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Molds -Yeast -Top 10 Microorganisms Important in Food Microbiology

Food molds and their importance:

  • Food microbiologists need to be familiar with microorganisms in foods to identify main types and compare results with other workers.
  • Food molds are not extensively studied in basic microbiology courses, but they play a significant role in food spoilage and production.

General characteristics of molds:

  • Molds are multicellular, filamentous fungi recognized by their fuzzy or cottony appearance on foods.
  • The mold thallus consists of branching filaments called hyphae, which collectively form the mycelium.
  • Hyphae can be submerged (growing within the food) or aerial (growing above the food).
  • Hyphae may be vegetative (involved in nutrition) or fertile (involved in reproduction).
  • Some molds produce sclerotia, which are dense masses of modified hyphae and are resistant to adverse conditions.

Morphological characteristics:

  • Molds can be divided into septate (hyphae with cross walls) and nonseptate (hyphae without cross walls).
  • Septate hyphae can grow through apical growth (tip cell division) or intercalary growth (division within the hypha).
  • Nonseptate hyphae have scattered nuclei and are considered multicellular.
  • Special structures aid in mold identification, such as rhizoids, foot cells, and dichotomous branching.

Reproductive parts or structures:

  • Molds can grow from transplanted mycelium and reproduce mainly through asexual spores.
  • Asexual spores include conidia (budded from conidiophores), arthrospores (formed by hypha fragmentation), and sporangiospores (formed within a sporangium at the tip of a sporangiophore).
  • Chlamydospores, or resting cells, can be formed by some molds and withstand unfavorable conditions.
  • Sexual spores are produced by certain molds, including oospores (Oomycetes), zygospores (Zygomycetes), ascospores (Ascomycetes), and basidiospores (Basidiomycetes).

Cultural characteristics:

  • The gross appearance of mold on food can indicate its class or order.
  • Mold growth can vary in texture (loose, fluffy, compact), surface appearance (velvety, dry, wet), color (pigments in mycelium and spores), and growth patterns (distinct zones).

Physiological characteristics:

  • Molds require less moisture than yeasts and bacteria, with an approximate moisture threshold for mold growth.
  • Most molds are mesophilic, thriving at ordinary temperatures, but some can grow at refrigeration or high temperatures.
  • Molds are aerobic and can tolerate a wide pH range, with a preference for acidic conditions.
  • Molds can utilize various types of food and produce hydrolytic enzymes for nutrient breakdown.
  • Certain molds produce compounds inhibitory to other organisms or have resistance to inhibitors.

Classification and identification of molds:

  • Classification and Identification of Molds:
  • Criteria for differentiation and identification of molds:
  • Differentiation based on septate or nonseptate hyphae.
  • Clear or dark (smoky) mycelium.
  • Colored or colorless mycelium.
  • Presence and type of sexual spores: oospores, zygospores, or ascospores.
  • Types of asexual spores: sporangiospores, conidia, or arthrospores (oidia).
  • Characteristics of the spore head: a. Sporangia: size, color, shape, and location. b. Conidia-bearing spore heads: single conidia, chains, budding conidia, or masses; shape and arrangement of sterigmata or phialides; gumming together of conidia.
  • Appearance of sporangiophores or conidiophores: simple or branched, types of branching; size and shape of columella at tip of sporangiophore; whether conidiophores are single or in bundles.
  • Microscopic appearance of asexual spores, especially conidia: shape, size, color; smooth or rough; one-, two-, or many-celled.
  • Presence of special structures or spores: stolons, rhizoids, foot cells, apophysis, chlamydospores, sclerotia, etc.
  • Molds of Industrial Importance:
  • Mucor:
    • Involved in food spoilage and manufacturing processes.
    • Common species: M. racemosus, M. rouxii.
  • Zygorrhynchus:
    • Similar to Mucor, with markedly unequal-sized zygospore suspensors.
  • Rhizopus:
    • Commonly known as bread mold.
    • Involved in the spoilage of fruits, vegetables, and bread.
    • Common species: Rhizopus stolonifer.
  • Absidia:
    • Similar to Rhizopus, but with small, pear-shaped sporangia.
  • Thamnidium:
    • Found on meat in chilling storage, causing “whiskers” on the meat.
    • Common species: Thamnidium elegans.
  • Aspergillus:
    • Widespread genus involved in food spoilage and food preparation.
    • Common species: A. glaucus, A. repens, A. niger, A. flavus-oryzae.
  • Penicillium:
    • Widespread genus important in foods.
    • Common species: P. expansum, P. digitatum, P. italicum, P. camemberti, P. roqueforti.
  • Trichothecium:
    • Common species: T. roseum.
    • Pink mold found on wood, paper, fruits, and vegetables.
  • Geotrichum (Oospora or Oidium):
    • Genus included with both yeastlike fungi and molds.
    • Common species: Geotrichum candidum.
  • Neurospora (Monilia):
    • Genus with confusion regarding classification.
    • Common species: Neurospora (Monilia) sitophila.
  • Sporotrichum:
    • Species include S. carnis.
    • Found growing on chilled meats, causing “white spot.”
  • Botrytis:
    • Important species: B. cinerea.
    • Causes grape disease and grows on various foods.
  • Cephalosporin:
    • Common species: C. acremonium.
  • Trichoderma:
    • Common species: T. vir.


