In this article we will discuss about:- 1. Types of Microorganism Associated with Food 2. Common Bacteria Present in Food 3. Popular Moulds and Yeast 4. Common Food Borne Yeasts.
Sources of Microorganisms in Foods:
The genera and species of organisms are the most important normally found in food products. Each genus has its own particular nutritional requirements, and each is affected in predictable ways by the parameters of its environment.
The primary sources of microorganisms in the food are explained below:
The soil contains the greatest variety of microorganism of any sources of contamination. The fertile soil contains large number of microorganisms which are contaminating the surfaces of plants growing on it and animals roaming over the land. Soil dust whipped by air current and soil particles are carried by running water to get into or on to foods.
The soil is important source of heat resistant spore forming bacteria. The most important types of organisms contaminating through soil are Bacillus, Clostriduim, E. coli, Enterobacter, Flavobacterium, Pseudomonas, Proteus, Leuconostoc, Chromobacterium, Acetobacter, etc.
Water is an important sources of microorganism especially coliforms which are indicator organisms for fecal contamination. Natural water contains the natural flora as well as the microorganism from soil, animals and sewage. Surface water like stream, pools and stored water like lake and large pond contain variety of microbial flora.
Groundwater from spring or wells has passed through layers of rock and soil contributes variety and kinds of microorganisms. The important kinds of bacteria in natural waters are Pseudomonas, Chromobacterium, Proteus, Micrococcus, Bacillus, Streptococcus, Enterobacter, and E. coli. The common waterborne bacteria are called coliforms. These are gram- negative, non sporulating facultative motile /non motile rods indicate water contamination. Hence these organisms are called indicator organisms. The bacterial flora of seawater is essentially gram-negative and gram-positive including many varieties.
Air is another source of contamination in food. Because it contributes dust, droplets, droplet nuclei, aerosols and suspended particle. Disease organisms especially those causing respiratory infections may spread by air. Microorganisms get into air on dust, or lint, dry soil, spray from streams, lakes, or oceans, droplets of moisture on walls, ceilings, floors, foods and ingredients.
The microorganisms in the air will not grow because the lack of nutrients but they will suspend in air for very long time. Fungal spores and bacterial spores are predominant in air, mould spores are more resistant to drying and persisting for very long time. Among bacteria cocci are predominant than rods. Yeasts are also present in air.
The number of microorganisms in the air may be depending on so many factors such as sunshine, location and the amount of suspended dust or spray. The number of organisms in air is increased by air currents caused by movements of people by ventilation and by breeze. Dry air contains more organisms than moist air. Rain or snow removes the organism from the air.
The natural surface flora of plants varies with the plant but generally include the species of Bacillus, Pseudomonas, Alcaligenes, Flavobacterium, Micrococcus, coliforms and lactic acid bacteria. The number of bacteria will depends on the plant and its environment and may range from a few hundred or thousand per square centimeter of surface to million.
The outer tissues of unwashed cabbages might contain 1 million to 2 million microorganisms per gram but washed and trimmed cabbage might contain fewer organisms. Exposed surface of plants become contaminated from soil, water, sewage, air, and animal so that microorganisms from these sources are added to the natural flora.
Plants are associated with the bacterial plant pathogens such as Corynebacterium, Curtobacterium, Pectobacterium, Pseudomonas and Xanthomonas; and fungal pathogens among several genera of moulds.
The sources of microorganisms from animals include the surface flora, the flora of respiratory tract and the flora of the gastrointestinal tract. Hides, hooves, and hair of animal contain large number of microorganisms from soil, manure, feed and water.
Similarly feathers and feet of poultry carry heavy contamination from soil and other sources. The skins of many meat animals contain Micrococcus, Staphylococcus, and β hemolytic Streptococcus. The feces and fecal contaminated products of animals can contain many enteric pathogens. Animal contributes their wastes and finally their bodies to the soil and water which contain large group of microbial flora.
