Microbiology, Lec 7.

Outline of lec. 7

Lactic acid bacteria

Yogurt Break
Endospore formers



Coryneform Bacteria


Review overall tree of life and what we have done so far.

Fig. 19.13

Gram-Positive Bacteria (Chapters 23 & 24)

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

Some methods for determining GC ratios are given on page 402.

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.



Ch. 23

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 homolactic fermenters produce 2 lactic acids per hexose (net 2 ATP), whereas the heterolactic fermenters 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 human body and foods. Some pathogens: e.g. S. pneumoniae (pneumonia), S. pyogenes (Strep throat and scarlet fever if S. pyogenes is carrying a specific lysogenic phage that carries the genes for an erythrogenic toxin).


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. (=Lactococcus 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

summary of yogurt overheads...

Temp. optimum for growth of L. delbruekii is about 37 degrees C
Temp. optimum for growth of S. thermophilus is about 50 degrees C
pH optimum for growth of L. delbruekii is 4
pH optimum for growth of S. thermophilus is about 5

Use the above info. to discuss how the yogurt making process enhances ecological succesion from S. thermophilus to L. delbruekii.



Low % GC gram + bacteria (cont.)

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 Formers

We have already mentioned these guys in lecture 2. See figure 3.47 for the 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.

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


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 nasty critters (C. botulinumand C. tetani) are specialists at amino-acid fermention. Does this help explain their pathogenicity?

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.

Mycoplasmas (Mollicutes)

Common in nature, mostly obligate symbionts (some pathogens)....
very small genome and cell size...
No cell wall, often pleomorphic morphology
e.g. Mycoplasma, Spiroplasma see Table 23.1 for others...

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

Desufotomaculum - endospore forming sulfate reducer.
Sporosarcina - forms distinctive tetrads



Ch. 24

The high GC group can be broken into about ten major genera and were traditionally divided into the Coryneform Bacteria and the Actinomycetes. Overall, the term Actinobacteria encompasses both of these groups.

Major genera of the Actinobacteria (Table 24.4)

Coryneform Bacteria

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 24.7).

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 guys 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. acnesis 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 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.). We will discuss M. tuberculosisand M. leprae (Fig. 24.9) 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 (Fig. 24.15).

Actinomyces - only actinomycetes that are facultative aerobes.

Frankia - Forms N2 -fixing root nodules on many trees including alders and Myrica. see Figures

Micrococcus- Aerobic cocci. Very common in air, on skin. Cells often in packets (fig. 24.6) yellow colonies.