  • Yeasts are unicellular fungi that reproduce by budding or fission and are commonly used in the production of various foods and beverages.
  • They play a beneficial role in the fermentation process for the production of bread, beer, wines, vinegar, and surface-ripened cheese.
  • Yeasts are also cultivated for enzymes and as food additives.
  • However, yeasts can be harmful when they cause spoilage of sauerkraut, fruit juices, syrups, molasses, honey, jellies, meats, wine, beer, and other foods.
  • The classification of yeasts is primarily based on their morphological and physiological characteristics.
  • Morphologically, yeasts can have different shapes, including spherical, ovoid, lemon-shaped, pear-shaped, cylindrical, triangular, or elongated into a false or true mycelium.
  • Yeasts reproduce asexually through multilateral or polar budding, where protoplasm bulges out of the cell wall and eventually forms a new yeast cell. Some yeasts can also reproduce by fission or a combination of fission and budding.
  • True yeasts (Ascomycotina) can undergo sexual reproduction, resulting in the production of ascospores, while “false” yeasts (Fungi Imperfecti) do not produce ascospores.
  • Yeasts can exhibit different cultural characteristics, but these characteristics are generally not useful for identification purposes.
  • Physiologically, yeasts have specific requirements for moisture, temperature, pH, and nutrient sources.
  • Yeasts generally require more moisture than molds but can tolerate higher solute concentrations, such as sugar or salt.
  • The temperature range for yeast growth is similar to that of molds, with an optimum around 25-30°C.
  • Yeasts typically prefer an acidic environment and grow best under aerobic conditions, although fermentative yeasts can also grow anaerobically.
  • Sugars are the primary source of energy for yeasts, and they can produce carbon dioxide and alcohol during fermentation.
  • Yeasts have varying nitrogen and carbon source requirements and may require accessory growth factors.
  • Yeasts can exhibit different physiological characteristics, and strains within a species can have specific adaptations or mutations.
  • The classification and identification of yeasts are based on factors such as ascospore formation, appearance of vegetative cells, method of asexual reproduction, growth as film or throughout a medium, color of macroscopic growth, and physiological characteristics.
  • The genus Saccharomyces is of significant industrial importance and is used in various food industries, including bread, beer, wine, and alcohol production.
  • Other important yeasts include species from the genera Schizosaccharomyces, Kluyveromyces, and Zygosaccharomyces, which have specific applications or are associated with spoilage in certain food products.
  • The classification and nomenclature of some yeast species have been revised and updated over time.
  • Genus Lactobacillus:
    • Rod-shaped bacteria that form chains.
    • They are microaerophilic, although some species are strict anaerobes.
    • Catalase-negative and gram-positive.
    • They ferment sugars to primarily produce lactic acid.
    • There are two types of Lactobacillus species:
      • Homofermentative lactobacilli: They produce lactic acid as the main product. Examples include L. bulgaricus, L. helveticus, L. lactis, L. acidophilus, L. thermophilus, and L. delbrueckii. These species thrive at temperatures of 37°C or above.
      • Heterofermentative lactobacilli: They produce lactic acid along with other volatile products. Examples include L. fermentum, L. brevis, L. buchneri, L. pastorianus, L. hilgardii, and L. trichodes.
    • Lactobacillus species are important in food fermentation, production of lactic acid, and exhibit heat resistance.
  • Genus Leuconostoc:
    • Heterofermentative lactic streptococci.
    • They ferment sugars to produce lactic acid, acetic acid, ethyl alcohol, and carbon dioxide.
    • Leuconostoc species are commonly used in lactic starters for products like buttermilk, butter, and cheese.
    • They are known for their contributions to flavor production, salt tolerance, rapid fermentation, and their ability to tolerate high sugar concentrations.
  • Genus Listeria:
    • Gram-positive, non-spore-forming rods with tumbling motility.
    • L. monocytogenes, a species within this genus, can cause food-borne disease outbreaks.
  • Genus Microbacterium:
    • Small, nonmotile, gram-positive rods.
    • They are catalase-positive and aerobic.
    • Microbacterium species produce lactic acid and exhibit resistance to adverse conditions.
    • Some species of Microbacterium are used in vitamin production.
  • Genus Micrococcus:
    • Spherical cells that can arrange in irregular masses, clusters, tetrads, or packets.
    • Gram-positive, aerobic, and catalase-positive.
    • They can utilize ammonium salts, ferment sugars, tolerate high salt concentrations, and withstand adverse conditions.
    • Certain species of Micrococcus can grow at low temperatures.
  • Coliform bacteria are a group of gram-negative, rod-shaped bacteria that belong to the family Enterobacteriaceae.
  • They are facultative anaerobes, meaning they can grow with or without oxygen.
  • Coliform bacteria are commonly found in the environment, including soil, water, and the intestinal tracts of humans and animals.
  • The presence of coliform bacteria in water is often used as an indicator of fecal contamination and the potential presence of other harmful microorganisms.
  • Most coliform bacteria are harmless, but some species, such as Escherichia coli (E. coli), can cause illness, including gastrointestinal infections.
  • Coliform bacteria are often used as indicators in water quality testing to assess the safety of drinking water.
  • The most commonly used test for coliform bacteria is the presence-absence test, which involves culturing a water sample in a selective medium and checking for the production of gas or acid.
  • The term “total coliforms” refers to the group of coliform bacteria as a whole, while “fecal coliforms” specifically indicates the subset of coliform bacteria associated with fecal matter.
  • The detection of coliform bacteria in food products can also be an indication of poor sanitation or improper food handling practices.

