Bacteria-HOST-PATHOGEN Relationships man

Any microorganism that is able to infect a host and produce disease is a pathogen. Bacteria can cause a multitude of different infections ranging in severity from inapparent to fulminating. The capacity of
the organism to cause disease reflects its relative pathogenicity. On this basis, bacteria can be
grouped into three major groups;

a) Frank / Primary Pathogens
These are those bacteria that when isolated from patients are
considered to be the agents of the disease. They always cause disease when they infect a host
e.g. Neisseria gonorrhoea, Yersinia pestis
b) Opportunistic pathogens
These rarely if ever cause disease in immunocompetent hosts but
can cause serious infection in immunocompromised hosts. Usually, they are members of the body's normal flora that take the opportunity offered by the reduced host defences to cause
disease e.g. Staphylococcus epidermidis, Pseudomonas aeroginosa, Mycobacterium avium
c) Non Pathogens
These rarely or never cause human disease. This categorization of non pathogens may change because of adaptability of bacteria and detrimental effects on resistance mechanisms of host e.g. Archaea, Lactobacillus acidophilus

Normal Flora
Generally, opportunistic and non pathogenic bacteria are found in living tissues as normal flora.
Normal flora (normal microbiota, commensals, indigenous organisms) refers to bacteria which do not normally cause diseases and can be found commonly in healthy individuals.
They may cause diseases
(1) in an immunocompromised host,
(2) after being introduced into different body sites,
(3) overgrow in favorable conditions.
A human first becomes colonized by a normal flora at the moment of birth and passage through the birth canal. In utero, the fetus is sterile, but when the mother's water breaks and the birth process begins, so does colonization of the body surfaces. Handling and feeding of the infant after birth leads to establishment of a stable normal flora on the skin, oral cavity and intestinal tract in about 48 hours. In a healthy animal, the internal tissues, e.g. blood, brain, muscle, etc., are normally free of microorganisms. However, the surface tissues, i.e., skin and mucous membranes, are constantly in contact with environmental organisms and become readily colonized by various microbial species that form the normal flora of these membranes.