Lecture #11: Growing and Killing Bacteria

Heat

– Heat Sterilization

– Pasteurization

Radiation

– Ultraviolet Radiation

– Ionizing Radiation

Filtering

– Depth Filters

– Membrane Filters

• Heat is the most widely used method of sterilization, the

killing of all organisms (including viruses).

• Sterilization must eliminate the most heat-resistant

organisms, usually bacterial endospores.

• An autoclave permits applications of steam heat under

pressure at temperatures above the boiling point of

water, killing endospores.

• Pasteurization does not sterilize liquids, but reduces

microbial load, killing most pathogens and inhibiting the

growth of spoilage microorganisms.

Sterilization and Pasteurization

Pasteurization

Reduces microbial populations

• Not a sterilization technique

• Kills most pathogens associated with food

• Less damaging to food products than heat

sterilization

• Flash Pasteurization = 71ºC, 15 seconds

• Bulk Pasteurization = 65ºC, 30 minutes

• Ultraviolet Radiation

– 220-300 nm

wavelengths

– Causes DNA

damage

– Surface

decontamination

(non-penetrating

radiation)

• Ionizing Radiation

– damage through

generation of highly

reactive ions (e-, H●,

and OH●)

– Absorbed radiation

dose measured in rads

or Grays

– Decimal reduction

doses can be

calculated

Depth Filters

– fibrous nature

– used to pre-filter liquids

– sterilization of air (HEPA)

• Standard Membrane Filter

– ~80% open pore

– traps filtrate on surface

– common heat-sensitive liquid

sterilization filter

• Nucleopore Membrane Filter

– formed by etching

polycarbonate film after

nuclear radiation

Sterilants

– gaseous infusion of chemicals

– e.g., formaldehyde, ethelene oxide

– sterilization of heat sensitive materials

Disinfectants

– kill most organisms (not endospores)

– e.g., cationic detergents

Sanitizers

• Antiseptics (germicides)

– antimicrobial agents that are safe for application to living tissue e.g., alcohol solutions

Synthetic Agents

– Growth Factor Analogs

• Sulfa Drugs

• Isoniazid

• Nucleic Acid Analogs

Quinolones

• DNA gyrase inhibitors

• e.g., nalidixic acid, Ciprofloxin

Antibiotics

– natural

– semisynthetic

Antibiotics

• Naturally occurring substances with

antibacterial properties

• Produced by fungi and bacteria

• Most are not usable as chemotherapeutics

• Semi-synthetic antibiotics are chemically

modified derivatives of antibiotics.

• They posses properties that increase their

efficacy

antibiotic synergism

effect of a combination of antibiotics is

greater than the sum of either antibiotic separately

antibiotic antagonism

interference of efficacy of one

antibiotic when coupled with a second antibiotic.

Target modification

β-subunit of RNA polymerase (rifampicin)

Antibiotic efflux

Tetracycline efflux in enteric bacteria

Antibiotic modification*

β-lactamase (penicillinase)

chloramphenicol acetyltransferase

Resistant pathways

Circumvention of folic acid synthesis (sulfonamides

Antibiotic impermeability

Penicillin in Pseudomonas aeruginosa