Clinical Micro Flashcards
Subacute sclerosing panencephalitis is associated with which slow virus ?
Measles virus
Which slow virus is associated with progressive rubella panencephalitis ?
Rubella virus - v rare complication
Progressive multi focal leukoenceohalopathy (PML) is associated with which slow virus ?
JC Polyomavirus
What are the criteria for slow viruses?
- prolonged incubation (months to years)
- clinical course of infection leading to death
- pathology limited to single organ - the brain
- limited to single host species
Which was the first transmissible spongiform encephalopathy to be described ?
Scrapie, from sheep and goats
First human transmissible spongiform encephalopathy to be described ?
Kuru
Differences in CJD and vCJD?
Onset: CJD= 55-70, vCJD= 14-52
Symptoms: CJD= dementia + myoclonus, vCJD= behavioural + ataxia
Clinical course: CJD=rapid, vCJD= insidious onset, prolonged course
Path: CJD= PrPSC in synapses vCJD= prominent florid plaques
Pathogenesis of sporadic CJD
Spontaneous Transformation of normal host-encoded prion proteins to aberrantly folded protease resistant forms - it is NOT transmitted
Pathogenesis of vCJD ?
Linked with BSE, Transmitted by eating infected meat, in particular parts of CNS
What is gerstman-straussler-Scheinker syndrome (GSS)
Rare inherited prion disease, characterised by adult onset of loss of memory and dementia, ataxia and pathological deposition of amyloid b plaques
(TSE)
Prions are resistant to inhibition by ?
- extremely heat resistant
- irradiation
- DNAse
- RNAse
Prions are susceptible to ?
- urea
- phenol
- other protein denaturing treatment
Classic epidemiological study
John snow, cholera
a removal of broad street pump in soho, London, broke cycle of infection
Outline Koch postulates (4)
- the microorganism must be present in all cases of the disease
- the pathogen can be isolated from hist and grown in pure culture
- the pathogen form pure culture must cause disease with typical symptoms (when inoculated in lab animal)
- the pathogen must be re isolated from artificially inoculated subjects
21st century version of Kochs postulates applied to virulence genes ?
- gene encoding virulence should be present in virulent strains
- should not be present or should be silent in a virulent strains
- disruption of gene -> avirulent (or intro =VV)
- gene must be expressed during infection and Abs should be raised
Outline the 3 basic steps in infection cycle
- Pathogen must encounter and adhere to host
- Multiply within hist
- Be dispersed to encounter another host
Examples of healthy carriers of pathogens ?
- neisseria meningitidis (back of nose and throat)
- streptococcus pneumoniae (nasopharyngeal)
Name the two clinical manifestations of leprosy and outline the pathological reason for the difference
- Paucibacilliary - stronger cellular immune response
- 5 skin lesions
Who discovered leprosy?
Dr Hansen, Norwegian physician,1873
What event bought leprosy to Europe ?
Soldiers which had joined the crusades returning home to Europe
Which world health programmes has helped Duce the cases of leprosy ?
World health assembly initiative
Average incubation of leprosy?
7 years
Which form of leprosy has high bacterial load ?
Lepromatous aka multibacilliary (MB)
Treatment of paucibacillary leprosy?
Rifampicin and dapsone - 6 months
Dapsone mechanism of action
- Inhibits folic acid production (but is not a sulphonamide)
- has anti inflammatory and immunomodulatory effects
Treatment of lepromatous leprosy ?
Rifampicin, clofazimine and dapsone (1 year)
What stain is used in the observation of mycobacterium ?
Ziehl-Neelson stain
Outline process of Ziehl-Neelson stain of mycobacterium
- slide flooded with string carbol fuschin
- slide heated until dye steams
- kept hot for 10 mins
- Carbol fuschin washed off
- slide flooded with mineral acid + acid
- this de colourises everything other than mycobacteria
- then stained with methylene blue
Describe cell wall of mycobacteria
- Peptidoglycan layer covalently linked to arabinogalactan which is linked to mycolic acids
- 60% lipids
Who first discovered M. Tuberculosis ?
German bacteriologist, Robert Koch, 1882
How many people die of TB each year worldwide ?
> 2m
How many new cases of TB occur worldwide per year ?
> 8m
What media does M. tuberculosis grow well on ?
Egg based e.g. Lowenstein-Jenson medium
Average time for M. tuberculosis to be grown on conventional culture ?
4-6 weeks
Which dye can be used as an alternative to Ziehl-Neelson and makes screening large numbers of slides easier ?
