Microbiology lec2

Medical MicrobiologyMedical Microbiology

Lecture 2Lecture 2

Dr. Saleh M Y OTHDr. Saleh M Y OTH

PhDPhDMedical Molecular Biotechnology and Infectious DiseasesMedical Molecular Biotechnology and Infectious Diseases

02/10/201002/10/2010IMS - MSUIMS - MSU

Infection: The growth of microorganisms in the host without causing damages to the host.

Definaations

Bacteria may be divided into three types based on their potential to cause diseases;

- pathogens, - commensals, and - nonpathogens.

Must to know: Morbidity, mortality, infections, parasite, host, vector, fomite, contagious (infectious) disease, epidemic, endemic, pandemic, Zoonosis, Epizootic, Attack rate.

Normal flora;Normal flora; (commensals, indigenous organisms): (commensals, indigenous organisms): Bacteria which do not normally cause diseases and can Bacteria which do not normally cause diseases and can be found commonly in healthy individuals. be found commonly in healthy individuals.

They may cause diseases They may cause diseases (1) in an immunocompromised host, or (1) in an immunocompromised host, or (2) after being introduced into different body sites, or (2) after being introduced into different body sites, or (3) overgrow in favorable conditions. (3) overgrow in favorable conditions.

They are considered as They are considered as opportunistic pathogensopportunistic pathogens. .

Nonpathogens:Nonpathogens: Certain bacteria, such as Certain bacteria, such as Lactobacillus acidophilusLactobacillus acidophilus, almost never cause disease., almost never cause disease.

Normal flora;Normal flora;

Frequently found on or within the body of healthy persons, protect the host by protect the host by competing with pathogens. competing with pathogens.

* * 101066 streptomycin-resistant streptomycin-resistant Salmonella Salmonella were were needed to infect an experimental animal. needed to infect an experimental animal.

* * Fewer than 10 cells were needed to infect an Fewer than 10 cells were needed to infect an animal pre-treated with streptomycin to suppress animal pre-treated with streptomycin to suppress the normal flora in the GI tract. the normal flora in the GI tract.

The composition of the The composition of the normalnormal flora varies in flora varies in different body sites. different body sites.

Ultimately, the flora composition is determined Ultimately, the flora composition is determined by by ecological factorsecological factors including the presence including the presence of receptors of host cell surface for bacterial of receptors of host cell surface for bacterial adherence, pH, oxygen, availability of adherence, pH, oxygen, availability of nutrients, water, host defense and personal nutrients, water, host defense and personal hygiene. hygiene.

Some tissues, organs are dense with normal flora, others are normally sterile

a. colonized sitesi. alimentary/intestinal tractii. upper respiratory tractiii. distal genitourinary tractiv. skin

b. normally sterile sitesi. bloodii. CSFiii. interstitial fluid and spacesiv. lymph

Disease; damage caused by presence of microorganisms or their products.

Colonization - presence of microorganisms without disease at that point.

This term applies to surfaces only, i.e., the blood cannot be colonized and host cells with intracellular infection are not colonized.

Carrier state; colonization with a pathogen

Pathogen; any organism that has the potential to cause disease

Pathogenicity:Pathogenicity: The ability of a bacterium to The ability of a bacterium to inflict damages on the host. inflict damages on the host.

Virulence:Virulence: The degree of pathogenicity The degree of pathogenicity associated with a bacterium. associated with a bacterium.

Virulence factorsVirulence factors: Bacterial products or : Bacterial products or strategies that contribute to virulence. strategies that contribute to virulence.

Adherence and colonization:Adherence and colonization:

Bacteria need to avoid being washed away by Bacteria need to avoid being washed away by non-specific host defense mechanisms (e.g., non-specific host defense mechanisms (e.g., saliva). saliva).

Most bacteria possess specific adherence factors which Most bacteria possess specific adherence factors which bind receptors on the surface of the host cells or tissue.bind receptors on the surface of the host cells or tissue.

The specific interaction between bacterial The specific interaction between bacterial adhesinsadhesins and host and host receptorsreceptors explains the observed tissue explains the observed tissue specificityspecificity of bacterial of bacterial infections. infections.

For example, For example, N. gonorrhoeaeN. gonorrhoeae adheresadheres better to human urogenital better to human urogenital epithelia than to other tissue. epithelia than to other tissue.

Pili and Afimbrial Adhesins

bacterium

host cell receptors

pili with adhesins at the tip

afimbrial adhesins

host cell receptors

bacterium

Pili - fimbrial Adhesins

Afimbrial Adhesins (No pili)

Adherence and colonization ……..Adherence and colonization ……..

