16.4: Microbial Hazards

Millions of people in the U.S. who get sick each year from microbial contamination of food think they have indigestion, “a bug,” or “stomach flu.” For most people, it’s “only” a short bout of misery, but it can be quite serious for the very young, the very old, and those with impaired immune systems (e.g., those taking immunosuppressant drugs because of an organ transplant).

Many kinds of microbes can contaminate food and make us sick (a few are listed in Table 16-2). Again, dose matters. The dose that can cause illness varies according to the microbe and a person’s vulnerability.

We can’t keep all microbes away. They are everywhere, in soil, air, and water, from the edge of space to the bottom of the sea. In fact, we ingest all kinds of microbes—friend and foe—all the time.* We want to avoid getting too big a dose of the disease-causing ones: Keep them out. Keep them from growing. Kill them. This can be hard to do, because a lot of the food we eat is made by someone else.

We can’t see, smell, or taste most of the microbes or the microbial toxins that make us sick, so it helps to know where these microbes lurk, how they get in food, what conditions favor their growth, and how they and their toxins can be destroyed.

Keep Them Out

Knowing how they get in food is key to keeping them out. Norovirus is the leading cause of disease outbreaks from contaminated food in the U.S. Most outbreaks occur in central food-service settings where food is prepared and served to large groups, e.g., nursing homes, cruise ships. Norovirus is extremely infectious—it only takes a small amount to make someone 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.

Staphylococcus aureus (“Staph”) causes millions of cases of food borne illnesses in the U.S. each year. Staph itself doesn’t makes us sick, but its toxin does. Many people carry staph—on skin, in nasal passages, in wounds and skin eruptions. They transmit it to food by direct handling (e.g., via an uncovered cut on the hand), or through the air by a sneeze or cough (a reason for plastic shields—“sneeze guards”—placed over salad bars).*

Hazardous microbes also get in food via fecal contamination. Worldwide, the most common route is sewage-contaminated food or drinking water. We saw this with dramatic effect among the tens of thousands of Rwandans who died of cholera in 1994. In the U.S., a source is food-preparers not washing their hands after using the toilet, as required by law. Another source is animal feces; carcasses can be contaminated in slaughterhouses.

Salmonella is another microbe that can infect animals and taint food by fecal contamination; when laying hens are infected, their eggs can contain salmonella.

In some cases, the animals themselves are infected. Pigs are sometimes infected with the parasite Trichinella spiralis. Eating inadequately cooked pork containing larvae-filled cysts can infect us (thorough cooking prevents this).

^{*In December, 2010, pastries from a gourmet bakery in Illinois were recalled when they were linked to 100 cases of Staph food poisoning from four separate events—three in Illinois (30 cases) and one in Wisconsin (70 cases). The pastries were sold retail, wholesale (to grocery stores, etc.), and online, so the recall was publicized nationwide.}

Keep Them From Growing

Even when disease-causing microbes get in food, the aim is to keep them from multiplying to amounts that make us sick. Microbes grow best when they have plenty of nutrients under moderate conditions—not too hot or cold (Figure 16.1), not too sugary, salty, acid, or dry. We don’t get microbial illness from candy (too sugary), pickles (too salty, too acid), or crackers (too dry). We limit microbial growth by drying apricots, sugaring strawberries to make jam, and heavily salting meat (whole grains of salt were known as corn in England; hence, corned beef).*

Food mixtures that contain mayonnaise, eggs, or milk products, and aren’t cooked or reheated after being made (e.g., potato salad, pastries filled with cream or custard). are more of a concern. Unless one takes precautions, microbes can get mixed in under conditions that favor microbial growth.

In a classic scenario, you invite friends to a barbecue. You cut the raw chicken while you boil potatoes. Using the same cutting board and knife, you cut up the warm potatoes and mix them with mayonnaise to make a bowl of potato salad. Even if you put it right in the refrigerator, it takes time for the salad to get cold, especially in the middle of the bowl.

