1. 1 Host-Parasite Interaction Overview  The first step in host-parasite interaction is the encounter between the microbes and the host  The methods can be: 1. direct contact 2. inhalation 3. ingestion 4. intimate sexual contact 5. arthropod vectors 6. contaminated needles and syringes The following steps are one of two possible situation: 1. The microbes will be lost without any significant interaction with the host 2. The microbe may establish a relationship with the host The relationship could be: 1. Commensalism 2. Symbiosis 3. Parasitism

2. 2 Infection of the host involves the production of structure and/or product that allow the microbe to adhere and colonize a host surface.  This include structure and products such as pili or fimbriae In parasitism, damage to the host may result from pathogen-encoded structures or products and/or the host response to the infection.  This damage results in the production of a set of signs and symptoms (e.g. inflammation, fever, sneezing, coughing, aches and pain) characteristic of a disease state.  Commensal organisms didn’t cause any damage to the host, → an important asset to the host (supplying important nutrients, occupying places). They are called normal or resident flora for the host.

3. 3 For a pathogen: 1. Exposure and adherence Begin with break or wound in the skin or on the mucous membran 2. Entry of the microbe or a microbial product(s) across an epidermal or epithelial cell surface to enter deeper well-vascularized tissue (lymph and blood circulation) 3. Colonization 4. Multiplication to increase cell number and/or the level of one or more cell products along with local and/or systemic spread/dissemination Specific adherence  Most pathogen do not adhere to all epithelial cells  Some macromolecule responsible for bacterial adherence are not covalently attached to the bacteria  Slime layer (polymer fiber) and capsule may be important for adherence to other bacteria as well as to host tissue  Fimbriae and pili may also function in the attachment process

4. 4 Invasion  Penetrate the epithelium to initiate pathogenicity  At the point of entry, usually at small breaks or lesions in the skin or in mucosal surface, growth is established  Growth may also begin on intact mucosal surfaces, especially ifthe normal flora is altered or eliminated  Pathogen growth may also be established at sites distant from the original point of entry Colonization and Growth  Temperature, pH, and the presence or absence of oxygen affect pathogen growth  Availability of microbial nutrients is most important. (vitamin, growth factors, trace elements)  After initial entry, some pathogens remain localized, multiplying and producing a discrete focus of infection such as the boil that may arise from Staphylococcus skin infection  If pathogen reaches the blood, it will be distributed to distant part of the body,usually concentrating in the liver or spleen

5. 5 Virulence Factors ♥ Virulence factor are structures or substances that aids pathogenic bacteria in the infection process and/or contribute towards the symptoms of disease ♥ Virulence factors are generally exported to the bacterial cell surface or beyond ♥ Virulence factors are not necessarily constitutively produced, and it appears that several are made only during the course of an infection and not when the pathogen is growing outside the host A few microorganisms are pathogenic solely because of the toxins they produce → do not need to gain access to host tissue 1. Exotoxin toxic protein released from the pathogen cell as it grows There are 3 categories: Cytolytic toxins, degrading cytoplasmic membrane integrity, causing lysis The AB toxins, the B component generally binds to a host cell surface receptor, allowing the transfer of the A subunit across the targeted cell membrane

6. 6 The superantigen toxins, stimulating large numbers of immune cells, resulting in extensive inflammation and tissue damage Cytolytic toxins  The toxins are called hemolysins  Some hemolysins attack the phospholipid of the host  -toxin Clostridium perfringens, Streptolysin O, Staphylococcus  -toxin AB Exotoxins - Diphtheria toxin : Corynebacterium diphtheria - Exotoxin A from Pseudomonas aeruginosa - Clostridium tetani and Clostridium botulinum Enterotoxins  Exotoxins whose activity affect the small intestine,generally causing massive secretion of fluid into the intestinal lumen resulting in vomiting and diarhea.  Vibrio cholerae, Salmonella enteritidis, Bacillus cereus

