Week 7 L1 biomed application, antimicrobial resistance

Question 1

The definition of infectious disease

Answer 1

• Harmful effects of one organism (‘pathogen’) on another organism (‘host’) in or on which they replicate
• Pathogens can transfer between hosts across or (more commonly) within species

Question 2

Penicillin use in the beginning

Answer 2

• Penicillin, part of the larger β-lactam group, showed immediate promise, but scaling up production was a challenge
• By 1944, Australia produced enough penicillin to allow some civilian use

Question 3

Antimicrobial resistance evolution: why did it happen?

Answer 3

• Individuals belonging to a given species of pathogen will vary genetically in numerous ways; this may include toxin resistance
• Use of antimicrobials creates a selective pressure favouring survival and reproduction of the most resistant individuals
• Over time, this will drive up the dose of an antimicrobial needed to clear an infection (minimum inhibitory concentration), or perhaps make it entirely ineffective

Question 4

mechanism of penicillin

Answer 4

• Penicillin mimics the structure of peptidoglycan, blocking the activity of enzymes which connect peptidoglycan molecules
• Penicillin uptake is affected by cell wall structure - the outer membrane of Gram-negative bacteria (e.g. E. coli) mostly blocks movement, but pores within it allow access

Question 5

Mechanism of penicillin resistance

Answer 5

• Three main mechanisms of penicillin resistance are known
• Target site alteration; some peptidoglycan-synthesis enzymes show much weaker binding to penicillin
• Reduced (net) uptake; higher levels of cell membrane proteins which remove penicillin from the cell increase resistance
• Degradation; β-lactamases break the β-lactam ring

Question 6

The origins of penicillin resistance

Answer 6

• Many bacteria had resistance to penicillin due to contact with Penicillum fungus
• The spread of resistance into previously susceptible species is promoted by the capacity for bacteria to exchange genetic material, e.g. through plasmids

Question 7

Bacterial plasmid exchange

Answer 7

• Plasmids carrying TEM-1 β-lactamase were first identified in E. coli and Salmonella species in the mid-1960s
• A decade later, they had also been found in Pseudomonas aeruginosa, Vibrio cholerae, Haemophilus species and Neisseria species

Question 8

The proliferation of antimicrobials

Answer 8

• Penicillin resistance spurred the development of alternative antimicrobials, often with naturally-occurring products as a start point
• Major antibacterial groups include:
• Diaminopyrimidines, quinolones (introduced in 1962) and sulfonamides (1935) - block DNA replication
• Aminoglycosides (1946), macrolides (1952) and lincosamides (1953) - inhibit protein synthesis
• Quinolones (1962) - block DNA unwinding and duplication
• Tetracyclines (1948) - inhibit protein synthesis or disrupt cell wall

Question 9

Novel antimicrobials, similar resistance
mechanisms

Answer 9

• Sulfonamides, an antimicrobial are now rarely used, in part due to the prevalence of mutations in their target, dihydropteroate synthase
• Aminoglycoside resistance largely occurs through enzymes which add functional groups to the antibiotics
• Efflux pumps play an important role in quinolone resistance in some bacteria

Question 10

The downsides of resistance to the microbe itself

Answer 10

• Mutations affecting an antimicrobial target protein are likely to reduce its activity
• However, plasmids can introduce an ‘alternative’ resistant form while leaving the native protein intact
• Expressing high levels of transport proteins or enzymes which metabolise antimicrobials may cause direct harms as well as requiring energy

Question 11

Slowing resistance development in microbes

Answer 11

• Removing an antibiotic from use may lead to loss of resistance over time - reducing use in agriculture is a popular goal
• Combination therapy can make formation of resistance less likely, especially where different mechanisms of resistance to each agent are favoured
• Specific common resistance mechanisms can be directly countered, e.g. βlactamase inhibitors, efflux inhibitors

Question 12

Antibiotic dosing

Answer 12

• The relationship between antibiotic concentration and level of resistance is affected by many factors
• Greater levels of antibiotic use will cause more intensive selection for resistance in commensal bacteria

Question 13

Antibiotic persistence

Answer 13

• In numerous populations of genetically identical bacteria, variation in lethality of a given antibiotic dose has been observed
• Under environmental stress, a portion of bacterial cells will enter metabolic dormancy, suspending many of the processes targeted by antibiotics
• Persistence is particularly common within biofilms

Question 14

Alternative treatments For microbe

Answer 14

• Antimicrobial development has slowed greatly - fewer than 50 candidate antibiotics in 2015, vs. more than 800 anti-cancer drugs
• Alternatives - or adjuncts - to antimicrobials could be considered
• Vaccines against several bacteria exist, with others in development
• Bacteriophages - viruses which infect bacteria -may be particularly useful in combatting biofilms