Microbial Food Spoilage- Types of microorganisms with examples

Food spoilage is a metabolic process that causes foods to be undesirable or unacceptable for human consumption due to changes in sensory characteristics.

• Spoilage of food is identified by off-colors, off-odors, softening of vegetables, fruits, and slime production.
• Spoilage may arise from insect damage, physical damage, and indigenous enzyme activity in food or by microorganisms (bacteria, viruses, fungi).
• The spoilage microbe’s common inhabitants are soil, water, or the intestinal tracts of animals or they are dispersed through the air and water.
• Food spoilage by microorganisms depends upon intrinsic (pH, water activity, nutrient content, oxidation-reduction potential, antimicrobial property) and extrinsic factors (temperature, relative humidity, pressure). 
• Different spoilage-causing microorganisms have different nutrients requirements.
• Microorganisms are the biological agents that cause foodborne diseases when consumed however the microorganisms not only cause spoilage, some of them are beneficial for food fermentation. 

Bacteria in food

• Food is most commonly spoiled by bacteria as it can grow in a wide variety of conditions however bacteria are used for beneficial fermentations of pickles, milk products, and some fermented vegetable products.
• Bacteria do not grow at a water activity level below 0.91 and require neutral pH (6.5-7) to cause food spoilage (e.g. milk, meat, green vegetables, fruits, etc.)
• Some bacteria are capable of spore formation so they are highly heat resistant and some are capable of producing heat-resistant toxins.
• The consumption of such spoiled food leads to foodborne illness.
• The most common bacteria that cause food spoilage are-
  • Gram-positive bacteria such as Staphylococcus aureus, Bacillus spp, Clostridium spp, Lactic acid bacteria (LAB), Leuconostoc spp, Streptococcus spp, Brochothrix spp, Weissella spp, Mycobacterium bovis, etc.
  • Gram-negative bacteria such as Salmonella spp, Shigella, Vibrio spp, Escherichia coli, Campylobacter jejuni, Yersinia enterocolitis, Brucella spp, Coxiella burnetii, Aeromonas spp, Plesiomonas shigelloides, etc.
• These bacteria cause off-odors and off-flavors, discolorations, gas production, slime production, and decreases in pH in food.

Fungi in food

• Fungi is the most abundant group of microorganisms that plays important role in food spoilage.
• Fungi are osmotrophic they obtain their nutrients by absorption.
• Fungi can be divided into mold and yeast.

Molds

• Molds are the most common food spoilage-causing microorganisms.
• Molds grow on the surface of food (they require free oxygen for growth) and in a wide range of pH values (from 2 to 8.5), but the majority of them prefer acidic pH.
• Molds can grow at very low water activity levels (0.7–0.8) on dried foods (e.g. grains, beans, peanuts, and some spices)
• The most common food spoilage causing molds are Mucor, Aspergillus spp, Rhizopus spp, Penicillium spp, Alternaria spp, Bothrytis, Byssochlamys, Fusarium spp. 
• This mold causes off-flavors, mycotoxins contamination, discoloration, rotting, and is externally visible in the food.

Yeasts

• Compared to bacteria and molds, yeasts play a minor role in food spoilage 
• Yeasts can grow with or without oxygen and are used for beneficial fermentation in bread and alcoholic drinks fermentation.
• They often spoil food that has high sugar or salt content (e.g. maple syrup, pickles, jams, soy sauce, and sauerkraut.)
• Yeasts require a water activity level of 0.90–0.95 for growth and they can grow in a wide range of pH (3 – 8) but in general, they prefer acidic pH (4.5-5.5).
• Most commonly food spoilage causing yeasts are Zygosaccharomyces spp, Saccharomyces spp., Candida spp, Dekkera spp 
• These yeasts cause a change in color, a change in texture, an unpleasant odor, or an undesirable taste in food.

Protozoa in food

• Protozoa are one-celled microorganisms without a rigid cell wall and the transmissible form of these organisms is termed cysts.
• Protozoan parasites are highly associated with foodborne and water-borne outbreaks of disease. The water and food act as a carrier for transmission of protozoan parasites from one host to another.
• The most common foodborne parasites are Giardia lamblia, Entamoeba histolytica,  Cyclospora cayetanensis, Toxoplasma gondii, and Trichinella spiralis.

Algae in food

• Algae are primary producers which are a source of different nutrients and they are usually of aquatic habitats.
• They contaminate the water source with their toxin and cause them to accumulate in fish and marine life. The toxic may or may not be harmful to marine lives. When such fish or other marine lives are consumed by humans, it leads to foodborne illness.
• The algae that cause poisoning are Gonyaulax catenella, Gonyaulax tamarensis, Gambierdiscus toxicus, Ptychodiscus brevis, Microcystis aeruginosa, Blue-green Algae.

Viruses in food

• Viruses are obligate intracellular parasites that cause a wide range of diseases in plants, animals, and humans.
• Viruses need specific living cells to replicate and therefore they cannot replicate in food or water. The water and food act as a carrier for transmission of viruses from one host to another.
• Foodborne viruses are quite stable outside the host and are acid-resistant.
• Some of the foodborne viruses are Norovirus, Hepatitis A virus (HAV), Hepatitis E virus (HEV), Astrovirus (AstV), Rotavirus (RV), Coronavirus, Sapovirus (SaV).

Prions in food

• Prions are infectious disease-causing agents which are the normal protein of a brain that gets misfolded that lacks genome resulting in a pathological, infectious conformation.
• Once misfolded, it can induce other normally folded proteins to become misfolded.
•  Prion diseases can affect both humans and animals. It can also get transferred from animal to human through the consumption of infected meat and meat products.
• Some examples of prion disease are Bovine spongiform encephalopathies’ (BSE), Scrapie, Chronic wasting disease (CWD), Creutzfeldt-Jacob Disease (CJD).

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