Review Questions

2.1 Write notes on moulds of importance in a food industry.

2.2 Describe the importance of Saccharomyces.

2.3 Explain the role of spore-forming bacteria in food microbiology.

2.4 Discuss the genera of bacteria in food bacteriology.

2.5 Write notes on Lactobacillaceae.

2.6 What are the characteristics that make coliform bacteria important in the spoilage of foods?

2.7 Write notes on pigmented bacteria in foods.

2.8 What are thermoduric bacteria? Give examples.

2.9 List the characteristics of Pseudomonas species that cause spoilage of foods.

2.10 Describe the organisms used in dairy industries.


2.1 Notes on molds of importance in the food industry:

  • Molds are a type of fungus that can grow on various food products, including fruits, bread, cheese, and meat.
  • Some molds are beneficial in the food industry, such as those used in the production of certain cheeses (e.g., blue cheese) and in the fermentation of soybeans to make soy sauce.
  • However, molds can also cause food spoilage and produce toxins called mycotoxins, which can be harmful if consumed.
  • Common molds of importance in the food industry include Aspergillus, Penicillium, and Fusarium species.

2.2 Importance of Saccharomyces:

  • Saccharomyces is a genus of yeast that plays a crucial role in the food industry, particularly in the fermentation process.
  • Saccharomyces cerevisiae, also known as baker’s yeast or brewer’s yeast, is widely used in baking, brewing, and winemaking.
  • It converts sugars into carbon dioxide and alcohol through the process of fermentation, leading to the leavening of bread, the production of alcoholic beverages, and the development of desirable flavors.
  • Saccharomyces boulardii, a strain of Saccharomyces, is used as a probiotic in the food industry and can provide health benefits by promoting gut health and preventing certain gastrointestinal infections.

