Encyclopedia of Environmental Science and Engineering, Volume I and II

MICROBIOLOGY 695

TABLE 4

Absorption maxima of chlorophylls from various sources

Organism Chlorophyll type Principal absorption maxima in nm

Green sulfur bacteria Bacterial Chlc

Bacterial Chld

660

650

Purple sulfur bacteria Bacterial Chla

Bacterial Chlb

820

1025

Non sulfur purple bacteria Bacterial Chla

Bacterial Chlb

820

1025

Green algae and Euglenids, higher plants Chla

Chlb

683

650

Diatoms, brown algae Chla 683

Pyrrophyta Chlc 620

Xanthophyta Chla

Chle

682

Cyano-, Chryso- and Rhodophyta Chla 683

classical microbiology. These laboratory methods, pioneered

by Winogradsky (1856–1953) and Beijerinck (1851–1931)

and refi ned by others, utilize specialized, restrictive, physi-

cal and chemical conditions to select and cause to predomi-

nate one or few types of organisms from a highly diverse

mixture. Hence this method is termed selective enrichment

culture. Once one understands how to manipulate these labo-

ratory systems, it is easier to analyze fi eld observations in

which specifi c conditions which result in microbial changes

can be recognized and, if necessary, altered. We will fi rst

explain the principles of selective enrichment techniques and

then look at the natural distribution of microorganisms and

their relationship to higher plants and animals. Highlights

of microbial characteristics which are useful taxonomically

have been described in the various sections listed under

Physical Characteristics of Microorganisms.

Selective Enrichment Methods

To determine whether a given sample of soil, water, or air

contains microorganisms capable of living under a particu-

lar set of conditions, one prepares a growth medium which

is selective for a property peculiar to those conditions. For

example, if organisms which can fi x atmospheric nitrogen

are sought, all non-atmospheric sources of nitrogen (such

as nitrites, nitrates, ammonia, amino acids) are eliminated

from the growth medium. If organisms which obligately fi x

carbon dioxide are required, all nonatmospheric (organic)

sources of carbon are eliminated from the growth medium

and, frequently, additional CO 2 is bubbled through the

medium.

On the other hand, it might be of particular interest to

determine if a certain weed-killer is biodegradable before

it is used under fi eld conditions. Many different approaches

to this important problem are possible. Thus more than one

mode of attack is described. The approach which is closest

to the general principle of revealing (selectively enriching

for) a minor population of desired microbial type among a

multitude of undesirable organisms makes use of an enrich-

ment medium in which the weed-killer is used as either

(a) the only source of organic carbon and nitrogen, (b) the only

source of organic carbon though other sources of nitrogen

are present, or (c) the only source of nitrogen though other

sources of organic carbon are present. Subsequent microbial

growth indicates biodegradability. The organisms may be

isolated and used as seed cultures for a percolation system

which is fed by the aqueous runoff from a fi eld which was

treated with the weed-killer. Thus the more public water-

ways fed by aqueous effl uents from treated fi elds would not

be polluted by potentially-toxic agricultural chemicals.

In order to prepare selective enrichment media, one

needs to provide the microorganisms with all their nutri-

tional requirements in proper proportions. Insuffi cient

quantities will not support growth and excesses are fre-

quently toxic. In addition conditions must be biased in some

way to insure that most of the undesired organisms will not

grow at all or will grow appreciably slower than the desired

organisms. It should be recognized that it is rare for any

single enrichment to select out only one microbial species.

Thus further purifi cation steps are required if one wishes to

isolate only one species uncontaminated with other living

things. An uncontaminated, single membered culture is

called a pure or axenic (a = absence of, xenos = strangers)

culture. The common nutritional requirements of microor-

ganisms, the quantities in which they must be supplied, and

the biological uses of each substance are shown in Table 5.

The selective enrichment techniques to be described are

most frequently used for the isolation of bacteria, yeast, and

certain prokaryotic algae. An outline of selecting properties

is given in Table 6.

A demonstration of factors involved during the natural

selection which takes place under fi eld conditions is shown

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