Pathogenesis Of Bacterial Infections Flashcards
Pathogenicity
Ability of microbial species to produce disease
Virulence
Relative degree of pathogenesis,which may vary b/w diff strains of same organism
Exaltation
Enhancement of virulence
Attenuation
Reduction of virulence
Route of transmission
•Streptococci
•Vibrio cholerae
•initiate infection by any route
•infective orally,not infective subcutaneously
•Contact transmission
•Droplet
•MDR organisms in hospitals like S.aureus,E.coli,Klebsiella etc nd Bacillus anthracis
•Meningococcus,C.diphtheriae,Pneumococcus
•Aerosol
•Ingestion
•Vector borne
•M.tuberculosis
•Salmonella nd Shigella, Vibrio nd diarrheagenic E.coli, Campylobacter, agents of food poisoning
•Rickettsiae,Borrelia
•Sexual
•Vertical
•Birth canal
•Gonococcus, Chlamydia trachomatis, Treponema pallidum, Haemophilus ducreyi
• Treponema pallidum
•Listeria, Streptococcus agalactiae
Infective dose
•factors on which it depends
Minimum inoculum size capable of initiating an infection
• Virulence-inverse relation
Host’s age and immune status
Ability of organism to survive in gastric acidity(Shigella✅ Vibrio❎)
Low infective dose
•Shigella-as low as 10 bacilli
•E.coli O157-H7(<10 bacilli)
•Campylobacter jejuni-500 bacilli
Large infective dose
•E.coli-10^6-10^8 bacilli
•Salmonella-10^3-10^6
•Vibrio cholerae-10^6-10^8
Adhesion
Initial event in pathogenesis
Mediated by adhesins-that bind to specific host cell receptors
ADHERENCE- prevents bac from being flushed away in secretions,facilitate bacterial invasions
•Fimbriae or pili
•Non pilus adhesins
• Bind to sugar residues(glycoprotein or glycolipids) on host cells
• M protein(Streptococcus pyogenes), lipoteichoic acid(gram positive cocci), cell surface lectin(Chlamydia)
Biofilm formation
Strong adherence to structures like catheters,prosthetic implants,heart valves
•Biofilm-grp of bacterial cells which stick to each other on a surface and are embedded within layer(slime layer) of a self produced matrix of extra cellular polymeric substance called glycocalyx
Invasion
•Highly invasive pathogens
•less invasive
Entry of bac into host cells
•spreading or generalised lesions (streptococcal infections)
•localised lesions (staphylococcal abscess)
*Pathogens causing fatal diseases but lack invasiveness , remain confined to site of entry, produce disease by elaborating a potent toxin(C.tetani)
Virulence factors helping in invasion
•Virulence marker antigen or invasion plasmid antigen in Shigella
•Enzymes-Hyaluronidase,collagenase,streptokinase,IgA proteases
Antiphagocytic factors (strategies to evade phagocytosis)
▪️Capsule-prevent phagocytes from adhering to bacteria
Produced by-Neisseria meningitidis
Streptococcus pneumoniae
Haemophilus influenzae
Klebsiella pneumoniae
▪️Cell wall proteins-help in invasion
•Protein A of Staphylococcus aureus binds to IgG nd prevents the activation of complement
• M protein of Streptococcus pyogenes
▪️Cytotoxins-interfere with chemotaxis or killing of phagocytes
S.aureus produces haemolysins nd leukocidins-lyse nd damage RBCs nd WBCs
•Facultative intracellular bac
• Obligate intracellular bac
•Salmonella typhi,Brucella legionella,Listeria,Nocardia,Neisseria meningitidis, Yersinia mycobacterium tuberculosis
•Mycobacterium leprae,Rickettsia,chlamydia,Coxiella burnetti
Endotoxins
•Lipid A portion of lipopolysaccharide
•integral part of cell wall of gram -ve
•released by cell lysis
•highly heat stable
•only large doses are fatal
•poorly antigenic
•no effective vaccine
•mode of action -⬆️IL-1 nd TNF-alpha
Mode of action of endotoxins
•Macrophage activation
•Complement activation
•Endothelial activation
•coagulation pathway activation
•platelet activation
•mast cell activation
•gram -ve septicemia
Exotoxins
•proteins
•secreted by both gram -ve nd gram +ve
•active secretion
• heat labile,destroyed at 60
•mostly enzyme like action
•more potent-small doses r fatal
•highly antigenic
•toxoid forms r used as vaccines
Exotoxins
•High potency-eg Botulinum toxin is most potent
•Used for vaccine-converted to toxoids by treatment with formaldehyde
Toxoids-lack toxicity,retain antigenicity
•Specific action on particular tissue-eg tetanus toxin for CNS
