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9
Phenotypic Properties of Source
Microorganisms and Their Genetically
Modified Derivatives
~ this chapter, we shift our attention from characterization
of the genetic modifications to characterization of the ecologically
important phenotypic properties of the unfamiliar microorganism
intended for field testing and eventual introduction into the environ-
ment. The most relevant phenotypic properties are those that relate
to the persistence of the introduced microorganism (or the genetic
material incorporated during its modification) and those that affect
the ecosystem. Of course, for a microbial application that is familiar,
such phenotypic information is likely to be already available or ren-
dered unnecessary by a safe history of cast use in the environment.
PERSISTENCE
Persistence can be viewed as survival of the introduced modified
microorganism or retention of particular genetic traits in new genetic
combinations resulting from gene transfer.
Persistence of the Microorganism
If an organism cannot persist in a particular environment, it
poses little threat of causing prolonged environmental impact. How-
ever, short-term responses may be seen, as in the use of Bacillus
99
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100
thuringiensis to control lepidopterous insect pests. An organism
that Is killed or does not persist might be viewed as similar to a
chern~cal treatment that produces no chemical residues. For exam-
ple, some organisms wiD be developed to control pests or degrade
pollutants and wiD be used once or reapplied, if necessary. On the
other hand, the utility of some applications of genetically modified
microorganisms wiD depend on their ability to persist in the environ-
ment. For example, genetically modified microorganisms introduced
into the rh~zosphere for plant growth promotion or disease control
will be most beneficial to farmers if the organisms remain active for
years.
Persistence and spread are particularly relevant, however, if
a proposed application is both unfamiliar and has some potential
for adverse environmental effects. ~ such cases, it may be difficult
to mitigate an adverse environmental effect simply by halting fur-
ther application. Therefore, the potential for persistence and adverse
effects should be considered together when establishing levels of con-
cern for proposed field tests involving unfamiliar microorganisms;
this is reflected in the framework presented in Chapter 11. For unfa-
m~liar applications, it Is prudent to evaluate potential adverse effects
prior to field testing (Vogel and McCarty, 1985; Tickle et al., 19893.
The persistence of an unmodified m~croorg~ism in its usual
habitat is largely predictable. Genetic modification could influence
persistence if it changed the fitness of the modified organism or sig-
nificantly altered its ecological niche. Fitness (the factor that reflects
the rate at which a particular type of microorganism Increases or de-
creases in number) might increase if a phenotypic change Increased
resistance to a noxious substance present in the environment or in-
creased the ability of the organism to metabolize a substrate in the
environment. It Is often possible to deterrrune the fitness of an or-
ganism through laboratory tests (Lenski, 1989), although field tests
may be needed.
Considerable evidence exists that nonindigenous bacterial pop-
ulations, including genetically altered strains, decline rapidly after
they are introduced into soil or aquatic environments (Scanferiato,
et al., 1989~. This supports the well-documented fact that long-
established microbial communities resist invasions by foreign organ-
isms (Liang et al., 1982~.
If persistence appears undesirable it may be avoided by choosing
strains with reduced survival, reduced reproductive capacity, low re-
sistance to a predictable change in the environment (such as seasonal
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101
heat or cold), or a tendency to lose the specific function of concern.
Biological confinement also can be provided by suicide genes (Molin
et al., 1987; Curtiss, 1988; OTA, 1988) or by incorporation of addi-
tional nutritional requirements. On the other hand, when persistence
is desirable, it may be fostered to achieve the benefits for the intended
function.
Persistence of the Genetic Modification
1
Gene transfer can affect persistence and may occur either to or
from the introduced microorganism. Only a new genetic combination
with higher fitness than the introduced or indigenous genotypes has
any likelihood of persisting.
Conjugation, transduction, and transformation are mechan~srns
of exchange of both chromosomal and extrachromosomal DNA be-
tween bacteria (Freifelder, l9g7; Lenski, 1987; Miller, 1988~. Sim-
ilarly, viruses can recombine when two or more types co-infect the
same cell (Hershey and Rotman, 1949~. Genetic material is ex-
changed most readily among clones of the same species, but ex-
change among groups that are more distantly related can occur as
well (Roberts et al., 1977; Morese et al., 19g6; Guerry and Colwell,
1977; Hememann and Sprague, 19893.
