Microorganisms in Food: Types and Their Roles

The same microscopic organisms can be a food's best friend or its worst enemy. One species of mold turns milk into blue cheese; a related one covers bread in fuzzy rot. Bacteria give yogurt its tang and turn cabbage into kimchi, yet close relatives spoil meat and cause food poisoning. Food microbiology is the study of this double life. To understand it, you do not sort microbes into "good" and "bad," because the same organism can be both. You sort them by the role they play in a particular food.

Figure: Microorganisms found in Food

Microorganisms are everywhere in food. They live on the surfaces of fresh produce, in raw milk and meat, and in the air and water used to process food. Some are deliberately added to make foods like cheese, yogurt, and bread. Others arrive by accident and spoil the food or cause illness.

Rather than labeling a microbe simply "good" or "bad," it is more accurate to ask what role it plays in a given food. The same organism can be helpful in one setting and harmful in another. This article groups the microbes found in food by type (bacteria, yeasts, molds, viruses, and parasites) and, within each, explains the roles they play.

The roles microorganisms play in food

Microbes in food fall into a few overlapping roles. The same species can appear in more than one, depending on the food and the conditions.

Food production. Some microbes are deliberately used to make foods and ingredients. Aspergillus niger is a mold used industrially to produce citric acid and enzymes. Saccharomyces cerevisiae (baker's and brewer's yeast) leavens bread and ferments beer and wine. Lactic acid bacteria such as Lactobacillus and Lactococcus are used to make cheese, yogurt, and other cultured dairy products.

Food preservation. Fermentation is itself a preservation method. As helpful microbes grow, they produce acids, alcohol, and other compounds that lower the pH or otherwise make the food hostile to spoilage and disease-causing organisms. Lactic acid bacteria preserve yogurt, pickles, and kimchi this way. This is covered in detail in a separate article on food preservation methods.

Food spoilage and illness. Unwanted microbes degrade food quality or cause disease. Pseudomonas species spoil refrigerated milk, meat, and fish. Some yeasts spoil high-sugar foods and beverages. Aspergillus flavus is a mold that produces aflatoxin, a dangerous mycotoxin. Many foodborne pathogens (covered in a separate article on food poisoning) also arrive as contaminants.

The rest of this article looks at each type of microbe in turn and the roles it plays.

Types of Food Microorganisms

The microbes found in food fall into four groups: bacteria, fungi (yeasts and molds), viruses, and parasites. They are found naturally on almost all fresh food, and some are deliberately added to make fermented and processed foods. The sections below cover each group and the roles it plays.

Bacteria

Bacteria are the most common and important microbes in food, playing roles in production, preservation, and spoilage.

Lactic acid bacteria (LAB) are the most useful group. As they grow, they produce lactic acid, which lowers the pH and inhibits spoilage and disease-causing bacteria. Some LAB also produce small amounts of hydrogen peroxide and bacteriocins, which further suppress competitors. This acid production is what preserves and flavors fermented foods such as yogurt, kimchi, miso, and pickles, and many of these bacteria act as probiotics.

Spore-forming bacteria, mainly Bacillus and Clostridium, matter because their spores survive heat that kills ordinary cells. This lets them survive cooking and some heat processing. Most spore-formers cause spoilage rather than disease, but a few are dangerous: Clostridium botulinum produces a deadly neurotoxin in improperly canned low-acid foods, and Bacillus cereus causes food poisoning. Some spore-formers are psychrotrophic, meaning they can grow slowly even in the refrigerator and spoil chilled foods.

Helpful bacteria

Lactic acid bacteria are the main helpful group. They preserve and flavor fermented foods and many act as probiotics, supporting gut health and lactose digestion.

• Lactobacillus ferments sugars into lactic acid, preserving food and suppressing spoilage organisms. Several strains are used as probiotics.
• Leuconostoc starts many vegetable fermentations (such as sauerkraut) and is used in cultured dairy products. It lowers pH early in fermentation.
• Streptococcus thermophilus and Lactococcus species are used to make yogurt and cheese.

Spoilage and harmful bacteria

• Pseudomonas species are major spoilers of refrigerated protein foods (milk, meat, fish), producing off-odors and slime through their enzymes. They are aerobic and grow well in the cold.
• Brochothrix thermosphacta is a non-spore-forming bacterium that spoils refrigerated and vacuum-packed meat, producing an off, cheesy odor. It does not form spores and does not cause nerve damage; its importance is as a meat spoiler.
• Clostridium botulinum is a spore-forming anaerobe that produces botulinum neurotoxin in improperly preserved low-acid foods, causing botulism. It is never a beneficial food organism. Its control is the central concern of safe canning.

