Module 7 - Epidemology, antibiotics, and antibiotic resistance

Question 1

Epidemiology

Answer 1

To carry out disease surveillance; to describe, analyse & understand the spread of disease

Question 2

Endemic

Answer 2

Constantly present - causes a low-level frequency of disease at regular intervals (colds, human flu, etc)

Question 3

Epidemic

Answer 3

Sudden increase above the expected (chickenpox etc)

Question 4

Pandemic

Answer 4

Increase simultaneously over a wide area (global) (AIDS, FLU, COVID-19, etc)

Question 5

Epidemics/pandemics in plants

Answer 5

Also pose a significant threat to local and global food security, exacerbating the problem of malnutrition (maize lethal necrosis, rice tungro, sweet potato virus, banana bunchy top, citrus tristeza, plum pox)

Losses caused by plant viruses are thought to cost global agriculture approximately $30bn (£22bn) a year

Question 6

Morbidity vs mortality rate

Answer 6

Morbidity - % of people who get it
Mortality - % of people who die due to it

Question 7

Two types of epidemic

Answer 7

Common source epidemic - Sharp rise to a peak and rapid decline (ie food poisoning)

Propagated epidemic - Slow rise and gradual decline (ie chickenpox rising in summer, falling in winter)

Question 8

Herd immunity: the definition and the effect on flu, polio, and the measles

Answer 8

Resistance of a population to infection - due to the immunity of the majority

Flu - 90% immunised - disease prevented
Polio - 70% immunised - less contagious
Measles - 90-95% immunised - disease prevented

Question 9

Antigenic shift

Answer 9

Minor antigenic variation due to mutations may alter haemagglutinin, neuraminidase, or amino acid sequences which may invalidate vaccines

Question 10

Types of epidemic control

Answer 10

Eliminate source - quarantine/destroy reservoir
Break connection between source and host
Raise the level of herd immunity - vaccination

Question 11

Zika virus: where was it first discovered, when was it first discovered, what are its symptoms, and how is it transmitted?

Answer 11

First isolated in the Zika Forest in Uganda in 1947

No or mild symptoms but if transmission occurs in utero then brain defects may occur (microencephaly and severe brain malformations)

Transmission:
* (Aedes aegypti) mosquito
* Sexual transmission
* Blood transfusion
* Vertical transmission can occur in utero

Question 12

Re-emergence of viruses: what are the ways it may occur and what examples are there?

Answer 12

1 - Demographics - move to cities (crowded)
2 - Transportation – bulk processing -> distribution -> speed of spread
3 - Economic development & changes in land use- eg build dam -> mosquitoes -> disease
4 - International travel (SARS/flu)
5 - Microbial adaptation (flu)
6 - Biological warfare (anthrax, plague, Ebola, botulinum toxin)
7 - breakdown of public health measures (cholera)

Question 13

Antimicrobials: what is the idea and why is the idea slay?

Answer 13

Idea formulated by Paul Ehrlich - basically an antibiotic

Question 14

Antibiotics: how do they work and what antibiotics are used to stop which process?

Answer 14

They have selective toxicity by targetting unique bacterial sites (prokaryotic cells):

• Cell wall synthesis inhibitors - penicillins (emphasis today)
• Protein synthesis inhibitors – aminoglycosides & macrolides
• Nucleic acid synthesis inhibitors – quinolones
• Folic acid biosynthesis- sulphonamides

Question 15

Types of antimicrobial methods

Answer 15

Bacteriostatic - growth inhibited by ribosome synthesis inhibitors (treatment must go on long enough for the immune defence to destroy the pathogen as it is only prevented from growing, not killed)

Bactericidal - killing pathogens, this is the preferred method

Question 16

Antimicrobials: cell wall synthesis inhibitors

Answer 16

Penicillin and cephalosporins

B-lactam antibiotics - they have a ß lactam ring (heterocycle ring with 3 carbons and a nitrogen) for activity which inhibits peptidoglycan biosynthesis

Question 17

Penicillin: what was the first type of penicillin produced, how does penicillin destroy pathogens, how is it broken down, and how prevalent is it in prescribed medicine?

Answer 17

Benzylpenicillin (Penicillin G)

Active against Gram +ve bacteria (ie pneumococci, streptococci target peptidoglycan biosynthesis) but does not (typically) penetrate Gram -ve Outer membrane

Destroyed by gastric pH (injection)

50% of all antibiotics prescribed

Question 18

How does penicillin destroy pathogens?

Answer 18

The B-lactam ring mimics the D-ala-D-ala at the end of the peptide in peptidoglycan, this addition prevents the completion of the peptidoglycan bridge which causes a buildup of peptidoglycan precursors which causes the bacteria to be digested by hydrolases

Question 19

Penicillin: which can affect Gram -ve bacteria and how?

Answer 19

Semi-synthetic penicillins - they have a modified R1 group (ie Amoxycillin & ampicillin) which allows antibiotics to pass through the outer membrane pores of Gram -ve bacteria

Question 20

Penicillin resistance: how does penicillin resistance work and what solutions do we have for it?

