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- 1.
- 2. PATHOGENS ■ Microbes thatcause diseases in host (animals, plants, human beings) PATHOGENECITY ■ Capability of microbial specie to cause disease PROCESS OF INFECTION
- 3. Types of microbesbased on infection ■ Virulent strain – if few cells of microbes cause disease then they are called as virulent strain ■ Avirulent strain – If microbial cells are not causing disease even in large number, it is called as avirulent strain ■ Attenuated strain – If virulent strain lose its virulence or pathogenicity then these are called as attenuated strain – Virulent strain lose its virulence when growing the microbe on other host sometime by culturing on laboratory media repeatedly. – Attenuated strains are used as vaccines to elicit immunity.
- 4. Characteristics of Pathogensor Virulent Strain ■ Pathogen must enter the host ■ Pathogen must metabolize and multiply in number in or on host tissues ■ Pathogen must resist host’s defense mechanism ■ Pathogen must damage the host
- 5. Infection ■ Infection – themost intimate way in which a microorganism may cause disease: – the host is invaded by the microorganisms which subsequently multiply in close association with the host’s tissues. ■ Infection can be through – Interaction of pathogen & host – Penetration through external / internal surfaces – Releasing certain virulent factor that act as antigen or it will produce certain toxins
- 6. Infection ■ Infection maybe – Localized – pathogen grow at port of entry – Generalized –pathogen distributed and grow throughout the body ■ On basis of virulence – Acute – disease for short time and is severe – Chronic –long lasting interaction of microbe & host causing mild disease for long time than it can be converted into severe form
- 7. Process of Infection ■Process of infection – 4 steps: – Microbial adherence – Penetration of epithelial cell layers – Growth in underlying tissues – Infection ■ Blood ■ Lymphatic fluid ■ tissues
- 8. Microbial adherence ■ Firststep in initiation of infection is usually adherence ■ Attachment of the pathogen to some surface of the host ■ Pathogen introduced directly into the tissues by wound, injection by an arthropod, or other similar means ■ May enter through exposed internal surfaces ■ After entry, attachment of host is selective ■ Various pathogens attach only to certain tissues ■ For example, – In case of bacteria, attachment by pili or by specific receptor protein or by tail spike or by hemagglutinin
- 9. Penetration to epithelialcell layer ■ Penetration into the body may be – Active directly penetrate by damaging cells; results in the formation of lesions (areas of damage) known as ulcers – Passive by any mechanism that is nothing to do with the properties of microorganism i.e by wounds, burns, lesions Example: ■ Soldiers wound in battlefield ■ Gas gangrene due to Clostridium perfringens (Gangrene is a type of tissue death caused by a lack of blood supply. Symptoms may include a change in skin color to red or black, numbness, swelling, pain, skin breakdown, and coolness. The feet and hands are most commonly affected. Certain types may present with a fever or sepsis) ■ Burns – may be infected by Pseudomonas aeruginosa ■ Arthropods – spirochetes may enter through bite of a tick or a body louse
- 10. Penetration to epithelialcell layer ■ After adherence, comes penetration to epithelial layer ■ But penetration is not always necessary for infection – Microorganisms merely multiply on epithelial surface and cause damage without penetration into the body – For example, Vibrio cholera, causative agent of diarrheal disease known as cholera multiplies on epithelial layer of small intestine where it produces a toxin that causes the loss of fluid from epithelial cells and kills the cells.
- 11. Growth in underlyingtissues ■ From epithelial layer, microbes will move or penetrate into underlying tissues ■ In tissues infection may be of 2 types – Localized infection – Generalized infection ■ Localized infection – E.g. infection by Staphylococcus aureus, where characteristic lesion is an abscess i.e., a walled-off cavity in the tissues containing the staphylococci, numerous white blood cells (that form pasty mass called pus) and dead, disintegrating tissue cells that are killed by toxins elaborated from staphylococci.
- 12. Generalized infection ■ Someinfections may not remain localized but may spread throughout the tissues ■ For example, – Anaerobic bacterium Clostridium perfringens, which cause wound infection gas gangrene. C. perfringens grows, produce toxins that kills healthy tissues This dead tissue becomes anaerobic, support growth of more clostridia, more toxins produced, more tissues killed, organism spread further Rapid tissue invasion – due to large amount of hydrogen gas by the bacteria; pressure of this gas separates connective tissue sheaths from muscle tissue, forming a space that is rapidly filled by clostridia containing fluid. Clostridia can invade quickly the entire length of a muscle. Amputation of affected limb is the only way to stop this spread to rest of body Growth in underlying tissues
- 13. Infection of LymphaticSystem ■ Many pathogenic microorganisms spread from initial site to lymphatic system ■ Microbes from tissues, and blood enter in lymph. ■ This lymph is collected into lymph vessels and then to lymph nodes ■ Lymph node contain cells called macrophages which engulf and destroy bacteria that enter the lymph node ■ Lymph node removes nearly all of the bacteria that enter it ■ And become inflamed, enlarged and sore when infected – condition called bubonic plague in which infection in lymph nodes occur
- 15. Infection of theBlood ■ Microbes enter into blood and circulate into whole circulatory system by entering through blood capillary or venule. ■ The presence of microbes in blood during circulation is called as bacteremia. ■ Once in the bloodstream, organism can be carried to various parts of the body and cause localized infections. ■ Neisseria meningitidis – present in nasopharynx can reach meninges (membrane that cover brain & spinal cord) and cause infection ■ Salmonella typhi enter into epithelium of small intestine, it then moves to lymph then to lymph nodes. If not destroyed there, move into blood and it will cause localized infection
- 16. Infection of tissues ■In some infections bacteria may actively multiply in the bloodstream and produce toxic products, these toxins move towards tissues where they produce infection – condition known as septicemia. ■ Septicemic infection begin as localized infection that later become generalized. ■ Septicemic infections may be – Chronic – Acute
- 17. REFERENCES ■ Pelczar Jr.;Chan, E.C.S.; Krieg, M.R. Microbiology, McGraw Hill, London. 1986. ■ Prescott, M. L., Microbiology, 5th Edition, ISBN: 0-07- 282905-2, The McGraw−Hill Companies, 2002.