Main Groups of Bacteria
GRAM + BACTERIA
Rod shaped Gram Positive bacteria
Lactic acid bacteria
Pleomorphic GRAM + Rods
The Coryneform Bacteria
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
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.
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).
Filamentous Gram +ve. Bacteria ( with high % G
The high GC group can be broken into about ten major genera that were divided into the Coryneform Bacteria and the Actinomycetes.
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
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.
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