Microbiology Flashcards
What is the function of bacterial replication?
To avoid elimination
What is required in order for pathogens to replicate?
- Tolerate host environment
- Evade host defences
- Compete successfully for nutrition
What is the O-antigen?
LPS on gram-ve bacteria
- Binds to TLR4 and leads to toxic shock
What is the bacterial capsule and what are its functions?
- Composed of protein subunits
- May allow better penetration through mucosa, adhesion to cells, avoidance of phagocytosis and inhibition of complemetnt
What is the function of bacterial Fc binding proteins?
Prevent interactions with Fc-receptors on phagocytes and so prevent phagocytosis
What are leukotoxins?
Toxins produced by pathogens that remove WBCs especially phagocytes
Which organism inhibits the phagolysosome?
Salmonella
Which organism is able to escape the phagolysosome?
Listeria
What are the consequences of caogulases?
- Clot blood
- Lead to damage to blood vessles
- If antibiotic goes through circulatory system will be inhibited
How may infection of a susceptible host with a pathogen occur?
- Entry through damaged epithelium leading to tissue invasion
- Adhrence to skin/mucous membranes
- engulfment by phagocyte cells on mucus membranes and survival of the pathogen in the phagocyte and transfer in tissues
List the factors that contribute to the ability to clear disease?
- Pathogen involved
- Immune suppression/modified physiology
- Site of infection
- Breaching of defensive barrier by accident or deliberately
Define persistent infection
Form of chronic, commonly involve silent and productive/clinical stages
Define latent infection
Overlaps with persistent, dormant until conditions are favourable for the pathogen
Define carrier state of infection
Infection present but does not suffer any symptoms, however can still spread disease
Describe the structure of Gram -ve bacteria
- Cytoplasmic membrane, thin cell wall, outer cytoplasmic membrane
- Outer membrane has lipopolysaccharides (LPS)
- Peptidoglycan can also trigger TLR, TLR4
Describe the structure of Gram +ve bacteria
- Single cytoplasmic membrane, thick cell wall, often have capsule
- Peptidoglycans in cell wall
Describe the structure of acid fast bacteria
- Similar to Gram +ve
- Cytoplasmic membrane, thick cell wall
- Mycolic acids in cell wall preventing penetration of disinfectants etc
Describe the structure of Mycoplasmas
- Single membrane, poor morphology, no cell wall
What are the sources of endo and exotoxins?
- Exo: living Gram +ve and -ve bacteria
- Endo: lysed Gram -ve bacteria
Where are endo and exotoxins found?
- Exo: released from the cell
- Endo: part of the cell
What is the chemical composition of exo and endotoxins?
- Exo: protein
- Endo: lipopolysaccharide
What is the heat sensitivity of exo and endotoxins?
- Exo: liable (60-80C)
- Endo: stable (250C)
Describe the immune reactions to exo and endotoxins
- Exo: strong
- Endo: weak
Is conversion to toxoids possible for exo or endotoxins?
- Exo: yes
- Endo: no
Are exo or endotoxins pyretic?
- Exo: no
- Endo: yes
Describe the enzymatic activity of exo and endotoxins
- Exo: mostly enzymatic activity
- Endo: no enzymatic activity
What is the molecular weight of exo and endotoxins?
- Exo: 10KDa
- Endo: 50-1000KDa
Compare the denaturing of exo and endotoxins
- Exo: on boiling, can get denatured
- Endo: cannot be denatured on boiling
Compare the specificity of exo and endotoxins
- Exo: specific to a particular bacterial strain
- Endo: non specific
Describe the antigenicity of exo and endotoxins
- Exo: high
- Endo: poor
Give examples of exotoxin producing bacteria
- Staph aureus
- Bacillus cereus
- Sterp pyogenes
- Vibrio cholera
- Bacillus anthracis
Give examples of endotoxin producing bacteria
- E. coli
- Salmonella typhi
- Shigella
Briefly describe the key features of Bacillus anthracis and Clostridium spp
- Spore forming
- Gram +ve
- Visualised using malachite green staining
- Heat needed to penetrate into spore cortex
- Spores resistant to disinfectants and heat
Give the stages of viral infection
- Acquisition
- Binding of receptor on cell and infection of cell
- Replication of proteins
- Release of more virus from cell
Define antibiotics
Low molecular weight microbial metabolites which can kill or inhibit the growth of susceptible bacteria
Define bactericidal
Kills the organism
Define bacteriostatic
Temporarily inhibits the growth of the organism
What is meant by the “magic bullet”?
