Microbial ecology is
the study of the relationship between microbes and their surrounding
(environments).
Microbe have developed
strategies which enable them to survive;
- Survival
and growth by structural adaptation eg alkaline soda lakes, saline lakes, hot
springs, desert soils.
- High reproduction rate allows
survival
- High nutrients enable rapid growth to
out compete other cells
Microbes and Body surface.
Bacteria are consistently associated with the body surfaces of animals.
There are many more bacteria cells on the surfaces of human (including the
gastrointestinal tract) than there are human cells that make up the animal. The
bacteria and other microbes that are consistently associated with an animal are
called the normal flora, or more properly the “indigenous microbiota” of the
animal.
These bacteria have a full range of
symbiotic interactions with their animal hosts. This relationship between
microbes and animals is called symbiosis. In biology, symbiosis is defined as
“life together” ie that two organisms live in an association with one another.
Thus, there are at least three types of relationship based on the quality of
the relationship for each member of symbiotic association.
1. Mutualism: - Both members of the
association benefit. For humans, one classic naturalistic association is that
of the lactic acid bacteria that live on the vaginal epithelium of a woman. The
bacteria are provided habitat with a constant temperature and supply of
nutrient (glycogen) in exchange for the production of lactic acid, which
protect the vagina from colonization and disease caused by yeast and other
potentially harmfully microbes.
2. Commensalism:- There is no apparent
benefit or harm to either member of the association.
A problem with commensal relationships is that if you look at one long
enough and hard enough, you often discover that at least one member is being
helped or harmed during the association. Consider our relationship with staphylococcus epidermidis, a consistent
habitant of the skin of humans probably; the bacterium produces lactic acid
that protects the skin from colonization by harmful microbes that are less acid
tolerance. But it has suggested that other metabolites produces by the bacteria
are an important cause of body odors (good or Lad, depending on your personal
point of view) and possibly associated with certain skin cancers.
3. Parasitism:-
Parasite refers to an organism that grows, feed and is sheltered on or
in a different organism while contributing nothing to the survival of its host.
In microbiology, the mode of existence of a parasite implies that the parasites
are capable of causing damage to the host. This type of a symbiotic association
draws our attention because a parasite may become pathogenic if the damage to
the host result in disease. Some parasitic bacteria live as normal flora of
humans while waiting for an opportunity to cause disease. Other no indigenous
parasites generally always cause disease if they associate with a non immune
host.
In human, some of the normal bacterial
flora (e.g staphylococcus aureus, streptococcus pneumonia, Haemophilus
influenza) are potential pathogen that live in a commensally or parasitic
relationship without producing disease. They do not cause disease in their host
unless they have an opportunity brought on by some compromise or weakness in
the host’s anatomical barriers, tissue resistance or immunity.
The host in a host-parasite
interaction is the animal that maintains the parasite. The host and parasite
are in a dynamic interaction, the outcome of which depends upon the properties
of the parasite and of the host. The bacterial parasite has its determinants of
virulence that allow it to invade and damage the host and to resist the
defenses of the host. The host has various degrees of resistance to the
parasite in the form of the host defenses.
A healthy animal can defend itself against pathogens at different stages
in the infections disease process. The host defenses may be of such a degree
that infection can be prevented entirely, or if infection does occur, the
defenses may stop the process before disease is apparent. At other times, the
defenses that are necessary to defeat a pathogen may not be effective until
infectious disease is well into progress.
Typically the
host defense mechanisms are divided into two groups.
1. Constitutive Defenses: Defenses common to all healthy animals. These
defenses provide general protection against invasion by normal flora or colonization,
infection, and infectious disease caused by pathogens. The constitutive
defenses have also been referred to as “natural” or “Innate” resistance, since
they are inherent to the host.
2. Inducible Defenses:
Defense mechanism that must be induced or turned on by host exposure to
a pathogen (as during an infection). Unlike the constitutive defenses, they are
not immediately ready to come into play until after the host is appropriately
exposed to the parasite. The inducible defenses involve the immunological
responses to a pathogen causing an infection. The inducible defenses are
generally quite specifically directed against an invading pathogen. The
constitutive defenses are not so specific, and are directed toward general
strategic defense. The constitutive defenses, themselves, may not be sufficient
to protect the host against pathogens. Such pathogens that evade or overcome
the relatively non-specific constitutive defenses are usually susceptible to
the more specific inducible defenses, once they have developed.