Auramine-Rhodamine stain - same principle but with fluorescent dyes
Earliest reports of TB
2400BC
Drug treatment for pulmonary TB
6 months of isoniazid and rifampicin with the first 2 months supplemented with pyrazinamide and ethambutol
Controlling factors of syphilis
Posey and promiscuity
What year did syphilis arrive in England ?
1498
What year did syphilis return to America ?
1800
Who inoculated himself with pus of a patient with gonorrhoea symptoms
John hunter, 1767
Who and when was the organism causing gonorrhoea discovered?
Albert Neisser, German physician and bacteriologist,1878
When and who by was the causative organism of chancroid identified ?
1889, August Ducrey, Italian physician and bacteriologist
Haemophilus ducreyi
When does primary syphilitic lesion appear?
Chancre appears at end of incubation period at site of inoculation
How long do chancre last ?
2 to 3 weeks then heal spontaneously
How long father primary syphilis does the secondary stage occur ?
6 to 8 weeks after primary chancre heals spontaneously
Symptoms of secondary syphilis
- generalised symptoms of infection e.g. Malaise
- generalised lymphadenopathy
- characteristic non itchy rash
- snail track ulcers in mouth, patchy ulcers, iritis
How long may seconds syphilis last?
Months to years
When does the latent stage of syphilis occur and how long does it last ?
After last lesions of secondary syphilis have healed, may last 3 to 30 years
What amount of untreated syphilis cases progress to tertiary syphilis ?
1/3
Physical appearance of congenital syphilis ?
- Hutchinson teeth
- mulberry molars
- saddle nose
- sabre shins
Symptoms of tertiary syphilis
- gummatous lesions of skin, bone, mucous membranes
- CNS= tabes dorsalis, generalised paresis of the insane
3 antibodies appearing in serum in syphilis infection ?
- lipoidophil or reagin antibody
- antibody to Reiter protein
- specific antibodies to T. pallidum antigens
Which antigen provokes the lipoidal antibody ?
Cardiolipin
How is Cardiolipin antigen produced ?
Prepared from alcoholic extract of beef heart muscle
The addition of which compounds make the Cardiolipin antigen test more sensitive ?
Lecithin and cholesterol
When do antibodies to Cardiolipin appear after infection with syphilis ?
1-3 weeks
What is added to the RPR test to aid visualisation as an improvement on the VDRL test ?
Carbon particles
Which specific treponemal test becomes positive first in syphilis infection ?
FTA-ABS (fluorescent treponemal Ab absorption)
How is FTA-ABS carried out ?
- treponemal antigens fixed to slide
- slide flooded with serum
- if anti-trep Abs present they will adhere to trep Ags that are stuck to slide
- slide washed and flooded with fluorescently labelled anti-human antiserum
- fluorescent Abs stick to human Abs which are stuck to the Ags
- final wash to remove unbound Abs
Which specific treponemal test remains positive despite therapy as a marker of last infection ?
TPHA
- treponema pallidum haemaglutination test
Which test has superseded TPHA test ? What modifications ?
TPPA
- treponema pallidum particle agglutination test
- RBCs replaced by gelatine particles to reduce risk of false positives and increase sensitivity
First line treatment for syphilis (primary, secondary and early latent)?
Benzathine penicillin (2.4 mega units IM, Single dose)
Second line treatment for syphilis (primary, secondary and early latent)?
Azithromycin (oral, single dose)
Paper on lab diagnosis of syphilis
Ratnam, 2005
Paper of treatment history and development of syphilis
Tampa et al, 2014
Paper on the history of syphilis?
Nguyen 2013
TB genetic predisposition study ?
Schurr, 2011
Paper on leprosy
Pinheiro, 2011
Paper on food spoilage
Rawat, 2015
Paper on wine making and brewing ?
McClure, 1976
Antibiotic definition and reference ?
‘A chemical compound made by microorganisms that inhibit or kill other microorganisms at low concentrations’ (Waksman,1945)
Most simple way to determine the antibiotic susceptibility ?
Disk diffusion test
7 targets of antivirals ?
- entry
- uncoating
- reverse transcriptase (retroviruses and DNA-RT)
- integration in to host genome
- protease inhibitors
- virus replication
- assembly
Examples of synthetic antibiotics
Sulphonamides and trimethoprim
Example of semi synthetic antibiotic
Ampicillin
How to determine MIC and MBC of antibiotics ?
Serial dilution broth then plate out
- MIC = conc of largest dilution that prevents growth in tube but not when plated out
- MBC = dilution of last plate to show no growth
Example of antibiotics which are antagonistic together and why ?
Penicillin and erythromycin (B-lactam and Macrolide)
- erythromycin inhibits protein synthesis and so inhibits the growing cell wall that is actively targeted by penicillin
Examples of synergistic pairs
- sulphonamides and trimethoprim
- penicillin and gentamicin (increase penetration)
Exact target of B-lactams ?