Pili (fimbriae): Pili (fimbriae): E. coliE. coli as an example.. as an example..

Most the strains of Most the strains of E. coli E. coli do not adhere to the do not adhere to the small intestine epithelia. small intestine epithelia.

They are normally found in large intestine. They are normally found in large intestine.

Pathogenic strains can colonize small intestine and Pathogenic strains can colonize small intestine and cause diarrhea. cause diarrhea.


Assignment (1): write in 6 pages about the pathogenic strains of E. coli.

The ability of the pathogenic strains of to cause The ability of the pathogenic strains of to cause diseases is intimately associated with the expression diseases is intimately associated with the expression of various pili. of various pili.

E. coliE. coli colonization factor antigen I (CFA) pili found in colonization factor antigen I (CFA) pili found in enterotoxigenic enterotoxigenic E. coliE. coli ( (ETECETEC) strains. ) strains.

ETECETEC strains adhere to small intestinal mucosa and strains adhere to small intestinal mucosa and produce symptoms by elaborating toxins that act on produce symptoms by elaborating toxins that act on mucosal cells to cause diarrhea.mucosal cells to cause diarrhea.


Assignment (1): write in 6 pages about the pathogenic strains of E. coli.

Nonfimbrial AdhesinsNonfimbrial Adhesins (Afimbrial Adhesins) (Afimbrial Adhesins)

- S. aureus- S. aureus: Lipoteichoid acid : Lipoteichoid acid

- Staphylococcus- Staphylococcus: Slime : Slime

- Group A streptococci- Group A streptococci: F protein (non-fibrillar, : F protein (non-fibrillar, mediates attachment to fibronectin). mediates attachment to fibronectin).

- Streptococcus pneumonia- Streptococcus pneumonia: Capsule: Capsule


Growth:Growth: Bacteria have Bacteria have two distinct phasestwo distinct phases in their life in their life cycle: cycle: activeactive or or vegetativevegetative stage and stage and inactive inactive or spore-forming or spore-forming stage. stage. Bacteria need nutrients to grow. Bacteria need nutrients to grow.

Iron is an essential growth factor for many Iron is an essential growth factor for many bacteria. bacteria.

At neutral pH, iron usually exists in a highly At neutral pH, iron usually exists in a highly insoluble ferric state (Fe3+). insoluble ferric state (Fe3+).

Within human body;Within human body;

transferrin, lactoferrin, ferritin and hemin bind transferrin, lactoferrin, ferritin and hemin bind most of the available iron. most of the available iron.

Many bacteria produce iron-chelating Many bacteria produce iron-chelating compounds call siderophores to help getting compounds call siderophores to help getting iron. iron.

Growth …..Growth …..

Within human body;Within human body;Siderophores are low molecular weight Siderophores are low molecular weight compounds that chelate iron with very high compounds that chelate iron with very high affinity. affinity.

The siderophores are excreted into the The siderophores are excreted into the medium and then the iron-siderophore medium and then the iron-siderophore complex is taken up by the cells through complex is taken up by the cells through receptors. receptors.


Within human body;Within human body;

Many bacteria also can Many bacteria also can useuse transferrin, transferrin, lactoferrin, ferritin or lactoferrin, ferritin or heminhemin as a source of as a source of iron. iron.


Mechanisms of Bacterial Pathogenesis

EXPOSURE TO PATHOGENS

ADHERENCE (ADHESION), COLONIZATION, GROWTH

INVASION

TISSUE DAMAGES, DISEASE

ESCAPE HOST DEFENSES

TOXICITY

Virulence factors are bacterial strategies or products which contribute to each of these steps leading ultimately

Functions and stages of pathogens and disease

1. encounter2. entry3. spread (+/-)4. multiplication5. evasion of host defenses6. damage7. outcome

a. transmission to new host (+/-)b. recovery or not

Growth and Toxins produced by pathogens Growth and Toxins produced by pathogens

Toxins: Toxins:

Toxicity Toxicity

Exotoxins:Exotoxins: Toxic proteins, many are excreted into the Toxic proteins, many are excreted into the medium; may damage host tissue away from where medium; may damage host tissue away from where bacteria infections occur. bacteria infections occur.

Endotoxins:Endotoxins: LPS, embedded in the G- organism outer LPS, embedded in the G- organism outer membrane, released during bacterial lysis. membrane, released during bacterial lysis.

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Microbiology lec2