留学生辅导 Food Microbiology &

Food Microbiology &
Helen Billman-Jacobe

Introduction to microorganisms in food-Bacteria

Ray and Bhunia Ch 2

Intended learning outcomes
Use the correct format for writing the names of microorganisms
Use the terms which describe the morphology and structures of bacteria Give examples of genera of bacteria that are important in food microbiology

Morphology and structure: bacteria
Bacteria are prokaryotes
They are small single cells, approximately 1 x 2-10μm in size
Usually have a single, circular DNA chromosome, 3-4 Mbp
Genome sits in the cytoplasm, so replication, transcription and translation all occur in cytoplasm
Some genetic information may also be contained in smaller circles of DNA, called plasmids (often able to transferred to other cells)
No organelles (ie. no nucleus, no mitochondria, no chloroplasts, no Golgi, etc).
Bacterial ribosomes are present but simpler than eukaryotes

Morphology and structure: bacteria
Bacterial cells have three morphological forms Cocci spherical
Bacilli rod shaped
Comma curved
The cells can be in groups such as
Tetrad a group of four
Clusters as illustrated by the staphylococci
chains of two or more cells
Chains of cocci are called streptococci
Bacilli aligned on their long side are described as palisades

Morphology and structure: bacteria
How would you describe the shape and arrangement of the bacterial cells in the image on the right?
Microbiology, OpenStax

Morphology and structure: bacteria
The bacterial cytoplasm is encased in a rigid cell wall on the surface and an inner plasma membrane
They may have hair like fimbriae which they used to attach to surfaces
The pilus is used as a channel to exchange genetic information between mating pairs of bacteria
Some bacteria are able to swim and they are called motile cells
Motile bacteria have flagella which propel them through liquid
Microbiology, OpenStax

Morphology and structure: bacteria
Bacteria that have flagella possess a special hook structure to anchor the flagella to the cell wall that this still allows the filament to rotate and act like propeller to push cells through liquid.
Motile bacteria use flagella to swim towards nutrients and away from toxic compounds
Microbiology, OpenStax

Directional movement
Microbiology, OpenStax

Microbiology, OpenStax

Morphology and structure: bacteria
The bacterial cytoplasm does not contain organelles Ribosomes are present
The chromosome occurs as a condensed mass of DNA called the nucleoid
The genetic material consists of a circular chromosome and plasmids
Some bacteria are able to form endospores
Microbiology, OpenStax

Morphology and structure: bacteria
Bacterial cells are surrounded by a cell wall.
The wall is comprised of peptidoglycan of the cell wall determines cell shape
Bacteria are grouped together as Gram-negative or Gram-positive depending on their response to the Gram stain
The structure of the cell wall differs markedly between Gram positive and Gram negative bacteria
This difference is one of the most important phenotypes used for classification and identification purposes, e.g. the Gram stain
Gram negative bacteria
Gram positive bacteria

Morphology and
structure: bacteria
Both Gram-positive and Gram-negative bacteria have an in a plasma membrane comprised of a phospholipid bilayer
Gram-positive cells have thick cell walls comprised of several layers of peptidoglycan and two types of teichoic acids.
Gram-positive cells retain the dye of a Gram stain and appear purple under the microscope
Gram positive cell wall
14 Tortora et al 2014

Morphology and
structure: bacteria
Both Gram-positive and Gram-negative bacteria have an in a plasma membrane comprised of a phospholipid bilayer
Gram-negative bacteria have walls comprised of a thin layer of peptidoglycan surrounded by an outer membrane.
The outer membrane provides an extra barrier property to gram negative cells
The thin layer of peptidoglycan is insufficient to retain the die of a Gram stain and the cells need to be counter stained. They appear pink under the microscope
Tortora et al 2014
Gram negative cell wall

Difference between Gram positive and Gram negative cell walls
Gram-positive cells have a cell wall consisting of many layers of peptidoglycan totaling 30–100 nm in thickness.
Microbiology, OpenStax

Peptidoglycan
Microbiology, OpenStax

Principles of the Gram stain
Alcohol washes stain out of Gram negative cells
Gram positive
Crystal violet stains cells purple
Iodine makes the CV less soluble so it sticks to cells walls
Safranin counterstain allows staining of Gram negative cells
Gram negative

Gram stain
Examine the image of Gram stained bacteria
Use scientific terms to describe the shape and arrangement of the cells

Important genera of bacteria
description
Genera (examples)
Gram-negative, aerobic/microaerophilic, motile, helical
Campylobacter
Gram-negative, aerobic, rods and cocci
Pseudomonas, Xanthomonas
Gram-negative, facultative anaerobic, rods
Escherichia, Klebsiella, Salmonella
Gram-positive, cocci
Staphylococcus
Gram-positive endo- spore forming rods and cocci
Bacillus, Clostridium
Gram-positive, non-sporing, regular rods
Lactobacillus, Listeria
Gram positive, non-sporing, irregular rods
Corynebacterium, Propionibacterium, Bifidobacterium

Morphology and structure: bacteria
Some bacteria are able to form endospores.
Endospore-forming bacteria are very important in food microbiology because the spores enable them to survive in the food environment
The image shows endospores in Bacillus staining bright blue
There is one endospores per cell
MDS/CSU. Gram stain of Bacillus ciculans

Optimum oxygen concentration
Microbiology, Openstax
Oxygen gradient

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