Microbial Morphology and Foodborne Pathogens

Foodborne pathogenic microorganisms

Foodborne disease is illness caused by pathogenic microorganisms or their toxins that enter the body through contaminated food or water. The causative agents belong to four main classes: bacteria, viruses, protists (protozoa), and helminths. Each disease must be linked to the correct class, because a disease can be caused by a bacterium in one case and by a protist or virus in another. Vector-borne diseases such as Lyme disease are not foodborne, even though they are caused by bacteria, because the agent is transmitted by an arthropod bite rather than by food. Noroviruses are a common viral cause of foodborne outbreaks, spread through food handled by infected persons. Legionellosis is a bacterial disease caused by Legionella pneumophila, and giardiasis is caused by the flagellated protist Giardia lamblia. Typhoid fever is caused by the bacterium Salmonella Typhi, and listeriosis is caused by the bacterium Listeria monocytogenes, so neither is a viral or fungal disease.

• Campylobacter jejuni — a Gram-negative, microaerophilic curved rod that causes bacterial gastroenteritis, often linked to undercooked poultry. Example: Campylobacter enteritis is a foodborne disease caused by a bacterium, not by a virus.
• Vibrio cholerae — a Gram-negative curved rod that causes cholera through contaminated water and seafood.
• Clostridium perfringens — an anaerobic, spore-forming bacterium whose enterotoxin causes food poisoning in cooked meat that is held at inadequate temperatures.

Mycotoxin-producing fungi

Mycotoxins are toxic secondary metabolites produced by filamentous fungi (molds) that grow on crops and stored foods. A mycotoxin-producing fungus is not automatically harmful for food use, since some species are used deliberately in fermentation or industry. Aspergillus niger is used industrially for citric acid production and does not form aflatoxins as a characteristic metabolite. Rhizopus oligosporus is the starter fungus of tempeh fermentation, so its presence in food is intended and beneficial. Aflatoxins are polyketide-derived mycotoxins produced mainly by Aspergillus flavus, and they contaminate groundnuts, maize, and other stored grains. Fusarium verticillioides, by contrast, produces fumonisins rather than aflatoxins.

Ergotism is a toxic condition caused by ergot alkaloids from the fungus Claviceps purpurea, which infects rye and other cereal grains and forms hardened sclerotia called ergots. Ingestion of contaminated grain causes gangrene and neurological symptoms. Clostridium perfringens, Rhizopus stolonifer, and Lactococcus lactis are not associated with ergot poisoning.

• Aflatoxin — a mycotoxin of Aspergillus flavus that contaminates stored crops such as groundnut and maize.
• Ergot alkaloid — a toxic alkaloid from Claviceps purpurea sclerotia on cereal grains.

Fungal cell structure and characteristics

Fungi are eukaryotic organisms, so they have a membrane-bound nucleus and linear chromosomes. Their nuclear chromatin contains histone proteins, around which DNA is wound to form nucleosomes, which is a feature shared with plants and animals but not with bacteria. The cell wall of most fungi is built mainly from chitin, a linear polymer of N-acetylglucosamine linked by beta-(1→4) bonds. Fungal cell walls therefore do not contain peptidoglycan, which is the characteristic wall polymer of bacteria. Pseudomurein, a distinct wall polymer, is found in some methanogenic archaea and not in fungi.

Example: When identifying an unknown cell type, the presence of histone-associated chromatin and a chitinous wall points to a fungus rather than a bacterium.