Exchange of genetic material
In this process , part of the genetic material of a donor cell is transferred to a recipient cell by one of the fallowing mechanisms :
a-Transformation
It is uptake of the naked free DNA of the donor bacteria by the recipient bacteria . The taken fragment is incorporated in chromosomal DNA and expressed as a new character in the recipient cell as seen in the figure.
b- Transduction
It is transfer of part of DNA of the donor bacteria to recipient bacteria by bacteriophage as seen in the figure below .
c-Conjugation
Conjugation occurs through mating of two bacteria cells one of them carries plasmid ( F + ) and the other is plasmid (F-) . Conjugation is the most common method occur in nature and it is a major phenomenon that is responsible for transfer o f antibiotic resistant in bacteria (fig.) .
Robert Koch was a pioneer in medical microbiology and worked on cholera , anthrax and tuberculosis , finally he proved the germ theory , in Koch s postulates , he set out criteria to proved if an organism is the cause of a disease and can be summarized as follows :
1- M.O must be present in every case of the disease but absent from healthy organisms.
2- The suspected M.O must be isolated and grown in a pure culture.
3- The same disease must result when the isolated M.O is inoculated into a healthy host.
4- The same M.O must be isolated again from the diseased host.
Mechanisms of bacterial pathogen
The steps when the pathogenic agents try to infect the host :
1-Adherence
2- Colonization
3- Evasion of host defenses
4- Causing disease
5- Exit from the host
Determinants of virulence
Pathogenic bacteria are able to produce disease because they possess certain structural biochemical or genetic traits that render them pathogenic or virulent such as :
1-flagella and pilli
2- capsule
3- enzymes such as hyaluronidase , streptokinase
4- toxin ( exotoxin & endotoxin)
5- lipopolysaccharid in outer membrane of G-ve bacteria .
Some of important definitions in medical microbiology
Pathogen : is a microorganism that is able to cause disease .
Pathogenicity : is the ability of M.O to produce disease in a host organism.
Virulence : refers to the degree of pathogenicity of the microbe.
Opportunistic pathogen
Some of the normal bacteria flora that live in a commensal relationship in a certain host without producing disease such as Staphylococcus aureus , Haemophilus influenza may causes disease if there is some compromise or weakness in the host s immunity .
Infection
The invasion of a host by M.O with subsequent establishment and multiplication of the agent ,an infection may or may not lead to cause disease .
Infectious disease
Any change from a state of health in which part or all of the host s body can t carry on it s normal functions because of the presence of an infectious agent or its product