Important words and
concepts from Chapter 14, Black, 1999 (3/28/2003):
by Stephen T. Abedon (abedon.1@osu.edu)
for Micro 509
at the Ohio State University
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(1)
Chapter title: Host-Microbe Relationships and Disease Processes
(a)
"Pathogens
have certain invasive capabilities, and you have a variety of defenses."
(b)
[host-microbe relationships and
disease processes (Google Search)] [index]
TYPES OF SYMBIOSES
(a)
A
symbiosis is when two organisms live intimately close, typically over longer
periods, often measured in generations for at least one of the organisms
(b)
If
one organism is substantially larger than the other organism, such that the
smaller organism lives in or on the other, then the larger organism is referred
to as a host and the smaller as a symbiont
(c)
Symbioses
are classified in terms of the benefits or costs accrued by the host in the
course of the symbiosis (in general we assume that symbiont gains from the
interaction)
(i)
Mutualism
(ii)
Parasitism
(iii)
Commensalism
(d)
[symbiosis and microbiology
(Google Search)]
[index]
(a)
A
symbiosis in which both host and symbiont benefit
(b)
For
example, in the mitochondrial-eukaryote endosymbiosis the mitochondria benefits
with a home, protection, provisioning of nutrients, and dissemination of
progeny mitochondria; the host benefits from cellular respiration (plus
additional biochemical tricks)
(c)
Other
mutualistic relationships abound including between hosts and otherwise
free-living microorganisms
(d)
[mutualism and microbiology
(Google Search)]
[biodiversity and mutualism in
ecosystems] [modeling mutualism]
[coevolution (and mutualism)]
[marine mutualisms] [index]
(a)
A
Commensalism is a symbiosis in which one of the participants
(typically the symbiont) benefits but the other organism (typically the host)
neither benefits nor is harmed
(b)
Due
to the complex nature of symbiosis it is often difficult to describe costs and
benefits with sufficient accuracy to make it possible to state with reasonable
certainty that a host neither benefits nor is harmed by its symbiont
(c)
Consequently,
commensalism is perhaps a rarely realized ideal
(d)
Nevertheless,
in the absence of evidence that a symbiont either hurts or helps its host, the
relationship is assumed to be one of commensalism
(e)
The
majority of the organisms living in or on your body probably
represent commensals, though this designation might require a consideration of microbial competition (a.k.a., microbial antagonism)
as something other than a benefit provided by the symbiont to the host
(f)
[commensalism and microbiology
(Google Search)]
[commensalism] [index]
(5)
Microbial competition (microbial antagonism)
(a)
A
microbe that fails to harm its host under ordinary circumstances and also does
not overtly help its host may still provide a helpful service to its host
(regardless, the symbiont gains by having a place to live as well as nutrients
found on or in the body)
(b)
This
is because such an organism is existing within a niche/location on or in the
host; the filling of any niche/location by a not-harmful bacterium serves to
prevent the filling of the same niche/location by a harmful bacterium
(c)
This
prevention of harmful bacterial growth by a non-harmful bacterium is called
microbial competition or microbial antagonism
(d)
(In
my opinion, it is a matter of semantics whether microbial competition makes a
symbiotic relationship a mutual one; that is, some argue that a
microbe has to live somewhere and therefore any effect such a microbe has on
preventing the growth of pathogens simply by filling this space/niche does not
constitute a direct benefit to the host—instead it is an indirect benefit—and
therefore the symbiotic relationship between host and a harmless microbe—that
is effecting microbial antagonism is a commensal one)
(e)
[microbial antagonism,
microbial competition
(Google Search)]
[index]
(a)
The
third category of symbioses is one where the host is harmed while the symbiont
gains (the latter, e.g., by having a place to live and something to eat)
(b)
Using
the term parasitism in terms of a symbiosis includes many pathogens (indeed,
all) including pathogenic bacteria, viruses, fungi, protozoa, helminths, and
arthropods
(c)
Note
that a more-narrow definition of parasite includes
only the latter three categories (protozoa, helminthes, and arthropods)
(d)
"Parasitism
encompasses a wide rage of relationships, from those in which the host sustains
only slight harm to those in which the host is killed. Some parasites obtain
comfortable living arrangements by causing only modest harm to their host.
Other parasites kill their hosts, thereby rendering themselves homeless."