Perhaps the most dramatic recombination mechanism is trance
formation, in which DNA from injured or dead cells (Kieft et al.,
1987) or extruded by living cells (Borenstein and Ephrati-~lizur,
1969; Orrego et al., 1978) is taken up by living cells of the same or
other species (Graham and Istock, 1979; Duncan et al., 1989~.
We know very little about the exchange of genetic "formation
among closely or distantly related rn~croorganisrns under natural
conditions. The possibility that these exchanges occur, however in-
frequently, must be taken into consideration in any planned mtroduc-
tion. However, In the extensive trials carried out by the Monsanto
Company and Clemson University in South Carolina, in which the
lacZY genes were used as a marker for a genetically altered strain of
Pseudomonas pnorescens, there was no evidence of exchange of the
genetic marker with other soil bacteria (Drahos et al, 1988~. Since
those tests were lirn~ted both in time and location, it is evident that
additional information must be generated on the frequency of gene
exchange, particularly in managed ecosystems (window et al., 1989~.
Effective genetic transfer between distantly related m~croorgan-
isms is far less frequent than between closely related microorganisms
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102
because of the improbability of each of a series of events: the in-
sertion of DNA from a distantly related organism into the recipient
organism; replication of the foreign DNA in the recipient; and set
lection favoring the new recombinant genotype. While we cannot
assume that any given transfer Is impossible in nature, a transfer of
DNA among closely related organisms is far more probable than a
transfer among distantly related ones. In principle, organisms ma-
nipulated ~ the laboratory to cross genetic barriers may be able in
the field to transfer genes secondarily to other organisms; but in fact,
even transfers between closely related microorganisms in nature are
infrequent and difficult to document.
Not aD genetic transfer Is undesirable; in most instances it proba-
bly wiD not matter. Unless the recipient organism has a selective ad-
vantage, the genetic transfer will have little or no consequence, there
will also be no consequence if an organism has a selective advan-
tage, but Is innocuous. Examples of genetic modifications that pose
little or no risk due to genetic transfer mclude ice- (ice-nucleation
deficient) Pseudomonas syrtngae, used to protect plants from frost
damage, and plants in the rhizosphere containing the [acZY marker
genes, used for monitoring their movement and persistence in soil.
PHENOTYPIC PROPERTIES AFFECTING
THE ECOSYSTEM
If an organism persists in the environment and has the potential
to spread, it is important to consider its phenotypic properties that
relate to its role in the ecosystem. These properties include compet-
itiveness, substrate utilization, environmental range, and host range
(if the organism is a pathogen or a symbiont). These properties are
also prime candiciates for genetic modification, since many microbial
applications depend on them. Examples include enhanced nitrogen
fixation, Stereo hos~range biocontro} agents, or beneficial soil bac-
teria that must out-compete indigenous m~croflora to be successful.
Competitiveness
Environments into which microorganisms are to be introduced
are diverse ~d often support a complex indigenous microflora. As
a result, competitive traits that are advantageous, such as a rapid
utilization of abundant substrates, high maximum specific growth
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103
rate, and antibiotic production, may be necessary if an introduced
microorganism is to colonize successfully in competition with native
· -
microorganisms.
Will the Introduced microorganism persist or spread to other
environments? It Is difficult to envision a specialized microorganism
faring unusually well in, for example, the plant rhizosphere because
the soil microbial populations there are dense and incredibly diverse.
However, when an organism is Introduced into an environment in
which the indigenous population is restricted in density and diversity,
or in which the introduced rn~croorganism has a monopoly on some
resource (for example, a substrate difficult to metabolize), then the
chances of proliferation to high density may be greater. However,
spread probably will be limited if the resource is less abundant outside
the target area.
It has been suggested that genetically modified microorganisms
will be competitively disadvantaged, relative to their wild-type coun-
terparts, because of burdens associated with carriage and expression
of additional functions. Indeed there are many we0-documented pa-
pers to support this suggestion (Lenski and Nguyen, 1988; Brill,
1985; Davis, 1987; Zund ~d I`ebek, i980; I.ee and Edlin, 1985;
Duval-~ah et al., 1981~. However, exceptions have been reported in
which increased fitness has resulted from carriage of foreign genes in
microorganisms (Hart] et al., 1983; Edwin et al., 1984; Bouma and
Lenski, 19883. Most of these experunents have been performed ~ the
laboratory and may not represent natural conditions. To date, how
expression of additional functions affects fitness has not been clearly
resolveci.
