Chapter 1: Apply knowledge of microorganisms and parasites in food and food environment

Food microbiology and parasitology form the foundation for understanding how microorganisms and parasites interact with food and the food environment. For Nutrition and Dietetics professionals in Kenya, mastering these concepts is essential for ensuring food safety, enhancing nutritional quality, and preventing foodborne illnesses. This chapter explores the fundamental terms in food microbiology and parasitology, followed by an in-depth classification of food microorganisms. The knowledge gained will support dietitians and nutritionists in managing food hygiene and assessing risks related to microbial contamination in various food settings, from hospitals to retail outlets.

1.1 Food microbiology and parasitology concepts

A dietitian meets microorganisms and parasites in every part of food work: a feeding plan for a ward, the storage of expressed breast milk, an enteral feed prepared in a kitchen, a household that reports repeated diarrhoea in children. To give sound advice, the dietitian first needs the vocabulary of food microbiology, because the same word can mean different things to a laboratory worker, a food handler and a family. This topic fixes the meaning of the seven terms used throughout the unit.

A parasite is an organism that lives on or inside another organism, the host, and takes its nutrients from it. Parasitology is the study of these organisms, and in food work it deals with the parasites that reach people through food and water, such as the cysts of protozoa and the eggs and larvae of worms. The food environment is everything around the food that can add or remove microorganisms: surfaces, utensils, water, air, pests, packaging and the people who handle the food. Contamination is the entry of anything harmful into food from that environment, whether it is a microorganism, a chemical or a physical object. These ideas appear in each of the terms below.

1.1.1 Meaning of terms in food microbiology

Each term below answers a different question: what is studied, what harms, how long before harm shows, what is being protected, and what goes wrong with the food itself. The terms are used again in later chapters of the unit, so they are defined here once and referred to by name afterwards.

1.1.1.1 Microbiology

Microbiology is the scientific study of microorganisms: organisms that are too small to be seen without a microscope and that are measured in micrometres (µm), one micrometre being one thousandth of a millimetre. A typical bacterial cell is about 0.5 to 5 µm long, a yeast cell is larger at about 3 to 10 µm, and a virus is smaller than either, measured in nanometres.

The subject is divided by the group of organism studied:

• Bacteriology: the study of bacteria.
• Virology: the study of viruses.
• Mycology: the study of fungi, including moulds and yeasts.
• Phycology: the study of algae.
• Protozoology, helminthology and entomology: the study of single-celled parasites, parasitic worms and the insects and mites that live on the body, which together make up parasitology.

Microbiology became a science in stages. Antonie van Leeuwenhoek described living microscopic cells in the seventeenth century using lenses he made himself. Louis Pasteur showed in the nineteenth century that microbes cause fermentation and spoilage and that heating can destroy them, which led to pasteurisation. Robert Koch then showed how to link a particular microbe to a particular disease. A dietitian uses the results of this science whenever advice on storage, cooking or feeding is based on how microbes grow and die.

1.1.1.2 Toxins

A toxin is a poison made by a living organism. Bacteria, fungi, algae and some plants and animals produce toxins, and several of them can be present in food even after the organism that made them has been killed. This is the reason a toxin can still cause illness in a meal that has been reheated until it is steaming hot.

Toxins are grouped by their origin:

• Bacterial toxins: the enterotoxin of Staphylococcus aureus, which is not destroyed by ordinary cooking, and the neurotoxin of Clostridium botulinum, which blocks nerve signals to muscles and is destroyed by boiling the food for long enough but not by chilling or freezing.
• Fungal toxins (mycotoxins): aflatoxins made by Aspergillus species on maize and groundnuts, and fumonisins made by Fusarium species on maize.
• Algal toxins: poisons made by microscopic marine algae that build up in shellfish and some reef fish.
• Plant toxins: natural poisons in foods such as raw kidney beans and in cassava that has not been processed properly.

A toxin can act within minutes to hours when it was pre-formed in the food, or it can be made inside the gut after the organism has been swallowed. The two routes are taught in the topics Food infection and Food intoxication.

1.1.1.3 Toxicant

A toxicant is a poisonous substance that is not made by a living organism. It enters food through human activity or from the environment. A toxin and a toxicant can cause the same kind of harm, but their origin differs, and so does the way each is controlled: a toxin is prevented by stopping the growth of the organism that makes it, while a toxicant is prevented by keeping the substance away from the food in the first place.

Examples of toxicants in food are:

• Heavy metals: lead, cadmium and mercury taken up from polluted soil and water, or released from poor-quality cookware and solder.
• Pesticide residues: chemicals sprayed on crops that remain on vegetables and fruit when the waiting period before harvest is not respected.
• Cleaning chemicals: detergent or disinfectant left on utensils or poured into unlabelled food containers.
• Process chemicals: an excess of curing salts such as nitrite added to meat, or fuel and oil contaminating food during transport.
• Veterinary drug residues: traces of drugs in milk and meat from animals slaughtered or milked too soon after treatment.

A dietitian who sees a cluster of cases that start within minutes of eating, with no fever, should consider a toxin or a toxicant before an infection, and should report the suspicion to the public health officer so that the food and the container can be examined.

