Microorganisms in food can be friends or foes. Bacteria, yeasts, molds, and protozoa all play roles in food production and spoilage. Understanding their characteristics helps you harness their benefits and prevent foodborne illness.
This section covers the major categories of microbes found in foods, how they're classified, and why each type matters for food safety and quality.
Microbial Classification
Prokaryotes and Eukaryotes
The most fundamental division in microbiology is between prokaryotes and eukaryotes. This distinction matters in food science because it affects how organisms grow, how resistant they are to preservation methods, and what kinds of problems (or benefits) they create.
Prokaryotes are single-celled organisms that lack a membrane-bound nucleus. Bacteria are the main prokaryotes you'll encounter in food microbiology.
- Genetic material sits in a single circular chromosome, free in the cytoplasm (no nuclear membrane)
- Ribosomes are smaller (70S) compared to eukaryotic ribosomes (80S)
- Cell walls contain peptidoglycan, a complex polymer that provides structural support and is the basis for Gram staining
Eukaryotes are organisms with membrane-bound organelles, including a true nucleus. Yeasts, molds, and protozoa are all eukaryotes relevant to food.
- Genetic material is organized inside a membrane-bound nucleus
- Ribosomes are larger (80S), found in the cytoplasm and on the rough endoplasmic reticulum
- Cell walls, when present, are composed of chitin (fungi) or cellulose (some other organisms), not peptidoglycan
Viruses and Prions
Viruses and prions don't fit neatly into the prokaryote/eukaryote framework, but they're still important food safety concerns.
Viruses are non-living infectious particles that require a host cell to replicate. They can't grow or multiply in food itself, but food can serve as a vehicle to transmit them.
- Each virus consists of genetic material (DNA or RNA) surrounded by a protein coat called a capsid
- Some viruses also have a lipid envelope derived from the host cell membrane (e.g., influenza virus)
- Viruses infect specific host cells by binding to receptors on the cell surface. Norovirus and Hepatitis A are the most significant foodborne viruses
Prions are infectious protein particles that cause fatal neurodegenerative diseases.
- Composed entirely of misfolded proteins called PrPSc (prion protein scrapie)
- They replicate by inducing normal cellular proteins (PrPC) to misfold and aggregate
- This accumulation of misfolded protein causes progressive brain damage. Bovine spongiform encephalopathy (BSE, or "mad cow disease") and Creutzfeldt-Jakob disease are prion diseases relevant to the food supply
- Prions are extremely resistant to heat, chemicals, and radiation, making them very difficult to destroy through normal food processing
Microbial Types
Bacteria
Bacteria are the most important group of microorganisms in food science, both as agents of spoilage and disease and as tools for fermentation.
Classification relies on several characteristics:
- Cell wall structure: Gram-positive bacteria have a thick peptidoglycan layer and stain purple in a Gram stain. Gram-negative bacteria have a thinner peptidoglycan layer plus an outer membrane and stain pink. This distinction affects their resistance to certain preservatives and antibiotics.
- Shape: cocci (spheres), bacilli (rods), spirilla (spirals)
- Oxygen requirements: aerobes need oxygen, anaerobes grow without it, and facultative anaerobes can do either
Some bacteria form endospores, which are dormant structures highly resistant to heat, chemicals, and radiation. Clostridium botulinum, the organism that causes botulism, is a classic example. Endospores are a major concern in canning and low-acid food processing because they can survive conditions that kill vegetative cells.
Bacteria can be beneficial or harmful. Lactic acid bacteria drive fermentation in yogurt, sauerkraut, and cheese. On the other hand, pathogens like Salmonella, Listeria monocytogenes, and E. coli O157:H7 cause serious foodborne illness.
Bacterial growth depends on temperature, pH, water activity (), and nutrient availability. Optimal conditions vary by species. Some are adapted to extreme environments: thermophiles thrive at high temperatures, while halophiles tolerate high salt concentrations. Controlling these factors through refrigeration, acidification, drying, and antimicrobials is the foundation of food preservation.
Yeasts, Molds, and Protozoa
Yeasts are single-celled eukaryotic fungi that typically reproduce by budding (a small daughter cell pinches off the parent cell).
- Yeasts are essential to fermentation. Saccharomyces cerevisiae is used to produce beer, wine, and leavened bread by converting sugars to alcohol and
- Some yeasts cause food spoilage, particularly in high-sugar or high-acid foods like fruit juices and jams
- A few species, such as Candida albicans, are opportunistic pathogens in humans, though they're more of a clinical concern than a food safety one
Molds are multicellular eukaryotic fungi that grow as networks of filamentous strands called hyphae. A visible mass of hyphae is called a mycelium.
- Molds reproduce by producing spores, which disperse easily through air and water. This is why mold contamination can spread quickly in storage environments.
- Some molds are deliberately used in food production. Penicillium camemberti ripens Camembert and Brie cheeses, and Aspergillus oryzae is used in soy sauce and miso production.
- Other molds cause spoilage or produce mycotoxins, toxic secondary metabolites that contaminate food. Aflatoxins, produced by Aspergillus flavus and A. parasiticus, are among the most potent natural carcinogens and are a major concern in grains, peanuts, and tree nuts.
Protozoa are single-celled eukaryotic organisms that can be free-living or parasitic.
- They move using cilia, flagella, or pseudopodia, depending on the species
- The most important foodborne protozoa are Cryptosporidium parvum and Giardia lamblia, both of which cause gastrointestinal illness and are commonly transmitted through contaminated water
- Toxoplasma gondii, found in undercooked meat (especially pork and lamb), is another significant foodborne protozoan parasite
- Proper food handling, thorough cooking, and effective water treatment are the primary defenses against protozoan infections