It also has been suggested that an introduced microorganism
might competitively exclude another potentially more valuable m~-
croorga~sm (Tie~je et al., 1989~. An example cited ~ that Bradyrhi-
zobium japonicum serogroup 123 is more competitive in some soils
than ~ others and may exclude other more elective nitrogen fixers
(Johnson et al., 1965; Moawad et al., 1984~. As no basis other than
field tests exists for judging field competition and no clear infor-
mation is available on what governs competitive ability among the
rhizobia, selection of inoculant strams has been made empirically.
Stram 123 forms a symbiotic relationship with soybeans resulting
in nitrogen fixation. In the infection process it outcompetes many
strains in the field that fix nitrogen more vigorously than 123 under
aseptic greenhouse conditions.
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104
Substrate Utilization
It may be desirable either to expand or to restrict the range
of substrates utilized for growth by an introduced microorganism.
Expansion will have most appeal in applications for removal of sta-
ble compounds from polluted environments. As introduced toxin-
degrading microorganisms proliferate, they and the indigenous m~-
croorganisms may co-exist owing to the removal of the toxic sum
stance (Lenski and Hatt~ngh, 1986~. Genetic modifications also can
be used to restrict substrate range, an appealing prospect for biolog-
ical confinement of certain introduced microorganisms.
Involvement In Ecosystem Processes
Microorgan~srns play roles in ecosystem processes important to
the sustained habitability of the planet, including major biogeochem-
ical cycles: carbon (lignin and cellulose decomposition), nitrogen (ni-
trogen fixation, vitrification, denitrification), sulfur (sulfur oxidation
and sulfate reduction), and the cycling of elements such as phos-
phorous, silicon, manganese, iron, and trace metals. Microorganisms
also may produce or use gases that include CH4, NO2, H2, and CO2,
and these gases may influence the earth's atmosphere. Field exper-
iments should have no detectable influence on these processes but
reasonable questions concerning potential adverse ecosystem ejects
should be evaluated prior to field testing.
lDuviron~nental Range
One of the classical criteria for choice of microorganisms for
applied purposes ~ their ability to function under a variety of cli-
matic and management conditions. The cI~rnatic range of a num-
ber of plant pathogens studied has been very restricted (for ex-
almple, Pseudomonas solanacearum; Kelman, 1953~. Another well-
documented example covers the restricted range of root nodule bac-
teria. Bradyrhizobi?`m japonicum, serogroup 123, dominates the root
nodules of soybeans grown in the north-central region of the United
States, despite a diversity of other strains of the same species found
in the soils (Ham, 1980~; however, serogroup 123 is rarely recov-
ered from nodules of soybeans growing in the southeast states of the
United States (Keyser et al., 1984~. Sirn~larly, Rhixobium trifolii TA1
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105
has been a successful inoculant of clover species in eastern Australia
but has been unsuccessful ~ western Australia (Parker et al., 1977~.
It should be pouted out that non-~digenous or exotic sapro-
phytic microorganisms have been introduced continually into soils in
the United States for decades without documented cases of harmful
responses. Examples of such introductions include those associated
with shipment of plant propagation materials such as seedlings and
bulbs that carry m~croorganisn~.
Host Range
Host specificity is well documented in symbiotic and pathogenic
associations between plants and rn~croorganisTns. Since relatively few
microorganisms enter these relationships, they clearly are specialized.
Tm recent years geneticists have identified not only the genes needed to
establish symbioses or pathogenesis, but also genes that determine
host range (DjorUjevic et al., 1987; Keen and Staskawicz, 19883.
From an agricultural perspective, it may be desirable either to restrict
or to expand the host range of symbiotic microorganisms.
With certain plant pathogens, the incidence of disease relates in
a complex manner to the population dynamics of the pathogen ~d
its interaction with susceptible host cultivars (Rouse et al., 1985).
Two strategies can be envisioned for biocontro! of plant disease: (1)
a biocontro] agent ~ introduced to colonize and occupy the target
habitat prior to the pathogen's appearance; or (2) the biocontro}
agent competitively displaces the pathogen. In the first case, biocon-
tro} agents with general features for rapid colonization of the leaf will
have broad appeal. In the second, the biocontro! agent will usually
need to be a specialized microorganism recognizing the same host
specific-signals as the pathogen and out-competing the latter.
