Taxonomy and Significance of Microorganisms in Food
Because
all of our foods originate from plant and animal sources, all foods will have
microorganisms associated with them that are involved in reducing the food to
inorganic compounds in order to perpetuate the gas and mineral cycles on earth.
When
we discuss the relative safety or potential for spoilage of a food, we need to
focus on two things:
1.
What are the total number of
microorganisms per gram or ml?
2.
What types of organisms are
represented in this number?
If we
know what kinds of organisms are associated with plant and animal foods in
their natural state, then we can usually predict the general types of microbes
that will occur on these foods at some later stage in processing and the types
of spoilage that may occur.
In the
past, classification of organisms into related groups was primarily based upon
numerical taxonomy, a system that arranged organisms on the basis of
phenotypical similarities and differences among them. Today, taxonomists try to group organisms phylogenetically,
a classification scheme that mirrors evolutionary (genetic) relationships.
Criteria
used to classify types of organisms include:
a.
Morphological/gram stain
b.
Biochemical characteristics
-e.g.
type of endospore, flagella, and types of substrates utilized.
This
is what the traditional numerical system was based upon. Although this information is still
valuable, it is now used primarily to support genetic similarities
c. Cell wall composition. e.g. type of peptidoglycan
monomer.
d.
Homology between 16S rRNA sequences:
-16S molecule (part of the
ribosome) about 1,500 bases long in bacteria
Because the 16S subunit
is an integral part of the ribosome and participates in protein synthesis in
all bacteria, some portions of this moleculesÕ sequence are highly
conserved. Sequence analysis of
divergent regions between 16S molecules serves as an excellent measure of
bacterial evolutionary divergence.
The subunits are collected and the enzyme reverse transcriptase is used
to obtain a DNA version of the molecule for sequencing. Computer analysis of sequence homology
between species is then performed to obtain an estimate of the relationship
between organisms. This technique
is so powerful that investigators have been able to identify disease-causing
bacteria which previously could not be grown in culture. This was done by using oligonucleotide
primers which anneal to portions of the 16S sequence which are conserved at the
kingdom level (e.g. in all prokaryotes) and then use PCR to amplify intervening
fragments of the 16S molecules from bacteria present in diseased tissue. The fragments were then sequenced to
identify the dominant bacterium in the tissue.
e. Serology; cell-wall antigens. e.g. group specific
antigens in streptococci.
f. Fatty acid profiles. Ratios of various fatty acids in the cell membrane are
characteristic of different species and some microorganisms produce unique
fatty acids.
g. Growth requirements
In
discussions of food microbiology, another useful way to group organisms is to
consider optimal growth temperatures, oxygen requirements, and particular
nutritional requirements or the nutrients that they are able to
metabolize. When combined with
true taxonomy, this system allows us to identify what species of microbes we
need to be concerned about in a particular food product or process. For example, we usually do not need to
worry about thermophilic species in refrigerated foods or whether species that
require O2 will spoil canned foods.
Primary Sources of Microorganisms in Food:
(HANDOUT
on Microbial Sources)
1. Soil and Water (Air and dust) - Grouped together because
of atmospheric cycling. Soil and
water are common sources of important pathogenic and spoilage microorganisms,
which is why it is important to thoroughly wash raw foods with good quality
water. Air and dust are important
sources of microorganisms during food processing and can influence food quality
in the home as well.
2. Plants and plant products - Although most soil and water
borne microbes will contaminate plants, very few types actually persist on
them. Those that persist, such as
lactic acid bacteria and some yeasts, must be able to adhere to the plant
material and to utilize it for growth.
3. Food Utensils - another important source for cross
contamination of raw and cooked foods e.g. cutting blocks, food trays where raw
food was held (BBQ plate).
4. Intestinal tracts of humans and animals - poor sanitation
practices (use of polluted water, poor personal hygiene) lead to contamination from
these sources. and many pathogens are transmitted by this route.
