Food Microbiology &
Helen Billman-Jacobe
Introduction to microorganisms in food
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Ray and Bhunia Ch 2
Intended learning outcomes
Explain how living organisms are classified
Describe the naming system used for microorganisms
Use the correct format for writing the names of microorganisms
Use the terms which describe the morphology and structures of yeasts and moulds Give examples of genera of yeast and moulds that are important in food microbiology
Introduction
Bacteria, yeasts, moulds and viruses are important in food for their ability to cause foodborne disease and food spoilage and to produce food and food ingredients.
Food spoilage: many bacteria, moulds and yeasts are able to grow in foods and can potentially cause food spoilage.
Viruses cannot grow in food and therefore do not cause spoilage but are important because they can cause foodborne disease.
Protozoa, parasitic worms and some algae are important in foods as they may pose a health hazard. They generally are not able to grow in food.
Bacteria are the largest group of organisms that affect food quality and safety.
A phylogenetic tree is a way of showing how different organisms are thought to be related to one another from an evolutionary standpoint.
Classification
Living cellular organisms are classified on the basis of phylogenetic and evolutionary relationships
They are placed in three kingdoms • Bacteria
Viruses are not included in this classification because they are not cellular, self replicating organisms
The Archaea include most organisms that live in extreme environments such as thermal pools. They are not important to food microbiology
Microbiology, OpenStax
Classification
The basic taxonomic group fo the classification of yeasts, moulds and bacteria is the species
Phyla fall into the three kingdoms, Bacteria, Archaea and Eucarya
Several classes make a phylum
Several orders make a class
Several families make an order
Several genera make a family
Species with similar characteristics form a genus Strains are variations within a species
Ranks above species genus and family are rarely used in food microbiology
Kingdom Phylum Class Order Family Genus Species Strain
Bacteria Proteobacteria Enterobacteriaceae Escherichia
Binomial nomenclature
Each species of bacteria, yeasts and moulds are given a two-part (binomial) name
The first part is the genus name and the second is the species name
The names are generally based on Latin words
Both genus and species are written in italics or are underlined
The first letter of the genus is written with a capital letter
The species is written all with lowercase letters
Bacterial species can be divided into subspecies if Binomial nomenclature assigns organisms
they show consistent differences Latinized scientific names with a genus and
Each strain of a species is identified with a strain species designation. 7
Lactococcus lactis subsp. lactis Lactococcus lactis subsp. cremoris Lactococcus lactis subsp. cremoris Lactococcus lactis subsp. cremoris Lactococcus lactis subsp. cremoris Lactococcus lactis subsp. cremoris Lactococcus lactis subsp. cremoris Lactococcus lactis subsp. cremoris Lactococcus lactis subsp. cremoris Lactococcus lactis subsp. cremoris Lactococcus lactis subsp. cremoris Lactococcus lactis subsp. cremoris Lactococcus lactis subsp. cremoris
KW2 MG1363 NBRC 100676 NZ9000
SK11 TIFN1 TIFN3 TIFN5
Nomenclature
The scientific names of bacteria are decided by the International Committee on Systematic Bacteriology of the International Union of Microbiological Association
When writing the name of a bacterial species in an article authors usually write both the genus and species names in full the first time and then abbreviate the genus name to its first letter
For example.
“Large numbers of Escherichia coli were found in the undercooked hamburger. Analysis of the E. coli isolates revealed they were the pathogenic strain O157:H7.”
It can be confusing when writing about several species with the same first letter in the genus name. In this case a three letter system may be used
For example “Lactobacillus lactis and Lactococcus lactis are both lactic acid producers. We recommend the use of Lac. lactis as a starter culture for cheese Lab. lactis for other dairy fermentations .”
The microbial world
Microbiology, OpenStax
The microbial world
Microorganisms are found in each of the three domains of life: Archaea, Bacteria, and Eukarya.
Microbes within the domains Bacteria and Archaea are all prokaryotes (their cells lack a nucleus)
Microbes in the domain Eukarya are eukaryotes (their cells have a nucleus).
Some microorganisms, such as viruses, do not fall within any of the three domains of life.