You have an ideal growth condition for any microbes that you may have mixed in, whether from raw chicken, a cut on your finger (a place where microbes like to grow), or unwashed hands that just changed a baby’s diaper. Potato salad is moist, neither too acid nor too salty; microbes are distributed throughout the salad, surrounded by nutrients to feast on; and the temperature stays cozy long enough for the microbes to grow to a number that can make you and your guests sick.

Some precautions you can take:

• Assume raw meat (including seafood) is contaminated, since you don’t know. After cutting it, don’t use that cutting board or knife for other foods that won’t be well-cooked, unless you thoroughly wash them (and your hands) with soap and hot water. A good habit is to make raw meat the last thing you cut, or have a cutting board used only for raw meat.
• When defrosting, storing, or marinating raw meat in the refrigerator, put it on a bottom shelf (or in a pan or dish) so it can’t drip on other food.
• Cool food like potato salad fast. Refrigeration cools from the outside in, so potato salad cools faster in a flat pan. You can transfer the potato salad to a bowl after it’s cooled. When you then put the bowl on the table, it will stay cold longer than in a flat pan. Don’t leave potato salad out of the refrigerator or picnic cooler for long. People can get sick from contaminated potato or macaroni salad that’s left out during a leisurely picnic.

^{*These procedures lessen risk, but don’t guarantee safety. Soon after St. Patrick’s Day 1993 in Cleveland, 15 people called the local health department about their illness, which was traced to microbial contamination of corned beef from a local deli. When the outbreak was reported in the local paper, 156 people called to report diarrhea and stomach cramps that occurred within 48 hours of eating food from the same deli.}

Table 16-2: Some Microbes That Can Contaminate Food or Drink

Kill Them

There are many chances to kill germs— disinfectants on food-processing equipment, chlorinated drinking water, pasteurized milk, cooked food. Heat doesn’t always have to be so high that it kills all the microbes. Heat used to pasteurize milk kills disease-causing ones, but not the ones that cause spoilage. Ultrahigh heat can sterilize milk (kill all microbes); this “fresh” milk can then be stored for months at room temperature.

Irradiation of food can be used for several purposes, e.g, to kill pests in spices, tea, and other crops (to replace post-harvest use of pesticides); to kill hazardous microbes in meat; to inhibit post-harvest sprouting of potatoes and onions; to delay ripening or spoiling of fruit. Irradiating a food doesn’t make it radioactive, as some consumers think.

We eliminate many microbial hazards by cooking, thus killing the microbes or destroying their toxins. People rarely get sick from food that is well-cooked and eaten soon after cooking, e.g., a bubbling hot casserole going from oven to table.

Often the problem is “simply” inadequate cooking of contaminated food, as is often the case when people get sick from turkey stuffing. Making stuffing is like making potato salad—meat is often prepared at the same time, and it’s easy to cross-contaminate. Furthermore, the cook may stuff the turkey the night before, since it’s often a part of a Thanksgiving feast, and there’s a lot to be done.

In the refrigerator, the center of the stuffing in a turkey stays for some time at a cozy temperature for microbial growth. In roasting, the turkey cooks from the outside in, meaning that the center of the stuffing may not get cooked enough.

If microbes have flourished there, they won’t be killed as they should. If you then leave the stuffing on the table for a long time, the microbes get another good chance to grow. Be cautious—stuff the turkey just before roasting or, better yet, bake the stuffing separately.

Botulism

Most microbial hazards in food are avoided by thorough cooking, but there are exceptions. The microbe that causes botulism isn’t destroyed by typical cooking times and temperatures, but its toxin is (it’s a protein and is denatured by cooking; Chap. 5). This is why very high heat is used in canning food.

Botulism is caused by a toxin made by Clostridium botulinum, a microbe common in soil. The microbe itself isn’t toxic. Botulism is uncommon in this country, but is worth discussing because of 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 as small as a grain of salt is enough to kill several people within an hour.

Symptoms usually appear 12 to 36 hours after ingestion, but 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 problems.† The toxin can impair breathing and cause death by suffocation.

