Microbiology, Lec 6.

Outline of lec. 6
Low %G+C GRAM + BACTERIA
Lactic acid bacteria

Streptococcus
Lactobacillus
Yogurt Break

Clostridium
Bacillus

Arthrobacter
Propionibacterium
Corynebacterium
Bifidobacterium

Mycobacterium
Streptomyces
Actinomyces
Micrococcus
Frankia

Gram-Positive Eubacteria (Chapter 20)

Before we dive into these guys too deeply we need to discuss an important method used in the classification of microbes (especially Gram + Bacteria).

% 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.

Some methods for determining GC ratios are given on pages 390 - 393.

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 critters). The validity of this division is also supported by 16S rRNA sequence data.

Fig. 20.1

LOW % G + C GRAM + GROUP:

Lactic acid bacteria are fermentative bacteria that can usually tolerate O2 but can't usually use O2 in their metabolism. 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. The homofermenters produce 2 lactic acids, whereas the heterofermenters produce 1 lactic acid and 1 ethanol (see figures 20.2 and 20.3).

One reason we know so much about these guys is that they are important in the food industry and medicine. (See Table 20.3)

Main genera:

Streptococcus (strept = Gr. for twisted) usually occur in chains (see Fig. 20.4). Very common inhabitants of the human body and foods. Some pathogens (e.g. S. pneumoniae). Homofermentative.

Lactobacillus. Usually rods (see picture from pg. 34 of: H. McGee. 1984. On food and cooking, the science and lore of the kitchen Collier Books, New York.) 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.

Link to Things you may want to know about Yogurt for exam #1

Other low % GC gram + cocci and rods

Staphylococcus (staphyl = Gr. for a bunch of grapes) See Figure 20.7.

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).

low GC gram + bacteria (cont.)

Endospore formers

We have already mentioned these guys in lecture 2. See figure 20.9 for a simplified life cycle of an endospore forming bacterium. They are very common in soil but rare in aquatic ecosystems. This makes sense if you view the endospore as an adaptation to environments that dry out occasionally. Most of the endospore formers are grouped into two genera, Bacillus and Clostridium.

Bacillus
All are aerobic or facultative aerobes. See table 20.7 for some common species.... e.g. B. thuringiensis ("Bt") is used as a commercial insecticide

Clostridium
A huge genus of strict anaerobes. They are often classified according to the substrates that they can ferment. Table 20.8 gives some examples. Note that some very nasty critters (C. botulinum and C. tetani) are in the amino-acid fermenting group.

Other low % GC genera that we may come back to later include:

Desufotomaculum - endospore forming sulfate reducer.
Sporosarcina - forms distinctive tetrads
Mycoplasma - No cell wall, e.g. M. pneumoniae

High % G + C GRAM + BACTERIA:

The high GC group can be broken into the Coryneform Bacteria and the Actinomycetes based on morphological features and 16S rRNA data.

Coryneform Bacteria

Phylogenetically these organisms may be intermediate between the low GC group and the Actinomycetes (See Bifidobacterium on Fig. 20.1). The main genera of the Corynebacteria are shown in Table 20.11.

Arthrobacter spp. are extremely common soil inhabitants that form rods when well fed and coccoid cells when hungry. The coccoid cells are very long-lived spore-like cells. (See Figure 20.23).

Propionibacterium spp. are so named because they ferment lactic acid (the waste product of the lactic acid bacteria) into propionic acid, acetic acid, and CO2. These guys are thus responsible for the flavor and holes of Swiss cheese. 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 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 mega group of mostly aerobic, mostly filamentous, gram + bacteria with high %G+C. We will touch upon a few genera today and come back to others later in the course.

Mycobacterium spp. form waxy colonies and group together in cord-like masses (see Fig. 20.26). We will discuss M. tuberculosis and M. leprae later in the course.

Streptomyces (over 500 described species). Millions to billions per gram of soil. Some form elaborate spore-bearing structures. They are the source of over half of the commercially produced antibiotics (e.g. tetracyclines, streptomycin, chloramphenicol) and some anti-cancer drugs.

Actinomyces - only actinomycetes that are facultative aerobes.

Frankia - Forms N2 -fixing root nodules on many trees including alders. Figures 25.16-25.19

Micrococcus- Aerobic cocci. Very common in air, on skin. Cells often in packets (fig. 20.18a) yellow colonies.