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Lecture Four

Main Groups of Bacteria


Rod shaped Gram Positive bacteria

Non-spore formers
Lactic acid bacteria
Endospore formers

Pleomorphic GRAM + Rods

The Coryneform Bacteria
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 the %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.


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 report 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.

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.

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


The End

Prepared by Barry Brazier