Answer 20

b-lactamases hydrolyse the b-lactam ring of penicillins rendering them inactive

Solutions:
* Combine with clavulanic acid - a b-lactamase inhibitor (Augmentin)
* Synthesise b-lactamase-resistant penicillins e.g. methicillin

Question 21

Carbapenem antibiotics: where are they obtained from, what do they do, how are they administered, and what are they used to treat?

Answer 21

Developed in the 1960s as b-lactamase inhibitors obtained as natural products from Gram +ve organisms

Acylate penicillin-binding proteins after entering Gram –ves via porins in the outer membrane

Administered intravenously- does not cross the gastrointestinal membrane

Used to treat b-lactamase-producing (i.e. penicillin) resistant bacteria

Question 22

Carbapenem resistance: the two types

Answer 22

1 - Three classes of enzymes which hydrolyse the b-lactam ring:

• Class A: Carbapenemases - hydrolyse β-Lactams
• Class B: metallo-β-lactamases (MBL) - makes bacteria resistant to a broad range of B-lactams
  (Enterobacteriaceae and environmental bacteria)
• Class C: β-lactamases - cleave the B-lactam ring of oxacillin which is resistant to all other B-lactamases
  (reservoir in environmental bacteria and deep sea microflora)

2 - mutations that reduce the influx or increase efflux of the drug

Question 23

MRSA: what is it, what treatments are there for it, how do they function, and (I have no clue but try remember this)?

Answer 23

Methicillin-resistant Staphylococcus aureus (includes beta-lactam antibiotics: penicillins (methicillin, dicloxacillin, nafcillin, oxacillin, etc.) and the cephalosporins)

Vancomycin (also a cell wall inhibitor) - binding to the D-Ala-D-Ala prevents cell wall synthesis in two ways: prevents the synthesis of the long polymers of N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) that form the backbone strands of the bacterial cell wall, and it prevents the backbone polymers that do manage to form from cross-linking with each other

MRSA strains contain the mecA gene which encodes penicillin-binding protein 2A with low affinity for b-lactam antibiotics and therefore transpeptidase enzymes are unaffected and can function in the presence of b-lactam antibiotics.

The large hydrophilic molecule is able to form hydrogen bond interactions with the terminal D-alanyl-D-alanine moieties of the NAM/NAG-peptides. Under normal circumstances, this is a five-point interaction

Question 24

Vancomycin: what is it and where is it obtained, when is it used, and what does it do

Answer 24

a glycopeptide antibiotic - a naturally occurring antibiotic isolated in Borneo in 1953 from Amycolatopsis orientalis

Drug of last resort (ie used to treat MRSA)

Inhibits cell wall synthesis in Gram +ve bacteria when it binds to the terminal D-Ala-D-Ala dipeptide and inhibits transpeptidase activity

Question 25

Vancomycin resistance: why is resistance different to normal resistance and how does it work?

Answer 25

Because vancomycin does not interact with cell wall biosynthetic enzymes but forms complexes with peptidoglycan precursors, its activity is not determined by the affinity for a target enzyme but by the substrate specificity of the enzymes that determine the structure of peptidoglycan precursors

Resistance to vancomycin is due to the presence of operons that encode enzymes for the synthesis of low-affinity precursors, in which the C-terminal D-Ala residue is replaced by D-lactate (D-Lac) or D-serine (D-Ser), thus modifying the vancomycin-binding target

Question 26

Aminoglycosides: what are they, what do they do, how are they administered, when are they used, why are they not always used, and which types of bacteria do they affect?

Answer 26

Bactericidals (ie kanamycin & gentamycin, streptomycin) derived from the Streptomyces genus

1) Binds to the 30S ribosomal subunit, preventing active ribosome formation
2) Appear to displace cations in the bacterial cell biofilm that are responsible for linking the lipopolysaccharide (LPS) molecules characteristic of Gram-negative bacterial cell walls, creating holes (lysis) in the cell that may kill the bacteria before the aminoglycoside even reaches the ribosome.

Since they are not absorbed from the gut, they are administered intravenously and intramuscularly.

Used when treatment is required before infecting agent has been determined (ie septicaemia)

Nephrotoxicity and ototoxicity (inner ear damage) during aminoglycoside treatment makes physicians reluctant to use these compounds in everyday practice. Only used for emergencies

Works on both Gram +ve and -ve bacteria (but only really used for -ve as there are many, less toxic alternatives for Gram +ve infections) but only effective on aerobic bacteria as they need energy derived from aerobic metabolism to cross membranes and enter the cell

Question 27

Tetracycline: what is it and where is it obtained from, what does it do, what is it used to treat, why are eukaryotic cells not affected, how does resistance occur, and what side effects are there?

Answer 27

Tetracycline is a broad-spectrum polyketide antibiotic produced by the Streptomyces genus of Actinobacteria, indicated for use against many bacterial infections

It is a protein synthesis inhibitor

It is commonly used to treat acne today, and, more recently, rosacea, and is historically important in reducing the number of deaths from cholera.