(e)
[parasitism and microbiology
(Google Search)]
[parasitism] [index]
INFECTIOUS DISEASE
(a)
"Disease
is a disturbance in the state of health (of the host) wherein the body cannot
carry out all its normal functions… When an infection causes
disease, the effects of the disease range from mild to severe… Disease, or
illness, is characterized by changes in the host that interfere with normal function."
(b)
[definition disease
(Google Search)]
[index]
(8)
How microbes cause disease
(a)
"Microorganisms
act in certain ways that allow them to cause disease. These
actions include gaining access to the host,
adhering
to and colonizing cell surfaces, invading tissues,
and producing toxins and other harmful metabolic products. However, host
defense mechanisms tend to thwart the actions of microorganisms. The occurrence
of a disease depends on whether the pathogen or the host wins the battle; if it
is a draw, a chronic, long-lasting disease may result."
(emphasis mine)
(b)
[how microbes cause disease
(Google Search)]
[index]
(a)
Contamination
is the converse of sterility, i.e., an environment that is
not sterile is one that is contaminated with microorganisms
(b)
Contamination
is the first step toward the occurrence of infectious disease,
i.e., a organism must be present (in the wrong place and at the wrong time as
far as the host is concerned) to start down a path leading to disease
(c)
Note,
however, that just because something is contaminated with a microorganism
(including contamination of yourself) does not mean that disease is occurring
or will occur
(d)
[microbe contamination,
microbial contamination,
bacterial contamination
(Google Search)]
[index]
(a)
Upping
the ante in terms of progression towards disease is
infection, the actual growth of an organism, particularly a pathogenic one, on
or within a host
(b)
Infection
can lead to disease (particularly if we define infection as a property solely
of pathogenic microorganisms) but does not necessarily do so since hosts
actively attempt to prevent the progression of infection to disease (see non-specific
host defences & host systems, basic principles of specific
immunity & immunization) [non-specific defenses of the
host] [humoral immunity]
(c)
So
long as a pathogen is prevented from invading tissues
(either by itself or with its toxins) disease may not occur (compare colonization
and invasion,
below)
(d)
[microbe infection, microbial infection,
bacterial infection,
viral infection (Google Search)] [index]
(a)
Pathogenicity
is an organism's capacity to cause disease; that is, whether or not it can
cause disease
(b)
Note
that this capacity is typically context dependent:
(i)
Pathogenicity
is higher in some environments (e.g., infection of the blood) than others
(e.g., presence in the lumen of the gastrointestinal tract), or
(ii)
Pathogenicity
is higher on some hosts but not on others depending on host susceptibility (in
general or specifically towards a particular pathogen)
(c)
Pathogenicity
can also depend on the number of organisms present, where many organisms have a
greater potential of bypassing host defenses than fewer organisms of the same
kind
(d)
A
commensal, more or less by definition at a given place and given time, has a
pathogenicity of effectively zero
(e)
For
an organism to be pathogenic it must be able to invade a host, multiply
in the host, evade host defenses, and harm the host in some way
(f)
[definition pathogenicity,
pathogenicity, capacity to cause disease
(Google Search)]
[index]
(a)
Virulence
is the degree of disease an organism has the potential to cause
(b)
That
is, a highly virulent pathogen can cause significant disease whereas an
avirulent microorganism can cause little or no disease
(c)
The
terms pathogenicity and virulence are closely related with (i)
pathogenicity referring to an organism’s binary ability to cause disease (or
not) given specific circumstances and (ii) virulence referring to the degree of
disease caused (also dependent on specific circumstances)
(d)
Evolution
of pathogen virulence:
(i)
The
virulence of a pathogen tends to increase (evolutionarily) when new hosts are
readily available (think childbirth fever in the days of Semmelweis)
(ii)
The
virulence of a pathogen tends to decline (evolutionarily) when new hosts are
difficult to acquire (think sexually transmitted diseases where a very sick
person will tend to not have sex and therefore not transmit the infection)
(iii)
Consider
this analogy: If you had access to a new car every day, at no cost to you, then
you would be less concerned about damaging one of those cars than when you are
responsible for and paying for the vehicle you drive; pathogens, too: if a
pathogen can be transmitted to a new host regardless of how much it damages
it’s current host, then there may be little selection against host damage,
a.k.a., host sickness, a.k.a., pathogen virulence (also, without a countering
force to select against virulence, i.e., few hosts available given disease, one
might expect selection for individual pathogens that reproduce the fastest
within the host environment, producing the most “baby” pathogens within the
infected host, and consequently, all else held constant, facilitating the most
host damage)
(iv)
[the evolution and maintenance of virulence in
microparasites] [index]
(e)
[definition virulence,
virulence -factor -factors,
virulence factors (Google Search)]
[index]
(a)
Attenuation
is a decline in virulence imposed on a pathogen by growing the
pathogen under conditions that decrease its adaptation to growth on a given
host; this is often done by growth in tissue culture or in otherwise non-host
species
(b)
That
is, by increasing a pathogen’s adaptation to one condition (e.g., tissue
culture), the pathogen’s adaptation to another condition (e.g., us) may be
reduced
(c)
Attenuation
is often employed in the development of live vaccines (e.g., Sabin oral polio vaccine)
(d)
[virulence attenuation
(Google Search)]
[index]
INDIGENOUS MICROBIOTA
(14)
Normal (indigenous) microflora
(microbiota)
(a)
"An
adult human body consists of approximately 1013 (10 trillion)
eukaryotic cells. It harbors an additional 1014 (100 trillion)
prokaryotic and eukaryotic microorganisms on the skin surface, on mucous membranes,
and in the passageways of the digestive, respiratory, and reproductive
systems."