2.3 Role of spore-forming bacteria in food microbiology:

  • Spore-forming bacteria, such as species from the genus Bacillus and Clostridium, have the ability to form dormant spores that can withstand harsh conditions, including heat, desiccation, and chemical disinfection.
  • In the field of food microbiology, spore-forming bacteria are of concern because their spores can survive food processing and sanitation procedures, leading to potential spoilage or foodborne illness.
  • The spores of certain species, like Clostridium botulinum, can produce toxins that cause severe food poisoning.
  • Proper handling, cooking, and storage practices are necessary to prevent the growth and survival of spore-forming bacteria in food products.

2.4 Genera of bacteria in food bacteriology:

  • Food bacteriology encompasses the study of various bacterial genera that can impact food safety and quality.
  • Some important genera in food bacteriology include:
    • Escherichia: Includes species like Escherichia coli, which can indicate fecal contamination in food and water.
    • Salmonella: Causes salmonellosis, a common foodborne illness.
    • Listeria: Includes Listeria monocytogenes, responsible for listeriosis, a serious foodborne infection.
    • Staphylococcus: Can produce toxins that cause food poisoning.
    • Campylobacter: Commonly associated with poultry and causes campylobacteriosis.
    • Clostridium: Includes species like Clostridium perfringens and Clostridium botulinum, which can cause foodborne illnesses.
    • Vibrio: Includes species like Vibrio parahaemolyticus and Vibrio vulnificus, associated with seafood-related infections.

2.5 Notes on Lactobacillaceae:

  • Lactobacillaceae is a family of bacteria that includes several genera, such as Lactobacillus and Lactococcus.
  • These bacteria are important in the food industry, particularly in the production of fermented foods and beverages.
  • Lactobacillus species are commonly used as starter cultures in yogurt, cheese, sauerkraut, and pickles, where they contribute to the fermentation process and impart desired flavors.
  • They convert sugars into lactic acid, helping to preserve the food and create a sour taste.
  • Lactobacillus acidophilus and Lactobacillus rhamnosus are examples of Lactobacillus species used as probiotics, providing potential health benefits to the consumer.

2.6 Characteristics that make coliform bacteria important in food spoilage:

  • Coliform bacteria, including Escherichia coli, can contribute to food spoilage due to several characteristics:
    • They are commonly found in the environment, including the intestines of humans and animals, and can contaminate food through poor hygiene practices.
    • Some coliform bacteria can produce enzymes that break down proteins and fats, leading to the degradation of food quality.
    • They can produce gas during fermentation, leading to off-flavors, bloating, and spoilage of packaged foods.
    • The presence of coliform bacteria in food can indicate unsanitary conditions and the potential presence of other harmful microorganisms.

2.7 Notes on pigmented bacteria in foods:

  • Pigmented bacteria are a group of bacteria that produce visible pigments, resulting in coloration in food products.
  • Some pigmented bacteria are harmless and may even contribute to the sensory appeal of certain foods, such as red-pigmented Serratia marcescens in Kombucha.
  • However, pigmented bacteria can also indicate food spoilage or poor hygiene practices.
  • Examples of pigmented bacteria associated with food include Chromobacterium, Serratia, and Pseudomonas species.
  • The growth of pigmented bacteria can lead to discoloration, off-odors, and off-flavors in food products.

2.8 Thermoduric bacteria and examples:

  • Thermoduric bacteria are a group of bacteria that can survive and even grow at elevated temperatures.
  • They are capable of withstanding heat treatments commonly used in food processing, such as pasteurization.
  • Examples of thermoduric bacteria include Bacillus cereus, Enterococcus faecalis, and Staphylococcus aureus.
  • These bacteria can pose a risk in heat-treated food products if not properly controlled, as they can multiply and cause spoilage or foodborne illnesses.

2.9 Characteristics of Pseudomonas species causing spoilage of foods:

  • Pseudomonas species are commonly associated with food spoilage, particularly in refrigerated or perishable food products.
  • Some characteristics that make Pseudomonas species problematic include:
    • They can grow at low temperatures, even in the refrigerator, leading to spoilage of stored food items.
    • Pseudomonas species produce enzymes that break down proteins and lipids, contributing to off-flavors, discoloration, and deterioration of food quality.
    • They can also produce slime, causing a slimy texture in affected food products.
    • Pseudomonas aeruginosa, a specific species within the genus, is a pathogenic bacterium associated with foodborne infections, particularly in immunocompromised individuals.