CHARACTERISTICS OF MICROBL\[ PATHOGENS
One of the concerns often raised about genetic modification of
rn~croorganisms is the possibility that inadvertent acquisition or loss
of one or a few functions Knight convert unrelated noupathogenic
organisms into potentially dangerous pathogens of humans, plants,
or animals. However, pathogenicity is controlled by a large portion
of the genome of prokaryotes. ~ certain plant-pathogenic bacteria,
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106
for example, many pathogenicity functions are carried on the chro-
mosome, while others are carried on very large plasmas. Thus, the
acquisition or loss of a single function cannot convert a nonpathogen
into a pathogen. In some cases, changes ~ virulence (increased ag-
gressiveness on specific hosts) may be increased by acquisition of
the ability to produce a toxin or by loss of an incompatibility func-
tion, but the recipient organism must already possess aD the wide
variety of genes that win allow it to Infect and colonize a particular
host. Endowing rhizosphere bacteria with the ability to produce Bt
toxin, as proposed by some biotechnology firms, does not represent
conversion of a nonpathogen to a pathogen in the sense described
above. Rather, a precise modification has been made that makes the
bacterium toxic to certain insect larvae.
Background
Prokaryotes preceded eukaryotes during evolution. Thus, Tom
the time higher plants and animals made their appearance on land,
they have been surrounded by ~rucroorganisrns. Parasitic or sym-
biotic relationships probably existed very early in evolution, but in
spite of such long coevolution, relatively few microbial species have
developer} the capacity to parasitize plants or animals (Sequeira,
1984~. For example, very few species of bacteria, representing only
five major genera, parasitize plants. Among the gram-positive bacte-
ria, which constitute a large portion of the soil ~rucroflora surrounding
plant roots, there are no plant pathogens of significance with the ex-
ception of a few members of the Corynebacteriae. Even fewer species
have developed a symbiotic relationship that allows them to mul-
tiply within plant cells. It is evident, therefore, that pathogenesis
and symbiosis are highly complex relationships that developed, in
relatively few instances, through eons of convolution. It is equally
evident that the evolution of a compatible relationship with a host
cannot be recreated by simply inserting or deleting a few genes in a
nonpathogen by recombinant DNA techniques.
Pathogen~cit~r Versus Virulence
A distinction should be made at this point between genes that
are involved in pathogenicity and those involved in virulence.
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107
Pathogenicity is an attribute (the ability to cause disease) of an
entire group, irrespective of the fact that particular strains or races
may not be pathogenic to a given host. Virulence is the relative
ability of an individual strain or race to cause disease under de-
fined conditions (Federation of British Plant Pathologists, 1973~.
Thus, virulence is a quantitative variable. While it may be am
propriate to refer to a particular strain as nonvirulent, the term
nonpathogenic may not be appropriate unless a wide range of hosts
has been tested.
Genes That Confer Pathogen~city
Pathogenic microorganisms must possess two general types of
genes: (1) those that are important for basic compatibility with the
host; and, (2) those that are specific for virulence on particular hosts.
To the first category belong housekeeping genes that control general
metabolism and that allow the pathogen to grow on the nutrients
available in the host. To this same group belong those genes that
allow the pathogen to degrade, detoxify, or bypass preformed or
induced substances that provide resistance for the host. Thus, one
can assume a priori that a pathogen of a particular host must be
able to utilize the nutrients available at the site of infection ~d
must be insensitive to substances that inactivate or destroy the vast
majority of the microorganisms that invade the host. The marked
specificity of certain strains of bacteria that colonize leaf surfaces,
for example, indicates a narrow adaptation to the nutritional and
toxic components of the host environment. Also, these bacteria must
be able to withstand intense solar radiation and extended periods
of desiccation. Once inside the leaf, those that are pathogens now
produce enzymes that degrade plant cell wads or toxins that may be
very specific in terms of the cellular targets affected in certain tissues
of particular hosts. These virulence functions are controlled by genes
that belong to the second category.
Gen~for-Gene interactions
interactions between pathogens and their hosts are often under
the control of apparently gen~for-gene systems (Flor, 1956; Barrett,
1985~. Such systems are common In plant-pathogen relationships,
where host resistance depends on single, dominant genes that are
sup erunposed on other, more general mechanisms of defense. The
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108
gene-for-gene concept accounts for the fact that for every major ret
sistance gene in the host there IS a corresponding gene for avirulence
in the pathogen. For many pathogens, there is a great deal of evi-
dence that av~rulence, rather than virulence, Is the positive function.
hnport ant consequences follow from this seemingly contradictory
statement. First, incompatibility of a pathogen on a particular host
may result Tom the interaction of products of an avirulence gene
of the pathogen and the corresponding resistance gene in the host
(EHingboe, 1982~. Second, mutations that elirn~nate the av~rulence
gene product may increase the pathogenic potential of an organism
(Staskawicz et al., 1988; MelIano and Cooksey, 1988~. Ultraviolet
treatment of plant parasitic fungi, for example, has long been known
to yield mutants that have a wider host range than that of the parent
line (Flor, 1958~.
