MFD Flashcards
Purpose of Koch’s postulates?
Scientific approach to finding the cause of infections
Koch’s Postulates
- Organism must be present in every case of disease and not present in health
- Organism must be isolated in pure culture
- Isolated organism must cause disease in suitable animal
- Organism must be re-isolated from infected animal
When is Koch’s postulates not possible to fulfil (relating to each step)?
- Asymptomatic carriers e.g. cholera
- Viruses could not be cultured in 1900s and prions are still hard to culture. Some infections are poly microbial
- Suitable animal may not be available and may not be representative of humans
Koch’s postulate cannot be applied when: (5)
No infecting species detected
Bacteria cannot be grown in culture
No suitable animal model available
More than one species of bacterium involved
The level of species rather than the presence of the species is important
How has Koch’s postulates changed in molecular biology era?
Positives
How has this development changed views on what infectious disease are?
DNA analysis is often used to determine the infectious species (PCR, hybridisation, sequencing)
- this circumvents need for isolation
- very sensitive and often detects even in absence of disease
- may be quantitative (can tell you how much there is)
Infectious disease now regarded as imbalances in microbiota
What is latex agglutination used for?
Antigen detection
How does latex agglutination work?
Latex article is coated in antibodies specific for the capsular polysaccharide antigen of the bacterium, this causes agglutination of the sample (positive result)
Why is latex agglutination better than growing on carbohydrate agar?
It is quicker and cheaper (don’t need to wait for it to grow) and very specific
Can be useful if patient has received antibiotic treatment so the organisms appear morphologically unidentifiably in sample and may not grow in culture
What type of antigen is latex agglutination used to detect?
Legionella in urne
Streptococcus Pneumoniae in CSF
Advantages of latex agglutination?
Quick
Cheap
Specific
What is critical pH of mouth?
Approx 5.5
why do people have different critical pH?
Critical pH dependent on calcium and phosphate concentration in saliva which is dependent on individuals saliva flow rate, this differs dramatically
Name the 3 plaque hypothesis?
Specific plaque hypothesis
Non-specific plaque hypothesis
Ecological plaque hypothesis
Specific plaque hypothesis
Disease caused by single microorganism
Non-specific plaque hypothesis
Disease caused by amount of plaque present
Ecological plaque hypothesis
Disease caused by interactions of specific micro-organisms with the microbial community
Normal pH of the mouth?
Approximately 6.2-7.5
What happens if HA left in pH7?
Why does the temperature rise?
HA normally lives at pH 6.5
Increase in pH will shift equilibrium to the left and increase bond making
Exothermic reaction = increase in temperature
Function of peptidoglycan?
Forms cell wall of bacteria
Provides structure and resists osmotic effects of cytoplasm
Involved in binary fission
Opportunistic pathogen
Pathogen that grows in an environment not normally available in the host e.g. weakened immune system
What agents aren’t killed by sterilisation?
Prions
How does antibiotic resistance occur?
Bacteria in human body, some are more resistant than others
Antibiotics kills the susceptible strain and the resistant strains live
Resistant strains replicate
Resistance spreads
Intrinsic resistance
Innate property of bacterium seen in all strains e.g. G-ve to beta lactic ring
Acquired resistance
Resistance selected by antibiotic use e.g penicillin and staph aureus
Cross resistance
Resistance to one AB leads to resistance to another (in the same class)
Multi-resistance
Resistance to several AB via independent mechanisms
Why is vancomycin and neomycin used int he agar to select for fusobacterium?
Fusobacterium have innate resistance to vancomycin and neomycin
Vanco = too large to cross the G-ve outer membrane
Neo = needs an active election transport chain for uptake, anaerobic bacteria do not have these
How is resistance transferred?
Mutation
Tranformation
Transduction
Conjugation
Transformation?
Cell lysis
DNA into environment
Taken up into bacteria and incorporated into their DNA
Transduction?
Viruses infect bacteria cell, incorporate parts of DNA into their genome, kill cell and spread
Conjugation?
Pili
Donor cell contains plasmid and pili, makes contact with host and transfers
Main mechanism of resistance (3)
Modification of AB
Modification of target
Preventing drug reaching target
Mode of action of penicillin
Inhibit transpeptidase forming cross linked in peptidoglycan cell wall (between NAM and NAG as glycine)
Bacteria swell and rupture
Why is amoxicillin better than penicillin?
Wider range of action
Effective against G-ve as it contains amino group to penetrate outer membrane
Longer duration of action
Mechanism of action for sulphonamides?
act as a false substrate for dihydropterate synthetase in folate synthesis
How are sulphonamides selective to bacteria?
Bacteria have to make own folate = intracellular
Humans get it from diet
Nature of sulphonamides?
a) Bacteriocidal
b) Bacteriostatic
c) Bacteriolytic
B Bacteriostatic
Mode of action of flouroquiolones?
