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)
Describe nucleoside reverse transcriptase (RT) inhibitors
Nukes
AZT - Zidovudine
Synthetic analogue of nucleoside thymidine –
when converted to tri-nucleotide by cell enzymes, it blocks RT by
- competing for natural nucleotide substrate dTTP
incorporation into DNA causing chain termination
Others ddI, ddC, d4T, and 3TC (2′,3′-dideoxy-3′-thiacytidine
Describe non nucleoside reverse transcriptase inhibitors ‘Non-nukes’
Non-competitive inhibitor of HIV-1 RT
Synergistic with NRTI’s such as AZT because of different mechanism
Describe pre and post exposure prophylaxis for HIV
PEP – within 72 hours post exposure - take for 28 days.
2x NRTIs + integrase inhibitor
PrEP – pre-exposure - blocks transmission 2x NRTIs (Truvada) two tablets 2 – 24 hours before sex, one 24 hours after sex and a further tablet 48 hours after sex - called ‘on-demand’ or ‘event based’ dosing
2 x NRTIs = Combination of Nucleoside RTIs emtricitabine (guanosine analog) \+ tenofovir (adenosine analog)
Describe the resistance to anti-virals
Use of single agents leads to rapid development of resistance
The drug binding site is altered in structure by as few as one amino acid substitution
Mutation rate - high
Viral load – high
resistance
describe HIV resistance to antivirals
Selection pressure and mutation frequency
Increased mutation rate seen in HIV.
They form a quasispecies within an individual patient:-
A viral swarm
The error rate in copying viral genome by reverse transcriptase enzyme is 1 base per 10 4-5 incorporations; lacks proof reading capacity. So, for HIV with 10 9-10 viruses produced every day, ALL possible viral variants would be produced
Hence use of combinations of antivirals
e.g. HAART
Antivirals for the influenza virus
Amantadine
Inhibit virus uncoating by blocking the influenza encoded M2 protein when inside cells and assembly of haemagglutinin
Now rarely used
Zanamivir and Oseltamivir (Tamiflu)
Inhibits virus release from infected cells via inhibition of neuraminidase
Oseltamivir –oral
Zanamivir- inhaled or IV - less likely for resistance to develop
Describe action of relenza (zanamivir) and Tamiflu (oseltamivir)
target and inhibit NA at highly conserved site (reduce chances of resistance via mutation)
prevent release of sialic acid residues from the cell receptor
preventing virus budding and release and spread to adjacent cells.
Neuraminidase inhibitors
Describe the influenza resistance to antivirals
Resistance sometimes only requires a single amino acid change - seen recently with swine flu (H1N1) and Tamiflu (oseltamivir)
Point mutation (H275Y; tyrosine replacing histidine)
Seen in immunocompromised patients; shed virus for weeks/months
Likely to be selected from among quasispcies during treatment
Transmissible and virulent
Remains sensitive to zanamivir;
Describe postexposure prophylaxis to viruses
-Emergency and require rapid treatment
Hep B
specific Hep B immunoglobulin (passive immunity)
+ vaccination
within 48 hours (HBV treatment includes antivirals 3TC/NRTIs)
Hep C
interferon- + ribavarin (anti-viral) for 6 months
within first 2 months of exposure
90% cure rate - now direct acting antivirals
HIV 80% protection i.e. no sero-conversion must be FAST – hours antiviral drug treatment – 28 days 2xNRTI + protease or integrase inhibitor
Describe the Hepatitis C virus
9.6 Kb RNA virus, enveloped; Flaviviridae family; identified in 1989
transmitted via blood – infectious (mother to baby)
increasingly common – high risk groups – drug users 20% +ve; – needles (sex?)
major cause of chronic liver disease
estimated 170 million people infected worldwide
occupational risk groups – healthcare workers
needle-stick risk – 3% to sero-conversion; chronic carriage almost certain (85%)
long incubation – 1 - 6 months
vaccination NOT available
prevalence in UK - ~6000 per year ( 95% are i/v drug users)
early treatment facilitates resolution
Ribavirin
Block RNA synthesis by inhibiting inosine 5’-monophosphate (IMP) dehydrogenase –
this blocks the conversion of IMP to XMP (xanthosine 5’-monophosphate)
and thereby stops GTP synthesis and, consequently, RNA synthesis
Treat: RSV and HepC (in combination with pegylated interferon)
Describe Hepatitis C virus and direct acting antivirals
relatively new class of medication
acts to target specific steps in the HCV viral life cycle
shorten the length of therapy, minimize side effects, target the virus itself, improve sustained virologic response (SVR) rate.
structural and non-structural proteins - replicate and assemble new virions
HCV - first chronic viral infection to be cured without IFN or ribavirin.
