Food Microbiology &
Helen Billman-Jacobe
Important groups of bacteria- function
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Ray and Bhunia Ch 2
Intended learning outcomes
Identify and describe different categories of microbes with requirements for growth Give examples of microorganisms for each category of growth requirements Describe bacterial sporulation
Explain the procedures and application of endospore staining
List some compounds produced by bacterial metabolic processes
Important genera of bacteria
description
-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, Micrococcus, Lactococcus, Leuconostoc, Streptococcus
Gram-positive endospore forming rods
Bacillus, Clostridium
Gram-positive, non-sporing, regular rods
Lactobacillus, Listeria
Gram positive, non-sporing, irregular rods
Corynebacterium, Propionibacterium, Bifidobacterium
Cell shape: cocci, rods, helical
Cell wall: Gram positive, Gram negative
Gaseous atmosphere affecting growth: Aerobic, anaerobic, microaerophilic, facultative Ability to form spores: spore-former, non-sporing
Temperature: is specific to each organism
Groups of bacteria by function
Characteristic
Acid they produce
lactic, acetic, butyric, propionic
Substrate they degrade
proteolytic, lipolytic, saccharolytic
growth temp
psychro, meso, thermo
extremophiles
thermo, aciduric
cellular products
gas,slime, spore
oxygen requirement
aerobe, anaerobe , facultative
Growth temperature
Three main ‘temperature groups’ of microbes
Psychrophiles
optimum ~ 55°C; range 45-70°C optimum ~ 35°C ; range 10-45°C
optimum ~15°C ; range -5 to +20°C
Two more groups are also important in food microbiology:
Psychrotroph – can grow at refrigerator temperature (0-5°C) regardless of their optimum growth temperature.
Thermoduric – can survive pasteurization heat treatments.
Optimum oxygen concentration
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Oxygen gradient
Effect of pH on growth
Most bacteria are neutrophiles : pH ~7
Escherichia coli, staphylococci, and Salmonella spp. are neutrophiles and do not do well in the acidic pH of the stomach.
Microorganisms that grow optimally at pH less than 5.55 are acidophiles.
Lactobacillus contribute to their acidic environment by producing lactic acid
Alkaliphiles, grow best at pH-8.0 -10.5.
Vibrio cholerae, the pathogenic agent of cholera, grows best at the slightly basic pH but can survive pH11.0
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Growth temperature
optimum growth temperature: the temp where growth rates are the highest
minimum growth temperature: the lowest temperature at which the organism can survive and replicate
maximum growth temperature: highest temperature at which growth can occur
permissive growth temperatures: range where growth can occur
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Growth temperature: : “middle loving” -moderate temperatures -~20 -45°C
-normal human microbiota
– Human pathogens
E. coli, Salmonella spp and Lactobacillus spp
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Growth temperature: Thermophiles and thermodurics
Thermophiles: “heat loving” -high temperatures
-~50 -80°C
-do not multiply at room temp
Thermophiles are widely distributed in hot springs, geothermal soils, and manmade environments such as garden compost piles
Thermodurics grow at lower temps but can survive (but not grow) at elevated temps
Growth temperature: Psychrophiles and psychrotrophs
Psychrophiles: “cold loving”
-low temperatures
-~0 – 15°C
-do not survive at room temp
Psychrotrophs: prefer ~25 °C but will grow at refrigeration temperature about 4 °C.
– responsible for the spoilage of refrigerated food.
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Sporulation
Some bacteria are able to form endospores when environmental conditions are unfavourable for growth
Sporulation is the process by which vegetative cells change into endospores
Endospores protect the bacterial genome in a dormant state
Endospores survive long periods without nutrients or water, as well as exposure to chemicals, extreme temperatures, and even radiation
Endospores present a practical problem in the food industry because they are very difficult to kill
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Endospore staining
Endospores are clear when cells are stained with the Gram stain.
Endospore staining uses two stains to differentiate endospores from the rest of the cell. Heat is used push the primary stain, malachite green, into the endospore.
Washing with water decolorizes the cell, but the endospore retains the green stain.
The cell is then counterstained pink with safranin.
Endospore-staining techniques are important for identifying Bacillus and Clostridium, two genera of endospore-producing bacteria that contain clinically significant species.
Endospore staining
Important groups of bacteria
description
-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, Micrococcus, Lactococcus, Streptococcus
Gram-positive endospore forming rods
Bacillus, Clostridium
Gram-positive, non-sporing, regular rods
Lactobacillus, Listeria
Gram positive, non-sporing, irregular rods
Corynebacterium, Propionibacterium, Bifidobacterium
Acid producers
lactic, acetic, butyric, propionic
Degradative
proteolytic, lipolytic, saccharolytic
Growth temperature
psychro, meso, thermo
Cellular products
gas, slime, spore
Oxygen requirement
aerobe, anaerobe , facultative
Extremophiles
thermo, aciduric
Cellular products – gas
Cellular products: gas, slime, spore
Gas-producing bacteria: some bacteria produce gas as a product of their metabolism. CO2, H2, H2S Example of a gas producer
Propionibacterium freudenreichii is used to make Swiss cheese
• lactate converted to form acetate, propionate, and carbon dioxide
• the carbon dioxide is responsible for forming the holes in the cheese
CREATIV STUDIO HEINEMANN/GETTY IMAGES
Cellular products – slime
Cellular products: gas, slime, spore
Slime-producing bacteria: Slime producers synthesise polysaccharides. The function of the polysaccharides
to provide a protective coating to the cell examples of slime producers
Xanthomonas campestris produces a polysaccharide gum called xanthan which is used as a thickening agent in food processing. It is often used to create a pleasant texture of ice cream
Xanthan gum is produced by fermentation of glucose, sucrose, or lactose
HBJ UoM 2014
Cellular products – slime vs capsules
Capsules and slime layers.
Many bacterial cells secrete extracellular material in the form of a capsule or a slime layer.
Slime is loosely associated with the bacterium and can be easily washed off.
A capsule is attached tightly to the bacterium
The ability to determine whether cells have capsules is an important diagnostic tool. Capsules do not absorb most basic dyes; therefore, a negative staining technique (staining around the cells) is typically used for capsule staining.
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: proteolytic, lipolytic, saccharolytic
Large molecules cannot be taken up by the bacterial cells but small hydrolytic products can be absorbed by
the cells and used as nutrients.
Proteolytic microorganisms secrete enzymes which can hydrolyse proteins. Lipolytic organisms produce extracellular lipases and hydrolyse triglycerides Saccharolytic organisms hydrolyse complex carbohydrates.
These organisms can be important in food spoilage
Billman-Jacobe et al 1995 AEM
Acid producers
Propionic acid is produced by Propionibacterium freudenreichii. It imparts a nutty flavour to dairy fermentations
Acetic acid is produced by Acetobacter aceti and is used for making vinegar this
Lactic acid bacteria produce large amounts of lactic acid from carbohydrates. This group are very important in food microbiology and are often referred to as LAB. The species are mainly from the genera Lactococcus, Leuconostoc, Pediococcus, Lactobacillus and Streptococcus.
LAB will be studied in more detail later in the course
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