Medical microbiology Flashcards
Why do most viruses not infect us?
Most of these don’t “infect” us:
They are adapted to non-human hosts
They are excluded by surface barriers
Innate Immunity prevents them establishing
Our adaptive immune response has seen something similar
What are the different sites of microbe entry?
Conjunctiva Respiratory tract Alimentary tract Urinogenital tract Anus Skin Scratch, injury Capillary Arthropod
What are the general patterns of viral infection?
a) Acute infection (a huge spectrum of disease and range outcomes). Resolution by immunity?
Influenza pathogenicity; different strains produce a huge range of outcomes
b) Latent, reactivating infection (chronic infection)
Human Herpes Viruses
Herpes simplex virus, first appears as primary gingivostomatitis, and then stays latent until it is seen as a cold sore.
HHV-3 causes chicken pox then reappears as shingles. Also called varicella virus Persistent infection (chronic infection)-has 2 types of patterns
¥ HIV; Virus infects CD4+ cells and weakens immune system
¥ HCV; Virus infects hepatocytes and damages liver
¥ Congenital Rubella; if infected in utero, virus is seen as self, baby is born immunotolerant and virus continues to replicate (and cause damage) in neonatal tissues
How does virus infection of a host lead to disease?
Many infections are apathogenic or associated with relatively mild symptoms; it is important to realize that from the virus’ point of view these are not always failed or resolved infections – a successful virus is one that replicates well enough to spread to the next host
Describe inapparent infections
Many infections are apathogenic or associated with relatively mild symptoms; it is important to realize that from the virus’ point of view these are not always failed or resolved infections – a successful virus is one that replicates well enough to spread to the next host
How does virus infection of a host lead to disease?
Pathogenesis results from cell and tissue damage caused by the viral infection. On most occasions the damage is limited by the host’s immune system
Eg, Ebola targets vascular endothelial cells.
Influenza A virus targets lung epithelia
RSV induces syncytia in lung epithelia. On some occasions the relative limited damage caused by the virus is made worse or even caused by the host’s immune system (= immunopathology)
eg, hepatitis C
Describe HCV virus
Chronic hepatitis is a disease of severe liver damage and loss of hepatocytes – caused by persistent HCV infection
HCV is non-cytopathic
Hepatitis associated with extensive liver infiltration of leukocytes
Pro-inflammatory cytokine levels very high
Viral clearance and disease is associated with generation and infiltration of CD8+ cells which attack infected cells and destroy them
HCV persistence is associated with the generation of HCV variants that are not recognised by CD8+ cells
Describe Dengue fever
Dengue virus infection is the most common mosquito-borne infection worldwide – even surpassing malaria
There are 2.5 billion people at risk of dengue due to living in an endemic area. There are an estimated 50–100 million infections per year, and 500,000 hospitalizations due to severe disease
The case fatality rate from severe dengue is 1 - 5%
There are 4 serotypes (1–4), all of which have the same clinical manifestations
Describe the immunopathology: Dengue virus
Severe dengue, which may include dengue shock syndrome (DSS), and hemorrhage
Greatest risk is a previous infection with a different serotype
Antibodies formed in response to a dengue infection are not cross-protective against other subtypes of the virus. In fact they may result in more severe disease due to a phenomenon known as antibody-dependent enhancement or ADE
Non-neutralizing antibodies coat virus, forming immune complexes which get internalised into mononuclear phagocytes through their Fc receptors; fixation of complement by circulating immune complexes results in release of products of the complement cascade leading to sudden increased vascular permeability, shock and death
Influenza virus
Influenza People of all ages are infected, usually only a serious problem in the old or children with asthma
Pathology
Mild URTI to severe LRTI
Lower respiratory tract infection causing damage to lung epithelia and viral pneumonia, often secondary pneumonia
Fever, often prolonged
Neurological (headache, malaise)
Myalgia
Infection generates powerful, long-live immunity
Easy to vaccinate against if you know what’s coming
Why do we need antivirals?
Quick killers e.g. influenza; ebola; MERS; SARS
Slowly, progressive chronic disease leading to cancer
hepatitis B [350,000,000 carriers]
hepatitis C [200,000,000 carriers]
human papilloma viruses
[cervical cancer, second commonest cancer in women]
Human immunodeficiency virus (HIV)
[40 million infected]
Acute imflammatory e.g. herpes
What do we use antivirals for?
Treatment of acute infection
Influenza ; Chickenpox; herpes infections -(aciclovir)
Treatment of chronic infection:
HCV, HBV, HIV (numerous different agents)
Post-exposure prophylaxis and preventing infection:
HIV (PEP)
Pre-exposure prophylaxis: HIV (PrEP)
Prophylaxis for reactivated infection: e.g. in transplantation
CMV (ganciclovir, foscarnet)
Describe the principle of antivirals as therapeutic agents
Selective toxicity Due to the differences in structure and metabolic pathways between host and pathogen
Harm microorganisms, not the host
Target in microbe, not host (if possible)
Difficult for viruses (intracellular), fungi and parasites
Variation between microbes
Why is it so difficult to develop effective, non toxic anti-viral drugs
Viruses enter cells using cellular receptors which may have other functions
Viruses must replicate inside cells – obligate intracellular parasites
Viruses take over the host cell replicative machinery
Virsues have high mutation rate - quasispecies
Anti-virals must be selective in their toxicity
i.e. exert their action only on infected cells
Some viruses are able to remain in a latent state e.g. herpes, HPV
Some viruses are able to integrate their genetic material into host cells
e.g. HIV
What are the considerations in developing safe anti-viral agents?