All the major HCV-induced enzymes - NS2-3 and NS3-4A proteases, NS3 helicase and NS5B RNA-dependent RNA polymerase (RdRp) are essential for HCV replication and are potential drug targets.
DAA with different viral targets, are synergistic in combinations
Why are some vial infections untreatable?
Because they are self limiting. Can give vaccinations for them. rabies dengue Common cold viruses Ebola HPV Arbovirsues
What antibiotics are cell wall inibitors?
B-lactams (penicillins, cephalosporins) and vancomycin.
Principle of antibiotic resistance
Bacteria acquire genes, and if you select with antibiotics for these clones of bacteria that have acquired the genes, they then spread throughout the population.
Describe the use of antibacterials
Widely used and misused drugs
20-50% questionable use
In hospitals - 30% of drug budget
~25% of patients have received antibiotics within the previous 24h
In ITU, 50% of patients are on antibiotics
50 million prescriptions per year
80% of human use is in the community
50% - respiratory infections
15% - urinary tract infections
What are antibiotics
Natural products of fungi and bacteria - soil dwellers
- natural antagonism and selective advantage - kill or inhibit the growth of other microorganisms
most derived from natural products by fermentation,
then modified chemically :increase in pharmacological properties
Increase in antimicrobial effect
Some totally synthetic - e.g. sulphonamides
Describe how we discovered Penicillin
Alexander Fleming
Diffusion of penicillin into agar caused
lysis of S.aureus. The plate had been left
in a room for several weeks at a temperature
that allowed growth of mould
without overgrowth of the bacteria.
Describe the principles of antibiotics 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
What is the therapeutic margin
active dose (MIC) versus toxic effect
narrow for toxic drugs - e.g. aminoglycosides, vancomycin
ototoxic, nephrotoxic
-the dose between when a drug can stop becoming therapeutic, and start causing host damage.
Describe microbial antagonism
Maintains flora - complex interactions
Competition between flora
Limits growth of competitors and PATHOGENS
how can we get a loss of flora?
bacterial or pathogen overgrowth
e.g. Antibiotic Associated Colitis :
(clindamycin, broad-spectrum lactams, fluoroquinolones)
- pseudomembranous colitis
Clostridium difficile (part of normal flora of 3% of population), causes pseudomembranous colitis of colon, so colon cannot absorb water, causing diarrhoea.
Ulcerations-inflammation
Severe diarrhoea
Serious hospital cross infection risks.
What do is equally as important as antibiotics to result in bacteiral clearance?
Immunity Antibiotics and immunity of individual result in bacteria clearance. Immunosuppression e.g. cancer chemotherapy, transplantations, myeloma, leukaemias, HIV with low CD4 Neutropenics, asplenics, renal disease, diabetes, alcoholics, Babies, elderly., ………..
What can antibiotics be classified by?
Classified by:-
Type of activity
Structure
Target site for activity
Bactericidal vs Bacteriostatic
Bactericidal:
Kill bacteria
Used when the host defense mechanisms are impaired
Required in endocarditis, kidney infection
Bacteriostatic:
Inhibit bacteria
Used when the host defense mechanisms are intact
Used in many infectious diseases
Varies for drug and species and concentration
Types of activity of bacteria
Spectrum of activity
Broad Spectrum Antibiotics:
Effective against many types
Example: Cefotaxime
Narrow Spectrum Antibiotics:
Effective against very few types
Example: Penicillin G
Describe the refinement of antibiotic activity
Antbiotics can be altered pharmacologically to have enhanced effect on some bacteria. Go from being first generation, to second generation, to third generations.
Molecular structure of antibiotics classificatin
Duff antibiotics have diff structures.
Structural mimics of natural substrates for enzymes
B-lactams have a beta lactam ring, mimics the active structure in penicillin.
Describe the structure of bacteria
Infolding of plasma membrane, capsule, cell wall, DNA coiled into nucleoid, basal body, ribosomes, cytoplasm, plasma membrane, pili, cytoplasmic inclusion,
Describe the different bacterial targets for current antibiotics used in the clinic
- Cell wall synthesis, eg, cycloserine, vancomycin, teichoplanin, penicillin.
