Food Safety Focus (3rd Issue, October 2006) – Food Safety Platform

Overview of Biological Hazards

Reported by Mr. Johnny CHU, Scientific Officer,
Risk Assessment Section, Centre for Food Safety


In the first issue of the newsletter, we have mentioned that a "hazard" can be classified as a substance or agent present in food with the potential to cause an adverse health effect to the consumer. Food hazards can be divided into three main categories: biological, chemical and physical. In this issue, we are going to give you some more information on biological hazards.

Biological Hazards

Biological hazards are biological agents that have the capacity to cause harmful effects in humans. Common biological hazards include bacteria, viruses and parasites.

Pathogenic Bacteria

Bacteria are living single-celled organisms and are generally considered to be the most important causative agents of foodborne illnesses. Bacteria grow fast in foods that are warm, moist, protein-rich and low in acid. Milk, shell eggs, poultry, fish, meat and shellfish are common food items that support the growth of bacteria. Most bacteria are not harmful to us while some can make people ill by living and multiplying inside human bodies (e.g. Salmonella, Listeria monocytogenes). Others (e.g. Staphylococcus aureus, Bacillus cereus) produce toxins in foods and people fall ill because of the toxins when they eat the foods. However, the mere presence of the organism in food may not cause the disease. The amount of organism present is important. The table below shows the top 3 pathogenic bacteria causing food poisoning in 2005 in Hong Kong:

Pathogenic bacteria Foods involved Number of bacteria required to cause disease in healthy adults*
Vibrio parahaemolyticus
  • Raw or under-cooked seafood
  • Ready-to-eat foods contaminated by raw seafood
Greater than 106 organisms per gram of food
  • Raw or undercooked egg and egg products
  • Undercooked poultry
Usually 102 to 103 organisms but sometimes as few as 15 to 20 organisms
Staphylococcus aureus
  • Ready-to-eat foods that have been contaminated and then kept at ambient temperature for a prolonged period of time
Greater than 105 organisms per gram of food are required to produce enough toxin

* Source: Bad Bug Book from the US FDA and Microbial Pathogen Data Sheets from the New Zealand Food Safety Authority

The total number of bacteria in a food is usually employed to indicate the sanitary quality of foods and itself is not considered as a food hazard. Likewise, occurrence of E. coli in food indicates direct or indirect faecal contamination of food and its mere presence may not mean it will cause harmful effects.
A particular type of E. coli, E. coli O157:H7 recently caused an outbreak of food poisoning involving consumption of raw spinach in the US . The infectious dose of the pathogen is low, probably as low as 10 organisms. In Hong Kong , foodborne disease involving E. coli O157:H7 is uncommon. From 1998 to September 2006, a total of six cases were reported.


Viruses are very simple and small organisms that cannot reproduce outside a living cell. Therefore, they do not multiply in or on foods. Viruses can contaminate food through foodservice workers' poor hygiene, and can be present in contaminated food and water supplies, or shellfish harvested from sewage-contaminated waters. The infective dose of most viruses is extremely small, sometimes as few as 10 virus particles. Two of the commonest foodborne viruses in Hong Kong are:


Parasites are organisms that live in or on another living organism, which is called the host. Parasites include single-celled organisms and worms. Human beings may be infected with single-celled parasites (e.g. Giardia lamblia) through consumption of contaminated water and food such as raw vegetables. Parasitic worms have more complex life cycles. Immature worms need to pass through an animal host (e.g. freshwater fish and snails) before it can infect the final host (e.g. human beings, dogs and cats). Human beings may get infected with parasitic worms through consumption of undercooked meat, freshwater fish and freshwater snails. Examples are tapeworms, Clonorchis sinesis and Angiostrongylus cantonensis respectively.

Simple Rules to Manage the Risk

Prevention of contamination is the most important control factor to enhance food safety. It is also important to keep foods at refrigerated temperature to prevent bacteria from growing to hazardous levels. Finally, foods should be cooked thoroughly before consumption in order to destroy pathogens that may be present.