bacterial growth control

thermal death point

• the minimal temperature at which all organisms are killed in a particular liquid

thermal death time

• the minimal time required to kill all bacteria in a particular liquid at a given temperature

types of heat for bacterial control

• moist heat (boiling water/ autoclave)

• dry heat

• pasturisation (mild heat/ HTST/ UHT)

ionising radiation

• food industry

• medical and lab equipment

• DNA destruction by breaking double strands and ROS

non-ionising radiation

• surface decontamination

• DNA damage (breaks/dimers)

filtration

• used to sterilise gases or liquids that can be damaged by heat

• the porosity of filters can be chosen for specific applications

• nucleopore, membrane and depth filter

chemical antimicrobial control

• effect on growth: bacteriostatic, bactericidal and bacteriolytic agents

sterilant

• completely eliminating or destroying all forms of microorganisms, including spores

disinfectants

• kill microorgansisms but not necessarily endospores

antiseptics and germicides

• inhibit growth or kill microorganisms

disc diffusion technique and spot assays

What are the disc diffusion technique and spot assays?

• The disc diffusion technique is a method used to determine the susceptibility of bacteria to antimicrobial agents. This technique involves placing filter paper discs soaked in antibiotics on an agar plate inoculated with the bacteria. After incubation, the effectiveness of the antibiotics is determined by measuring the diameter of the inhibition zone around each disc.

• Spot assaysinvolve applying small volumes of different solutions containing potential antimicrobial agents directly onto an agar surface that has been inoculated with bacteria. This method allows for the quick assessment of the effectiveness of multiple agents by observing the growth inhibition in the spots.

minimum inhibitory concentration (MIC)

• the lowest concentration of a drug inhibiting the visible growth of a test organism after overnight incubation

minimum bactericidal concentration (MBC)

• the lowest concentration of a drug killing >99.9% of a test organism after overnight incubation

phenolic compounds

• first used in 1867

• aromatic derivatives (originally used to control odour in sewage)

• local anaesthetic at low concentration

• antibacterial (but toxic) at high concentration: disrupts cytoplasmic membrane, denature proteins

alcohols

• denature proteins

• lipid solvent

• disrupting cytoplasmic membrane

• active concentration between 60-85%

aldehydes

• alkylating agents

• modifies proteins and DNA causing cell death

quaternary ammonium compounds

• interacts with phospholipids of the cytoplasmic membrane (cationic agents)

halogen releasing agents: 2 types

1. chlorine-releasing agents: sodium hypochlorite (house bleach), formation of chlorinated bases in DNA → oxidation of proteins

2. iodine-releasing agents (iodine/iodophors): very powerful, but stain. Target DNA and proteins

history

• louis pasteur (1822-1895): formally demonstrated the theory of germs

• robert koch: established the casual relationship between a microbe and a disease

koch’ postulate

1. the microorganism must be found in all organisms suffering from the disease, but not in healthy organisms

2. the microorganisms must be isolated from a diseased organism and grown in pure culture

3. the cultured microorganism should cause disease when introduced into a healthy organism

4. the microorganism must be reisolated from the inoculated, diseased experimental host and identified as being identical to the original specifc causative agent

the major classes of antibiotics

4 ways bacteria resist antibiotics

• drug inactivation

• target modification

• efflux/ impermeability

• bypass

what causes antibiotic resistance

1. antibiotic misuse in human theraputics

2. farming (animals fed with antibiotcs at subtheraputic doses)

3. agriculture: treatment of diseases in trees and plants

4. aquaculture

5. pets

what are the properties of an ideal antibiotic

1. target: selective toxicity and must inhibit an essential process/ virulence

2. stability and effectiveness: pharmacokinetics and pharmacodynamics

3. cost

beta-lactams

• beta-lactams inhibits peptidoglycan polymerisation mediated by D, D-transpeptidases

• beta-lactams are structural analogs of D-Ala-D-Ala-C terminal residues in the peotide stem

• they are used by PBP as substrates and inactivate these enzymes irreversibly

resistance to beta-lactams

• beta-lactamases

• low affinity PBPs/overexpression

• efflux systems (gram negative bacteria)

• alternative pathways