How Bacteria Attack and Spread Infection in the Human Body or Host?

Firstly we will have to know about the Endogenous infection-if the hos defense of the patient is compromised, the microorganisms invade or contaminate an unusual site(urinary tract infections due to E.coli , pneumonia due to streptococcus pneumonia and wound infections and abscesses following abdominal or pelvic surgery due to  bacteroides spp). Exogenous infection-is a consequence of the transmission of new species or strains.Other infected persons(or even domestic animals in the case of food poisoning) either directly or indirectly are the most common source of infection for susceptible individuals.
       It is very important to know that about host, now i am writing the host which are available in the human body: skin, mouth and oro pharynx, esophagus and stomach, upper small intestine, lower small intestine, liver, pancreas, gallbladder, large bowl, outer ear, nose, naso pharynx, trachea and bronchi, alveoli,vagina, endocervix, urethra.
      The majority of microorganisms have nothing to do with human disease. They live in soil, water, vegetables, animals and plants.They fix nitrogen, degrade dead biological material, ferment fruit producing alcohol and vinegar,. Many bacteria live on and in the human body. They can be very helpful to us, but some of them are harmful and can cause infections.The production of clinical disease due to bacteria is a consequence of a complex interplay of vacterial properties and host factors.
      Colonization of the host is a prerequisite of any infection and this involves either attachment of bacteria to epithelial surface . M-proten layer of streptococcus pyogenes or direct inoculation through a wound. But, it is important to remember that colonization does not necessarily lead to infection. Colonization by highly pathogenic microorganisms without the outbreak of infection is also possible.
      The size of the inoculum is also important, requiring that approximately 10 by 10 organisms be ingested in the case of vibrio sholerae, 10 by 6 in the case of salmonella spp. After colonization, local infection may ensue at the portal of entry of the organism. Alternatively, organisms may invade, for example spread widely through the lymphaics or bloodstream and cause deep-seated infection. Common sites of invasion are the respiratory and gstrointestinal tracts. Hosts with lowered
resistance are very susceptible to infections by opportunistic pathogens from their own flora and also from the environmental flora.
     This type of host is called a compromised host.Normaly body flora-bacteria that coexist with the host and that are usually harmless and often beneficial. Host-an organism on (in) which bacteria live, feed or reproduce.Compromised host-the host whose defenses against bacterial invasion are weakened.Environmental flora-bacteria that live on non-living objects and in the air. Colonization-the presence of microorganisms at a body site doing no harm to the host cell(normal skin flora).Opportunistic bacteria-bacteria that  take advantage of a host,s lowered resistance to become pathogenic.