Single drug that is able to clear infection/pathogens without affecting the host cells
Give examples of differences between bacterial and eukaryotic cells
- Ribosome structure
- Bacterial cell walls
What is the consequence of a drug target that is similar to host biology?
More undesired interactions
Give an example of how killing a pathogen may lead to adverse effects
- Gram -ve bacteria release lipid-A as they die
- This can cause sepsis/toxic shock
Give examples of adverse reactions to antimicrobials
- Direct host toxicity
- Toxic interference with other drugs
- Interferences with protective host flora
- Tissue necrosis at injection site
- Impairment of host immune function
- Hypersensitivity
- Enzyme induction/inhibition
Give examples of side effects from antimicrobials
- Nephrotoxicity (aminoglycosides)
- Neurotoxicity
- Tendon damage (quinolones)
- Liver disturbances (rifampicins)
Outline the secondary effects due to rifampicin
- Metabolised by liver, induces P450 pathway
- Increases rate of metabolism of many other drugs normally cleared by the liver
- Can cause redness of body fluids
Give examples of possible side effects due to rifampicin specifically
- Hepatotoxicity
- Respiratory
- Cutaneous (flushing, pruritus, rash, redness, watering of eyes)
- Abdominal (nausea, vomiting, abdominal cramps +/- diarrheoa)
- Flu-like symptoms (chills, fever, headache, arthralgia, malaise)
List the potential routes for antimicrobial administration
- IV
- Oral
- IM
- Per rectum
- Topical
- Nebulised (sprays for lungs)
Why is oral administration of antimicrobials contraindicated in ruminants?
Will destroy the rumen microflora
What is meant by time dependent activity?
Time exposed more significant than the concentration, time that serum concentration exceeds the MIC
Describe concentration dependent activity antimicrobials
- Concentration increase leads to killing increase
- Requires high concentration at drug binding site in order to be effective
- Maintaining this level between doses may not be beneficial
What physiochemical features facilitate penetration into cells?
- Highly lipophilic
- Low ionisation
Give examples of acidic (polar) antibiotics with low lipophilicity
- Penicillin
- Cephalosporins
- Beta-lactamase- inhibitors
Give examples of basic antibiotics with low lipophilicity
- Polymyxins
- Aminoglycosides (gentamycin, spectinomycin, tobramycin, streptomycin, amikacin)
Describe the tissue distribution characterstics of polar antibiotics with low lipophilicity
- Do not readily penetrate intact natural body barriers
- Effective concentration in CSF, milk, other transcellular fluids not always achieved
- Adequate concentrations may be achieved in joints, pleural and peritoneal fluids
- Penetration may be aided by acute inflammation
Name a weak- acid antibiotic drug of moderate to high lipophilicity
Sulphonamide
Give examples of weak-basic antibiotic drugs with moderate to high lipophilicity
- Trimethoprim
- Lincosamides (lincomycin, clindamycin)
- Macrolides (erythromycin, tylosin, spiramycin, tilmicosin)
Give examples of amphoteric antibiotic drugs with moderate to high lipophilicity
Tetracyclines (tetracycline, chlortetracycline, oxytetracycline)
Give examples of highly lipophilic, low ionisation antibiotics
- Chloramphenicol
- Fluoroquinolones (enfrloxacin, norfloxacin, ciprofloxacin)
- Lipophilic tetracyclines
- Metronidazole
- Rifampicin
Describe the tissue distribution characteristics of antibiotics with moderate to high lipophilicity
- Cross cellular membranes more readily than polar, enter transcellular fluids to greater extent
- Some trapping may occur (some ionisation)
- Penetrations of CSF and ocular fluids affected by plasma proteins binding as well as lipophilicyt
Describe the tissue distribution characteristics of highly lipophilic, low ionisation antibiotics
- Cross cellular barriers easily
- Penetrate into difficult transcellular fluids e.g. prostatic fluid and bronchial secretions
- All penetrate into CSF except tetracyclines and rifampicin
- All penetrate into intracellular fluids
What natural barriers exist in the host that may prevent penetration of antibiotics into tissues?
- BBB, poor penetration so require high doses to reach brain
- Lesions and pathologies may have poor perfusion and so poor drug delivery
When is the use of a bactericidal antibiotic indicated?
Where infections cannot be controlled by or eradicated by host mechanisms
When is the use of bacteriostatic antibiotics indicated?