Inhibit DNA gyrase in G-ve and DNA topioisomerase in G+ve
stop DNA replication
Mode of action for macrolides
Block protein synthesis by blocking translocation on large ribosomal subunit
Mode of action for tetracylines
Small subunit Block elongation (join gin of AA)
Structural features of archaea
Prokaryote
No nucleus, single celled, no internal organelles, coiled ring of DNA
How to take sample of archaea organisms from mouth?
Anaerobes so sub-gingival swab
Difference between specificity and sensitivity?
Specificity = proportion of people who test negative and remain disease free Sensitivity = proportion of people who test positive and develop disease
Calculating specificity and sensitivity from table
Disease
Yes No
\+ve a b
Test
-ve c dSensitivity = a/a+c Specificity = d/d+b
2 molecular components contributing to fighting fungal infections
Azoles
Polyenes
How can dental clinicians avoid spread of antibiotic resistance?
Follow up to date prescription guidance
Don’t give as a preventative measure
Prioritise local measures
Inform patient on options, risks and risks on over prescribing
Why is saliva hypotonic?
Resorption of Na+ and Cl- is greater than secretion of K+ and HCO3- in salivary duct
Duct cells are impermeable to water so water stays within the lumen reducing the concentration of the solute in the saliva
Why do B lactase only work on growing bacteria?
Transpeptidase is not active in quiescent bacteria
Function of MacConkey agar
Positive test, Negative test
Differentiate between bacteria that can ferment lactose and those that cannot
Select for G-ve enterobacteria using bile salts
If fermenting lactose = produces acid = pH indicator turns red
If fermenting proteins = produce alkali substance = pH indicator = yellow
Why does number of CFU not equal total number of bacteria?
Some cells may be dead/non-vaibale/unable to grow on particular medium
Some cells clump together or form chains = may appear as one CFU
3 red complex bacteria?
P. gingivalis
T. denticola
T. forsythia
Orange complex bacteria?
Fusobacterium
Obligate anaerobes
Environment?
Unable to live in areas where there is oxygen
Subgingivally
2 bacteria in Mutans Streptococci?
Strep. Mutans
Strep. Sobrinus
Example of bacteria that does beta haemolysis?
Appearance on agar?
Streptococcus pyogenes
Transclucent
Example of bacteria that does alpha haemolysis?
Appearance on agar?
Streptococcus Gordonii
Red with greenish tinge
Example of bacteria that does gamma haemolysis?
Appearance on agar?
Staphylococcus epidermis
No haemolysis = red
4 labels on batch curve of bacteria?
Lag = slow growth because small number of bacteria Exponential = growth increases quickly Stationary = growth = death = limited availability of nutrients Death = too many bacteria and absence of nutrients = reduction
2 compounds causing halitosis?
Volatile sulphur compounds e.g. hydrogen sulphide
Protein and amino acid metabolites e.g. skatole/cadaverine
What is a plasmid?
Small circular string of DA in cytoplasm, containing useful but not essential genes for survival
Endotoxin
Bacterial virulence factor = LPS = associated with cell wall of G-ve
When cell dies Lipid A from LPS = pro-inflammatory
Definition of neurotoxin
Exotoxin that destroys nervous tissue
2 types of exotoxins secreted by bacteria?
Tetanus toxin and botulinum
Faculatative anaerobe
Can use oxygen to make ATP but when oxygen is not present can switch to fermentation
How do Mutans polymers support plaque?
Increase adhesion and survival of mutant streptococci
What is the Snyder test used to determine?
Susceptibility to caries - selects acidogenic bacteria e.g. lactobacilli
Which carbohydrate is fermented in Snyder test?
Glucose
How does fluoride affect critical pH?
Reduce so the environment can be more acidic before dissolution
Main virulence factor of Aa?
Leukotoxin LPS adhesions (fimbriae) Cytolethal distending toxin Proteases
FAA agar?
Identifies?
Fastisious anaerobe agar?
Non-selective anaerobes
How can FAA be made selective fro fusobacterium?
Add vancomycin and neomycin = resistant to both of these (too large and doesn’t have the transporter)
Blood agar?
Identifies?
Nutrient-rich, non-selective
TYCS agar?
Identifies?
Tryptone Yeats Cysteine
S. mutans and other streptococci by having high sucrose levels = glucans
Distinguished between S. mutans and S. sanguinis
MSB agar?
Identifies?
Mitis Salivarius Bacitracin
Distinguish between S. Mitis and S> salivarius
What enzyme tests distinguish
a) E.coli
b) S. aureus
c) S. salivarius
a) -ve urease, +ve catalase, -ve DNAase
b) -ve urease, +ve catalase, +ve DNAase, +ve coagulase
c) +urease, -ve catalase, -ve DNAse
Virulence factors of
a) T. Forsythia
b) T. denticola
c) P. gingivalis
d) S, aureus
e) Aa
f) C. albicans
a) Glycosylated S layer to hide from immune cells
b) Flagellum and majro sheath protein
c) proteases, LPS
d) Exotoxins
e) leukotoxin
f) hyphae formation and phenotypic switching
On blood agar what will appear red when looking at it? Bacteria?
