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Food Safety Focus (24th Issue, July 2008) – Food Safety Platform

Biological Hazards in Food – Pathogenic Bacteria (Part I)

Reported by Dr. Ken CHONG, Scientific Officer,
Risk Assessment Section, Centre for Food Safety

Previously in this column, we have introduced the topic of biological hazards in food which include bacteria, viruses and parasites. They are the biological agents that can cause harmful effects in humans. In this issue and the coming three issues, we are going to take a closer look at these agents.

Bacteria in Food

In summer, there are often more news reports on food poisoning cases caused by bacteria. Bacteria are locally the most common cause of food poisoning. However, not all the bacteria in food can make you sick, some are even added intentionally during the preparation of foods such as probiotics in yoghurt. Other than those desirable bacteria, there are also spoilage and pathogenic (being able to cause disease) bacteria in food.

Food Spoilage by Bacteria

Spoilage bacteria in food are able to grow in large number, decompose the food and cause taste/smell changes in food, which affect the quality of food. Microbial spoilage of food occurs in a relatively sudden manner, because bacteria grow by multiplication, i.e. one bacterium can become two in just 15 minutes under the best conditions. Under the same condition, one bacterium can multiply to over 16 million in just six hours! Spoilage bacteria do not normally cause illness, however, when consumed in very large numbers, they can cause gastrointestinal disturbance. Refrigeration of perishable food is one of the common ways to retard the spoilage process.

Foodborne Pathogenic Bacteria

Pathogenic bacteria can cause illness and lead to food safety problems. They are able to invade your body or produce toxins to cause illness. Pathogenic bacteria in food frequently need to multiply to a certain amount to cause disease. The number of organisms required to make individuals ill is called the infective dose. Hot weather during summer can promote the growth of foodborne pathogenic bacteria, which mostly prefer to grow in a warm, humid environment. The bacteria themselves and/or their toxins can make you sick. Once enough bacteria or their toxins have been swallowed, there is a delay called the incubation period (interval between the consumption of food containing bacteria/toxins and the onset of symptoms of the illness), which ranges from hours to days depending on the types of bacteria and the amount of bacteria/toxin ingested. In that period, bacteria can multiply in the gut and/or invade your body. The table below shows the infective dose and incubation period of some common foodborne pathogens in ready-to-eat food:

Pathogenic bacteria Incubation period Estimated infective dose
Salmonella spp. 6 to 48 hours (usually 12 to 36 hours) As few as 15 to 20 organisms; 100 000 to 1 000 000 organisms are more likely to cause disease
Listeria monocytogenes 1 to 90 days (mean 30 days) for high risk group; 11 hours to 7 days for general population (median 18 hours) ~100 to 1 000 organisms for high risk group; consuming food containing >100 000 organisms per gram for general population
Vibrio parahaemolyticus 4 to 74 hours (mean 12 to 46 hours) ~200 000 to 30 000 000 organisms
Staphylococcus aureus 30 minutes to 7 hours (mean 2 to 4 hours) >100 000 organisms per gram of food to produce enough toxin
Bacillus cereus Emetic (cause vomiting): 1 to 6 hours
Diarrhoeal: 10 to 12 hours
>100 000 organisms per gram of food to produce enough toxin or cause illness
Clostridium perfringens 6 to 24 hours (usually 10 to 12 hours) ~100 000 000 organisms

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

You may find that some organisms need to grow to considerable number in foods to produce toxins which are an important weapon of some foodborne pathogenic bacteria to make you sick.

Toxin Production by Pathogenic Bacteria

Pathogenic bacteria can produce toxin in two ways – secreting it into food or releasing it inside your body. Staphylococcus aureus and Bacillus cereus are notorious for being able to produce heat-resistant toxins in food. You may notice that the incubation period of these pathogens are relatively short, which reflects the rapid effect of direct consumption of toxins. On the other hand, some pathogenic bacteria like Clostridium perfringens, primarily release toxin inside your body. Some of the pathogenic bacteria may release a small amount of toxin within your body to help them to invade. Some bacteria can both secrete toxin into food and release toxin inside your body. For example, Bacillus cereus is able to produce emetic and/or diarrhoeal toxins: emetic one (heat-resistant toxin mentioned before) in food that can cause vomiting; diarrhoeal one released inside the body can cause diarrhoea. Although all these bacteria are known to be able to produce toxin, not all strains of them can produce toxin.

We will continue discussing characteristics of bacteria in the next issue.