WHAT ARE MICROORGANISMS
Microorganisms are tiny living things that are found all around us.They are too small to be seen by the naked eye.
They live in water, soil, and in the air. The human body is home to millions of these microbes too.
Some microbes make us ill, others are important for our health. The most common types are bacteria, viruses and
fungi. There are also microbes called protozoa. These are tiny living things that are responsible for diseases such as
toxoplasmosis and malaria.
Microorganisms comprise prokaryotes as well as some eukaryotic organisms, and these include bacteria, fungi,
viruses, protozoa, and algae. Food can be defined as anything that, when taken into the body, serves to nourish or
build up the tissues, or to supply body heat: essentially, an aliment or nutrient. This definition of food encompasses
all items that are consumed by man and other animals. Water, beverages, and all edible solid material are food. Food
sources are plankton, seaweed, fish, crustacea, mushrooms, molluscs, meat, poultry, fruits, vegetables, edible algae
and fungi, as well as grains.
Microbes are in contact with food whether it is on the farm or in the forest, garden, lake, pond, river, or sea. Food in
storage, industrial processing machines, supermarkets, warehouses, brewery holdings, restaurants, hotels and public
dining places all have, at one stage or another, harbored or been in contact with microorganisms.
A number of microorganisms are in themselves food or can be exploited and used as food. Some are contaminants
on food and food products, and a number can also cause food spoilage or produce toxins in foods leading
to foodborne illnesses in man and animals. Some microorganisms are also capable of transforming a food's
properties in a beneficial way such as the various food fermentation processes. Since there are a large number of
different microorganisms with diverse habitats and different attributes, food microorganisms will be classified here
in terms of the way in which they are associated with foods in different habitats, cause food spoilage, contaminate
food, or harm the consumer by the production of noxious metabolites. Microorganisms are ubiquitous and can
survive and grow at different temperatures at which food is either kept or processed. They exist from the coldest of
salty ponds in the frozen wasters of the polar regions to the boiling water of hot springs. Actively growing bacteria
may occur at temperatures in excess of 100 °C in thermal volcanic vents at the bottom of the deeper parts of the
oceans, in acid wastes from mines, or in alkaline waters of soda lakes. Microbes have been isolated from black
anaerobic silts of estuarine mud and also have been found in the purest waters of the biologically unproductive or
oligotrophic lakes. This means that, irrespective of the environment, microorganisms can find a way of adapting,
surviving, and multiplying at different temperatures, and they exist in a similar manner on food.
MICRO-ORGANISMS AND FOOD
The foods that we eat are rarely if ever sterile, they carry microbial associations whose composition
depends upon which organisms gain access and how they grow, survive and interact in the food over
time. The micro-organisms present will originate from the natural micro-flora of the raw material and
those organisms introduced in the course of harvesting/slaughter, processing, storage and distribution .
The numerical balance between the various types will be determined by the properties of the food, its
storage environment, properties of the organisms themselves and the effects of processing. In most
cases this microflora has no discernible effect and the food is consumed without objection and with no
adverse consequences.
In some instances though, micro-organisms manifest their presence in one of several ways:
(i) They can cause spoilage
(ii) They can cause food borne illness
(iii) They can transform a food’s properties in a beneficial way – food fermentation.
Food Spoilage/Preservation
From the earliest times, storage of stable nuts and grains for winter provision is likely to have been a
feature shared with many other animals but, with the advent of agriculture, the safe storage of surplus
production assumed greater importance if seasonal growth patterns were to be used most effectively.
Food preservation techniques based on sound, if then unknown, microbiological principles were
developed empirically to arrest or retard the natural processes of decay. The staple foods for most parts
of the world were the seeds – rice, wheat, sorghum, millet, maize, oats and barley – which would keep
for one or two seasons if adequately dried, and it seems probable that most early methods of food
preservation depended largely on water activity reduction in the form of solar drying, salting, storing in
concentrated sugar solutions or smoking over a fire.
Food Safety
In addition to its undoubted value, food has a long association with the transmission of disease.
Regulations governing food hygiene can be found in numerous early sources such as the Old Testament,
and the writings of Confucius, Hinduism and Islam. Such early writers had at best only a vague
conception of the true causes of foodborne illness and many of their prescriptions probably had only a
slight effect on its incidence. Even today, despite our increased knowledge, ‘Foodborne disease is
perhaps the most widespread health problem in the contemporary world and an important cause of
reduced economic productivity.’ (WHO 1992.) The available evidence clearly indicates that biological
contaminants are the major cause. Fermentation Microbes can however play a positive role in food.
They can be consumed as foods in themselves as in the edible fungi, mycoprotein and algae. They can
also effect desirable transformations in a food, changing its properties in a way that is beneficial.
MICROBIOLOGICAL QUALITY ASSURANCE
Food microbiology is unashamedly an applied science and the food microbiologist’s principal function is
to help assure a supply of wholesome and safe food to the consumer. To do this requires the synthesis
and systematic application of our knowledge of the microbial ecology of foods and the effects of
processing to the practical problem of producing, economically and consistently, foods which have good
keeping qualities and are safe to eat.