There are, of course, many examples of positive-control func-
tions that also specify virulence of pathogens. Of particular interest
are the host-specific toxins produced by some pathogenic fungi that
have highly specific targets in particular varieties of the plant host.
A°particular receptor for the toxin appears to confer susceptibility.
The toxin Tom Helminthosporium maydis, for example, affects only
the m~tochondria of hybrid corn lines carrying Texas male sterile cy-
toplasm (Yoder, lOSO). Other species of Helminthospori?`m produce
toxins that affect varieties of oats and sugar cane. Artificial hybrids
among species of Helminthosporium may exhibit combined virulence
to each of the hosts affected by the parental strains, as specified by
the toxins they produce.
GENERAL P:EIENOTYPIC CHARACTERISTICS OF
PATHOGENS
-One of the primary characteristics of pathogens is their ability
to colonize the host. For many rnicroorganisrns, the ability to attach
to the host surface, or to specific host cells in particular tissues,
is the first step ~ colonization (Costerton et al., 1981~. Microor-
ganisms have developed highly specialized structures (extracellular
polysaccharides, fimbriae, appressoria) that provide adhesion to spe-
cific host surfaces. Once attached, colonization depends not only
on the ability of the organism to utilize nutrients at the site, but
also on the ability to compete with other organisms for those same
nutrients. A high degree of competitiveness, therefore, characterizes
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109
a successful pathogen. Saprophytes share many of these properties
with pathogens, but they may be unable to colonize particular eco-
logical niches withm a host because of their inability to resist a wide
range of host defense systems.
A series of nonspecific and specific mechanisms for resistance
present a hostile environment that limits the growth of the vast ma-
jority of microorganisms that gain access to an uncompromised host
(Falcone et al., 1984; Mims, 1982~. With the exception of cornrnensal
species, only those organs s that have an array of virulence fac-
tors are capable of growing in animal or plant host environments.
Extracellular pathogens of anunals, for example, owe their virulence
to their ability to resist or inhibit phagocytosis. The production
of capsular polysaccharides Is important in this regard. Intracellu-
lar pathogens, on the other hand, are highly adapted to ingestion
by phagocytes and even use phagocytes as a means for dispersal
within the host. There are evident analogies between plant and ani-
mal pathogens in this regard. Whether intracellular or extracellul=,
the ability to resist, inhibit, or degrade host defense compounds
when colonizing the host is therefore one of the key properties of
pathogens. Some plant pathogens, for example, are capable of chem-
ically degrading phytoalexins, antimicrobial compounds produced by
plants upon challenge by potential pathogens. Others are insensitive
to phytoalexins from particular hosts. Yet others grow in the host in
such a way that they electively bypass the tissues where phytoalexins
accumulate.
Pathogens can establish themselves in the host because of their
ability to produce a large series of compounds, including hydrolytic
and proteolytic enzymes, toxins, polysaccharides, and growth regula-
tors that affect the host in ways that ultunately favor multiplication
or transmission of the pathogen. The enzymes that destroy cell mem-
branes or cell walls, the toxins that affect the normal metabolism of
the cell, and the growth regulators that influence the ways host cells
grow all have an unp act in determining pathogenesis. As a result,
the host is damaged much more than would be expected from the
strictly energetic drain on its metabolism.
Pathogens must also have an elective mechanism for spreading
to new hosts. Reproductive structures must be formed that can
persist in unfavorable environments or to reach new hosts before
they perish. Most pathogenic organisms depend on their hosts or
on Rectors for survival, but many are capable of surviving for long
periods ~ soil or water (Brubaker, 1985~. The chemotactic property
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110
of m~croorganisrns often assumes ~rnporta~ce in this respect since
many an~rnal pathogens must move through intercellular spaces to
reach target tissues.
