Characteristics Of Microorganisms

Morphological Characteristics

One of the first step in the identification of bacteria in food is microscopic
examination to ascertain the shape, size, aggregation, structure and staining reactions of the bacteria present. The following characteristics may be of
special significance:
Encapsulation: The presence of capsules or slime may account for sliminess or
ropiness of a food.

Characteristics Of Microorganisms


Most capsules are polysaccharides of dextrin, dextran or
levan and they serve as a source of reserve nutrients and increase the resistance
of bacteria under adverse conditions.
Formation of Endospores: Bacteria of genera Bacillus, Clostridium,
Sporosarcina etc have the ability to form endospores. Endospores are formed
at an intracellular site and are resistant to heat, ultraviolet light and dessication.

Lysis of the vegetative cell releases the free endospore, which may remain
dormant with no detectable metabolism for years. Sporulation usually appears
in the late logarithmic phase of growth, possibly because of nutrient depletion
or product accumulation. The acquisition of heat resistance is closely related to
the formation of dipicolinic acid and the Ca2+ uptake. Germination is favoured
by conditions that are favourable for growth.
Formation of Cell Aggregates: 

It is characteristic of some bacteria to form
long chains or of others to clump under certain conditions. It is more difficult
to kill all bacteria in intertwined chains or sizable clumps than to destroy
separate cells.
Cultural Characteristics
Bacterial growth in and on foods often is extensive enough to make the food
unattractive in appearance or otherwise objectionable. 

Pigmented bacteria
cause discolouration on the surfaces of foods; films which may cover the
surfaces of liquids; growth may make surfaces slimy; or growth throughout the
liquids may result in undesirable cloudiness or sediment.
Physiological Characteristics
Most bacteria may be placed into one of three groups based on their response
to gaseous oxygen. 

Aerobic bacteria thrive in the presence of oxygen and
require it for their continued growth and existence. Other bacteria are
anaerobic, and cannot tolerate gaseous oxygen, such as those bacteria which
live in deep underwater sediments, or those which cause bacterial food
poisoning. The third group are the facultative anaerobes, which prefer growing
in the presence of oxygen, but can continue to grow without it. 

Bacteria may also be classified both by the mode by which they obtain their
energy. Classified by the source of their energy, bacteria fall into two
categories: heterotrophs and autotrophs. Heterotrophs derive energy from
breaking down complex organic compounds that they must take in from the
environment − this includes saprobic bacteria found in decaying material, as
well as those that rely on fermentation or respiration. 

The other group, the autotrophs, fix carbon dioxide to make their own food
source; this may be fueled by light energy (photoautotrophic), or by oxidation
of nitrogen, sulfur, or other elements (chemoautotrophic). 

chemoautotrophs are uncommon, photoautotrophs are common and quite
diverse. They include the cyanobacteria, green sulfur bacteria, purple sulfur
bacteria, and purple nonsulfur bacteria. The sulfur bacteria are particularly
interesting, since they use hydrogen sulfide as hydrogen donor, instead of
water like most other photosynthetic organisms, including cyanobacteria.

Microbe is a term for tiny creatures that individually are too small to be seen
with the unaided eye. Microbes include bacteria (back-tear-ee-uh), archaea
(are-key-uh), fungi (fun-jeye) and protists (pro-tists). You’ve probably heard of
bacteria and fungi before. 

Archaea are bacteria-like creatures that have some
traits not found in any true bacteria. Protists include primitive algae (al-gee),
amoebas (ah-me-buhs), slime molds and protozoa (pro-toe-zoh-uh). We can
also include viruses (vye-rus-is) as a major type of microbe, though there is a
debate as to whether viruses can be considered living creatures or not.

General Characteristics

The term “mold” is a common one applied to certain multicellular, filamentous
fungi whose growth on foods usually is readily recognized by its fuzzy or
cottony appearance. The main part of the growth commonly appears white but
may be coloured or dark or smoky. Coloured spores are typical of mature mold
of some kinds and give colour to part or all of the growth. The thallus, or
vegetative body, is characteristic of thallophytes, which lack true roots, stems
and leaves. 

Morpohological Chactacteristics

 The morphology, i.e. the form and structure, of molds, as judged by their
macroscopic and microscopic appearance, is used in their identification and
Hyphae and Mycelium: The mold thallus consists of a mass of branched,
intertwined filaments called hyphae (singular hypha), and the whole mass of
these hyphae are known as the mycelium.
Reproductive Parts or Structures: Molds can grow from a transplanted piece
of mycelium. Reproduction of molds is chiefly by means of asexual spores.
Some molds also form sexual spores. 

Culture Characteristics

 The gross appearance of a mold growing on a food often is sufficient to
indicate its class or order. Some molds are loose and fluffy; others are
compact. Some look velvety on the upper surface, some dry and powdery, and
others wet or gelatinous. Some molds are restricted in size, while others seem
limited only by the food or container. Pigments in the mycelium – red, purple,
yellow, brown, gray black, etc. – are characteristic, as are the pigments of mass
of asexual spores; green, blue-green, yellow, orange, pink, lavender, brown,
gray, black, etc. 

Physiological Characteristics

 The physiological characteristics of molds will be reviewed only briefly here
and will be discussed in more detail subsequently.
Moisture Requirements: In general most molds require less available moisture
than do most yeasts and bacteria. It has been claimed that below 14 to 15
percent total moisture in flour or some dried fruits will prevent or greatly delay
mold growth.
Temperature Requirements: Most molds would be considered mesophilic i.e.
able to grow well at ordinary temperature. The optimal temperature for most molds is around 25 to 30°C, but some grow well at 35 to 37°C or above, e.g.
Aspergillus spp. And some at still higher temperatures. 