Where the animal is healthy otherwise and only need to control the infection to allow the host to regain control
What is required in order for bacteriocidal antibiotics to function?
Bacterial growth, hence cannot be used in conjunction with bacteriostatics
Why might bacteriostatic drugs be ineffective?
- Nature/site of infection e.g. endocarditis (site)
- Host immunosuppression e.g. cat with existing feline leukaemia infection
Which antibiotics stem from Bacillus spp?
- Colistin (polymyxin E)
- Polymyxin B
- Bacitracin
Which organism is the source of the largest number of antibiotics?
- Streptomyces spp
- Source of: Streptomycin, tetracycline, Neomycin, Rifamycin, Vancomycin, Linicomycin, Eyrthromycin, Kanamycin, Amphoteracin B, Chloramphenicol
Which antimicrobials stemp from Penicillium spp (fungi)?
- Penicillin G
- Griseofulvin (anti-fungal)
Which antibiotic stems from Cephalosporium spp.?
Cephalosporins
What control methods other than antibiotics should be used?
- Prevention (vaccination, biosecurity, infection control plans)
- Removal of infected animals, euthanasia of infected animals to prevent spread of untreatable conditions
What are the goals of antimicrobial therapy?
- Eliminate infections without toxicity in the host
- Prevent infections in situations where the risk is high
What is MIC?
Minimum inhibitory concentration
- the concentration required at a site of infection to achieve bacterial inhibition
What is MBC?
Minimum bactericidal concentration
- the concentration required at a site of infection to kill the bacteria
What factors may affect the ability to achieve the required concentrations of antimicrobials?
- Pharmacological properties where antimicrobials distribute
- Elimination from the host (method and rate)
- MIC/MBCs achievable depending on area and drug
- Bacterial resistance mechanisms may increase MIC/MBC to unachievable amounts
What is required in order for an antimicrobial to be effective?
- Distribution to the right site
- Be above MIC/MBC
- Come into contact with the organisms
Outline how E. coli may respond differently to antibiotic treatment depending on site of infection
- If in abscess and growing anaerobically, are more resistant
- Do not express oxidative transport systems in anaerobic conditions
- Reduces entry of antibiotic into the bacterial cell
Give examples of how location of pathogens may affect antibiotic efficacy
- Intracellular organisms not affected by extracellular drugs
- Milk proteins may bind to antibiotics
- Local pH may reduce disassociation of some antibiotics and reduce distribution
- Poor blood supply and good epithelial barriers may reduce drug access
List factors that may restrict access of, or concentration of antimicrobials to site of infection
- Abscess formation, pus
- Foreign bodies
- Oedema fluid
- Factors that may bind the drug
What are MRLs?
Maximum Residue Levels
What is the significance of MRLs?
- Restrictions on use of antimicrobials in food producing animals as residues may enter food
- With-holding periods before animal or products can enter food chain
- Differs for different drugs
List factors that affect the MRL
- Drug persistence profile
- Drug dosing schedule
- Route of dose (e.g. oral vs injection into tissue)
List the classifications of antibiotics
- Inhibition of protein synthesis
- Inhibition of cell membrane function
- Inhibition of cell wall synthesis
- Interference with other pathways
- Inhibition of DNA dependent RNA polymerase
- Disruption of DNA structure
Briefly describe the bacterial cell wall structure
- Peptidoglycan unique to bacteria (good target)
- Polymer of sugars and amino acids forming mesh-like cell wall
- Sugars form alternating residues of beta-(1,4) linked N-acetylglucosamine N-acetylmuramic
- N-acteylmuramic acid cross linked by peptide chains of 3-5 amino acids
- Peptidoglycan serves structural role, counteracts osmotic pressure of cytoplasm
What do beta lactam antibiotics target?
Penicillin binding protein (transpeptidase) which stops peptide cross links in cell wall
What do peptide antibiotics target?
Directly interact with cell wall D-Ala-D-Ala moieties
Prevent synthesis of NAG/NAM polymers
What do bacitracin antibiotics target?
Cyclic peptides that interfere with dephosphorylation of isoprenyl carriers for cell wall synthesis
What do teixobactin antibiotics target?