30S ribosomes only affected - eukaryotic cells safe

Resistance to the tetracyclines results from changes in the permeability of the microbial cell enveloaseape.

Can stain developing teeth (even when taken by the mother during pregnancy) Can cause permanent teeth discolouration (yellow-grey-brown); infancy and childhood to eight years old

Question 28

Macrolides: what are they, what are the ideas behind the mechanism, what is the effect of macrolide, who is it used to treat, and why?

Answer 28

Macrolides are protein synthesis inhibitors (bacteriostatic)

Mechanism of action of macrolides is not fully understood, possibly inhibition of bacterial protein biosynthesis by preventing peptidyltransferase from adding the peptidyl attached to tRNA to the next amino acid and inhibiting ribosomal translocation or possibly premature dissociation of the peptidyl-tRNA from the ribosome.

The addition of an incoming tRNA and its attached amino acid to the nascent polypeptide chain is inhibited

Used in individuals with allergies to penicillin, generally used to treat Gram +ve infections with limited use of Gram –ve (resistance is common)

Question 29

Nucleic acid synthesis inhibitors: what types are there and how do these inhibitors work?

Answer 29

Fluoroquinolones (synthetic)

Bind to DNA gyrase, preventing it from supercoiling DNA and inhibiting DNA replication

Question 30

Antimetabolites: what are they, what were they used for and what are they used for now, how is resistance reduced, and how often are adverse effects seen?

Answer 30

Sulphonamides and Trimethoprim (bacteriostatic) Inhibit dihydropteroate synthase and dihydrofolate reductase - both required for folate synthesis

Used in wars on wounds before penicillin, now used to treat acne and UTIs

Both drugs used in combination to reduce the development of resistance

Adverse side effects in 3% population

Question 31

Teixobactin: what is it and how does it work?

Answer 31

Peptide-like antibiotic which targets Gram +ve bacteria

Binds to lipid II (a peptidoglycan precursor) and forms fibrillar oligomers with Lipid II which obstruct peptidoglycan biosynthesis and causes membrane defects by displacing polar lipid head groups as membrane thickness is reduced

Question 32

Malacidin

Answer 32

Bind to calcium; the calcium-bound molecule then appears to bind to Lipid II

Question 33

Retinoids

Answer 33

Vitamin A analogues can kill MRSA

Question 34

How old is antibiotic resistance?

Answer 34

β-lactamases arose >2 billion years ago, (pre-dating the divergence of Gram +ve and Gram -ve bacteria)

Antibiotic resistance genes are therefore predicted to have been present in environmental bacterial populations for millions of years.

Question 35

Superbugs

Answer 35

Pathogenic bacteria resistant to most usual antibiotics:

1 - penicillin-resistant pneumococci (Streptococcus pneumoniae)
2 - multidrug resistant mycobacteria (Mycobacterium tuberculosis)
3 - methicillin (meticilin) resistant Staphylococcus aureus (MRSA) sensitive to vancomycin but resistant to all others
4 - vancomycin-resistant MRSA resistant to all usual antibiotics (VRSA – rare but worrying)
5 - Commensals - vancomycin-resistant enterococci (VRE) (rare) (Enterococcus faecalis) commensal that transfers vancomycin resistance to MRSA

Question 36

Reasons for antibiotic resistance

Answer 36

1 - Natural resistance (intrinsic or innate):
* lack of target structure (ie no wall)
* impermeable to the antibiotic

2 - Sensitive bacteria develop resistance (acquired):
* enzymatic inactivation of antibiotic
* modification of the target
* organism pumps out the antibiotic (efflux mechanism)

Question 37

How does antibiotic resistance develop?

Answer 37

1 - Selection pressure of antibiotic
2 - Rapid cell division
3. transfer of resistance genes; species to species; genus to genus (HORIZONTAL GENE TRANSFER)

Question 38

Transfer of R-genes

Answer 38

Methods involving a narrow host range (between members of the same species):
* transduction (from bacteriophages)
* transformation (from naked DNA)

Methods involving a broad host range: between different genera (most important):
* Conjugation-transfer of plasmids

Question 39

Conjugation mechanism

Answer 39

1 - Donor cells (males) contain plasmid
2 - Recipient cells (females) do not
3 - Mating pairs form (top left)
4 - Pilus retracts & R plasmid transfers to recipient cell (plus resistance genes)
5 - Mating involves cell-to-cell contact

Question 40

Resistance mechanisms: three mechanisms

Answer 40

1 - Resistance to ß-lactams (penicillins):
* Cleave the ß-lactam ring

2 - Resistance to aminoglycosides (eg streptomycin):
* enzymes inactivate antibiotics by adding groups (eg acetyl, adenyl) - modified antibiotic reduces transport into the cell

3 - Active efflux:
* pumps out the antibiotic immediately using cytoplasmic membrane proteins

Question 41

Resistance mechanisms: modifications of the target

Answer 41

One example is Resistance to ß-lactams (penicillins):
* Transpeptidase (penicillin binding protein: PBP) has an altered shape preventing penicillin binding but still allowing D-ala-D-ala binding - cross-linking of PG not inhibited & normal PG is produced