Microbes that colonize the human body during birth or shortly thereafter,
remaining throughout life, are refer to as normal flora. Normal flora can be
found in many sites of the human body including the skin (especially moist areas
such as groin and between the toes), respiratory tract (particularly the nose),
the urinary tract, and digestive tract (primarily the mouth and colon). On the
other hand, areas of the body such as the brain, the circulatory system and
lungs are intended to remain sterile (microbe free).
The human body provided may unique environments for different bacterial
communities to live. Individual microbe may be carrying out important functions
within our bodies that we have not yet discovered.
Whether a host-microbe relationship
is “positives” or “negative” depends on many factors. And in most cases the relationships
well actually remain positive. The host provides a niches and nutrition for the
colonizing microbe and the microbe occupies a space that a potential parasite
or pathogen might otherwise colonize. In these cases microbial communities may
even aid in digestion or synthesize nutrient for the host.
However, life is not always perfect,
and in certain situations good-standing members of your normal flora can cause
disease or invading pathogens can displace them, the result will be disease.
Life on the surface, the skin:
Human skin is not a
particularly rich place for microbes to live. The skin surface is relatively
dry, slightly acidic and the primary source of nutrition is dead cells. This is
an environment that prevents the growth of many microorganisms, but a few have
adapted to life on our skin.
Propionibacterium
acnes is a gram positive bacterium that inhabits the skin. P. acnes are anaerobes, so they live in
pores and glands where oxygen levels are lower. As the name implies, P. acnes
causes the common skin condition called acnes.
Although acne out breaks can result
in emotional and physical discomfort, the infection is not life threatening. A
point complemented by P. acnes
performing an important role through occupying niches that might otherwise be
colonized by more dangerous pathogens.
Another prominent member of the skin
flora is staphylococcus epidermidis.
This is a highly adapted gram positive bacterium that can survive at many sites
throughout the body. S. epidermidis
can cause life threatening disease in hospital patients when invasive medical
devices such as catheters are used. In such cases, S epidemics form antibiotic resistance biofilms along the catheter
and enter the blood stream causing systemic infection that can be fatal.
Nose
The human nose is home to the
infamous Gram positive bacterium staphylococcus
aurous, best known for its role in hospitals where it is a major cause of
surgical wound and systemic infection.
Mouth
500-600 different kinds of bacteria thrive on mucus and food remnants
in the mouth. A predominant member of this community is the Gram positive
bacterium streptococcus mutans. It
grows in biofilms on the surface of teeth (plague) where it consumes sugar and converts
it to lactic acid.
Lactic acid erodes the
enamel on the surface of teeth, which leads to the formation of cavities.
STOMACH
The stomach has a highly acidic (pH 1-2) environment. One organism that
has been discovered living in the human stomach is the gram negative bacterium
called Halicobacter phylori.
It creates a less acidic
microenvironment. The bacteria achieve this by burrowing into the stomach’s
mucosal lining to a depth where the pH is essentially neutral. In addition H pylori produce an enzyme called urease
to convert urea produced by the stomach into ammonia and carbon dioxide. H pylori is the causative agent of
gastric ulcers.
Compared to the stomach, the small
intestine is a relatively hospitable environment. However, the small intestine
presents microbes with a new challenge – high flow rates. This makes it
difficult for bacteria to colonize, the small intestine because they get washed
out very quickly. As a result the concentration of bacteria in the small
intestine remains relatively low and human enzymes carry out most of the
digestion processes.
In the colon, the show rate of food
movement, gives bacteria in the colon time to reproduce so that they reach very
high concentration (1012-1013 bacteria per ml). The colon
is a holding tank for bacteria that participate in the end stages of food
digestion. For it is here that bacteria are presented with polysaccharides that
cannot be broken down by human enzyme. The process of polysaccharide
degradation in the colon is referred to a colonic fermentation. Polysacchande
fermentation results in the production of acetate, butyrate and propionate,
which are used as a source of carbon and energy by mucosal cells of the colon.
There is also evidence that E coli
within the colon produce vitamin K, which the human body requires for the
process of blood clothing.
REFERENCE
Hentschel, U.M. Steinert and J. Hacker (2000). Common molecular mechanisms of symbiosis and pathogenesis trends
microbio/8:226-23.1
Kenneth Todar (2009). The nature of Host-parasite relationships between Bacteria and animals. University of
Wisconsin madison department of Bacteriology.
Lansing M. Prescott, John P. Harlern Donald A. Klein (1999) general microbiology MC Graw-Hill companies,
inc. 4th edition pg 400, 405-407
Michael J. Pelczar, JR., E. C.
S. Chan, Noel R. Krieg (1999). Tata McGraw-Hill
Publishing Company Limited New Delhi 5th Edition pg
687