Thousands of different kinds of microbes are present in all eco-logical niches Some are beneficial ones, others are opportunists and some are harmful ones.
Infection is the establishment of the organisms in the tissues re-sulting in injury or harmful effect to the host. Infections may be endog-enous or exogenous. Endogenous infections are contracted from the host himself from the normal flora. Many areas of the body have nor-mal commensal flora. They have many functions. They provide barrier to the infection by competing for nutrition with pathogens. Some pro-duce vitamins which are useful for the host. Some produce colicins to act against pathogens. Generally they do not cause any infection. But there are exceptions. Streptococcus mitis is the normal flora of the mouth. It produces infection in the previously damaged heart valves through blood stream after tooth extraction . Streptococcus faecalis causes infective endocarditis and the source is the urinary tract and intestine of the host. Exogenous infections are derived from man, ani-mals and soil. Man gets the infections from patients suffering from dis-eases. Some persons may be carriers for the pathogens and they may transmit the diseases to others without getting affected.
Animals are important sources of infection. Such infections are known as zoonotic diseases. Spread of these diseases is usually from animal to animal . Man may be infected as an end host as in rabies. In some cases the infection may spread from man to man as in pneumonic plague.
Soil has also a role in the transmission of infection. Soil is the reservoir for the spores of Clostridium species and Bacillus anthracis.
There are five main routes by which a host may become infected.
1. The respiratory route
2. The alimentary tract
3. Genital tract
4. The skin and mucous membrane
Organisms causing respiratory infections are as follows. Streptococcus pneumoniae, Haemophilus influenzae, Mycobacterium tuberculosis, Bordetella pertusis are some of the bacterial pathogens. Common cold virus, influenza virus, adno virus are some of the viruses producing respiratory infections.
The intestinal diseases like cholera, bacillary dysentery, the enteric fever and bovine tuberculosis are contracted when the organisms are ingested. But in the case of entero virus infections (poliomyelitis) and Hepatitis though the organisms enter through gastro intestinal system, the effects are seen elsewhere in the body.
Organisms may be acquired from the skin as in the case of herpes virus infection or through wounds as in tetanus. Wounds may be formed from trauma or thorn pricks or needle stick injury. Organ-isms may also be introduced through animal bite as in the case of rabies or by insect bites as in dengue, malaria, filariasis, and yellow fever.
Syphilis , gonorrhea, hepatitis B and AIDS are some of the sexu-ally transmitted diseases. Treponema pallidum, Neisseria gonorrhoeae, Hepatitis B virus and Human Immunodeficiency Virus are the etiologic agents respectively.
Bacteria like T.pallidum, Viruses like rubella, cytomegalovirus, parasite like Toxoplasma gondii are some of the organisms that enter through placenta and cause disease in the newborn.
Pathogenicity refers to the ability of the organism to cause dis-ease. Virulence is the quantitative measure of this property. Virulence is not generally attributable to a single property but depends on several parameters related to the organism, the host and their interaction.
Microbes first enter the body, survive then multiply, elaborate many factors and produce the disease.
Pili are useful for the attachement of the organisms on the epithe-lial cells.
Capsules down regulate the secretion of cytokine. They inhibit leukocyte accumulation. They also induce the suppressor T cells and inhibit lymphoproliferation
3. Intracellular residence
The following microorganisms reside intracellularly and try to avoid host defense mechanisms. They are M.tuberculosis, M.leprae, S.typhi, T.gondii, L.donovani, H.capsulatum.
4. Production of enzymes
Some enzymes like proteases, DNAses, and phospholipases are produced and they help in disruption of cell structures and to hydrolyse host tissues. In Aspergillus species proteases help in invasion.
Bacteria produce both exotoxins and endotoxins which play and important role in the pathogenesis of disease
Exotoxins are produced by some organisms like C.diphtheriae, C.tetani, C.botulinum. The exotoxin produced by V.cholerae acts on the intestine and is called enterotoxin. The toxin produced by one type of Escherichia coli causes acute gastroenteritis.
Endotoxins are lipopolysaccharide cell wall of gram negative bacteria. They induce production of cytokines by different cells of im-mune system. Coagulation system and complement system are acti-vated. They also affect various organs like kidney, heart and lungs leading to organ failure.
6. Antigenic variation
Microorganisms evade the host immune responses by changing their surface antigens. N.gonorrhoeae very often changes its outer mem-brane protein. Antigenic drift and shift are common in influenza viruses. Trypanosoma brucei are covered with thick protein coats which un-dergo antigenic change during infection. Some organisms produce sur-face proteins that are similar to host proteins or coat themselves with host proteins that they are mistaken for part of the host itself
The distinction between the commensal and the organisms asso-ciated with disease is subtle. The definition of normal flora or pathogen is derived from the resultant complex interaction between the organism and its host.