So many pathogenic organisms are present in animal and animal products including Brucella sps, Mycobacterium tuberculosis, Coxiella burnetti, Salmonella typhi and paratyphi, Listeria monocytogens, Campylobacter jejuni, P haemolytic Streptococcus, enteopathogenic E. coil, Clostridium perfringens and Clostridium bolutinum.
The microbial flora on the hands and outer garments of handlers generally reflect the environment and habits of individuals and the organisms may be those from soil, water, dust and other environmental sources. Additional important sources are those that are common in nasal cavities, the mouth and on the skin, and those from the gastrointestinal tract that may enter food through poor personal hygiene practices. When vegetables are harvested in containers and utensils, it is expected to find some of the surface organisms on the products to contaminate contact surfaces.
As more and more vegetables are placed in the same containers, a normalization of the microbial flora will takes place. Similarly, the cutting block in a meat market along with cutting knives and grinders are contaminated from initial samples, and this process leads to contribute meat-borne organisms.
The different physiochemical and biological characteristics of bacteria make them important in food and which are very useful for the complete study of bacteria in food biotechnology.
The bacteria which are important in food biotechnology are explained briefly:
Gram-negative bacteria oxidize ethanol to acetic acid. They are rod shaped motile bacteria found on fruits and vegetables.
They are gram-negative, facultative anaerobic, psychrophilic rods and which are commonly found in fish, frog and other mammals.
They are gram-negative rod shaped bacteria present in feeds, soil, water and dust. The important species are Alcaligenes viscolatis, which produces ropiness in milk and Alcaligenes metalcaligenes gives a slimy growth on cottage cheese.
These organisms are gram-negative aerobic rods .The important species is Alteromonas putrefaciens.
It is a gram-positive bacterium predominant in soil.
It is a gram-positive spore forming aerobic or facultative anaerobic organism. Different species may be mesophilic or thermophilic, proteolytic or non proteolytic gas forming or not. The important species are Bacillus subtilis and Bacillus stearothermophilus (these are hyper thermophlic and spore producing bacteria).
It is also a gram-positive, bacteria produces orange red pigment and helps ripening.
These are gram-positive non spore forming bacteria found in many food items.
It is gram-negative, spiral shaped rods. The species Campylobater jejuni is associated with gastroenteritis in human.
These are gram-positive, spore forming rods. These are obligate (strict) anaerobes. Many species are capable of fermenting carbohydrate and produce acids and gases. Clostridium botulinum (causes botulism). Clostridium perfringens (causes gas gangrene and food poisoning) are the most important species in food biotechnology.
These are gram-positive, rod-shaped bacteria that are sometimes involved in the spoilage of vegetable and meat products. Most are mesotrophs and the important species, Corynebacterium diphtheriae, causes diphtheria in humans.
These enteric bacteria are slow lactose-fermenting, gram-negative rods that typically produce yellow colonies.
It is a gram-negative rod and inhabitants of the soil, fresh water and the rumen. It is a sulfur oxidizing bacteria.
It is a gram-negative, non-sporulating, motile and facultative anaerobic bacteria commonly referred as coliform and which are indicator organisms.
These are enteric gram-negative coliform bacteria like E. coli.
These are gram-negative enteric rods especially associated with plants. Erwinia carotovora is the most important organism responsible for spoilage of food.
These are gram-negative rods characterized by their production of yellow to red pigments on agar and by their association with plants. Some are mesotrophs, and others are psychrotrophs, where they participate in the spoilage of refrigerated meats and vegetables.
It is a gram-negative, rod shaped bacterium which can oxidize ethanol to acetic acid and can cause ropiness in beer.
These are gram-negative enteric rods important in the spoilage of refrigerated meat and vegetable products.
These are obligate halophiles and causes discoloration on the foods high in salt such as salted fish.
These are gram-negative, non-sporulating, non-motile and facultative anaerobic bacteria commonly referred as coliforms and which are indicator organisms. Klebsiella pnuemoniae is the causative organism for bacterial pneumonia in human.