1.1.1.4 Incubation period

The incubation period is the time between swallowing the cause of a foodborne illness and the first symptoms. It is a useful clue because it points to the likely cause: a short period suggests a pre-formed toxin, while a longer one suggests that an organism had to multiply in the gut first.

The periods are ranges because they depend on the dose swallowed and on the person: young children, older adults and people with weak immunity may fall ill sooner or more severely. When a dietitian takes a history after a suspected outbreak, the incubation period is worked out like this:

1. Ask each ill person when they ate each food in the last few days.
2. Ask when the first symptom appeared.
3. Subtract the meal time from the onset time for every person.
4. Compare the range of results with the table above, and look for the one meal that fits.

For example, if six members of a household develop vomiting between 2 and 5 hours after a shared meal of rice and stew, the short period points to a pre-formed toxin and the dietitian looks first at how the rice was cooled and kept.

1.1.1.5 Food microbiology

Food microbiology is the study of the microorganisms that live in food, spoil it, make it, or contaminate it and cause illness. It looks at them from three sides: the harmful ones (pathogens and toxin producers), the spoiling ones, and the useful ones used to make foods such as fermented milk and bread.

Microorganisms reach food from several sources: soil on root crops, water used for washing, dust and air, the intestines of animals during slaughter and milking, insects and rodents, and the hands, clothing and equipment of food handlers. Each source is a point at which a food-safety measure can be placed.

Food microbiology gives the dietitian the knowledge to:

• advise a household which foods need to be kept cold and which are safe at room temperature;
• plan the safe preparation and storage of therapeutic and enteral feeds, which support rapid growth of microorganisms when they are warm;
• recognise from the pattern of illness whether a food or the water supply is the likely cause;
• explain why fermented and dried foods keep well and why cooked starchy foods left in a warm pot do not;
• read the results of laboratory counts and tests, which are taught in the chapter on identification and enumeration.

1.1.1.6 Food safety

Food safety is the assurance that food will not cause harm to the consumer when it is prepared and eaten as intended. It covers three kinds of hazard: biological hazards such as bacteria, viruses and parasites, chemical hazards such as toxins and toxicants, and physical hazards such as glass, stones and metal fragments. A food can be nutritious and still unsafe, so the dietitian treats safety as part of every diet plan.

The World Health Organization summarises safe handling in five keys, which are simple enough to teach to a caregiver or a community health promoter:

1. Keep clean: wash hands before handling food and often during preparation, and clean surfaces and utensils.
2. Separate raw and cooked: keep raw meat, poultry and fish away from other foods, with separate boards and knives.
3. Cook thoroughly: cook until the food is steaming hot throughout, and bring soups and stews to the boil.
4. Keep food at safe temperatures: do not leave cooked food standing at room temperature, and refrigerate promptly.
5. Use safe water and safe raw materials: choose fresh, undamaged food, and treat or boil drinking water when its source is doubtful.

In Kenya, food safety is regulated under laws such as the Public Health Act and the Food, Drugs and Chemical Substances Act, which county public health officers enforce through inspection of premises and food. Food safety is different from food security: food security is about having enough food, food safety is about the food being fit to eat.

1.1.1.7 Food spoilage

Food spoilage is any change that makes food unacceptable to the consumer in smell, taste, appearance or texture. It can be caused by microorganisms, by the food's own enzymes, by chemical changes such as the oxidation of fats, or by damage from pests and handling. Microbial spoilage is the focus of this unit, and it happens when bacteria, yeasts and moulds use the food as a source of energy and leave products that change it.

Foods are grouped by how fast they spoil. Perishable foods, such as fresh milk, meat, fish and cooked rice, spoil within hours or days unless kept cold. Semi-perishable foods, such as potatoes and eggs, last for weeks. Stable foods, such as dry maize and dried beans, keep for months when they stay dry.

Spoilage and food poisoning are not the same. Spoiled food usually looks, smells or tastes wrong and is rejected, but it does not always cause illness. Food that contains disease-causing organisms may look and smell completely normal. A dietitian therefore teaches that appearance and smell are not a test of safety, and that cooked food kept too long in a warm place is discarded even when it seems fine.

Chapter Summary

This chapter provides a comprehensive overview of key concepts in food microbiology and parasitology, beginning with definitions that clarify terms such as microbiology environment, toxins, toxicants, incubation period, food microbiology, food safety, and food spoilage. It then explores the classification of food microorganisms into prokaryotes and eukaryotes, setting the foundation for understanding their structural and functional differences. The chapter further details the basic types of food microorganisms including bacteria, viruses, fungi, and algae, alongside parasites such as protozoa, helminths, and ectoparasites, highlighting their relevance to food. The roles these microorganisms play in both food safety and spoilage are examined, emphasizing their impact on public health and food quality. An introduction to microscopy covers the types of microscopes, the parts of a light microscope, and their application in food microbiology. Finally, the chapter addresses the proper use, care, and maintenance of microscopes, ensuring accurate observation and study of microorganisms. Throughout, the chapter integrates knowledge of the physiology, genetics, and behavior of food microorganisms to underpin practical applications in food safety and quality control.