FOOD MICROBIOLOGY

A variety of intrinsic and extrinsic factors will influence what microbes will do to food.

Intrinsic factors - Factors inherent to the food

pH - Low pH will select for fungi; neutral pH will promote bacterial growth. Low pH is sometime used to preserve foods from bacterial attack.

moisture content - drying is used to preserve foods. Salt and sugar have the same effect.

oxidation-reduction potential - after cooking foods are more anaerobic. Sausages and canned foods are anaerobic also.

physical structure of the food - grinding increases the surface area and also increases the amount of available oxygen. It also releases nutrient rich juices and disperses microbes throughout the food. Freeze-thaw treatment also liberates juices, destroying the integrity of the food and increasing the available nutrients.

Extrinsic Factors - Environmental factors

Temperature Humidity

O2

CO2

Food Preservation

1. Filter to physically remove microbes

2. Low temperature, below 5^o C but above freezing (refrigerator temp), slows down but doesn't stop microbe growth. Psychrophiles and psychrotrophs will grow. Freezing is a good way to preserve but freeze-thaw has its dangers (see above).

3. High temperature:

a.) Pasteurization - high temperature treatment to REDUCE microbial population and ELIMINATE PATHOGENS. For milk, the elimination of Mycobacterium tuberculosis has historically been the main concern. There are three common pasteurization procedures:

Low Temperature Holding (62.8^oC for 30 minutes)

High Temperature Short Time (71^oC for 15 seconds)

Ultra High Temperature (141^oC for 2 seconds).

The main reason for pasteurizing milk today is to prolong shelf life.

b.) Canning - This is really a form of sterilization, the treatment is very similar to autoclaving: steam under pressure.

4. Dehydration:

Drying of fruits, vegetables and meats is one of the oldest food preservation techniques.

The use of high concentrations of salt and sugar works by the same principle, reducing the free water concentration.

Freeze drying or LYOPHILIZATION is the newest technique in this category.

5. Chemicals and Radiation:

Organic acids such as PROPRIONIC, SORBIC and BENZOIC ACIDS are frequently used to inhibit the growth yeasts and molds in a variety of foods.

Sulfites are used on fruits and vegetables to inhibit bacteria and fungi.

Sodium nitrite is used on meats to inhibit the growth of Clostridium botulinum.

Ultraviolet radiation has been used to sterilize the surfaces of food handling equipment.

Gamma radiation is increasingly being used to preserve a variety of foods including fresh fruits like strawberries.

Food-borne Diseases

These diseases can be divided into two categories. The FOOD-BORNE INFECTIONS and the FOOD-BORNE INTOXICATIONS.

FOOD-BORNE INFECTIONS:

In the food-borne infections, LIVE ORGANISMS are ingested and these infect the gastrointestional tract releasing toxins, damaging the intestinal epithelium and causing gastroenteritis (anorexia, nausea, vomiting, diarrhea and abdominal pain).

Salmonella sp.

The Salmonellae are commonly found on poultry and eggs and are responsible for at least 50,000 cases of gastroenteritis a year in the US.

Symptoms usually start 24-48 hours after eating tainted food with nausea, vomiting, cramps and diarrhea. Fever is present about half the time. The diarrhea usually gets better after 3-4 days and the disease resolves itself after a week.

Salmonella typhi causes TYPHOID FEVER a particularly severe disease because this organism invades the circulatory system of the victim.

Campylobacter jejuni

This organism is a common intestinal organism of many animals. It causes between 4 - 30% of infectious human gastroenteritis in the US.

Most people become infected by eating partially cooked poultry and sometimes by carrier pets. Symptoms start 1 to 7 days after ingesting less than 500 bacterial cells. Severe abdominal pain is common followed by diarrhea often containing blood and pus. Fever is commonly present and symptoms may last up to two weeks.

A rare complication of Campylobacteriosis is an ascending paralytic disorder called Guillaine-Barre syndrome.

Escherichia coli

A number of strains are capable of causing a variety diarrheal diseases.

Enterotoxigenic E. coli (ETEC) is a main cause of Traveler's Diarrhea and infantile diarrhea in developing countries.

Enteropathogenic E. coli (EPEC) is a killer of bottle fed infants in developing countries.

Enterohemorrhagic E. coli (EHEC) is commonly carried by cattle and spread to humans through milk and undercooked hamburger. These strains which include O157:H7 can cause severe diarrhea and intense cramps 4 days after ingestion. The diarrhea may become bloody. In children under 5 the disease may progress to HEMOLYTIC UREMIC SYNDROME, a severe complication which may require dialysis and can lead to death. This is the organism that caused the "Jack in the Box" epidemic in the Pacific Northwest USA several years ago and the extensive epidemic in Japan during the summer of 1996. It is interesting to note that several outbreaks have been associated with vegetables. Lettuce was implicated in a recent outbreak in the mid-west US and daikon (radish) sprouts are suspected in the Japan epidemic. Can you think of any scenarios to explain how vegetables could become contaminated with this intestinal organism?

Listeria monocytogenes

This organism is commonly passed to humans via unpasteurized milk or milk products such as cheese.

In 1985 there was an outbreak in California involving 86 cases and 29 deaths caused by Mexican-style soft cheese.

The organism comes in through the gastrointestinal tract but quickly invades the bloodstream and may cause meningitis.

FOOD-BORNE INTOXICATIONS

In the food-borne intoxications the organisms grow up in the food and produce powerful TOXINS which are then ingested and cause illness

Staphylococcal food poisoning

Staphylococcus aureus can produce a heat resistant enterotoxin which causes acute diarrhea and powerful vomiting (projectile vomiting) 1 to 5 hours after ingestion. The toxin is known to be a SUPERANTIGEN. These substances cause T-cells to over-produce CYTOKINES or LYMPHOKINES.

Clostridium botulinum

This organism, a strict anaerobe, can produce a very powerful neurotoxin which blocks the synapses of the motor neurons. Botulism thus results in FLACCID PARALYSIS and high mortality due to respiratory and cardiac failure.

The most common vehicle is improperly canned foods - since this organism is a endospore former it may survive if canning temperatures are inadequate.

INFANTILE BOTULISM is a food infection which can be caused by feeding infants honey. There are approximately 100 cases of this per year in the US. Infants present with constipation and they are listless and weak.

Viral Infections

A number of viruses can cause gastroenteritis and can be spread through contaminated food and water. Noroviruses have a very low infectious dose (maybe as few as 10 virus particles) and these viruses have been the culprits behind behind the massive diarrhea and vomiting outbreaks on cruise ships.