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.