Bind to highly conserved motif of lipid II (precursor of peptidoglycan) and lipid III (precursor of cell wall teichoic acid)
- NB, recent, not commercial yet
List the antibiotics that come under the wall targeting group
- Beta-lactams
- Peptide antibiotics (glycopeptides, vancomycin)
- Bacitracin
- Teixobactin
Compare the activity of beta-lactam antibiotics in Gram +ve and Gram -ve bacteria
- Direct access to cell wall in Gram +ve bacteria
- Require porin to get to wall through outer membrane of Gram -ve bacteria
Describe the mechanism of action of beta-lactam antibiotics
- Binds to cell receptors PBP (penicillin binding protein)
- Inhibits transpeptidation of cell wall peptidoglycan, damaging integrity of binding proteins
- Peptide precursors accumulate
- This triggers autolysin activity (needed to weaken bonds to allow cell growth)
- Positive pressure on weakened cell wall leads to cell lysis
What is the mechanism of resistance to beta-lactam antibiotics?
Mutation in transpeptidase (PBP) or presence of beta-lactamases that degrade beta-lactams
What are the 2 structural forms of beta-lactams?
Penicillin
Cephalosporins
Describe penicillin
- Organic acids
- Ionised in serum so spread well in ECF, poor at crossing membranes
- 3 groups: narrow spectrum, penicillinase stable, braod spectrum
- Low resistance in Gram +ve bacteria
- Polar hydrophilic molecules, circulate in blood and ECF without entering cells, short half lives, large molecules, largely excreted in urine
Name the narrow spectrum penicillins
- Penicillin-G (benzylpenicillin-G)
- Penicillin-V (phenoxmethyl-penicillin)
Name the penicillinase stable penicillins
- Methicillin
- Cloaxacillin
What are the penicillinase stable penicillins resistant to?
S. aureus beta-lactamase
Name the broad spectrum penicillins
- Ampicillin
- Carbenicillin
Describe cephalospporins
- Beta-lactam ring attached to 6-membered dihydrothiazine ring
- Resistant to beta-lactamase produced by Staphylococcus
- Used on patients hypersensitive to penicillin
- 3rd and 4th generaition are protected
What group of bacteria are resistant to cephalosporin?
Extended Spectrum Beta-lactamases
Describe the mechanisms of action of bacitracin
- Interferes with dephosphorylation of isoprenyl pyrophosphate carrier (bactophenol)
- Bactophenol is carrier molecule that transports components of peptidoglycan cell wall across inner membrane
- If inhibited, cannot build cell wall
Describe the structure of bacitracin
- Cyclic polypeptide based agent
- Cyclic structure with long chain attached
Describe the spectrum of activity of bacitracin
- Narrow
- Gram +ve bacteria only
Give an example of a side effect of bacitracin
- Nephrotoxicity when used systemically
- Limited to use topical and ophthalmic preparations
Outline the use of vancomycin
- Protected group under good antimicrobial stewardship
- Last line of defence in multiple resistant staphylococcal infections
List the membrane targeting antibiotics
- Polypeptides (polymyxins, Colistin)
- Cationic peptides (newer, e.g. PEptivet)
What are the consequences of disruption of bacterial cell membrane integrity?
- Leakage of essential ions
- Removal of proton motive force
- Damage to cytoplasmic conditions
- Significant disruption of cell homeostasis
Describe the structure of polymyxins
Cyclic peptide with long hydrophobic tail
Describe the mechanism of action of polymyxins
- Act on Gram -ve bacteria
- Disrupt bacterial cell membrane by interacting with its phospholipids
- Interact with LPS on Gram -ve outer membrane
Describe the use of polymyxins
- Voluntary ban of vet use
- Resistance develops by mutation of LPS, inheritable
Describe the administration of polymyxins
- Pooly absorbed from GI tract, given by injection
- Some nephrotoxicity with systemic use
- Well tolerated if topical
Describe the pharmacokinetics of polymyxins
- Bind moderately to plasma proteins
- Bind extensively to muscle
- Diffuse poorly
- Slow excretion by glomerular filtration
Describe the spectrum of activity of antimicrobial peptides
- Broad
- Active against clinically relevant antibiotic-resistant strains
- Gram -ve and Gram +ve
Describe the mechanism of action of antimicrobial peptides
- Small peptides, insert into membrane and disrupt
- Gram -ve: peptide fist interacts with LPS on outer membrane, permeabilised allowing peptide to be captured inside
- Gram +ve: peptide attracted to techoic acid and other anionic groups found externally on peptidoglycan layer
What facilitates the intrinsic resistance of eukaryotic cells to antimicrobial peptides?
- High content of zwitterionic phospholipids and cholesterol
- These are absent in bacteria