Early diagnosis is important. Botulism is a medical emergency. Antitoxin is available, and mechanical ventilation can be used when breathing is impaired.

Most cases of botulism are from eating home-canned, low-acid food (e.g., green beans, corn, spinach) where the canning temperature was too low and/or the heating time too short. The microbe is left alive in ideal conditions: low in acid, salt, and sugar, and stored at room temperature with oxygen kept out (botulinum is anaerobic—it grows in the absence of oxygen). Because canned food is already cooked, it’s often eaten cold (e.g., canned green beans added to a salad) or simply warmed (temperature too low to destroy the toxin).

The microbe or its toxin doesn’t necessarily make the food look, smell, or taste unusual. So if contamination is even suspected (haphazard processing, bulging of the canning lid, etc.), discard the food without tasting. “If in doubt, throw it out”—it isn’t worth the risk.

The toxin (unlike some other toxins and the microbe itself) can be destroyed by thorough cooking, but this isn’t advised as a way of making it safe to eat.

Commercially canned foods rarely cause botulism—only four deaths have been reported in the U.S. since 1926. The latest was in 1971 from canned vichyssoise made by a small company. Vichyssoise is a creamy potato soup (non-acidic), usually served cold. Had it been adequately heated, the toxin would have been destroyed.

Concern with botulism in commercially processed food is pretty much confined to processed meats (e.g., frankfurters, sausages, ham), and nitrite is added to hamper the production of the toxin (see nitrosamine section in Chap. 12). The name botulism comes from the Latin word for sausage (botulus).

Botulism from sausages used to be quite common. When animals were slaughtered, sausage was a way to use scraps of meat. It wasn’t a tidy job; the microbe could easily get mixed in with the ground meat and seasonings stuffed into an animal’s cleaned-out intestine (you’ll be happy to know that artificial casings are now used instead).

The casing keeps oxygen low (the growth con­­dition for botulinum), and the sausage was then heated enough to cook the meat but not enough to kill the microbe, which could then grow and make toxin during storage. Sausage was often only warmed before eating, or eaten cold.

^{*Botulinum toxin can be used in biological warfare by sabotaging food supplies or by an aerosol attack. Iraq filled and deployed more than 100 munitions with the toxin before the Gulf War.
†The toxin blocks muscle contraction by interfering with the release of the neurotransmitter acetylcholine (Chap. 15). It is thus used to treat facial tics (injecting a bit of it into the muscle keeps it from twitching) and, cosmetically, to relax facial muscles that cause wrinkles.}

Infant Botulism

Botulism is usually caused by ingesting the toxin, but in infant botulism, the microbe itself is ingested and the toxin is made in the infant’s own digestive tract. It’s seen mostly in infants 2 to 3 months old. An infant’s stomach isn’t as acid as that of an adult or older child. This aids the growth of the microbe and its production of toxin. It doesn’t take much toxin to be a large dose for a small infant.

The only food directly implicated in infant botulism is honey (another source is soil that contains the microbe). It’s advised that infants less than a year old not be fed honey.

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.

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 9,000 people got sick and at least nine died. Most cases were traced to fresh radish sprouts served in school lunches. (Sprouts from contaminated seeds are a problem when eaten raw; seeds don’t sprout if heated to kill microbes. Raw alfalfa sprouts in salads and sandwiches are a source of outbreaks in the U.S.) 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, where it destroys platelets and red blood cells and causes kidney failure.

Antibiotics can do more harm than good— they typically work by destroying the bacterial cell wall, releasing the toxin. Vaccines and drugs are being developed for humans and livestock.

In meat, hamburger 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 a 1,000 pieces of beef is contaminated. If all 1,000 pieces are ground together, all patties made from that batch (and the grinding equipment) will be contaminated.* (If they aren’t combined, as with a steak or roast, the odds of getting the contaminated piece is 1 in 1000.)