(b)
The
vast majority of those organisms do not cause disease except under special
circumstances (movement to different body sites or decline in immunity)
(c)
Such
microorganisms are collectively called normal microflora (as well as a variety
of other names listed in the heading to this section such as indigenous
microflora and normal microbiota)
(d)
Most
of these organisms are considered commensals
(e)
These
organisms may have more-or-less permanent
associations with their hosts or more transient
associations
(f)
See Table 14.1, Major normal
microflora (unless otherwise noted, bacteria) of the human body
(g)
See Figure 14.3, Locations
of resident microflora of the human body
(h)
[normal flora, normal microflora, normal microbiota, indigenous flora, indigenous microflora,
indigenous microbiota
(Google Search)]
[index]
(15)
Microbe-free regions of the
body
(a)
Note that normal microflora are not present on all
regions of the body and that, in fact, the majority of the body is microbe free
(except for the endosymbionts within our own cells) except under unusual (e.g.,
disease-state)
circumstances
(b)
In particular, all of the interior of the body is microbe free (i.e.,
all but the gastrointestinal tract and the lower regions of the urogenital
tract)
(c)
See Table 14.2, Body
tissues, organs, and fluids that are normally microbe-free
(a)
The
resident microflora are the more-or-less permanent members of normal microflora
(b)
[resident microflora
(Google Search)]
[index]
(a)
The
transient microflora are present only under unusual (i.e., not usual) circumstances and only transiently (hours to months)
(b)
[transient microflora
(Google Search)]
[index]
(a)
Opportunists
are members of the normal microflora that do not usually cause disease but can
be pathogenic under certain circumstances, i.e.,
(i)
Host
immunosuppression
(ii)
Transfer
to other parts of the body
(iii)
Elimination
of microbial antagonism
(b)
Among
opportunists are members of the genera Escherichia,
Enterobacter, and Serratia
(c)
[opportunistic pathogen
(Google Search)]
[index]
ASSIGNING CAUSE
(a)
The
means, or logic, by which a specific microorganism is classified as the cause
of a disease is called Koch postulates
(b)
There
are four of these postulates and generally all four postulates must be
satisfied before an organism is considered by Koch’s postulates to be the cause
of a disease
(c)
These
postulates include:
(i)
Evidence
that the causative agent is always present when the specific disease is present
(ii)
Successful
isolation of the causative agent in pure culture
(iii)
Ability
of pure-cultures of the organism to cause disease in a healthy organism
(iv)
Successful
isolation of the causative agent from the experimentally infected host (i.e.,
indication that replication of the presumptive causative agent has occurred
(d)
See Figure 14.4,
Demonstration that a bacterial disease satisfies Koch's postulates
(e)
Koch's
postulates are not foolproof and a number of complications can arise in
assigning a specific disease to a specific causative agent
(i)
There
may exist more than one cause of a specific disease
(ii)
It
might not be possible to grow the causative agent in pure culture
(iii)
There
might not exist a suitable experimental host
(iv)
The
disease might result from more than one causative agent acting in unison
(f)
[Koch's postulates (Google Search)] [Robert Koch (MicroDude)] [index]
KINDS OF DISEASE
(a)
Disease
may be classified in a number of ways, many of which we will subsequently
discuss, including:
(ii)
Communicable disease
(iii)
Noncommunicable disease