Food and Nutrition

Microbiological hazards in food

Major microbiological hazards in food includes:

  • Bacteria
  • Virus
  • Parasite


Bacterial hazards are defined as those bacteria that, if they occur in food, may cause illness in humans, either by infection or intoxication. They can  be grouped into spore formers and non-spore formers.

Food-borne infections are caused by swallowing live pathogens that grow within the body, usually in the intestinal tract. They differ from food-borne intoxication, which is a condition caused by swallowing preformed toxins (i.e., toxins produced by microorganisms in the food before it is eaten).

Certain bacteria have dormant stage in their life cycle i.e. spore (e.g. Clostridium and Bacillus spp.) & it is very resistant to chemicals, heat and other treatments, normally lethal to non-spore forming ones. As they are dormant, spores are not hazardous as long as they stay spores.

However, if they survive a processing step designed to kill non-spore forming bacteria, they may become a hazardous in the food if they are allowed to grow.  When spore formers are a concern, processing steps used to control them are often much more severe than when only non-spore formers need to be controlled.


Bacteria in food


Spore formers

  • Clostridium botulinum
  • Clostridium perfringens
  • Bacillus cereus

Non spore-formers

  • Campylobacter
  • Listeria monocytogenes
  • Shigella (S. dysenteriae)
  • Streptococcus pyogenes
  • Salmonella (S. typhimurium, S. enteriditis)
  • Pathogenic Staphylococcus aureus
  • Pathogenic Escherichia coli ( coli 0157:H7)
  • Vibrio ( V. cholerae, V. parahaemolyticus, V. vulnificus)
  • Yersinia enterocolitica

Following are some bacterial hazard in food:

  • Clostridium botulinum: Causes an intoxication that affects the central nervous system and causes shortness of breath, blurred vision, loss of motor capabilities and death.


  • Listeria monocytogenes: Causes an infection with mild flulike symptoms. Severe forms of listeriosis are possible in people with weakened immune systems, causing septicemia, meningitis, encephalitis and stillbirths.


  • Salmonella spp: Causes an infection with symptoms such as nausea, vomiting, abdominal cramps, diarrhea, fever and headache. Death is possible in people with weakened immune systems.



They are very small particles that cannot be seen with a light microscope and cannot reproduce by themselves. Although they are alive, viruses differ from other microorganisms in what they need to live and how they multiply.

Viruses exist in foods without growing, so they need no food, water or air to survive. They do not cause spoilage. Viruses cause illness by infection. They can infect living cells and reproduce inside the host cell using material from it and only grow once they enter a suitable host.

Only some virus consider humans a suitable host. They can survive in human intestines, frozen foods and contaminated water for months. They can be found in people who were (previously infected but are no longer ill and also in people who show no outward signs of illness (carriers).

Transmission of viruses to foods is usually related to poor hygienic practices.  People who have viruses shed them when they defecate.  Food handlers with viruses can transmit them to food under unhygienic conditions. This route can also result in contamination of food with bacteria.

Following are viral hazards found in food:

  • Hepatitis A virus: Causes fever and abdominal discomfort, followed by jaundice.


  • Norwalk virus: Causes nausea, vomiting, diarrhea and abdominal pain (gastroenteritis). Headache and low-grade fever may also occur.


Parasites (Worms and Protozoa)

Parasites are organisms that need a host to survive, living on or within it. Although thousands of parasites exist worldwide, only about 20 percent can be found in food or water, and less than 100 are known to infect people through consumption.

There are two types of parasites that can infect people through food or water: parasitic worms and protozoa.

  • Parasitic worms include roundworms (nematodes), tapeworms (cestodes) and flukes (trematodes). These worms vary in size from barely visible to several feet in length.
  • Protozoa are single-cell animals, and most cannot be seen without a microscope.


They have the opportunity to infect humans when people eat them along with the food. Some parasites may be transmitted through food or water that is contaminated by fecal material shed by infected hosts.


Fish spoiled by parasite


Methods of preventing transmission of parasites to foods by fecal contamination include:

  • Proper disposal of human feces.
  • Good personal hygiene practices by food handlers.
  • Elimination of insufficiently treated sewage to fertilize crops.

Parasitic infections are normally associated with raw or undercooked foods because cooking of foods eliminates almost all foodborne parasites.  In some cases, freezing can be used to destroy parasites in food.

Proper host (organisms that can be infected by parasites) and suitable environment (water, temperature, salinity, etc.) are two factors most important to parasitic survival.


Some examples of parasitic hazards  in food:

  • Giardia lamblia: Protozoan parasite that causes diarrhea, abdominal cramps, flatulence (intestinal gas) fatigue, nausea and weight loss. Illness may last for one to two weeks, but chronic infections can last months to years.


  • Entamoeba histolytica: Protozoan, cause dysentery (severe, bloody diarrhea).


  • Ascaris lumbricoides: Roundworm, causes intestinal and lung infection.


  • Diphyllobothrium latum: Tapeworm, attaches itself to the intestinal wall and can grow to 3 to 7 feet. Symptoms include abdominal pain, diarrhea, flatulence and cramping.