- Folic acid metabolism, trimethoprim, sulfonamides.
- Cell membrane, colistin, daptomycin.
- Protein synthesis
50S inhibitors, erythromycin, clindamycin
30S inhibitors, tetracycline, streptomycin, getamicin. - DNA and RNA processing
Quinolones,
-DNA gyrase, DNA directed RNA polymerase. - Forming free radicals, Metroonidazole, nitrodurantoin.
Difference between gram positive and gram negative bacteria
Gram positive
- Peptidoglycan on outside of membrane
- Peptidoglycan makes majority of cell wall, but also contains lipoteichoic acids, and integral proteins.
Gram negative
-peptidoglycan found in periplasmic space between outer and inner membrane. To get to it, you need porins, and selective transport mechanisms, found in outer membrane.
-Different structure of cell wall between them both.
How do bacterial cell wall inhibitors work?
To make peptidoglycan, there are two dimers, one with 5 peptide amino acids attatched. This contains to D-ALA, D-ALA terminal amino acids.
An inhibitor like cycloserine inhibits reactions involved in incorporation of alanine into cell wall precursor.
An enzyme in the bacterial cell breaks off the terminal D-ALA, and attaches the remaining D-ALA to adjacent chain, causing the formation of peptidoglycan. Vancomycin inhibitor binds to the terminal D-ALA D-ala residues, and prevents incorporation of sub unit into growing peptido glycan chain.
Bacitracin prevents dephosphorylation of phospholipid carrier which prevents regeneration of carrier necessary for synthesis to continue.
Penicillins and cephalosporins (beta-lactams) bind to and inhibit enzymes which catalyse this link. Peniclilin forms covalent enzyme-inhibitor complex with transpeptidase, as it is a structural mimic of the 5 amino peptide chain.
Antibiotics are often structural mimics of natural substrates for bacterial enzymes.
Describe folic acid synthesis inhibitors
bacteria can make their own folic acid. So we can inhibit this process, sulfonamides and dapsone block the first enzyme.
Trimethoprim antibiotic blocks the enzyme dihydrofolate reductase.
Gives the bacteria selective toxicity.
When do we use antibiotics
Treatment of bacterial infections
Prophylaxis - close contacts of transmissible infections
carriage rates ( ~80% in outbreaks)
e.g. meningitis
- prevention of infection e.g. tuberculosis
- peri-operative cover for gut surgery - people with susceptibility to infection.
Inappropriate use - viral sore throats - patient pressure
Dose of antibacterial MIC
This will depend upon the age, weight, renal and liver function of the patient and the severity of infection
Depend on the susceptibility of the organism
Will also depend upon properties of the antibiotic i.e. enough to give a concentration higher than the MIC (minimum inhibitory concentration)
! at the site of infection
Antibiotic combinations
BEFORE an organism identified in life-threatening infections
e.g. endocarditis, septicaemia
Polymicrobial infections e.g. abscess, G.I. perforation
anaerobes and aerobes
Less toxic doses of an individual drug possible
Synergy e.g. penicillin and gentamicin
Co-trimoxazole (sulphonamides + trimethoprim)
reduce antibiotic resistance e.g. Tuberculosis
Beta lactams : Penicillins
Basic penicillins e.g. benzylpenicillin(PenG), penicillin V
Active against streptococci, pneumococci, meningococci, treopnemes.
Most strains of Staphylococcus aureus are resistant.
Anti-staphylococcal penicillins e.g. flucloxacillin
narrow spectrum, G+ves, beta-lactamase resistant, less potent that PenG
Not MRSA
Pen G benzlypenicillin (G= gold standard);
not acid stable i/v or i/m good for some G-ves as well as G+ves
penV phenoxymethlypenicillin
oral (more acid stable than penG)
less active v G-ves, but same activity v G+ves as PenG
Broader spectrum penicillins e.g. ampicillin
Spectrum of activity is similar to basic penicillins but also includes some Gram-negative organsims and also enterococci
Anti-pseudomonal penicillins e.g. piperacillin
extended spectrum beta-lactam antibiotic
also G+ve, G-ve, anaerobes
Beta-lactam/beta-lactamase inhibitor combinations
e.g. co-amoxiclav (Augmentin)
Spectrum like amoxicillin plus activity against some Gram-negatives and Staph aureus
Why is antibiotic resistance a global concern?
