Bacterial growth control

L7

What are the 3 physical methods used to eradicate bacteria?

• heat

• irradiation

• filtration

How can heat be used as a physical antimicrobial control?

• moist heat (for spores eg, 15 mins at 121ºC under pressure)

• dry heat (direct flaming, incineration, >150ºC for 2hours)

• pasteurisation (HTST=72ºC , 15 sec; designed to kill 99.999% of viable micro-organisms in milk, UHT=140ºC , 2-5 sec)

What do thermal death point and thermal death time describe?

min temp at which organisms are killed in 10 mins in a particular liquid

min time needed to kill bacteria in a particular liquid at a given temp.

What are 2 types of radiations can be used as a physical antimicrobial control?

• ionising radiations (DNA destruction for food/medical industries)

• non ionising radiations (surface decontamination, DNA damage)

How can filtration be used as a physical antimicrobial control?

• to sterilise gases (air) or liquids that can be damaged by heat - immobilises bacteria

• porosity of filters adapted to need (1mm to 0.01 um)

Name 3 types of filters used for bacterial control

• nucleopore filter

• membrane filter

• depth filter

Describe the difference between bactericidal & bacteriostatic agents

kill bacteria /vs/ stops their growth by stopping cell proliferation, eventually die after not being able to move any more or immune system recognition

Would optical density decrease when bacterial growth was controlled by bacteriostatic, bactericidal or bacteriolytic agents?

bacteriolytic is the only one that destroys the dead cells, the other just kill the bacteria but absorbance/OD would stay the same

What do bacteriolytic agents do ?

kill and destroy bacterial cells

What are two settings in which chemical antimicrobial control is used?

• when destroying m-o on objects - sterilants, disinfectants like alcohol or ethylene oxide

• when inhibiting/killing m-o on living tissues - antiseptics, germicides like handwash

What are 4 ways to measure antimicrobial activity?

• disc diffusion technique

• spot assays

• min inhibitory concentration (MIC) - min drug needed to inhibit visible growth

• min bactericidal concentration (MBC) - min drug killing over 99.99% of organism

Name some examples of chemicals used for antimicrobial control

triclosan + chlorhexidine in hygiene products, phenolic compounds to control odour in sewage, alcohols, aldehydes, halogen releasing agents, “quat compounds“

How are MIC and MBC determined?

using broth dilution methods, where a standard number of microorganisms are incubated with serial dilutions of an antimicrobial agent then look visually

What are 3 classes of chemical agents used as antibacterials?

sterilants, disinfectants, antiseptics and germicides

How do alcohols act as an antibacterial?

Denature proteins; lipid solvent, disrupting cytoplasmic membrane, Active concentration between 60-85%

How do aldehydes act as an antibacterial? Name some examples of aldehydes

Alkylating agents; modify proteins and DNA causing cell death

formaldehyde, glutaraldehyde

How do quaternary ammonium compounds (quats) act as an antibacterial?

Interact with phospholipids of the cytoplasmic membrane (cationic detergents)

How do halogen releasing agents act as an antibacterial? What are the 2 main types of these agents?

targets DNA and oxidates proteins

chlorine releasing agents like household bleach + iodine-releasing agents

What are 2 major therapeutic strategies for bacterial diseases?

antibiotics and vaccination

What did Louis Pasteur contribute to the concept of antibiotics?

demonstrated the theory of germs in 1860s - need air for bacterial growth

What did Robert Koch contribute to the concept of antibiotics?

Koch’s postulates (~1890) establish the causal relationship between a microbe and a disease

List Koch’s 4 postulates (1890s)

• m-o must be found in all organisms suffering from the disease, but not in healthy organisms.

• m-o must be isolated from a diseased organism and grown in pure culture.

• cultured m-o should cause disease when introduced into a healthy organism.

• m-o must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

Who discovered penicillin?

Alexander Fleming

What were the first 2 attempts at antibiotics? Were they successful? Name 2 people and their agents that were successful.

Emmerich and Low with Pyocyanase and Paul Ehrlich with Salvarsan - unsuccessful bc of toxicity and side effects

Fleming with Penicillin and Domagk with Prontosil red (only works on gram positive cocci)

What defines the classes of antibiotics? Name some

what they target

• cell wall

• dna

• rna elongation

• protein synthesis

• cytoplasmic membrane

• dna dirceted rna polymerase

What’s the difference between antibiotics and beta lactams?

broad class of drugs targetting growth of bacteria // specific type of antibiotic distinguished by a unique ring structure (the beta-lactam ring)

What are 2 types of problems that are rising from antibiotic resistance?

• health: by 2025, 10 million deaths due to resistance

• economic : 75 trillion pound cost

What are 5 societal explanations for antibiotic resistance?

• antibiotic misuse in human therapies (eg viral infections)

• farming (animals fed with antibiotics to promote growth)

• agriculture (treating plants)

• aquaculture

• pets

What are the properties of an ideal antibiotic?

• target: selective toxicity + must inhibit an essential process or inhibit virulence

• stability and effectiveness: pharmacokinetics and pharmacodynamics

• cost

What does pharmacokinetics mean?

what does the body do to the antibiotics?

What does pharmacodynamics mean?

what do the antibiotics do to the body?

Describe the mode of action of β-lactams

inhibit peptidoglycan cross-linking by irreversible inactivation D,D-transpeptidases (D-Ala-D-Ala homologs, form covalent intermediate)

Describe 3 mechanisms conferring resistance to β-lactams

• degradation of antibiotic (lactamase)

• efflux/impermeability (gram -)

• target modification (affinity/overexpression)

• bypass (modification of the biosynthetic pathway)

What’s the beta-lactam paradigm?

beta-lactam antibiotics work by inhibiting bacterial cell wall synthesis, by binding to and inactivating penicillin-binding proteins (PBPs), which are essential for peptidoglycan cross-linking

What are D,D transpeptidases and their role?

penicillin binding proteins

enzymes essential for synthesizing and maintaining the bacterial cell wall, specifically the peptidoglycan, by cross-linking peptide chains

What do beta lactams target in bacteria? How does this work?

Penicillin Binding Proteins

by binding to the PBP enzyme, blocking substrates like peptidoglycans, a key component of the bacterial cell wall

What does it mean clinically that beta lactams are structural analogs of  D-Ala-D-Ala C-terminal residues?

they can bind to PBP just like peptidoglycans would when polymerised = this blocks this from happening, breaks the cell wall

How does inactivation by beta lactamases lead to antibiotic resistance?

enzyme destroys beta lactams so they no longer hinder dvpt of bacterial cell wall ie bacteria are intact

How does a mutation in penicillin binding proteins lead to antibiotic resistance?

= reduced PBP affinity for beta lactams bc beta lactams no longer the right shape to fit into this new protein morphology

How does secretion of the antibiotics lead to antibiotic resistance? In which bacteria can this happen?

specialised transporter proteins called efflux pumps, pump antibiotics out of their body = lower concentration of antibiotics within the cell

How does modification of the synthetic pathway targeted by beta lactams lead to antibiotic resistance?

beta lactams target the shape of PBP so bacteria respond by changing the enzyme responsible for peptidoglycan integration into the cell wall which is beta lactam insensitive ie antibiotics useless

What explains bacteria gaining more than one type of gene that confers antibiotic resistance?

through conjugation, this gene is interesting bc allows bacteria to

Name 2 types of efflux pumps in gram negative bacteria. What do they make bacteria resistant to?

 MexAB-OprM system and MexEF-OprN system

carbapenem (usually last resort as major weapon) and imipenem (both types of antibiotics)