Bacterial spoilage is slowed by lowering water activity, lowering pH, refrigeration, and proper heat processing.

Yeast

Yeasts are single-celled fungi, larger than bacteria, that usually reproduce by budding. They are used to ferment alcoholic beverages and leaven bread. In spoilage, yeasts break down sugars and produce gas and off-flavors, and some produce enzymes (lipases, proteases) that alter flavor and appearance. Because yeasts tolerate acid, sugar, and salt, foods that are acidic, sugary, or salty (where bacteria struggle) are especially prone to yeast spoilage.

Helpful yeasts

• Saccharomyces cerevisiae ferments alcoholic beverages, leavens bread, and is used to produce biofuel. Some strains (*S. boulardii*) are used as probiotics.
• Candida humilis is used with lactic acid bacteria in sourdough as a leavening agent.
• Debaryomyces hansenii is a salt-tolerant yeast used in cheese and fermented meat ripening.

Spoilage yeasts

Some yeasts spoil food rather than help it. Because they tolerate acid, sugar, and salt, they spoil foods where bacteria cannot easily grow, such as fruit juices, syrups, jams, and pickled products, causing fermentation, gas, and off-flavors. Zygosaccharomyces is a well-known spoiler of high-sugar and acidic foods.

Yeast spoilage is controlled by chemical preservatives such as sodium benzoate, along with refrigeration and reduced oxygen.

Mold

Molds are fungi that can grow within a wide range of water activity (aw), pH values, and temperature making them highly adaptable to different environments. They are multicellular in structures, visible to the naked eye, and generally grow from spores present in the air. Edible mold is essential in the cheese industry, used to produce “blue cheese.”

However, not all types of molds are safe for consumption. Molds, when present in food, utilize carbohydrates, organic acids, protein, and lipids, allowing them to grow on acidic foods (fruits and juices) and on foods with intermediate moisture (such as bakery products), where bacteria cannot easily grow.

Helpful molds

• Penicillium roqueforti gives blue cheeses (such as Roquefort, Stilton, and Gorgonzola) their veins and flavor.
• Penicillium camemberti forms the white rind on soft cheeses such as Camembert and Brie.
• Rhizopus oligosporus is the starter culture for tempeh, binding cooked soybeans into a firm cake.

Spoilage and toxin-producing molds

• Mucor and Rhizopus are fast-growing molds that spoil bread, fruit, and other foods. (Rhizopus oligosporus is the exception, used deliberately to make tempeh.)
• Aspergillus flavus produces aflatoxin, one of the most dangerous mycotoxins, on stored grains and nuts. Aflatoxin is a potent liver carcinogen.
• Penicillium expansum produces the mycotoxin patulin in rotting apples and apple products.
• Alternaria spoils fruits and vegetables and can produce mycotoxins and trigger allergies.

Note on Botrytis: Botrytis cinerea is used deliberately in winemaking as "noble rot" to make certain sweet dessert wines. On most other crops the same mold is simply a destructive spoiler. It is not used to make cheese.

Mold growth is controlled by lowering the food's moisture (water activity) and keeping storage areas dry.

Viruses

Unlike bacteria and fungi, viruses cannot grow in food. They need a living host to multiply, so they do not spoil food by growing in it. Instead, food acts as a carrier: viruses contaminate food through infected handlers, contaminated water, or contact, and cause illness when eaten. (One food-relevant use of viruses is bacteriophages, which are applied to some foods to kill specific bacterial pathogens such as Listeria.) The important foodborne viruses are:

1. Rotavirus is a contagious virus that degrades salads and shellfish. It causes diarrhea, vomiting, and fever in humans. It can also cause dehydration.
2. Norovirus plays a significant role in spoiling fresh fruit and leafy vegetables. It causes gastrointestinal illness and dehydration.
3. The consumption of contaminated food and water can transfer hepatitis A. This virus directly affects the liver.

Cleaning food before consumption, handwashing, and proper cooking can prevent food spoilage from viruses and protect us from foodborne illnesses.

Parasite

Parasites are organisms that live on or in a host. The foodborne ones are protozoa (single-celled, such as Toxoplasma and Cryptosporidium) and helminths (worms, such as tapeworms and roundworms). They reach humans through undercooked meat and fish, unpasteurized milk, and contaminated water.

Parasites in food are essentially all harmful. Like viruses, they do not grow in the food itself; food and water simply carry them to the human host.

The important foodborne parasites include:

1. Roundworms (nematodes) such as Trichinella (from undercooked pork) and Anisakis (from raw fish) infect humans through food. Hookworm infection can cause anemia and malnutrition.
2. Tapeworms (cestodes) infect the human intestine and are acquired from raw or undercooked beef (Taenia saginata) and pork (Taenia solium).

