Microbiology Flashcards
(25 cards)
Healthy periodontium
G+ve, aerobic bacteria predominate in health
Low numbers, high diversity
Streptococcus, Actinomyces
Main nutrition sugars from host diet
Bacteria found in gingivitis vs periodontitis
Gingivitis
Increased plaque biomass → host inflamm response → increased GCF flow - increased nutrient supply for plaque bacteria
Non-bleeding gingivitis:
- Actinomyces Naeslundii and A. Israelii
Bleeding gingivitis: assoc w black pigmented bacteria (Prevotella denticola)
Over time,
Decreased redox potential by decreased O2 due to bacterial metabolism - more anaerobic
Supragingivally along gumline: overgrowth of commensal bacteria
Subgingival decreased redox potential due to gingival swelling
Move towards capnophilic and G-ve anaerobic bacteria w orange complex bacteria (Fusobacterium nucleatum)
Periodontitis
- Collapse in diversity, increase in number
- Increasing GCF flow and deepened pocket (reduced redox potential) → promotes growth of g-ve obligate anaerobes which metabolise amino acids (asaccharolytic)
- Porphyromonas gingivalis, Treponema denticola, Tannerella forsythia and Aggrebacter actinomycetemcomitans in aggressive forms
- Energy generated by AA metabolism liberates NH3 → increased alkalinity in pocket
- Bacteria form proteases which digest proteins in periodontium and GCF
Bacterial factors involved in the aetiology of PD
Attachment
- Cell surface adhesins - eg fimbraie (P.g.)
Multiplication at susceptible site
- Enzymes to obtain nutrients - proteases, haemolysin (P.g.)
- Production of bacteriocins
Evasion of defences
- Capsule production (P.g.)
- Leukotoxin production (A.a.)
- Ab-specific proteases
- Complement-degrading proteases
Direct Tissue damage
- Proteolytic enzymes - Collagenase,
Hyaluroniase
- Trypsin-like protease (A.a.)
- Gingipains (P.g.)
- Bone-resorbing factors
- LPS, cytotoxins
- Metabolic pdts: short chain fatty acids, ammonia, volatile sulphur cmpds
Indirect Tissue damage
- Inflammatory response to plaque antigens
Ecological plaque hypothesis
Keystone pathogen hypothesis
IMPEDE model.
Ecological plaque hypothesis:
Disease is a result of imbalance in microflora due to ecological stress which enriches oral pathogens
Microenvironment is modified to favour pathogens
Keystone pathogen hypothesis
Keystone pathogens (P. gingivalis) trigger inflammation even if present in low numbers
Cause normal microbiome to become dysbiotic and disease-provoking in susceptible host
Manipulates native immune response
Inflamm byproducts (degraded collagen peptides & heme-containing cmpds) sustain dysbiotic microbiota - must modulate host in adjunct to AB measures
IMPEDE: inflammation-mediated polymicrobial-emergence and Dysbiotic-exacerbation model
Inflammation as the principal driver of periodontitis
Inflammation
Polymicrobial diversity
Dysregulated inflammation & pocket formation
Inflammation-mediated dysbiosis & further tissue damage
P. gingivalis
Porphyromonas gingivalis
Attachment to host
- Fimbriae of P. gingivalis binds to and invades of epithelial cells
Multiplication at disease state
- Haemolysin - releases haemoglobin frm erythrocytes
- Lysine-x protease - produces hemin from haemoglobin
- Proteases degrade components of immune system: immunoglobulins, complement, cytokines
Evasion
- Capsule
Direct tissue damage
- Gingipains: lys-x and arg-x - surface adhesins that degrade collagen, fibrinogen, fibronectin, lamelin and host protease inhibitors TIMPs
Who produces leukotoxins
Role of leukotoxins
Aggrebacter actinomycetemcomitans produces leukotoxins:
- Lyses human neutrophils, monocytes, and lymphocytes
Treatment of periodontal diseases
Plaque control
Antibiotic therapy
Slow release of high doses of ABs locally
Define apical periodontitis and the ultimate goal of endodontic (root canal) therapy
Apical periodontitis: an inflammatory process in the periradicular tissues, primarily caused by a polymicrobial infection within the root canal system
Goal of endo therapy:
Elimination of causative agents, thus providing a sterile environment conducive to healing.
Complications from apical periodontitis
Pain on biting
Tooth mobility
Tooth loss
Routes of entry of bacteria
Caries
Mechanical exposure
Trauma
Anachoresis (bacteraemia)
Periodontal pocket - (when pocket reaches apex)
Bacterial succession in endodontic infection
Endo Microflora initially is predominantly G+ve facultative anaerobes and some G-ve
On exposure of root canal system to environment:
- G-ve microflora enters from gingival sulcus
- Autogenic succession occurs: by consuming away the oxygen available, bacteria are driving the change in the environment to favour G-ve anaerobes
- Redox potential decreases over time, allowing completely obligate anaerobes to establish
Anatogonistic and synergistic relationships
Antagonistic:
- Competition for nutrients
- Production of toxic metabolites
- H2O2, ammonia, sulphur cmpds, acids, nitrite
- Bacteriocins (antimicrobial cmpds produced by bacteria)
Synergistic:
- Prevotella oralis needs syn. Rs
- Enterococcus faecalis did not rely on SRs for survival
Where are microbes thought to be located in an endodontically infected tooth?