These are gram-positive, rod shaped, microaerophilic bacteria typically occur on most vegetables, along with some of the other lactic acid bacteria. Their occurrence in dairy products is common. It can ferment the carbohydrate lactose and can produce acids and gas. The important species are Lactobacillus bulgaricus, Lactobacillus delbrueckii, Lactobacillus pentosus, Lactobacillus acidophilus and Lactobacillus casei.
They are gram-positive bacterium. The most important species is Leuconostoc mesenteroides.
These are gram-positive and non-sporing rods. The most important organism is Listeria monocytogens which are causing the spoilage to fish and fish products.
These are gram-positive cocci and are inhabitants of mammalian skin and can grow in the presence of high level of salt.
These short gram-negative rods they do not form acid from glucose. The most important species is Moraxella bovis.
These are gram-positive rods. The most important species are Mycobacterium tuberculosis (causative agent of tuberculosis), Mycobacterium leprae (causative agent of leprosy) and Mycobacterium bovis.
These are gram-positive, homofermentative cocci that exist in pairs and tetrads. These are lactose fermenting bacteria.
These enteric gram-negative rods are aerobes that often display pleomorphism. These organisms are motile and typically produce swarming growth on the surface of moist agar plates. They are typical of enteric bacteria in being present in the intestinal tract of humans and animals. One important species is Proteus vulgaris.
These are gram-negative rod shaped motile bacteria. These are typical soil and water bacteria and they are widely distributed among fresh food especially vegetables, meats, poultry, and seafood products. These are one of the largest food borne bacteria.
These are coccobacilli and can cause phosphorescence of meat and fish.
These are gram-positive, small, nonmotile rod shaped bacteria can ferment carbohydrates. These bacteria are commonly present in cheese and cheese related products.
These are gram-negative enteric bacteria are considered to be human pathogens. The most important pathogen is Salmonella typhi and Salmonella paratyphi. These organisms cause enteric fever.
These gram-negative rods that belong to the family Enterobacte- riaceae are aerobic and proteolytic and produce red pigments on culture media and in certain food. Serratia liquefaciens and Serratia marcescens are the most prevalent of the food borne species; it causes spoilage of refrigerated vegetables and meat products.
Shigella is a gram-negative, non-sporulating, motile and facultative anaerobic bacteria and can cause bacillary dysentery in human. Shigella dysenteriae is the causative organism. Other Shigella species are Shigella flexneri, Shigella boydii and Shigella sonnei.
These gram-positive, cocci occur in the form of grape like clusters and which include Staphylococcus aureus, causes several diseases in human including food borne gastroenteritis.
These are also gram-positive cocci which occur in the form of long or short chains. It will cause pyogenic infection in human (Streptococcus pyogens).
These are gram-negative straight or curved rods. The most important organism is Vibrio cholerae which causes the disease cholera.
These are gram-negative rods and include the causative agent of human plague, Yersinia pestis and some species are causing food borne gastroenteritis
Fungus are eukaryotic spore producing organisms capable reproducing both sexual and asexual manner. Fungi are commonly classified into two: unicellular fungi—yeast and multicellular fungi—mould. Fungi belong to plant kingdom – myceteae. These are multicelluar, nonphotosynthetic organism with chemoheterotrophic mode of nutrition. Fungi lacks roots, stem, or leaves and devoid of chlorophyll.
Moulds regarded as eumycetes or true fungi. Moulds are filamentous fungi that grow in the form of a tangled mass that spreads rapidly and may cover several inches of area in 2 to 3 days. The total of the mass or any large portion of it is referred to as mycelium. Mycelium is composed of branches or filaments referred to as hyphae. Groups of hyphae are referred as mycelium. Fungi are greatest importance in food multiply by ascospores, zygospores and conidiospore.