Proper handling and cooking practices can prevent the spoilage of food from parasites.

What controls microbial growth in food

Whether the microbes in a food grow quickly, slowly, or not at all depends mainly on three factors. These are covered fully in the article on food spoilage, and summarized here.

Temperature. Microbes are grouped by preferred temperature: psychrophiles and psychrotrophs (cold-tolerant, growing even in the refrigerator), mesophiles (grow best around 30 to 40°C, and include most foodborne pathogens), and thermophiles (heat-loving, above 45°C). Refrigeration slows most microbes but does not stop psychrotrophs, which is why chilled food still spoils.

pH. Every microbe has a preferred pH range. Most bacteria prefer near-neutral pH (around 6.5 to 7.5), while yeasts and molds tolerate acidic conditions much better. This is why acidic foods (fruit, juice) are spoiled mainly by fungi, and near-neutral foods (meat, milk) mainly by bacteria. Lactic acid bacteria are unusually acid-tolerant, which is what lets them dominate fermentations, though even they stall as the pH drops toward about 3.5.

Water activity (aw). This measures the free water available to microbes. Most spoilage bacteria need an aw above about 0.91; molds grow much drier, some xerophilic molds down to about 0.65. Two pathogens sit at the edges: Staphylococcus aureus can grow down to about 0.86, while Clostridium botulinum needs a high aw of at least 0.94.

For the full mechanism of how these factors drive spoilage, see the article on food spoilage.

How to Remember

Sort by role, not by "good" or "bad."
The same microbe can help one food and ruin another. Saccharomyces makes bread and beer, but also spoils juice. Ask what the organism is doing in this food: producing, preserving, or spoiling. That question does the sorting for you.

Who spoils what: pH picks the spoiler.
Near-neutral foods (meat, milk) are spoiled by bacteria. Acidic foods (fruit, juice) are spoiled by yeasts and molds, because fungi tolerate acid and bacteria do not. Match the food's pH to the likely spoiler.

The three heat groups by the "-phile."
Psychro = cold-lovers (grow in the fridge), meso = middle (body-temperature range, most pathogens), thermo = heat-lovers. The prefix is the temperature.

LAB are the workhorses.
Lactic acid bacteria appear in almost every helpful food fermentation: yogurt, cheese, pickles, kimchi, sauerkraut. Their trick is one thing: they make lactic acid, which preserves and flavors at the same time.

Key exam facts in one table

Where Students Get Confused

Is a microbe "good" or "bad"?
Neither, on its own. The same organism can be helpful or harmful depending on the food and conditions. Saccharomyces cerevisiae makes bread and beer but also spoils fruit juice. Always ask what role it plays in the specific food rather than assigning it a fixed label.

Do viruses spoil food the way bacteria do?
No. Viruses cannot grow in food; they need living cells to multiply. Food simply carries them until eaten. So viruses cause foodborne illness (like norovirus) but do not "spoil" food by growing in it.

Are all spore-forming bacteria dangerous neurotoxin producers?
No. Most spore-formers cause spoilage, not disease. Only a few are dangerous, most notably Clostridium botulinum (botulinum toxin) and Bacillus cereus (food poisoning). The spore matters because it survives heat, not because every spore-former makes a toxin.

Why do fruit and juice grow mold while meat grows bacteria?
Because of pH. Fruit and juice are acidic, and yeasts and molds tolerate acid far better than most bacteria. Meat and milk are near-neutral, which suits bacteria. The food's acidity selects which microbe spoils it.

Is Clostridium botulinum ever a beneficial or probiotic organism?
No. This is an important correction: C. botulinum is never beneficial in food. It produces one of the most potent toxins known. Any list of helpful food bacteria that includes it is wrong.

References

1. Jay JM, Loessner MJ, Golden DA. Modern Food Microbiology. 7th ed. New York: Springer; 2005.
2. Adams MR, Moss MO, McClure P. Food Microbiology. 4th ed. Cambridge: Royal Society of Chemistry; 2015.
3. Ray B, Bhunia A. Fundamental Food Microbiology. 5th ed. Boca Raton: CRC Press; 2014.
4. Hungaro HM, Peña WL, Silva NBM, Carvalho RV, Alvarenga VO, Sant'Ana AS. Food Microbiology. In: Encyclopedia of Agriculture and Food Systems. Elsevier; 2014. https://doi.org/10.1016/B978-0-444-52512-3.00059-0
5. Lorenzo JM, Munekata PE, Dominguez R, Pateiro M, Saraiva JA, Franco D. Main groups of microorganisms of relevance for food safety and stability. In: Innovative Technologies for Food Preservation. Elsevier; 2018:53–107.