IMPORTANT MICROORGANISMS IN FOOD
A. Important Mold Genera
Molds are important in food because they can grow even in conditions in which many bacteria
cannot grow, such as low pH, low water activity (Aw), and high osmotic pressure. Many types of
molds are found in foods.5 They are important spoilage microorganisms. Many strains also produce
mycotoxins and have been implicated in foodborne intoxication. Many are used in food
bioprocessing. Finally, many are used to produce food additives and enzymes. Some of the most
common genera of molds found in food are listed here. Aspergillus. It is widely distributed and
contains many species important in food. Members have septate hyphae and produce black-colored
asexual spores on conidia. Many are xerophilic (able to grow in low Aw) and can grow in grains,
causing spoilage. They are also involved in spoilage of foods such as jams, cured ham, nuts, and
fruits and vegetables (rot). Some species or strains produce mycotoxins (e.g., Aspergillus flavus
produces aflatoxin). Many species or strains are also used in food and food additive processing. Asp.
oryzae is used to hydrolyze starch by a-amylase in the production of sake. Asp. niger is used to
process citric acid from sucrose and to produce enzymes such as b-galactosidase. Alternaria.
Members are septate and form dark-colored spores on conidia. They cause rot in tomatoes and
rancid flavor in dairy products. Some species or strains produce mycotoxins. Species: Alternaria
tenuis. Fusarium. Many types are associated with rot in citrus fruits, potatoes, and grains. They form
cottony growth and produce septate, sickle-shaped conidia. Species: Fusarium solani. Geotrichum.
Members are septate and form rectangular arthrospores. They grow, forming a yeastlike cottony,
creamy colony. They establish easily in equipment and often grow on dairy products (dairy mold).
Species: Geotrichum candidum. Mucor. It is widely distributed. Members have nonseptate hyphae
and produce sporangiophores. They produce cottony colonies. Some species are used in food
fermentation and as a source of enzymes. They cause spoilage of vegetables. Species: Mucor rouxii.
Penicillium. It is widely distributed and contains many species. Members have septate hyphae and
form conidiophores on a blue-green, brushlike conidia head (Figure 2.1). Some species are used in
food production, such as Penicillium roquefortii and Pen. camembertii in cheese. Many species
cause fungal rot in fruits and vegetables. They also cause spoilage of grains, breads, and meat.
Some strains produce mycotoxins (e.g., Ochratoxin A). Rhizopus. Hyphae are aseptate and form
sporangiophores in sporangium. They cause spoilage of many fruits and vegetables. Rhizopus
stolonifer is the common black bread mold.
B. Important Yeast Genera
Yeasts are important in food because of their ability to cause spoilage. Many are also used in food
bioprocessing. Some are used to produce food additives. Several important genera are briefly
described next.6 Saccharomyces. Cells are round, oval, or elongated. It is the most important genus
and contains heterogenous groups. Saccharomyces cerevisiae variants are used in baking for
leavening bread and in alcoholic fermentation. They also cause spoilage of food, producing alcohol
and CO2. Pichia. Cells are oval to cylindrical and form pellicles in beer, wine, and brine to cause
spoilage. Some are also used in oriental food fermentation. Species: Pichia membranaefaciens.
Rhodotorula. They are pigment-forming yeasts and can cause discoloration of foods such as meat,
fish, and sauerkraut. Species: Rhodotorula glutinis. Torulopsis. Cells are spherical to oval. They cause
spoilage of milk because they can ferment lactose (e.g., Torulopsis versatilis). They also spoil fruit
juice concentrates and acid foods. Candida. Many species spoil foods with high acid, salt, and sugar
and form pellicles on the surface of liquids. Some can cause rancidity in butter and dairy products
(e.g., Candida lipolyticum). Zygosaccharomyces. Cause spoilage of high-acid foods, such as sauces,
ketchups, pickles, mustards, mayonnaise, salad dressings, especially those with less acid and less
salt and sugar (e.g., Zygosaccharomyces bailii).
C. Important Viruses
Viruses are important in food for three reasons.8–10 Some are able to cause enteric disease, and
thus, if present in a food, can cause foodborne diseases. Hepatitis A and Norwalk-like viruses have
been implicated in foodborne outbreaks. Several other enteric viruses, such as poliovirus, echo
virus, and Coxsackie virus, can cause foodborne diseases. In some countries where the level of
sanitation is not very high, they can contaminate foods and cause disease.
Some bacterial viruses (bacteriophages) are used to identify some pathogens (Salmonella
spp., Staphylococcus aureus strains) on the basis of the sensitivity of the cells to a series of
bacteriophages at appropriate dilutions. Bacteriophages are used to transfer genetic traits in some
bacterial species or strains by a process called transduction (e.g., in Escherichia coli or Lactococcus
lactis).
Finally, some bacteriophages can be very important because they can cause fermentation
failure. Many lactic acid bacteria, used as starter cultures in food fermentation, are sensitive to
different bacteriophages. They can infect and destroy starter-culture bacteria, causing product
failure. Among the lactic acid bacteria, bacteriophages have been isolated for many species in the
genera Lactococcus, Streptococcus, Leuconostoc, and Lactobacillus; no bacteriophage of
Pediococcus is yet known. Methods are being devised to genetically engineer lactic starter cultures
so that they become resistant to multiple bacteriophages (see Chapter 13). D. Important Bacterial
Genera Bacterial classification is changing rapidly.1–3 In Bergey's Manual of Systematic
Bacteriology, published between 1984 and 1988, more than 420 bacterial genera are listed in 33
sections on the basis of their differences in characteristics. Since then, many other genera have
been created, such as Lactococcus (former N-group or dairy Streptococcus) and Carnobacterium
(some species previously included in Lactobacillus). In the ninth edition of Bergey's Manual of
Determinative Bacteriology (1993), more than 560 genera are listed in 35 groups. Of these, Table
2.1 lists 48 genera whose species are frequently associated with spoilage, health hazard, and
bioprocessing of food. Species of other genera besides these 48 can also be found in food, but their
relative significance is not well established. Many species names in several genera are also no longer
valid and thus not included in the current Bergey's Manual of Determinative Bacteriology. In this
text, only species and genera currently approved and listed in Bergey's Manual are used.