Due to alpha haemolysis = S. Gordonii
Which antibiotic causes fluorescence?
Tetracycline
What is calculus?
Plaque depositions that irritate the human and cause inflammation
What is added to toothpastes to reduce calculus formation?
Anti-bacterial agents
MIC
MBC
Minimum inhibitory concentration
Minimum bactericidal concentration
Sections of LPS
Lipid A = inflammatory response
O antigen
What bacteria brows on NaCl agar?
Staphyloccus
How is cholera transmitted?
Food or water that have been contaminated with faeces from contaminated person
What do you clean clinical surfaces with?
Disinfectant
What would you clean a blood spillage with?
Wipe area with water and detergent until visibly clean and the use sodium hypochlorite (bleach)
What produce black colonies on blood agar?
P. gingivalis due to porphoryn pigments containing heam
Main etiological factor of caries
Microbial dysbiosis, bacterial fermentation
2 main spore forming bacteria
Bacillus anthracsis, Clostridium difficile
What component of enamel pellicle functions by stabilising calcium and phosphates with oral environment?
Proline rich proteins
Slatherin
What inhibits growth by binging enzyme co-factor?
Lactoferrin
When sugar in low quantities, what is a virulence factor of mutans?
Antigen I and II can bind lots of structures with high affinty
Action of antigen I/II on streptococci?
Mediate adhesion in pellicle and binds bacteria e.g. p. gingivalis and Aa
What is chlorhexidine?
Disinfectant and antiseptic used for disinfection fo skin and surfaces
What is autoclaving unable to kill?
Prions
What procedure is used to treat a dental hand piece?
Steralisation through autoclaving (heat and pressure)
Prion
Misfolded proteins which form aggregates = highly resistant
What salivary element is a potent inhibitor of candida albicans?
Competence stimulating protein of S. mutans inhibit hype formation
Lactobacillus
What component of s. gordonii promotes binding to Candida albicans hyphae form?
Antigen I/II
What contributes to pathogenicity of Staph Aureus?
Exotoxin production
Results in toxic shock syndrome
What bacterial component causes an acute inflammatory response?
LPS (G-ve)
What is CLED agar?
Cysteine, lactose, electrolyte deficienent agar
Why is CLED used to culture urinary microbes?
Cysteine and lactose promote the growth of the pathogens but lack of electrolytes prevents the swarming of proteus species
What are the selective agents of MacConkey agar? How do they work?
Bile salts and crystal violet iodine stain
Inhibits growth of Gram +ve
Selects for gram -ve
Because G-ve have outer membrane which decreases the permeability to bile salts - bile salts can’t cause protein aggregation
What is the product of tryptophan metabolism? Consequence?
Skatole
Halitosis
2 products of protein breakdown and the consequence?
Cadervine, Putrescine
Halitosis
What is BANA test used for?
Halitosis diagnosis by detecting presence of periodontal pathogens
What bacteria will test positive in a BANA test?
Red complex bacteria
P. gingivalis
T. denticola
T. forsythia
Which bacteria is a spirochete?
T. denticola
Units of Ksp (solubility product?
(Moles/L) to the power of the number of products e.g. HA 10 Ca 6 PO3 2 OH (moles/L)18
3 most outer layers of bacteria?
Capsule
Cell wall
Plasma membrane
Function of capsule
Protect against phagocytosis
Function of cell inclusion bodies?
Nutrient reserve and protect against osmotic shock
Function of Fimbriae
Function of Flagellae
Structural difference
Fimbriae = adhesion of biofilm
Flagellae = motility
Fimbriae are much smaller and in higher numbers
Microorganism responsible for stomach ulcers
Helicobacter pilori
Microorganism responsible for chicken pox?
Varicella zoster virus (type of herpes)
Microorganism responsible for cold sores?
Herpes simplex
Genetic structure of
a) Herpes
b) HIV
a) Double stranded DNA
b) Single stranded RNA
What is a retrovirus?
Example?
An RNA virus that inserts its DNA into the host cell changing the genome of that cell
HIV
What 2 substances act as a buffer?
Saliva and GCF
What and how does staphylococcal agar select for?
Staphyloccus bacteria = high salt content
How does candida agar select for candida?
Low pH
Chloramphenicol which inhibits bacterial growth
Chromogenic substances that are cleaved by certain enzymes in the candida = different colours
Why does S. Aureus appear gold?
Contains staphyloxanthin
What makes some E coli pathogenic?
Possession of O antigen 157 or H antigen 7