The major identification features of moulds in food are listed below:
1. Whether the hyphae is septate or nonseptate.
2. Mycelium is clear or dark.
3. Mycelium is colored or colorless.
4. Sexual spores present or not. The most important type of sexual spores are zygospore, ascospore, oospore and basidiospore.
5. Types of asexual spore – sporangiospore, conidiospore, arthrospore, blastospore and chlamydospore.
6. Characteristics of spore head.
7. Features of sporangia – size, color, shape and location.
8. Features of spore head and conidia— single, chain, budding, masses, shapes and arrangement of sterigmata.
9. Appearance of asexual spores— single or branched, type of branching, size and shape of collumella at the tip of sporan- giophore, conidiophores are single or bundles.
10. Microscopic appearance of asexual spores (conidia) — shape, size, color, smooth or rough, one or two or many celled.
11. Presence of special structures (or spore) — stolons, rhizoids, foot cell, chlamydospores, sclerotin, etc.
Class – Zygomycetes
Genus – Rhizopus, Mucor, etc.
These fungi have nonseptate hyphae and asexual reproduction is by sporangiospore. But sexual reproduction is by special types of sopres called zygospore. It will grow rapidly compared to other group.
Class – Plectomycetes
Genus – Eurotium, Aspergillus, Eupenicillium, Penicillium, Emericella, Neurospora, etc.
They possess septate mycelium and will reproduce both sexual and asexual manner. The sexual reproduction by the formation of sexual spore called ascospores and asexual reproduction is by conidiospore.
Class – Coelomycetes
Genus – Alternaria, Aureobasidium (Pullularia), Botrytis, Cladospo- rium, Fusarium, Geotrichum, Helminthosporium, Trichothecium, Mo- nascus, Cephalosporium, Sporotrichum, Trichoderma, etc.
Yeasts are unicellular fungus. It can be differentiated from bacteria by their larger size, oval, elongated-spherical cell shape.
Yeasts are classified into two:
1. True yeast- Ascomycotina
2. False yeast- Deuteromyces or fungi imperfecti
Classification and Identification of Yeast in Food:
The classification and identification of yeast based on –
1. Whether ascospore are produced
2. If they are spore forming
(i) Method of production of ascospore -without conjugation of yeast cell (parthenogenetically) or with the conjugation of yeast cell.
(ii) Appearance of ascospore – shape, size and color
(iii) Usual number of ascospores per ascus – one, two, four or eight.
3. Appearance of vegetative cell – shape, size, color and inclusions
4. Method of asexual reproduction
(ii) Binary fission
(iii) Combined budding and fission
(iv) Arthrospore (oidia)
5. Production of pseudomycelium, mycelium or no mycelium
6. Growth pattern— growth throughout the media or film over the surface of liquid
7. Color of the colony
8. Physiological characteristics
(i) Carbon or nitrogen sources
(ii) Vitamins requirement
(iii) Oxidative or fermentative – film yeast are oxidative, other yeasts are fermentative
(iv) Lipolysis, urease activity, acid production and production of starchy material
1. True Yeast:
This type of yeast produces ascospore and arthrospores. Vegetative reproduction of such types of yeast is taking place by fission and budding.
Family – Saccharomycetaceae.
This family is classified into two subfamilies:
(i) Kluyveromyces- Kluyveromyces marxianus, K. fragilis and K. lactis
(ii) Pichia – It is the largest fungal yeast
(iii) Saccharomyces – S. cerevisiae
(v) Torulaspora – T. delbreuckii
(vi) Zygosaccharomyces – Z. rouxii
ii. Subfamily B – Schizosaccharomycetoideae:
Schizosacchciromyces- S. pombe
False yeast is reproducing by budding. These are fungi imperfecti because their sexual stages of cell division are not yet identified.
Division – Deuteromycotina:
Family – Cryptococcaceae:
(i) Candida- C. albicans, C. guilliermondii, C. lipolytica
(ii) Cryptococcus- C. neoformans
(iii) Rhodotorula- R. mucilaginosa (pigment producing yeast), R. glutinis