Root canal
Lateral/accessory canals
Dentine tubules
Extra-radicular
Found in a biofilm → chemo-mechanical preparation thus essential:
- Instrumentation to disrupt biofilm (challenges - anatomical complexities of root canal system - complete cleaning impossible)
- Irrigation (sodium hypochlorite; challenges - infiltration into dentinal tubules problematic & toxic if extruded into ST)
- Medication (CaOH - pH 12.5 strong antimicrobial, inhibits inflammatory root resorption, induces apical hard tissue barrier)
Secondary endodontic reinfection -
- Resistant species: Enterococcus faecalis (G+ve facultative anaerobe, source: contaminated fermented food eg cheese), Candida species (resist alkalinity via proton pumps + biofilm formation)
- Extra-radicular bacteria: Actinomyces & propionibacterium
Nucleotide metabolism
Nucleoside = ribose sugar + nitrogenous base
Nucleotide = ribose sugar + base + phosphate
Deoxyribonucleotide = deoxyribose sugar + B+P
Nucleotide synthesis:
De novo pathway: from molecules/atoms
Salvage pathways: recycling from nucleic acid breakdown
Cancer chemotherapy target
Targets cancer via DNA snthesis via the nucleotide synthesis of thymine
- Pyrimidine analogs destroy thymidylate synthase (catalyst for attachment of methyl group to form dTMP - unique to DNA synthesis)
- Folate analogs – interfere w formation of methyl group, targeting dihydrofolate reductase
Fluorouracil
Suicide/Mechanism-based inactivator (irreversible enzyme inhibitor)
- I2 (Fluorouracil) + E → EI1 → EI2 (Inactive enzyme)
- Fluorouracil is converted to deoxy-fluorouracil monophosphate, which binds irreversibly (covalently) to the enzyme - destroying further enzyme catalysis
- However side effects on dividing noncancerous host cells
Streptococcus
Characteristics
Virulence factors
Diagnosistical tests (brief)
G+ve, facultative anaerobe
3 types haemolysis
Alpha - incomplete lysis of RBCs and Hb breakdown
Beta - complete lysis of RBCs & Hb breakdown
Gamma - no haemolysis
Virulence factors: S. pyogenes (GAS)
- Adhesins
- Capsule contains Group specific carbohydrate (hyaluronan) - helps evasion of immune system phagocytosis
- M protein
- fimbriae-like extensions with double a-helix - rare shape in bacteria, common in mammalian proteins - induces anti-M Abs and cytotoxic T cells → pathogenesis of rheumatic fever
- Inhibit phagocytosis by inhibiting complement tagging of bacterial surface
- Adheres to host
- F proteins - adherence - fibronectin-binding proteins help in attachment
Secreted factors
- Streptokinase - promote spread of GAS through tissues - used therapeutically to stop blood clots
- Hyaluronidase - dissolves hyaluronic acid, promoting mvmt of bacteria thru tissue
- Superantigens
Eg Strep M protein & SPEs (streptococcal pyrogenic exotoxins)
Induces huge immune response (T cell activation) with pathological consequences = toxic shock
ID tests:
Cultivation of sample on blood agar plate for B-hemolytic colonies
Followed by antigen test, antibiotic resistance testing
Oral Streptococci belonging to the viridans group are associated with what type of human diseases?
Name several diseases associated with Group A Streptococci (GAS)
Explain the mechanism associated with Rheumatic fever.
Dental Mgmt , Treatment of Streptococci
- Caries
Endocarditis - GAS
URT: pharyngitis (strep throat), otitis media
Skin: scarlet fever, impetigo, cellulitis - Rheumatic fever
GAS antigen mimics M protein on heart
Immune cells attack heart cells, causing heart valve tissue damage and scarring - B-lactam, so penicillin. If allergic, cephalosporins. For severe invasive GAS infections, tx w clindamycin (ribosomal Ab) which decreases protein synthesis and elaboration of toxins.
Identification:
Staphylococci and Streptococci have the same cell morphology when viewed under a microscope. What biochemical test can be used to identify Staphylococci?