Acquisition of Violence
It ~ evident from this discussion that pathogenicity depends on
an impressive array of different characteristics that relatively few
microorganisms have acquired through extended coevolution with
particular hosts. It is unlikely, therefore, that minor genetic mod-
ifications can convert a nonpathogen into a pathogen. It also is
clear that factors for virulence are not exchanged ~nd~criminately
among microbial populations. Although certain unique properties of
pathogens are encoded by plasmas and may be transferable under
certain conditions, only a limited portion of the whole array of genes
required for virulence would be acquired by the recipient organism.
These plasmas, therefore, can confer virulence only to related strains
that are already highly adapted to particular ecological niches on or
in the host. For example, most of the genes necessary for tumori-
genicity of Agrobacterium tumefaciens, the crown gall pathogen, are
located in a plasmid (Ti). Acquisition of the Ti plasrn~d by strains
of Agro bacterium radio la cter, a nonpathogenic soil inhabitant , auto-
matica~y converts them into the pathogen A. tumefaciens. There is
a great deal of evidence, however, that the two species are essentially
identical except for the presence of the plastnid (Nester and Kosuge,
1981~.
Similarly, E. cold is a normal component of human add some
animal intestinal microflora, but certain strains are pathogenic and
can cause different types of diarrhea or urinary tract infections.
Certain commensal strains of E. cold become capable of causing
disease upon acquisition of plasmas conferring enterotoxicity and
adhesiveness (McConnell et al., 1981~. These pathogenic strains,
however, often have short persistence tunes, presumably because
they are less fit than strains lacking these elements (Duval-~ah et
al., 1981~.
In these examples, virulence is increased only when factors Dom
very closely related species are acquired. When changes are eRected
between unrelated species, pathogenicity is rarely acquired. For
example, transfer of pectin lyase genes from Erwinia chrysanthemi,
a potato soft-rot pathogen, to E. coZi does not convert the latter into
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111
a pathogen, even though it is now capable of rotting potato tuber
slices in the laboratory (CoHmer and Keen, 1986~.
The overall conclusion of this discussion is that, as a rule, an
unrelated Iriicroorganism does not become a pathogen merely because
it has received a portion of the DNA of a pathogenic species.
SUMMARY POINTS
1. E ram the standpoint of assessing potential effects of unfamil-
iar application of microorganisms ~ managed ecosystems, the most
relevant phenotypic properties are those that relate to the persis-
tence of the microorganism (and its genetic modification) and those
properties that may have adverse effects. Such assessments are not
necessary, however, if a proposed microbial application is familiar
and has a safe history of usage in the environment.
2. Key phenotypic properties include the fitness of a genetically
modified microorganism relative to its unmodified counterpart; the
potential for gene transfer between the introduced rn~croorg~sm
and the indigenous microflora; the physiological tolerances of the
introduced microorganism; the competitiveness of the introduced
microorganism; the range of substrates available to the introduced
microorganism; and, if applicable, the pathogenicity, virulence, and
host range of the introduced microorganism.
3. Persistence of genetic modifications may occur in either of
two ways. The introduced microorganism itself may survive and
propagate to form a sel£sustaining population. Alternatively, the
modification may persist in new genetic combinations resulting from
gene transfer. The potential effects of undesired persistence can be
avoided by the use of appropriate strategies for biological confinement
of the introduced microorganism and its genetic material.
4. Microorganisms intended for environmental introduction
may be modified for either highly specialized or more generalized
phenotypic properties, such as competitiveness, substrate utiliza-
tion, physiological tolerance, and host range (either pathogen or
symbiont). Persistence after introduction is most likely for rn~croor-
ganisms having generalized properties and for rn~croorgan~sms hav-
ing properties that allow exploitation of previously unfilled ecological
niches.
5. Pathogenicity is controlled by a large portion of the genome
of rn~croorganisms. This is so because a pathogen must be able
to cross a large number of potential barriers to be successful. The
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112
successful pathogen must be able to attach to appropriate sites on or
in the host, to compete for nutrients within the host, to resist various
host defenses, to survive and persist when the host is not present,
and to be effectively transmitted to new hosts. Therefore, acquisition
of a small number of genes from a pathogenic source cannot convert
an unrelated, nonpathogenic microorganism into a pathogen.
6. However, if the recipient is closely related to the pathogenic
source, or if the recipient is itself a pathogen, increased virulence for
particular hosts may result. In these special cases, the recipient al-
ready contains a large complement of genes related to pathogenicity.
Representative terms from entire chapter:
phenotypic properties
