MICROBIAL INFECTIONS OF HUMANS(HUMAN MICROBIOLOGY CONTD..)
Evolutionary success means that the organism continues to propagate. As with the number of offspring
produced by all animals, the numbers produced depend on the likelihood that they will survive for
sufficient time to produce new offspring. With microbes, the more organisms that are shed, the greater
the chances of infecting a new host. Sexually transmitted infections are organisms that do not tolerate
drying and the route of transmission is quite demonstrably one in which the organisms are transmitted
without any time spent outside of the body. Correspondingly, there will be less need to release as many
organisms as would those organisms that risk desiccation during transmission. The stability of the infective particle is thus another parameter of relevance.
STABILITY OF THE MICRO-ORGANISM IN BETWEEN HOSTS:
Airborne transmission runs both the risk of the organisms drying out and failing to not encounter a new
host whilst the particles are suspended in the air. Gram positive organisms resist desiccation more
readily than Gram negative bacteria, but the most stable infective form is the endospore. Tetanus,
anthrax and botulism spores are all known to be stable in the environment (typically, soil) for years, if not decades.
THE INFECTIVE DOSE:
Obviously, it suits the micro-organism to have a low infectious dose so as to efficiently infect new hosts.
This number may be reduced if there are breaches in the host defences or other compromising events
that lower the resistance of the host. A well-recognised example is the reduction in infectious dose for salmonellosis in patients with achlorhydria (absent or reduced gastric acid production).
TRANSMISSION AND VIRULENCE:
From these three factors, the reproductive fitness of the organism (the efficiency with which the
organism is maintained in the population) is directly linked with its transmission between hosts. In
evolutionary terms, natural selection will favour the organism that is able to produce sufficient infectious
offspring. To do this the organism needs to multiply and thus may inflict damage on the host (termed
virulence). The relationship between transmission and virulence is unresolved but it can be argued that
the route of transmission will influence the numbers shed. Vertical transmission will require fewer infectious particles than horizontally transmitted infections because the vertically transmitted microbe
will depend on the host and the host’s reproductive success to maintain the organism. In horizontally
transmitted infections the numbers of potential new hosts will be much greater and the invading microbe
does not need to concern itself with the host’s ability to reproduce, therefore the organisms can be more
virulent so as to produce more infective particles. Since all parasites will exploit their hosts so as to best maintain their own reproductive fitness, the
transmissibility of an organism may be coupled to its virulence. When the organism is able to multiply
within its host it will produce more infectious offspring and increase the chances of infecting a new host.
It is possible that this will damage the host. People with cholera will excrete millions of Vibrio cholerae
bacilli in the fluid stools. If the microbe is transmitted by a vector (e.g. yellow fever virus and mosquito)
then the immobilisation of the host is probably beneficial. It is easier to get bitten lying ill in bed than
being healthy and mobile! If organisms can replicate sufficiently and maintain opportunities for
transmission, then the microbe and host may maintain a stable relationship but, if the host seeks to
eliminate the microbe or more than one microbe is competing for the same niche (more correctly, both microbes have the same resource requirements), then an increase in virulence is required to overcome the competition. Thus evolutionary pressures will force the microbe to evolve to a level of virulence that trades transmission opportunities with keeping the host alive.
THE NORMAL FLORA:
The term “normal microbial flora” denotes the population of microorganisms that inhabit the skin and
mucous membranes of healthy normal persons. It is doubtful whether a normal viral flora exists in humans.
The skin and mucous membranes always harbor a variety of microorganisms that can be arranged into two
groups: (1) The resident flora consists of relatively fixed types of microorganisms regularly found in a given area at a given age; if disturbed, it promptly reestablishes itself. (2) The transient flora consists of non-pathogenic or potentially pathogenic microorganisms that inhabit the skin or mucous membranes for hours, days, or weeks; it is derived from the environment, does not produce disease, and does not establish itself permanently on the surface. Members of the transient flora are generally of little significance so long as the normal resident flora remains intact. However, if the resident flora is disturbed, transient microorganisms may colonize, proliferate, and produce disease.
