1: Microbial world


It is the study of interactions between humans and the microorganisms with which they coexist. The microorganisms involved are classified, according to the nature of their interactions with humans, on a spectrum that varies from beneficial to harmful.


Classification of microorganisms:

♣ Saprophytes: Free living microbes that live on dead or decaying organic matter. They are found on soil and water. They are generally unable to invade the living body.


♣ Parasites: Microorganism which lives on living host and derives nutrition and shelter from the host, without any benefit to it and may cause harm.


♣ Commensals: Organisms that routinely colonize body surfaces without doing harm and are often referred to as the normal microbial flora.


♣ Pathogens: Organisms that damage the human host either by direct invasion and injury or by the production of harmful toxic products.


♣ Opportunistic Pathogen: Commensals and saprophytes can produce disease when host resistance is lowered.


Host-microbe interactions.

The pathogenic potential of many organisms is variable. It is influenced by both the intrinsic properties of the microorganism and the state of health of the human host.


♣ Host defenses and natural immunity: It is a multifactor system of protective mechanisms that prevent entry of microorganisms into normally sterile areas and limit the spread of those invaders that overcome the first line of defense.

These mechanisms may be weakened by a variety of insults, including direct physical trauma, systemic diseases, drugs, and toxins.

When normal defenses are impaired, the person loses the ability to combat infection and the injury caused by pathogens, even those with low intrinsic virulence. In such cases, the compromised host often succumbs to infection.


♣ Pathogenecity: It is the ability of microbes to produce disease.


Virulence: It is the relative intrinsic ability of a microorganism to cause disease. Organisms of high virulence have evolved efficient mechanism for circumventing normal host defenses. Virulent organisms are adept at gaining entry and doing damage even when the inoculum is small.



Pathogens vary in size and biologic complexity. Some are able to extract sufficient nutrients from an inanimate environment and, hence may be cultured on artificial media. Others are incapable of growth outside living host cells and are referred to as obligate intracellular parasites.


Pathogens are divided into four major groups.

Human Immunodeficiency Virus
Human Immunodeficiency Virus

♣ Viruses: They are the smallest intact organisms with demonstrated pathogenic potential. They are too small to be seen with a light microscope. Viruses are obligate intracellular parasites that depend entirely upon the host cell’s synthetic machinery for reproduction. Viruses contain only one type of nucleic acid, either DNA or RNA, but not both. After entering a host cell, a virus sheds its coat and releases viral nucleic acid into the cell. Under the direction of the viral genes, the host cell diverts its activities toward producing new viral components, which are then assembled within the cell into new virus particles (virions). The virions are released, additional cells are infected, and the cycle is repeated.


Gram Positive cocci arranged in cluster
Gram Positive cocci arranged in cluster

♣ Bacteria: They are larger and more complex than viruses. Most bacteria are visible under the light microscope. Bacteria are termed prokaryotes because, unlike higher organisms, they lack a true cell nucleus. Since no nuclear membrane is present, the genetic material, in the form of a nucleoid, lies within the cytoplasm. Unlike viruses, bacteria possess both DNA and RNA. Bacteria reproduced by binary fission. Many pathogenic bacteria are capable of independent growth and thus, may be cultured on artificial media. Some bacteria, however, lack the ability to produce important metabolites and are obligate intracellular parasites. These organisms must be grown in tissue culture if their recovery is necessary.


Gram Positive Yeast-Candida cells
Gram Positive Yeast-Candida cells

♣ Fungi: They are larger than bacteria and have a more advanced cell structure. As eukaryotic organisms, their genetic material is separated from the cytoplasm by a nuclear membrane. Some fungi reproduce by budding (yeasts), whereas others form growing colonies of attached organisms (molds). Most pathogenic fungi exist in the nature as environmental saprophytes, and human infection does not appear to be necessary for their life cycle.


♣ Parasite: It is a general term often used in a narrow sense to refer to a variety of protozoan and multicellular eukaryotic organisms capable of causing disease.

Round Worm
Round Worm

Many parasites undergo complex life cycle that may involve several host species, including humans. The parasitic diseases remain major health problem and source of economic drain.