The potential for widespread contamination is even worse when the day’s leftover hamburger is added to the next day’s batch, as was the case at Hudson Foods. In 1997, more than 16 people in Colorado got sick from tainted hamburger that was traced to the company. Federal inspectors found substandard meat-processing procedures, bacterial testing, and bookkeeping (to track the various lots of beef).

The company recalled a record 25 million pounds of hamburger, leaving 1 of every 4 Burger Kings in the U.S. without hamburger for more than a day. Hudson customers also included Safeway, Walmart, and Boston Chicken.

Hamburger chains place huge orders for ground beef (McDonald’s buys about a billion pounds a year). 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 160°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 160°F.) Consumers should look for a hamburger patty that isn’t pink/red inside.

Fast-food burgers are popular among young children, who are particularly vulnerable to the toxin. So it isn’t surprising that they were the ones who died in the 1993 outbreak. However, some of them had not eaten the hamburgers.

They probably were infected by person-to-person transmission via an infected child’s dirty diaper (also the presumed source of the 1998 outbreak involving 26 children playing in a pool at a water park in Georgia; a 2-year-old boy died). When people ingest the microbe, it’s passed in their stool. Person-to-person transmission is a common mode of infection.

Also keep in mind that cattle infected with E. coli O157:H7 often don’t get sick themselves. Cows wander—and drop their feces along the way. People have gotten sick from swimming in a lake where an infected cow had wandered to the lake’s edge.

Major outbreaks cause a media blitz, but small outbreaks occur regularly. One outbreak in 1994 was traced to raw hamburger from a local grocery in Ft. Bragg, California. Ten people got sick; an 85-year-old woman died. Because contaminated raw beef is a common cause of infection, experts recommend irradiating raw beef to kill the microbe.

There are several variants of E. coli that cause disease. A particularly virulant one (O104:H4), traced to fresh sprouts, caused an epidemic in Europe in May-June, 2011, with more than 3900 cases and at least 46 deaths. In July, 2011, the European Food Safety Authority reported that a single lot of fenugreek seeds from an exporter in Egypt was the most likely source of the contaminated sprouts.

^{*We saw this combined batch effect when hemophiliacs were infected with HIV via clotting factor made from blood pooled from thousands of donors (Chap. 7). Likewise, food made in big batches, as in restaurants and school cafeterias, can raise risk. If 1 egg in 10,000 is infected with salmonella, and you scramble 200 eggs together to make scrambled eggs, the risk of getting a contaminated serving is much higher than if you scramble yourself only one egg. But if the eggs are thoroughly cooked, even contaminated eggs aren’t a problem. Many restaurants and institutions now use only pasteurized eggs.}

Tracking Microbial Contamination

To be effective in preventing illness, epidemics must be stemmed quickly. But an epidemic is hard to avert; it isn’t generally recognized—or even considered to be one—until many people get sick. In epidemics of years past, patients were interviewed to look for what they ate in common in order to track the source of contamination. This required a lot of time, money, and footwork, and was basically detective work that took place well after the start of an epidemic.

If a patient in Ohio and another in Iowa get severe diarrhea, how can we tell if these are just isolated cases? One method stems from DNA technology. Bacteria mutate fast (Chap. 10). Their DNA sequences match if they came from the same source. So if you culture the stool from the Ohio patient, and the DNA of the culprit bacteria matches that from the Iowa patient’s stool, you know to act fast on what looks to be the start of a widespread epidemic. Many health departments now use PulseNet, a national molecular subtyping network for food-borne diseases.

Even if an epidemic can’t be halted, this technology can trace back the source. In the summer of 1998, restaurant-associated infections with Shigella sonnei occurred in Minnesota, California, Massachusetts, and Canada. DNA fingerprinting of the microbe isolated from stool samples implicated parsley from a farm in Mexico.

The municipal water used to chill the parsley and to make ice to pack the parsley for shipping wasn’t chlorinated and was vulnerable to contamination. The restaurants had used fresh parsley (sometimes chopped in large batches in the morning and left out all day) in pasta dishes, chicken sandwiches, coleslaw, etc.