2.10 Organisms used in dairy industries:

  • Various organisms play essential roles in the dairy industry for the production of dairy products:
    • Lactic acid bacteria, such as Lactococcus and Lactobacillus species, are used as starter cultures in the production of yogurt, cheese, and fermented dairy products. They help convert lactose into lactic acid, which contributes to the characteristic flavors and textures of these products.
    • Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus are commonly used together as starter cultures for making yogurt. They work synergistically to ferment lactose and produce the desired acidity and texture in yogurt.
    • Propionibacterium freuden



#1. The fungus known as bread mold is

c. Rhizopus

The fungus known as bread mold is scientifically classified as Rhizopus. Rhizopus is a common genus of filamentous fungi that belongs to the class Zygomycetes. It is commonly found in soil, decaying plant material, and various food sources, including bread.

When bread becomes exposed to moisture and warmth, it creates an ideal environment for the growth of Rhizopus. The spores of Rhizopus are present in the air and can easily settle on the bread’s surface. Once the spores find suitable conditions, they germinate and produce hyphae, which are thread-like structures that extend and spread across the bread.

Rhizopus hyphae grow rapidly and release enzymes that break down the complex carbohydrates present in the bread into simpler molecules. These enzymes help Rhizopus obtain nutrients from the bread. As the hyphae continue to grow, they form a network called mycelium, which becomes visible as fuzzy, black patches on the bread’s surface. These patches are the characteristic signs of bread mold.

Rhizopus reproduces both sexually and asexually. Asexual reproduction occurs through the production of sporangia, which are spherical structures that contain spores. The sporangia develop at the tips of specialized hyphae called sporangiophores. When the sporangia mature, they burst open, releasing countless spores into the surrounding environment. These spores can be easily dispersed through air currents, facilitating the spread of Rhizopus to new food sources.

While Rhizopus is responsible for bread mold, it is important to note that other fungi, such as Penicillium and Aspergillus, can also cause mold growth on bread under certain conditions. However, in the specific context of the question, Rhizopus is the most commonly associated fungus with bread mold.

#2. Pigmented Propionibacter causes

The correct answer for the multiple-choice question is c. color defects in cheese.

Pigmented Propionibacterium species are known to cause color defects in cheese. These bacteria are commonly found in certain types of cheese, such as Swiss cheese. During the aging process, Propionibacterium produces carbon dioxide gas, which forms characteristic holes or “eyes” in the cheese. Additionally, some strains of Propionibacterium can produce pigments that contribute to the yellow or orange coloration often seen in certain types of cheese. This color development is an important characteristic in the maturation of cheese and contributes to its flavor profile.

#3. The fungus known as bread mold is

The correct answer for the multiple-choice question is c. Rhizopus.

The fungus commonly known as bread mold is Rhizopus. Rhizopus is a genus of fungi that includes species like Rhizopus stolonifer, also known as black bread mold. It is a common mold that can grow on bread and other food items, particularly in warm and humid conditions. The presence of Rhizopus can lead to the growth of fuzzy, black spore-bearing structures on the surface of bread, resulting in its characteristic moldy appearance.

#4. Pigmented Propionibacter causes

The correct answer for the multiple-choice question is c. color defects in cheese.

Pigmented Propionibacterium species are known to cause color defects in cheese. These bacteria are commonly found in certain types of cheese, such as Swiss cheese. During the aging process, Propionibacterium produces carbon dioxide gas, which forms characteristic holes or “eyes” in the cheese. Additionally, some strains of Propionibacterium can produce pigments that contribute to the yellow or orange coloration often seen in certain types of cheese. This color development is an important characteristic in the maturation of cheese and contributes to its flavor profile.

#5. Red pigmentation on spoilt food is caused by

The correct answer for the multiple-choice question is d. Serratia.