The normal flora refers to all the microbes present on sites on the body. In terms of number and
diversity, the normal flora is a rich collection of microbes. Essentially, all external surfaces are colonised
to some degree with microbes. The skin obviously is a large site that houses organisms, but the mucous
membranes of the respiratory tract, genito-urinary tract and gastrointestinal tract (and others) must be
viewed as involuted external surfaces that lead to the body surface at some point. The mucous
membranes harbour the largest number of organisms. Estimates of the numbers of bacteria in the colon
are in the order of 10^14/g of faeces.
The association of the bacteria with the host surface will vary. Some appear virtually permanent whereas
others may be only transient occupants. Several mechanisms exist that regulate the density of the normal
flora. The flushing action of urine flowing over the mucous membranes of the urethra is going to remove
organisms that have not developed appropriate means of attachment. Similarly, the continual shedding
of skin and epithelial cells will also act as a means of eliminating organisms. The sites that organisms
occupy are highly sought after and therefore organisms adopt various mechanisms to secure their
The organisms that make up the normal flora is not a clearly defined list but large-scale studies of people
have provided a core of organisms that are routinely isolated. The cast list will vary depending on the
body site. Clearly, organisms occupying the skin will have a number of characteristics suited to that
environment and these will be different to those found in the throat or the colon. It is not unusual to find
that certain people may harbour organisms that are considered pathogenic. Neisseria meningitidis may
be found in the throat of at least 1 per cent of the population without any associated disease. Not
surprisingly, the normal flora will be affected by the activities of the host. People living in tropical
countries, eating diets that are predominantly vegetarian will have an intestinal flora that varies from
people living in northern hemispheres and eating a packaged diet. Whilst there will be variations in the exact number of different species present, there will be a core of bacteria that are almost universal; that
is, which are present all of the time. In addition, there will be a number of species that appear
temporarily (transients), some of which may well be pathogenic.
The development of the normal flora starts immediately the foetus is delivered. The passage through the
genital tract provides the child with its first encounter with large numbers of organisms. The mother will
also provide a steady supply of intestinal skin and respiratory tract-derived organisms that will begin to
establish themselves in the growing child. The two main sites that are colonised with normal flora are those that are exposed to the outside world: the skin and the mucous membranes.
ROLE OF THE RESIDENT FLORA:
The microorganisms that are constantly present on body surfaces are commensals. Their flourishing in a
given area depends upon physiologic factors of temperature, moisture, and the presence of certain nutrients and inhibitory substances. Their presence is not essential to life, because “germ-free” animals can be reared in the complete absence of a normal microbial flora. Yet the resident flora of certain areas plays a definite role in maintaining health and normal function. Members of the resident flora in the intestinal tract synthesize vitamin K and aid in the absorption of nutrients. On mucous membranes and skin, the resident flora may
prevent colonization by pathogens and possible disease through “bacterial interference.” The mechanism of
bacterial interference is not clear. It may involve competition for receptors or binding sites on host cells, competition for nutrients, mutual inhibition by metabolic or toxic products, mutual inhibition by antibiotic materials or bacteriocins, or other mechanisms. Suppression of the normal flora clearly creates a partial local void that tends to be filled by organisms from the environment or from other parts of the body. Such organisms behave as opportunists and may become pathogens. On the other hand, members of the normal flora may themselves produce disease under certain circumstances. These organisms are adapted to the noninvasive mode of life defined by the limitations of the environment. If forcefully removed from the restrictions of that environment and introduced into the bloodstream or tissues, these organisms may become pathogenic. For example, streptococci of the viridans group are the most common resident organisms of the upper respiratory tract. If large numbers of them are introduced into the bloodstream (eg, following tooth extraction or tonsillectomy), they may settle on deformed or prosthetic heart valves and produce infective endocarditis. Small numbers occur transiently in the bloodstream with minor trauma (eg, dental scaling or vigorous brushing). Bacteroides species are the commonest resident bacteria of the large intestine and are quite harmless in that location. If introduced into the free peritoneal cavity or into pelvic tissues along with other bacteria as a result of trauma, they cause suppuration and bacteremia. There are many other examples, but the important point is that microbes of the normal resident flora are harmless and may be beneficial in their normal location in the host and in the absence of coincident abnormalities. They may produce disease if introduced into foreign locations in large numbers and if predisposing factors are present.
Cited By Kamal Singh Khadka
Assistant Professor In PBPC, PU, PNC, LA, NA.
SOME SUGGESTED REFERENCES:
www.ncbi.nlm.nih.gov › NCBI › Literature › Bookshelf