-Can stages of infection be targeted?
Cellular receptor may have other important function
Viral enzymes may be very similar to host
Blocking cellular enzyme may kill cell
Describe the virus life cycle
- Recognition
- Attachment
- Penetration
- Uncoating
- Transcription
- Protein synthesis
- Replication
- Assembly, and envelope
- Lysis and release, or budding and release.
Describe the modes of action of selected anti-virals
Preventing virus adsorption onto host cell
Preventing penetration
Preventing viral nucleic acid replication (nucleoside analogues)
Preventing maturation of virus
Preventing virus release
gIve example of some antivirals
Amantadine Acyclovir, Ganciclovir, Ribavarin, AZT Interferons, HIV protease inhibitors RSV -guanosine analogue Zanamivir-influenza release
Give examples of selective toxicity viral targets?
Discovery of virally encoded enzymes sufficiently different from human counterparts
e.g.
Thymidine kinase and HSV/VZV/CMV Protease of HIV Reverse transcriptase of HIV DNA polymerases Neuraminidase of influenza virus
Act as selective targets with minimal effect on host enzymes or processes
What virus can cause muco-cutaneous lesions?
Herpes simplex type 1
What are the 4 types of herpes viruses
Herpes viruses include: Herpes simplex (HSV), Varicella Zoster Virus (VZV) Cytomegalovirus (CMV) Epstein-Barr virus (EBV)
What are the different antivirals we can use for herpes virus
aciclovir
IV/oral/topical
For HSV, VZV treatment/prophylaxis
CMV/EBV prophylaxis
ganciclovir IV/oral For CMV Foscarnet IV/local application For CMV cidofovir IV for CMV
What virus causes chickenpox, and then reactivates to cause shingles
Varicella Zoster virus
Describe the selective toxicity of aciclovir
Aciclovir is activated to active drug
Substantially more in infected cells. Requires 2 viral enzymes
= selectively activate ACV
= selectively inhibited
Accounts for low toxicity
Why is aciclovir so effective and safe?
HSV thymidine kinase (TK) has 100x the affinity for ACV compared with cellular phosphokinases
Aciclovir triphosphate has 30x the affinity for HSV DNA polymerase compared with cellular DNA polymerase
Aciclovir triphosphate is a highly polar compound - difficult to leave or enter cells (but aciclovir is easily taken into cells prior to phosphorylation)
DNA chain terminator
what can aciclovir be used as a treatment for?
Treatment of encephalitis
Treatment of genital infection
suppressive therapy for recurrent genital herpes
Varicella zoster virus
Treatment of chickenpox
Treatment of shingles
Prophylaxis of chickenpox
CMV/EBV
Prophylaxis only
Shingles (zoster)
Describe ganciclovir
Active for CMV
- reactivated infection or prophylaxis in organ transplant recipients
congenital infection in newborn
retinitis in immunosuppressed
Structurally similar to aciclovir
CMV does not encode TK but has UL97 kinase
Inhibits CMV DNA polymerase
what virus does aciclovir not work on?
CMV, because it doesnt have the tyrosine kinase needed to activate the drug in the first step.
Describe other anti-herpes virus agents
Foscarnet:
Selectively inhibits viral DNA/RNA polymerases and RTs
No reactivation required
Binds pyrophosphate binding site – a structural mimic
used for CMV infection in the immunocompromised
e.g. pneumonia in solid organ and bone marrow transplants.
May be used because of ganciclovir resistance (TK mutants)
Cidofovir
Chain terminator - targets DNA polymerase
Competes with dCTP
Monophosphate nucleotide analog
Prodrug – phosphorylated by cellular kinases to di-phosphate
drug active against CMV; but MUCH MORE nephrotoxic
Treatment of retinitis in HIV disease
Describe the resistance to antivirals in herpes viruses
Two main mechanisms
Thymidine Kinase mutants
DNA polymerase mutants
If occurs in TK, drugs not needing phosphorylation are still effective (e.g. foscarnet, cidofovir)
If occurs in DNA polymerase, all drugs rendered less effective
VERY RARE in immune competent patients (low viral load)
Describe the structure of HIV
Envelope protein, gp120 with transmembrane gp41
Membrane associated matrix protein Gag 17
Nucleocapsid protein Gag p24
Reverse transcriptase
Viral envelope
ds RNA genome
Name the 7 steps in the life cycle of HIV
- Attachment with binding of viral gp120 via CD4 and CCRX
- Reverse transcription of RNA into ds DNA
- Integration into host chromosome of proviral
- Transcription of viral genes
- Translation of viral mRNA into viral proteins
- Virus assembly and release by budding
- Maturation
Describe anti-HIV drugs
- Anti-reverse transcriptase inhibitors
nukes -nucleoside/nucleotide RT inhibitors
non-nukes -non-nucleotide RT inhibitors (allosteric) - Protease inhibitors - multiple types
- Integrase inhibitors – POL gene - protease, reverse transcriptase and integrase (IN)
with the 3´end encoding for IN (polynucleotidyl transferase) - Fusion inhibitors – gp120/41 - biomimetic lipopeptide
Treatment-HAART
(combination of drugs to avoid resistance)
