6.2 An Overview of Food Microbiology

Micro-organisms are invisible to the naked eye and can be found everywhere. There are four main categories that micro-organisms can be grouped into:

1. Essential

   These microbes are found in our gastrointestinal tract (and help in digestion and vitamin absorption).

2. Useful

   Humans use these microbes to make products: beer and cheese for instance.

3. Spoilage

   These microbes change the color, the aroma, and the taste of food, but don’t necessarily render food inedible (e.g., blue cheese).

4. Harmful

   These are microbes that can potentially cause foodborne illnesses - for instance, Campylobacter jejuni and Clostridium botulinum.

6.2.1 What is a foodborne illness?

A foodborne illness is the disease of an infectious agent or a toxic agent that is caused by the consumption of contaminated food or water.

We say that a foodborne illness is an infection if and only if it is caused by ingesting food or water that has been contaminated with the disease-causing pathogen (e.g., Salmonella).

Otherwise, a foodborne illness is toxin-based if it is caused by the ingestion of food that has been contaminated with chemical or biological toxins. For instance, pesticide poisoning, mushroom poisoning, and staphylococcal poisoning are all examples.

6.2.1.1 Incubation periods of foodborne illnesses

The incubation period of any disease refers to the period of time between catching an infection and symptoms appearing. Depending on the pathogen and the nature of the disease, the incubation period is highly variable.

For instance, staphylococcal food poisoning has an incubation period between 1 - 6 hours whereas Salmonella infections have an incubation period of roughly 12 - 72 hours after the ingestion of contaminated food!

6.2.1.2 Infective dose

The infective dose is the number of pathogens that are necessary to demonstrate infection in 50% of people (i.e., the ID50 - the median infective dose).

Figure 6.6: Effect of Temperature on * Cases in Singapore

Figure 6.6 shows the infective dose of various foodborne illness-causing microbes.

6.2.1.3 Pathogenesis

There are two main mechanisms: local and distant.

1. Local mechanism

   Here, the bacteria either directly transfers proteins into host cells or act via bacterial surface-bound molecules.

2. Distant mechanism

   Pathogenesis involves the secretion of a factor that diffuses prior to exerting pathogenic effects.

6.2.2 Typhoid Mary

Mary Mallon was an Irish immigrant who was born in 1869 and moved to the US in 1884.

Figure 6.7: Typhoid Mary on a Print Publication

Mary was a healthy carrier of Typhus, hence her nickname “Thphoid Mary”. Many became infected through Mary’s cooking due to her ignorance and denial of being ill.

Mary was ultimately quarantined on two separate occasions (for a total of 26 years) and died alone, evidently having found consolation in her religion.

6.2.3 Common symptoms of foodborne illness

Figure 6.8: General Symptoms of Foodborne Illness

Figure 6.8 shows five general symptoms of foodborne illnesses.

However, it is worth noting that the symptoms presented in figure 6.8 can be more severe in individuals with weaker immune systems, for instance:

1. The elderly
2. Pregnant women
3. Patients with chronic illnesses (e.g., HIV, cancer, diabetes, etc.)
4. Patients with a high gastric pH
5. Patients undergoing antibiotic treatments (this alters the microflora in the intestines) and surgery (this prohibits normal gastric emptying and deproves intestinal mobility)

Figure 6.9: Notifiable Foodborne Illnesses in Singapore

The above graphic (generated using data from the MOH) also display a list of notifiable foodborne illnesses in Singapore. Again, the data suggests that Salmonella infections are the most common out of all reported foodborne illness cases.