5. Food handlers - microbiota on hands, garments, etc.
reflects the habits of the individual.
This can include microorganisms from virtually any environmental source.
6. Animal feeds - very important source of Salmonella in poultry and of Listeria monocytogenes (from silage) in dairy and meat animals.
7. Animal hides - e.g. microbiota of raw milk influenced by
that of the udder.
(HANDOUT - Differences between Types of MO)
(HANDOUT - Common Food-borne bacteria)
Synopsis of Food-borne Molds and Yeasts:
Fungi
are an equally important group of microorganisms in food microbiology. They are utilized for the industrial
fermentation of beer and wine, production of enzymes (e.g. rennetÕs for cheese,
pettiness to clarify fruit juices and wine, amylases for bread industry, etc),
antibiotics, vitamins, and organic acids.
The baking industry and the certain types of cheese also depend on
fungal activities. Some fungi are
pathogens of plants, humans, or other animals. Other important fungal activities include the decay of
complex plant and animal remains or environmental pollutants.
Fungal pathogens cause about
70% of all major crop diseases!
Some of these outbreaks have been catastrophic:
-Potato blight fungus, Phytophthora
infestans, Ireland 1840, 1 million died of starvation, an equal number of
people were forced to emigrate to the U.S. or Europe.
-Great Bengal Famine, 1943, 2 million died of starvation due to a
severe outbreak of Helminthosporium
oryzae on the rice crops.
Fungal
Morphology:
Molds are filamentous
fungi. The individual filaments
are called hyphae and the mass of
these filaments is termed a mycelium. The mycelium is a multinucleate mass of
cytoplasm enclosed within a rigid, branch system of tubes. These tubes represent a protective
structure that is homologous to a cell wall of a unicellular organism. A mycelium usually arises from the germination
and outgrowth of a single reproductive cell, or spore. Upon germination, the spore puts out a
long threadlike hyphae which branches repeatedly as it elongates to form the
mycelium. The hyphae of higher
molds are compartmentalized by membrane divisions called septa. Septa are
usually perforated and allow passage of cytoplasmic fluid and even nuclei
between different parts of the hyphae.
Yeasts
are unicellular fungi that multiply by budding or by binary fission (like many
bacteria). Some fungi are
dimorphic; they are molds under certain environmental conditions and yeast-like
at others (e.g. Candida albicans, M
at 20-25oC
and Y at 37oC).
Unlike bacteria, fungi are
eukaryotic. They may reproduce by
both sexual or asexual means, but in either case spores are usually
produced. Sexually produced spores
are derived from the nuclei of parental cells. In general, the parent and the spores are haploid. Two nuclei from the parental cells fuse
to form a diploid zygote nucleus.
Haploid spore nuclei are derived from reductive nuclear division
(meiosis).
Molds: Most common habitat is soil and spores
are widely dispersed in dust.
(HANDOUT - Fungi of greatest importance to foods)
Physiology of molds and
yeasts:
Physiologically, fungi are
more adaptable to severe environmental stress than other microorganisms. They are more tolerant to osmolarity
extremes (osmophiles spoil syrup and jams), pH (acidophiles - acid tolerances
range within strains or species from pH 1.8 to 8.0), and drying (xerophiles -
some can grow at aw < 0.7).
Molds are strictly aerobic
(discuss Ca2+ lactate on cheese) but Yeasts can
utilize sugars such as glucose anaerobically (fermentation) or aerobically
(respiration). Under anaerobic
conditions, the major end products of yeasts are ethanol and carbon dioxide via
the alcoholic fermentation. Under
aerobic conditions, complete oxidation of glucose by yeasts accumulates acids,
alcohols, esters, glycerol, aldehydes and other products. Fungi exhibit a wide growth temperature
range but 20 to 30oC is optimal for most species. Some fungi can grow at 0oC and a few thermophilic molds grow as
high as 62oC.