7. 7 ToxR and Cholera  ToxR is a transcription regulatory protein produced by Vibrio cholerae. It stimulates the transcription of virulence genes that are in the ToxR regulon  The toxR gene product that controls cholera A dan B chain production and virulence factors  Cholera toxin is an AB toxin consists of one A subunit and five identical B subunits.  In the gut, the B subunit binds specifically to GM1 ganglioside and targets the toxins specifically to the intestinal epithelium  The toxic action is a function of the A chain which crosses the cytoplasmic membrane and activates adenyl cyclase, enzyme that converts ATP to cAMP  cAMP is a mediator of many different regulatory systems in cells including ion balance.  Increasing cAMP induce secretion of chloride and bicarbonate ion from the mucosal cells into the intestinal lumen → secretion of large amounts of water into the intestinal lumen

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9. 9 2. Endotoxins  Many Gram negatif bacteria produce toxic lipopolysaccharides as part of the outer layer of their cell envelope. These lipopolysaccharides are called endotoxins  Endotoxins are bound and released in large amount only when the cell lyse  Endotoxins cause a variety of physiological effect: fever, diarrhea, rapid decrease of lymphocyte, leucocyte, release of cytokines  Large doses of endotoxins can cause death from hemorrhagic shock and tissue necrosis Quorum Sensing Many bacteria sense and respond to the presence of and communicated with, other bacteria in their environment. This interactions are cell density dependent and may occur between individual cell of a single species and/or with other bacteria → quorum sensing Each member in the population releases a signal molecules called an autoinducer (AI) or pheromone, which can be sensed by neighboring cell

10. 10 Once a threshold of autoinducer is reached, the bacteria respond by activating a transcriptional regulator protein that induces or represses a set of genes → this allows bacteria to gauge the cell density of their own population and/or other bacteria. In Vibrio fischeri, AI is produced by the luxI protein and binds to the luxR protein; the luxR-AI complex than transcriptionally activates genes necessary for bioluminescence as well as LuxI itself  It is now apparent that many pathogen link quorum sensing to virulence factor secretion  In the early of an infection there are likely sufficient nutrients to support growth, but as cell members increase, nutrients became limiting → produce virulence factor to cause the release of nutrients from host tissue / cells → by that point, they can withstand the host defenses  The mechanisms used by Gram + and Gram – bacteria are fundamentally different

11. 11 Gram – bacterial pathogen  Most quorum sensing systems identified in Gram – bacteria use N-acyl homoserin lactones (AHL) as their autoinducer (AI) molecules.  This molecules bind to and activate a transcriptional regulator or R protein which regulates the expression of various target genes  The best characterized quorum sensing system related to virulence are those of Pseudomonas aeruginosa In humans, it is a frequent opportunistic pathogen of patients who : 1.are immunocompromised (cancer or AIDS patients) 2.are suffering from burns 3.have indwelling medical devices 4.are undergoing prolonged aggressive antimicrobial therapies or 1. are suffering from cystic fibrosis

12. 12 Virulence genes controlled by quorum sensing include lasA (staphylolytic protease), lasB (elastase), apr (alkaline protease), toxA (exotoxin A) and xcp (encode a type II secretion system) The las system is composed of the LasR transcriptional activator and LasI autoinducer synthase, which synthesizes the cognate AI for this system Gram + Bacterial Pathogens  The quorum sensing systems of Gram + bacteria are fundamentally different from those of Gram – bacteria  They do not produce AHL and use sensor-kinase/ regulator two component signal transduction systems  Instead of AHL molecules, they produce peptide signals that interact with the sensor-kinase, which than phosphorylates the cognate regulator protein

13. 13 A good example is in regulating of Staphylococcus aureus virulence  The regulation of virulence factors is under the control of two regulatory loci: agr (accessory gene regulator) and sar (staphylococcal accessory gene regulator)  The agr locus contains two divergently transcribed operons encoding two polycistronic mRNAs referred to as RNAII dan RNAIII  The agrBDCA operon (RNAII) encodes AgrC, a signal transducer protein and AgrA, a response regulator.  AgrB and AgrD are involved in producing the quorum sensing signal molecules, an octopeptide cleaved from AgrD and exported extracellularly by the AgrB protein.  During quorum sensing, AgrC binds to the peptide signal molecule, phosphorylates itself (autophosphorylation) and then phosphorylates AgrA  Phospho- AgrA then induces RNAIII expression, which goes on to stimulate the expression of numerous secreted products