Coagulase test
Other tests to identify S. aureus
- Catalase test (hydrogen peroxide → water + O2)
Staphylococcus: catalase positive
Streptococcus: negative - Coagulase test: differentiates btw S. aureus (+ve) and S. epidermidis (-ve). Coagulase - an extracellular protein binds to prothrombrin → activated thrombrin
- Deoxyribonuclease and haemolysin tests
Staphylococcus
Characteristics
Virulence factors
Treatment & MRSA
G+ve bacteria, facultative anaerobe
S. aureus → mainly nasal passages, pigments golden/yellow
S. epidermidis → skin, pigments white
Associated infections
Pyogenic infections: boils, abscesses, impetigo, wound infections, pneumonia, root canal infection, osteomyelitis, angular stomatitis/cheilitis, septicaemia (sepsis)
Toxin-mediated: toxic shock, scalded skin syndrome, food poisoning (from enterotoxins)
Infected catheters, infected prostheses, endocarditis, meningitis, septicaemia
Virulence factors:
Adhesins - laminin and fibronectin - attach to extracellular matrix proteins of epithelial and endothelial surfaces in blood clots
Capsule of polysaccharide - impedes phagocytosis
Coagulase - causes localised clotting - protection from phagocytosis
Endotoxins - damage membranes
a-toxin
Causes release of pro-inflammatory cytokines
→ Septic shock in severe infections
B-toxin
A. sphingomyelinase damages sphingomyelin-rich membranes
Leukocidin
Kills leukocytes
→ assoc w necrotising skin infections
Proteases
Hyaluronidase - breakdown of CT for spreading bacteria
Catalase - protects against phagocytic oxidative burst
B-lactamase - penicillin resistance
Staphylokinase - dissolves fibrin clots → spread bacteria
Pyrogenic toxin Superantigens (PTSAgs)
Enterotoxins - diarrhoea, vomiting when ingested - food poisoning
Toxic shock syndrome toxin (TSST-1)
Treatment:
No vaccine - alternative: biological control replacement therapy with less virulent strain (502A) in nasal mucosa of newborns
antibiotics, penicillin
However, MRSA!
Methicillin resistant Staphylococcus aureus
Multiple drug resistance to beta-lactam antibiotics, antiseptics and disinfectants (quartenary ammonium compounds) → removal difficult from hospitals
Mycobacterium tuberculosis characteristics
No Gram classification. Obligate aerobe, facultative intracellular parasite grows in macrophages.
Predisposing factors for MTB infection
Close contact with large pops of people
Poor nutrition
IV drug use, alcoholism
HIV infection (400x rate)
Virulence factors
Cell wall is lipid-rich affecting permeability
Mycolic acids prevent attack by lysozyme, oxygen radicals, complement
Cord factor inhibits PMN migration
Slow generation time - immune system may not be triggered
TB infection VS disease
TB Infection
TB disease in lungs
MTB present
present MTB
Tuberculin skin test +ve
+ve tuberculin skin test
Chest Xray normal
Cavity in chest xray
Sputum smears and cultures negative
Positive sputum smears and cultures
Not infectious
Not defined as case of TB
INFECTIOUS
Defined as case of TB
MTB disease symptoms
Fever, night sweats, weight loss, persistent cough (>/3wks), constant tiredness and loss of appetite
Diagnosis usually chest x-ray
Stages and Patient outcomes when exposed to TB bacteria
Stage 1
Droplet nuclei inhaled
MTB reach alveoli of lungs
MTBacteria taken up by alveolar macrophages
Macrophages are not activated and can’t destroy MTB (because of lipid shell)
Stage 2
MTB multiples exponentially in inactivated alveolar macrophages
Stage 3
Lymphocytes infiltrate: T and B cells surround infected macrophages forming granuloma
T cells secrete cytokines activating macrophages → destroy bacteria
T cells can directly kill infected macrophages
Latent infection
Stage 4
Some inactivated macrophages surround tubercle - growth of MTB continues
If invade blood supply line, hematogenous spread → miliary tuberculosis → secondary lesions in other parts of body (high fatality)
Disease waxes and wanes - tissue necrosis balanced by healing and fibrosis
Stage 5: rare - host usually controls infection at some point
Caseous centres of tubercles liquify, aiding MTB growth
Rapid extracellular MTB growth
Antigen load → necroses nearby bronchi walls → rupture, cavity formation → rapid spread through lungs
Treatment of TB
Directly Observed Therapy (DOT)/DOTS-Plus
4 antibiotics
HCW observes pt taking medication
MDR-TB
- From lack of direct observation of pts
- Requires extensive chemotherapy: more toxic and expensive, less effective, w greater side effects, and tx lasting up to 2y
DOTS-Plus: Second-line anti-TB treatments
General clinic Infection control measures to limit exposure of TB to staff
HCWs: education, training and counseling; screening for TB infection and disease
Air exhaust ventilation & air cleaning via air filtration or UV germicidal irradiation - prevention of spread and reduce droplet nuclei concentration
Use of personal respiratory protective equipment
Patients’ medical history, symptoms of active TB should be referred promptly for medical evaluation.
