Certificate IV in Food Technology
General Microbiology
Lecture Four
Main Groups of Bacteria
GRAM + BACTERIA
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Lecture
Rod shaped Gram Positive bacteria
Non-spore formers
Lactic acid bacteria
Streptococcus
Lactobacillus
Endospore formers
Clostridium
Bacillus
Mycoplasma
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Pleomorphic GRAM + Rods
The Coryneform Bacteria
Propionibacterium
Corynebacterium
Actinomycetes
Mycobacterium
Gram +ve Positive Rod shaped bacteria
Ratio of Nucleic Acid Base pairs in Bacterial DNA
An extra Classification tool for microbes (especially for Gram + Bacteria) is relative amounts of the bases found in bacterial DNA. i.e. the %age of G and C and %age of A and T.
eg. mol% (G + C) = (G + C)/(G + C + A + T) x 100 /1
or simply the percentage of guanine (G) and cytosine (C) in the DNA of the organism.
This is a quick method of
grouping organisms that relates to their genotype rather than phenotype but it
is very course gauge and not a complete picture
if an organisms genes. Two organisms can have similar %G+C but completely
different DNA sequences. GC ratios can be used to show how different organisms
are but not always useful to shoe how similar they are. The
information is only useful in conjunction with other tests
Gram + bacteria can be divided into
two groups using GC ratios
:
the Low GC group (mostly single rods) and
. the High GC group (mostly filaments)
The validity of this division is also supported by 16S rRNA sequence data.
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Single Gram + ve Rod shaped Bacteria (with low % G C) :
Lactic acid bacteria are fermentative bacteria that can usually tolerate O2 but can't usually use O2 in their metabolism. They are facultative anaerobes. Most of these live in rich environments (like milk) and have lost the ability to synthesize many amino acids and vitamins they need. Such organisms are referred to as being fastidious as they require complex media with any nutrients to grow.
We can further sub-divide lactic acid bacteria by how many types of molecules that they produce from the hexose sugar that they ferment.
Homofermentative which produce mainly lactic acids ( homo- meaning only one product)
Heterofermenters produce a mixture of products: lactic acid, ethanol and CO2
Many lactic acid bacteria are important in the food and medicine.
Main genera:
Streptococcus usually occur in chains . Very common inhabitants of the human body and foods. Some are pathogens (e.g. S. pyogenese). These are Homofermentative.
Lactobacillus. Usually rods and can live at lower pHs than Streptococcus spp. and thus are important in later stages of food fermentations (e.g. in sauerkraut and yogurt). They may be Homo- or heterofermentative.
Staphylococcus. Usually grape like clusters
S. aureus is a common inhabitant of the nose face and hands and some strains can cause food poisoning and toxic shock syndrome (TSS).
Endospore Forming Rods
Very common in soil and rare in aquatic ecosystems.. Most of the endospore formers are grouped into two genera, Bacillus and Clostridium.
Bacillus
All are aerobic or facultative
aerobes
e.g. B. cerus. B. coagulans
and B. stearothermophilus and B. anthracis are
problems in food
Clostridium
A huge genus (probably soon to be many genera...) of strict anaerobes. They are often distinguished by the substrates that they can ferment. Some very pathogenic (C. botulinumand C. tetani)
Endospores
Endospores survive for years in
a dormant state. There are some recent claims that endospores can survive for
millions of years . See this article on a Bacillus
sp. that may have been revived from 250 million year old salt
formations....
Endospores are a constant problem in the food industry because the make it
difficult to sterilize food because endospores are resistant to heat, radiation
and many disinfectants.
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Filamentous Gram +ve.
Bacteria ( with high % G C)
The high GC group can be broken into about ten major genera that were divided into the Coryneform Bacteria and the Actinomycetes.
Coryneform Bacteria
Propionibacterium spp. are so named because they break down lactic acid (the waste product of the lactic acid bacteria, see above) into propionic acid, acetic acid, and CO2. These bacteria are thus responsible for the flavor and holes of Swiss cheese.....
3 lactic acid ------ 2 propionic acid + 1 acetic acid + 1 CO2
Some species are very common in cattle rumens and on human skin. P. acnes is found in the sebaceous glands of all humans and are one reason that skin has a low pH, which inhibits pathogenic organisms. Excess growth can cause acne.
Corynebacterium spp. are common aerobic organisms of soil. One species, C. diphtheriae causes diphtheria, but only when the bacterium itself is infected by a specific phage (= a virus).
Bifidobacterium bifidus (Fig. 24.18) is an anaerobic bacterium that ferments a specific amino sugar found in breast milk and is therefore one of the initial colonists of the intestines of human babies.
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The
Actinomycetes.
This is a large group of mostly aerobic and include:
Mycobacterium spp. form waxy colonies and group together in cord-like masses
M. tuberculosis and is the cause of TB is a major reason for pasteurising milk and inspecting meat.
M. leprae is the cause of Leprosy