Red pigmentation on spoilt food is commonly caused by the bacterium Serratia. Certain species of Serratia, such as Serratia marcescens, can produce a red pigment called prodigiosin. When present on spoiled food, the prodigiosin pigment can result in a distinctive red or pink discoloration. This bacterium is often associated with food spoilage and is known to thrive in various environments, including refrigerated conditions. It is important to practice proper food handling and storage to prevent the growth of spoilage bacteria like Serratia.

Genus Serratia Many species produce a pink or magenta pigment and may cause red
discolorations on the surface of foods. S. marcescens is the most common species.

#6. Aeromonas hydrophila is a

The correct answer for the multiple-choice question is c. human pathogen.

Aeromonas hydrophila is a bacterium that is known to be a human pathogen. It can cause a range of infections in humans, including gastrointestinal infections, wound infections, and septicemia. This bacterium is commonly found in aquatic environments, such as freshwater and estuarine systems. While Aeromonas hydrophila primarily affects humans, it can also cause infections in certain animals. It is important to handle and cook food properly, especially seafood, to reduce the risk of contamination and infection by this pathogen.

#7. E. carotovora causes

The correct answer for the multiple-choice question is b. bacterial soft rot.

E. carotovora, also known as Erwinia carotovora, is a bacterial species that is commonly associated with causing bacterial soft rot. This bacterium can infect a variety of plant tissues, leading to the breakdown and decay of the affected tissues. It can cause soft, mushy, and discolored areas in fruits, vegetables, and other plant-based materials. Bacterial soft rot can be detrimental to agricultural crops and stored produce, causing significant economic losses.

#8. The yellow-orange pigment of which genus causes discoloration on the surface of meat?

The correct answer for the multiple-choice question is a. Flavobacterium.

The yellow-orange pigment that causes discoloration on the surface of meat is typically associated with the genus Flavobacterium. Certain species within this genus, such as Flavobacterium lutescens, produce a pigment that can result in the yellow-orange discoloration of meat. This pigmentation is often considered a sign of meat spoilage and can be an indicator of bacterial contamination. It is important to handle and store meat properly to prevent the growth of spoilage bacteria and the development of discoloration.

#9. The genus Photobacterium causes

The correct answer for the multiple-choice question is a. phosphorescence of meat.

The genus Photobacterium is known for causing phosphorescence in meat. Certain species of Photobacterium, such as Photobacterium phosphoreum, possess bioluminescent properties. When these bacteria grow on meat, they can produce light, leading to a phenomenon known as phosphorescence. This can give the meat a shiny or glowing appearance, and it is considered a spoilage indicator as it can indicate the presence of bacterial contamination. It is important to note that phosphorescent meat is not safe for consumption and should be discarded.

Phosphorescence. This rather uncommon defect is caused by phosphorescent or luminous
bacteria, e.g., Photobacterium spp., growing on the surface of the meat.

Genus Photobacterium The genus includes coccobacilli and occasional rods which can be
luminescent. They are not widespread; however, P. phosphoreum has been known to cause
phosphorescence of meats and fish.

#10. Dairy mold is

Oospora (Geotrichum) spp. Oospora (Geotrichum) lactis, called the dairy mold, grows on
soft cheeses and during ripening sometimes suppresses other molds as well as surfaceripening bacteria. The curd gradually becomes liquefied under the felt. O. rubrum and O.
crustacea produce a red coloration, and O. aurianticum forms orange to red spots. O.
caseovorans causes “cheese cancer” of Swiss and similar cheeses. Bumps of growth become
filled with a white, chalky mass.

The correct answer for the multiple-choice question is b. Geotrichum.

Dairy mold is commonly associated with the genus Geotrichum. Geotrichum species are commonly found in dairy products, such as cheese and yogurt, and they can contribute to the ripening and flavor development of these products. They are filamentous fungi that can appear as white or off-white mold on the surface of dairy items. While some species of Geotrichum are considered beneficial in dairy production, others may cause spoilage or off-flavors if they grow excessively

#11. Trichothecium is commonly called

Trichothecium The common species, T. roseum ( Figure 2.14), is a pink mold which grows on
wood, paper, fruits such as apples and peaches, and vegetables such as cucumbers and
cantaloupes. This mold is easily recognized by the clusters of two-celled conidia at the ends of
short, erect conidiophores. Conidia have a nipplelike projection at the point of attachment, and
the smaller of the two cells of each conidium is at this end.


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