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Certificate IV in Food Technology

General Microbiology

Lecture Four


Rod shaped Gram Positive bacteria

Non-spore formers

Lactic acid bacteria
Endospore formers


Pleomorphic GRAM + BACTERIA

The Coryneform Bacteria

Gram-Positive Bacteria

Ratio of Nuceic Acid Base pairs in Bacterial DNA

An extra Classification of microbes (especially for Gram + Bacteria).
mol% G + C = (G + C)/(G + C + A + T) x 100
or simply the percentage of guanine (G) and cytosine (C) in the DNA of the organism.
This is a quick method to group organisms, but remember that two organisms can have similar %G+C but completely different DNA sequences. Therefore it is only useful in conjunction with other tests and for groups that differ by say greater than 10% or so in G+C.
Using %GC we can divide most of the gram + bacteria into two groups: the High GC group (mostly filamentous) and the low GC group (mostly single-celled). The validity of this division is also supported by 16S rRNA sequence data.



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 them live in rich environments (like your throat) and have therefore lost the ability (through evolutionary time) to synthesize many amino acids and vitamins. Such organisms are sometimes referred to as being fastidious
We can further sub-divide them based on how many lactic acid molecules that they produce for each hexose sugar that they ferment. Homofermentative which produce mainly lactic acids
Heterofermenters produce 1 lactic acid, 1 ethanol and 1 CO2 (net 1 ATP per hexose fermented) (see figures 9.14 & 23.14).
One reason we know so much about these guys is that they are important in the food industry and medicine.
Main genera:
Streptococcus (strept = Gr. for twisted) usually occur in chains (see Fig. 20.4). Very common inhabitants of the humanbody and foods. Some pathogens (e.g. S. pneumoniae). 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). Homo- or heterofermentative.
More Low % GC gram + bacteria
Staphylococcus (staphyl = Gr. for a bunch of grapes) See Figure 23.17.
S. aureus is a common inhabitant of the nose and some strains can cause skin infections, especially in infants. S. aureus can also 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.
All are aerobic or facultative aerobes
e.g. B. cerus. B. coagulans and B. stearothermophilus and B. anthracis are problems in food
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 survive for years in a dormant state. There are some recent claims that endospores can survive for millions of years - but these claims are controversial. Read this article on a Bacillus sp. that was(?) revived from 250 million year old salt formations....
For more information on bacteria isolated from 40 million year old amber see: Greenblatt, C.L. et al. 1999. Diversity of microorganisms isolated from amber. Microbial Ecol. 38: 58-68:




The high GC group can be broken into about ten major genera and were traditionally divided into the Coryneform Bacteria and the Actinomycetes. Major genera of the Actinobacteria (Table 24.4)

Coryneform Bacteria
Propionibacterium spp. are so named because they ferment 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
(1 ATP per 3 lactic acids fermented)
Some species are very common in cattle rumens and on human skin. P. acnes is found in the sebaceous glands of all humans (up to 10,000,000 per square cm of skin) and are one reason that skin has a low pH, which inhibits pathogenic organisms.
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.
The Actinomycetes.
This is a large group of mostly aerobic, mostly filamentous, gram + bacteria with high %G+C.
Mycobacterium spp. form waxy colonies and group together in cord-like masses
. M. tuberculosisand the cause of TB is a major reason for pasteurising milk and inspecting meat.
M. leprae the cause of Leprosy

The End

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