- Transpeptidases aka PBPs involved in transpeptidation (cross linking) of peptidoglycan by preventing cleavage of the terminal D-alanine residue
- also stimulate activity of autolysins
6 antibiotics (2 classes) targeting the cell wall?
- B-lactams
- glycopeptides
- fosfomycin
- cycloserine
- isoniazid
- teixobactin
Mechanism of action of glycopeptides ?
Inhibit transglycosylation and so prevents elongation of peptidoglycan chain and therefore inhibits subsequent transpeptidation as well
Mechanism of resistance in B-lactams ?
- B-lactamases
- modified PBPs with lower affinity (target mod)
Which mechanism of resistance in B-lactams is more common in Gram positive bacteria ? Give example
Altered PBP
- mutant PBP2a in S. aureus by mutation in MecA gene -> MRSA (lower affinity)
Mechanism of resistance in glycopeptides
Reprogramming if D-ala D-ala substrate
- e.g. To D-ala D-Lac in vancomycin resistance
*resistance rarer than in other antibiotic classes
Mechanism of action and resistance in fosfomycin
- inhibits MurA, 1st enzyme in PG synthesis pathway
- resistance = sub mutation in active site (cysteine->aspartate), target mod unable to bind
*resistance develops rapidly so not used much clinically
Mechanism of action of clycoserine ?
- inhibits 2 steps in PG synthesis
- competitive inhibitor of L-ala racemase converting L-ala to D-ala
- inhibits D-ala D-ala synthetase
Mechanism of resistance in clycoserine?
Over expression of D-ala racemase gene (upregulation of target)
Clinical indication for cycloserine ?
Drug resistant TB
- or other resistant infections where all other therapies failed (salvage therapy)
- neurotoxic
Mode of action of isoniazid
Inhibits synthesis of mycolic acids
Mechanism of action of Teixobactin?
Bids to lipid II, precursor if PG, Preventing cell wall synthesis
What antibiotic class is the newly developed (2015) antibiotic with Gram negative activity ?
Ceftolozane
*administered with B-lactamase inhibitor (tazobactam)
How does ceftolozane evade the usual resistance mechanism of P. aeruginosa ? (4)
- particularly high affinity for PBP
- low affinity for class C B-lactamases (cephalosporinases)
- enters OM through alternative porin (not OprD)
- not a substrate of the usually up regulated efflux pump
Mechanism of action of polymyxins?
Binds to LPS in gram negative outer membrane and subsequently disrupts outer and inner membranes
Which antibiotics disrupt membrane integrity ? (3)
- polymyxins
- Gramicidin
- possibly metronidazole as secondary target after DNA gyrase
What antibiotic class is gentamicin ?
Aminoglycosides
Which is the only class of antibiotics which inhibit protein synthesis that are bacteriocidal ?
Aminoglycosides
Most common new traits conferring resistance to antibiotics ? (3)
- Alteration of drug target
- Encoding of new enzymes which hydrolyse drug
- Prevention of binding to target e.g. Efflux pump
Main mechanism of resistance effecting all aminoglycosides ?
- decreased uptake/accumulation of drug
- bacterial expression of enzymes modifying drug
Which 3 classes of antibiotic exhibit cross resistance due to a shared target on the 50S ribosome (23S)
Macrolides, lincosamides and streptogrammins (MLS antibiotics)
Which organisms are intrinsically resistance to aminoglycosides ?
Anaerobic bacteria - lack oxidative metabolism to drive uptake
Why is the claim that a drug with 2 targets less likely to develop resistance false ? E.g. teixobactin
Most clinically relevant resistance is modification of the drug not the target
- therefore as one drug, the chances are no less likely than other
- combo therapy more likely to be effective
Usual mechanism of resistance in mycobacteria ?
- Point mutation in chromosomal DNA
- target modification
Define conjugation
Transfer of genetic material between two bacteria by direct cell-cell contact via bridge-like structure, sex pilus
Describe HGT by transformation
Transfer of genetic material in which free DNA (I.e. From lysed cell), is taken up and incorporated into the genome of another bacterium
Describe HGT by transduction
DNA transferred from one bacterium to another by a viral vector e.g. Bacteriophage
3 processes errors may occur in to produce mutational resistance ?
- polymerisation
- exonucleolytic proofreading
- strand-directed mismatch
2 mechanisms by which resistance increases in prevalence ?
- clonal expansion
- horizontal dissemination
2 classes of antibiotic acting on the 30S ribosome
- Aminoglycoside
- tetracycline
