harmless; some are even helpful. E. coli strain O157:H7 is one of several unusual strains that make a potent toxin that causes serious illness, especially in young children and older adults.

The first recognized outbreak of E. coli O157:H7 was in 1982 when at least 47 people in Oregon and Michigan got sick after eating McDonald’s hamburgers. There have been many outbreaks since, including one involving three classes of kindergarten children who drank unpasteurized milk during a trip to a Canadian dairy farm, and another in 1993 traced to fresh apple cider—apples can fall on soil that has animal droppings (feces) or manure fertilizer. The most common route of infection is via fecal contamination, particularly from cows (E. coli O157:H7 doesn’t sicken cows).

A 1993 outbreak in Washington, Idaho, California, and Nevada killed four children and made hundreds of people ill; most cases were traced to Jack-in-the-Box hamburgers. In a 1996 outbreak in Japan, more than 9000 people got sick and at least nine died; most cases were traced to fresh radish sprouts served in school lunches. (When eaten raw, sprouts from contaminated seeds are a problem; seeds won’t sprout if heated to kill microbes.) Also in 1996, about 70 people got sick and a 1-year-old girl died from drinking Odwalla juice that contained unpasteurized, contaminated apple juice.

The toxin damages the lining of the colon and its blood vessels, causing severe cramps and bloody diarrhea. In about 5% of the cases, the toxin enters the bloodstream, destroys blood cells, and causes kidney failure.

Hamburger meat is more likely to be contaminated because it’s usually made by combining meat from many cattle and from many slaughterhouses to get a homogeneous product with a designated fat content. Suppose 1 of 1000 pieces of beef is contaminated. If all 1000 pieces are ground together, all patties made from that batch will be contaminated. If the pieces aren’t combined, as with a steak or roast, the odds of getting the contaminated piece is 1 in 1000.

Figure 21-2: Effects of temperature on bacteria that cause food-borne illnesses.

Hamburger chains place huge orders for ground beef. Beef from many sources is combined and the patties shipped to various franchises. If a batch is contaminated, a widespread outbreak can occur (the contaminated Jack-in-the-Box hamburger meat was traced to a single processor).

Contamination won’t cause disease if the patties are thoroughly cooked. All parts of the patty must be cooked to 155°F to kill the microbe; 140°F was the standard for Jack-in-the-Box hamburgers. (A contaminated steak would be contaminated only on its surface—only the surface has to be cooked to 155°F.) Consumers should look for hamburger patties that aren’t pink inside.

Botulism

Botulism is the food-borne illness caused by the toxin made by Clostridium botulinum. Botulism is unusual in this country, but is worth some discussion because it has some particularly interesting aspects. The toxin is a nerve poison that affects muscle action, and is one of the most potent poisons known. An amount weighing as little as a grain of salt is enough to kill several people within an hour. Botulism poisoning is a medical emergency.

The symptoms usually appear 12 to 36 hours after eating the contaminated food, but the time can vary from 4 hours to 8 days. Neurological symptoms usually start in the head area, and then move downward. Early symptoms include double vision, dry mouth, drooping eyelids, and speech and swallowing difficulties. The toxin can affect respiration, and cause death by suffocation. Early diagnosis is important. An antitoxin is available, and mechanical ventilation can be used when breathing becomes difficult.

---

Local injection of botulism toxin is used medically (to treat facial twitches and excessive underarm sweating) and cosmetically (to deaden facial muscles that cause wrinkles).

---

In the U.S., most cases of botulism occur from eating improperly home-canned foods of low acidity (e.g., green beans, corn, spinach). When defective canning methods have failed to kill the botulinum, the conditions are ideal for their growth. They thrive and produce toxin where there’s no oxygen (as in the canned food) and where acidity is low, and they grow best at room temperature (the storage temperature for canned goods). In 2014, two young women were sickened from a jar of home-canned botulinum-contaminated pesto sold at a farm stand in California.⁴

---

Home-canning is actually home-jarring; the foods are “canned” in jars.

---

The presence of the bacteria or the toxin doesn’t necessarily make the food look or smell unusual. So if there’s even a suspicion of contamination (e.g., haphazard processing, bulging of the canning lid), throw the food out—without tasting. Botulinum toxin (unlike some other toxins) can be destroyed by thorough cooking, but this isn’t recommended as a way of ridding the food of the toxin. Throw it out; it’s not worth the risk.