~ 25,000 people die every year across Europe because of infections related to Antimicrobial Resistance (AMR)
In USA, MDR infections cost the health-care system ~20 billion US$ annually and generate more than 8 million additional hospital days
Antibiotic resistance:
Increases mortality
challenges control of infectious diseases
threatens a return to the pre-antibiotic era
increases the costs of health care
jeopardizes health-care gains to society
Describe the ‘Superbug’
Drug resistant bacteria
are NOT MORE pathogenic
We just have fewer antibiotic options
for treatment
Describe the mechanisms of antibiotic resistance
- Altered or new target
e.g.
Ribosome
Porin
PBPs –
peptidoglycan synthesis
DNA gyrase
RNA polymerase
Mcr1 & colistin - Drug inactivation
e. g. beta-lactamase - Metabolic by-pass
e.g. vancomycin
D-ala-D-lac - Efflux pump
- Overproduction of target
- trimethoprim - Intrinsic impermeability
Mechanisms of resistance
Natural resistance
Genetic Mechanisms - acquired
Non-Genetic Mechanisms (growth phases)
Describe the antibacterial paths to resistance
Directed at antibiotic itself -
Degrading the drug
Modifying the drug
New or Altered target
antibiotic no longer binds
e.g. PBPs - PBP2a in MRSA
Altered transport
Actively pumping drug out - efflux pump
porins no longer influx drug
Metabolic by-pass
metabolic change D-ala-D-lac and vancomycin
Describe the difference between natural resistance as opposed to acquired.
Drug must reach target - natural barriers, porins, export pump
G+ve peptidoglycan - highly porus - no barrier to diffusion
G-ves outer membrane - barrier resistance advantage
Porins single mutation - multiple resistance
Genetic mechanisms
Chromosome-mediated Due to spontaneous mutation: in the target molecule in the drug uptake system Mutants are SELECTED ; they are NOT induced
Plasmid-mediated
Common in Gram-negative rods
Transferred via conjugation
Multidrug resistance
The mutant is out there-we select it. As bacteria are constantly mutating.
Genetic mechanisms of resistance Production of drug-inactivating enzymes
Altered target structures – new or mutations
Alteration of membrane permeability
Altered influx or efflux - new pumps or mutations or altered gene expression
Gene transfer in bacteria summary
Mechanism for genetic heterogeneity and evolution
Rapid, cross-species
Virulence (toxins), drug resistance, antigens (immune evasion)
- Transformation
- fragment of DNA from another bacterial cell enters bacterial chromosome. - Transduction
- Fragment of DNA from another bacterial cell (former phage host, is transmitted into bactera) - Conjugation
- Sex pili form, allow the transfer of bacterial DNA.
Describe resistance to Beta-lactams by gram positive and gram negative bacteria
Gram + ve
ß-lactamase (Penicillinase), destroys beta lactam ring
Alteration of the transpeptidase enzyme
(PBP)
Gram - ve
ß -Lactamase (Penicillinase), destroys beta lactam ring.
Alteration of porins
Augmentin/co-amoxiclav
- combination of both
- binds to and inactivates beta lactamases. No antibacterial activity of its own.
Describe the different ways of beta lactam resistance in gram negative bacteria
- Porin mutates or new porin type
Multi-resistant, so beta lactam cannot enter bacteria - PBP - mutates or bacteria
acquires a new PBP (penicillin binding protien) - Bacteria acquires a beta lactamase enzyme.
Mechanisms by which bacteria become resistance to penicillin
Produce penicillinases / beta lactamases that cleave the beta lactam ring
- penicillin is inactivated
Acquire alternative forms of / or mutations in penicillin binding proteins (PBPs)
- penicillin can’t bind
Acquire alternative forms of / mutations in porins,
- penicillin cannot get into cell
Acquire alternative forms of / mutations in efflux pumps
- penicillins are pumped out faster
What is the only effective treatment of MRSA
vancomycin
Describe vancomycin resistance
Acquisition of van operon by transposition
-Makes D-ala, D-lactate, prevents vancomycin binding.
Non genetic mechanisms of resistance
Inaccessibility to drugs
(e.g., abscess, TB lesion)
Stationary phase/vegetations and biofilms
(non-susceptible to inhibitors of cell wall synthesis)
How to prevent/overcome antibiotic resistance
- Control use not in animal feeds
complete course [DOTS for TB]
appropriate prescribing - New or modified drugs few in past 25 years
- Combination therapy different targets
overcome mutation rates - Infection control individual - ward - society
Re-establish susceptible fora?