A number of molds are
psychrotophic or psychroduric i.e. they grow fairly well at temperatures of
refrigeration, and some can grow slowly at temperatures below freezing.
Growth has been reported at as low as – 5 to 10°C. A few are thermophilic; i.e.
they have a high optimal temperature.
Oxygen and pH Requirements Molds are aerobic; i.e. they require oxygen for
growth; this is true at least for the molds growing on foods. 

Most molds can
grow over a wide range of hydrogen-ion concentration (pH 2 to 8.5), but the
majority are favoured by an acid pH.
Food Requirements: Molds in general can utilize many kinds of foods, ranging
from simple to complex. Most of the common molds possess a variety of
hydrolytic enzymes, and some are grown for their amylases, pectinases,
proteinases, and lipases.
Inhibitors: Compounds inhibitory to other organisms are produced by some
molds, such as penicillin from Penicillium chrysogenum and clavacin from
Aspergillus clavatus. 

Certain chemical compounds are mycostatic, inhibiting
the growth of molds (sorbic acid, propionates, and acetates are examples), or
are specifically fungicidal, killing molds.
Initiation of growth of molds is slow compared to that of bacteria or yeasts, so
that when conditions are favourable for all these organisms, molds usually lose
out in the competition. After mold growth is under way, however, it may be
very rapid

Like mold, the term “yeast” is commonly used but hard to define. As used here
it refers to those fungi which are generally not filamentous but unicellular and
ovoid or spheroid and which reproduce by budding or fission.
Yeasts may be useful or harmful in foods. Yeast fermentations are involved in
the manufacture of foods such as bread, beer, wines, vinegar, and surfaceripened cheese, and yeasts are grown for enzymes and for food. Yeasts are
undesirable when they cause spoilage of sauerkraut, fruit juices, syrups,
molasses, honey, jellies, meats, wine, beer, and other foods. 

Morphological Characteristics

 Form and structure: The form of yeasts may be spherical to ovoid, lemonshaped, pear-shaped, cylindrical, triangular, or even elongated into a false or
true mycelium. They also differ in size.
Reproduction: Most yeasts reproduce asexually by multilateral or polar
budding, a process in which some of the protoplasm bulges out the cell wall;
the bulge grows in size and finally walls off as a new yeast cell. 

A new species
or yeasts reproduce by fission, and one reproduces by combination of fission
and budding.
Sexual reproduction of “true” yeasts (Ascomycotina) results in the production
of ascospores, the yeast cell serving as the ascus. The ascospores may differ in
colour, in smoothness or roughness of their walls, and in their shape (round,
oval, reniform, bean or sickle-shaped, hemispherical, angular, fusiform, or

“False” yeasts, which produce no ascospores or other sexual spores, belong to
the Fungi Imperfecti. Cells of some yeasts become chlamydospores by
formation of a thick wall about the cell, for example, Candida, Rhodotorula,
and Cryptococcus. 

Cultural Characteristics

 For the most part, the appearance of massed yeast growth is not useful in the
identification of yeasts, although growth as a film on the surface of liquid
media suggests an oxidative or film yeasts, and production of a carotenoids
pigment indicates the genus Rhodotorula. 

However, the appearance of the
growth is important when it causes coloured spots on foods.
Yeasts are oxidative, fermentative, or both. The oxidative yeasts may grow as a
film, pellicle, or scum on the surface of liquid and then are termed film yeasts.
Fermentative yeasts usually grow throughout the liquid and produce carbon

Physiological Characteristics

 Most common yeasts grow best with a plentiful supply of available moisture.
But since many yeasts grow in the presence of greater concentration of solutes
(such as sugar or salt) than most bacteria it can be concluded that these yeasts
require less moisture than the majority of bacteria. 

Most yeast require more
moisture than molds, however, on the basis of water activity or aw yeasts may
be classified as ordinary if they do not grow in high concentrations of solutes,
i.e. in a low aw, and as osmophilic if they do. However limits of aw for
ordinary yeasts tested thus far ranges from 0.88 to 0.94.

 The range of temperature for growth of most yeasts is, in general, similar to
that for molds, with the optimum around 25°C to 30°C and the maximum
about 35°C to 47°C. Some kinds can grow at 0°C or less. The growth of most
yeasts if favoured by an acid reaction in the vicinity of pH 4 to 4.5, and they
will not grow well in an alkaline medium unless adapted to it. 

Yeasts grow best
under aerobic conditions, but the fermentative types can grow anaerobically,
although slowly.
In general, sugars are the best source of energy for yeasts, although oxidative
yeasts, e.g., the film yeasts, oxidize organic acids and alcohol. 

Carbon dioxide
produced by bread yeasts accomplishes the leavening of bread, and alcohol
made by the fermentative yeasts is the main product in the manufacture of
wines, beer, industrial alcohol, and other products. The yeasts also aid in the
production of flavors or “bouquet” in wines. 

Nitrogenous foods utilized vary from simple compounds such as ammonia and
urea to amino acids and polypeptides. In addition, yeasts require accessory
growth factors.
Microorganisms, namely, bacteria, yeasts and molds can be found in any

The eight environmental sources of organisms to foods are: soil
and water, plants and plant products, food utensils, intestinal tracts of humans
and animals, food handlers, animal feeds, animal hides, air and dust. 

we see that the microorganisms are beneficial to the humans in many ways,
there are many microorganisms that are the causative agents for food borne
diseases. e.g. Staphlococcus aureus and Clostridium botulinum cause food
borne intoxication whereas Salmonella, E.coli, Campylobacter, Listeria, Yersinia, Bacillus etc cause food borne infections. Molds are responsible for
causing food intoxication by production of mycotoxins, which are lethal for the
human body.

Leave a Comment