Botulism contamination rarely occurs with commercially-prepared foods. Since 1926, there have been only four deaths from commercially canned foods reported in this country. The latest death occurred in 1971 from canned vichysoisse produced by a small company. Vichysoisse is a creamy potato soup (non-acidic), usually served cold. If it had been heated, at least some of the toxin might have been destroyed, and the death might have been avoided.

Nitrite as a Food Additive to Prevent Botulism

Microbe	Symptoms	Onset*	Common Source
Noravirus	Vomiting, diarrhea, stomach pain	12-48 hours	Infected vomit, stool
Staphylococcus aureus	Vomiting, diarrhea, similar to stomach flu	2-6 hours	Food handler’s skin cut or nasal fluid
Salmonella	Nausea, fever, vomiting, diarrhea	5 to 72 hours	Raw poultry and raw eggs
Shigella	Stomach pain/cramps, nausea, vomiting, diarrhea, fever, bloody stool	1 to 7 days	Infected food handler with poor hygiene (fecal contamination)
Clostridium botulinum	Trouble swallowing, double vision, progressing paralysis, medical emergency	12 to 36 hours	Improperly home-canned foods of low acidity
Listeria monocytogenes	Fever, vomiting, headache, can cause miscarriage	4 to 21 days	Soft cheeses, unpasteurized milk
Yersinia enterocolitica	Nausea, diarrhea, fever, flu-like	2 to 3 days	Unpasteurized milk, unchlorinated water
Trichinella spiralis	Fever, weakness, flu-like	Days to months	Inadequately cooked infected pork and wild game
Hepatitis A virus	Flu-like, jaundice, fatigue	2 to 7 weeks	Shellfish from contaminated waters, infected food handler

^{*Approximate time after eating that symptoms appear.}

Table 21-2: Some Microbes that Cause Food-Borne Illnesses

In commercially processed food, concern about botulism is mainly confined to frankfurters, sausages, bacon, ham, bologna, etc. Botulinum grows particularly well in processed meats. (The word botulism derives from botulus, meaning sausage in Latin.) Nitrite added to the processed meats inhibits the production of botulinum toxin, and also gives these meats their distinctive taste and pinkish color.

There’s been some concern with this use of nitrite as a food additive. Nitrite can combine with other substances called amines (amines are natural components of food) to form nitrosamines, which are very potent carcinogens for animals. Nitrosamines in food haven’t been proven to cause cancer in humans, but their effect on animals suggests that they might increase the human risk of cancer of the esophagus and stomach. Nitrosamine levels in nitrite-cured meats are low, but these meats are our main dietary source of nitrosamines.

---

Plant foods contain nitrates and nitrites—recall that these are plant nutrients taken from the soil.

---

Nitrite as a food additive presents an example of the complexity of risk assessment:

• Nitrite itself hasn’t been shown to cause cancer in any animal. (If it had, it would be prohibited as a food additive under the Delaney Clause.)
• The remote risk of cancer from any nitrosamines formed must be weighed against the immediate risk of botulism, particularly because processed meats often are eaten cold, or may not be thoroughly heated before eating.
• Nitrites and nitrates (which can be converted into nitrites) naturally occur in vegetables and saliva, in larger amounts (but in lower concentrations) than those in processed meat.
• Oscar Mayer sells wieners with “no added nitrates or nitrites...so you can enjoy the great taste and quality you expect, with no guilt.” Instead, celery juice is added; celery is exceptionally high in nitrites/nitrates.
• The nitrosamine content of processed meat can be increased by cooking at a high temperature (e.g., frying bacon).
• Nitrosamines can be formed in the stomach from nitrites and amines in food.

---

Nitrosamines are found in cigarette smoke. Tobacco is a plant (and thus contains nitrates/ nitrites). Burning the tobacco provides the high temperature that helps form nitrosamines.

---

The aim, then, is to minimize the formation of nitrosamines. Vitamin C (“ascorbate”) is thus added to nitrite-containing meats, since vitamin C interferes with the chemical reaction that forms nitrosamines. The hazard of nitrite/nitrosamines from processed meats is considered to be very small, but it’s prudent to limit the amount we eat —these meats are also high in fat and salt.

Infant Botulism

Infant botulism isn’t really considered to be a food-borne illness—it isn’t the toxin in food that causes the disease. Rather, the toxin is formed in the infant digestive tract from ingested botulinum. Ingesting the botulinum itself isn’t normally harmful, but the infant stomach isn’t as acid as the adult’s, aiding botulinum growth and toxin production. Also, because the infant is small, a small amount of toxin can be a large dose.

Most cases of infant botulism are in infants 2 to 3 months old. The only food directly implicated has been honey, and it’s advised that infants less than a year old not be fed honey.⁵