Morphology and structure: yeasts and moulds
Yeasts and moulds are eukaryotic organisms and belong in the taxonomic group Fungi.
http://etc.usf.edu/clipart/25700/25744/mushroom_25744_md.gif
Morphology and structure: yeasts
Yeasts are unicellular eukaryotes
Yeast cells are 20-100μm compared to bacteria 1-10μm
The cell wall is composed of carbohydrates and does not have peptidoglycan
The cell membrane lies within the cell wall and contains sterols
The cytoplasm contains organelles (mitochondria and vacuoles ) which a membrane bound
The DNA is linear and enclosed in a nuclear membrane Cell division occurs by mitosis (a sexual reproduction) Sexual reproduction may occur by meiosis
Bacteria in counting chamber under x40 mag
Yeast cells in counting chamber under x40 mag
Morphology and structure: yeasts
Budding yeasts reproduce by mitosis in asymmetric manner
A bud forms on the parent cell and develops into a mature cell and eventually detaches
The cells can have scars where the progeny have detached
Fission yeasts reproduce by binary fission rather than budding. Binary fission occurs in a symmetrical manner
Yeast cells in counting chamber under x40 mag
. Blaisedell Our bodies and How We Live (Boston: Ginn &, 1904) 91
Morphology and structure: yeasts
Sometimes the detachment of progeny cells of budding yeast is delayed
Short chains of un-detached cells may be observed These groups of cells are called pseudo-hyphae
14 Dodge Elements of Continental Geography – Dodge’s Geography by Grades: Book II (Chicago, IL: Rand McNally & Company, 1904) 14
Morphology and structure: yeasts
Yeast cell walls have a plasma membrane consisting of a phospholipid bilayer
The lipids in a yeast membrane contain ergosterol.
This sterol does not occur in plant, animal or bacterial cells The rigidity of the yeast cell wall is due to the glucan / chitin outer layer
Important genera of yeasts
Saccahromyces species
S. cerevisiae :
• Used in beer and bread making
• “top-fermenting yeast” in beer making. • First eucaryote to be sequenced.
S. pastoranus:
“bottom fermenting” yeast
Schizosaccharomyces pombe
is a brewing yeast originally isolated from African millet beer.
Fission yeast forms a rod-shaped cell
grows by elongation and divides by fission.
doubling time of 2-4 hours
Used as a model organism for cell biology and genetics
Morphology and structure: yeasts and moulds
These cells have been stained with simple stain using crystal violet. The staining aids in the visualisation of cells under a microscope.
17 HBJ UoM 2015
Morphology and structure: moulds
Moulds are multicellular fungi
Moulds are non-motile, filamentous and branched The cell wall is composed of chitin (rigid)
The body of the mould is called the thallus
A thallus is made up of filaments called hyphae
A mass of hyphae is called a mycelium
. Curtis, Nature and Development of Plants ( : Company) 228 18
Morphology and structure: moulds
Hyphae are long filaments of cells
Hyphae grow by elongating at the tips
Septate hyphae: Cells are divided by cross-walls (septa).
Coenocytic (Aseptate) hyphae: Long, continuous cells that are not divided by septa.
. Curtis, Nature and Development of Plants ( : Company) 228 19
Morphology and structure: moulds
There are different types of hyphae
Vegetative hyphae: Portion that obtains nutrients.
Reproductive or Aerial Hyphae: Portion connected with reproduction.
. Curtis, Nature and Development of Plants ( : Company) 228 20 . Blaisedell Our bodies and How We Live (Boston: Ginn &, 1904) 85
Morphology and structure: moulds
Some fungi can reproduce asexually by fragmentation or production of asexual spores
Two types of mould spores can be produced asexually
–Conidiospores (conidia)-a unicellular or multicellular spore that is not enclosed in a sac
Lactophenol cotton blue stain, x 1000 HBJ
Medical Microbiology, 1990, Murray, et al.
Morphology and structure: moulds
Some fungi can reproduce asexually by fragmentation or production of asexual spores
Two types of mould spores can be produced asexually ––Sporangiospores–formed within a sporangium (sac).
Lactophenol cotton blue stain, x 1000 HBJ
Medical Microbiology, 1990, Murray, et al.
Important genera of moulds: oryzae is used to make sake and tempe
Aspergillus niger is grown to produce a variety of enzymes for food processing
Important genera of moulds: Penicillium
Examples of Penicillium species used to make cheese
Penicillium roqueforti Penicillium caseicolum Penicillium camemberti
Morphology and structure: yeasts and moulds
Take a moment to examine this microscope image
Use appropriate scientific terms to describe the structures
Is this a yeast or a mould?
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