Controlling Microbes

Not Too Hot to Handle

11

Looking AheadOn completing this chapter, you should be able to:

• Summarize factors that influence the effectiveness of agents used for microbial control

• Explain some of the physical methods of control used to achieve sterilization and destroy all forms of microbes

• Compare the chemical methods of microbial control to the physical methods with respect to the anticipated objectives

• Identify some of the important chemical agents used to slow the growth of microbes on skin and on objects

Looking Ahead

On completing this chapter, you should be able to:• Explore the advantages and disadvantages of using antibiotics

to control microbes in the body• Identify some of the important antibiotics used to treat disease

and indicate how these drugs achieve their antimicrobial activity

• Discuss the problem of antibiotic resistance with reference to its cause and implications

Physical Methods of Control

Heat is a great sterilizing agent

Boiling cannot inactivate spores

Radiation is a great sterilizing agent

Deinococcus radiodurans resists high levels of radiation, too, though!

Physical Methods of Control

Heat methods– Denature and inactivate proteins– Drive off necessary water– 100 °C steam from boiling water (moist heat)

• Cannot inactivate spores– Pressure

• Autoclave• 15 psi• Allows higher water and steam temperatures• 121 °C steam now capable of inactivating spores

Physical Methods of Control

Physical Methods of Control

Heat methods• Pasteurization

• 62.9 °C for 30 minutes (hold method)• 71.6 °C for 15 to 30 seconds (flash method)• 82 °C for 3 seconds (ultraflash method)• Used to kill pathogens in milk, wine, fruit juice• Does not inactivate spores• Protects against Mycobacterium tuberculosis,

Coxiella burnetii

Physical Methods of Control

Heat methods• Dry heat

• 160 to 170 °C for at least 2 hours• Oxidation of proteins• Necessary for materials that cannot be

autoclaved or pasteurized

Physical Methods of

Control: Heat

Physical Methods of Control

Radiation• Ionizing radiation

• X rays• Gamma rays• About 10,000 times more energetic than UV light• Sterilizing• Creation of oxygen and hydroxyl free radicals that

inactivate proteins and DNA

Physical Methods of Control

Radiation• Electron Beams

• Room Temperature Treatment• Can pass through packaging to sterilize

contents• Ultraviolet radiation

• Results in mutations• Effective against spores, since no repair

mechanism

Physical Methods of Control

Drying• Also known as desiccation• Water required for microbes to survive• Removal prevents many enzymatic processes• Not effective to inactivate spores• Effective for storage of

• Cereals• Grains• Other foodstuffs normally stored in pantries

Physical Methods of Control

Drying• Lyophilization• Osmotic drying

• Salt• Sugar• Spices

Physical Methods of Control

Filtration and refrigeration• Filtration

• Heat-sensitive solution passed through filter• Pores in filter prevent passage of microbes

• Pores can be chosen based on size of microbe

• 0.2 m to 0.5 m pores prevent passage of many bacteria

• Does not prevent passage of viruses • Solution is not truly sterilized

Physical Methods of Control

Filtration and refrigeration• Refrigeration

• Slows down enzymatic reactions• Only slows microbial growth• Refrigerated foods are not sterile

Chemical Methods of ControlDisinfection and antisepsis

Practiced for thousands of years

Medicinal chemistry started in the 1800s

1860s: Joseph Lister

• Principles of antisepsis in surgery

• Diminished incidence of common infections that occurred during surgery

Chemical Methods of Control

General principles• Disinfectants

• Kill microbes on inanimate objects• Antiseptics

• Kill microbes on body surfaces• Ideal agent

• Soluble in water• Kills all microbes and inactivate infectious agents• Stable over time• Nontoxic to humans and animals

Chemical Methods of Control

General principles• Ideal agent (cont’d)

• Uniform composition• Combine with organic matter other than

microbes• Highest efficacy at room or body temperature• Efficiently penetrate surfaces• Not corrode or rust metals• Not damage or stain fabrics• Readily available in useful quantities• Cost effective

Chemical Methods of Control

Alcohols and aldehydes• Alcohols

• 70% ethyl alcohol (ethanol)• Isopropyl alcohol (isopropanol)

• Aldehydes• Formaldehyde (formalin)• Glutaraldehyde

Chemical Methods of Control

Halogens and heavy metals• Halogens

• Iodine• Tincture (2% iodine in ethanol)• Iodophor (iodine plus detergent)

• Betadine®• Wescodyne®

• Chlorine• 5% sodium hypocholorite (bleach)

Chemical Methods of Control

Halogens and heavy metals• Heavy metals

• Silver (as silver nitrate)• Mercury (as Merchurochrome®,

Merthiolate®, or thimerosal)• Copper

• Copper sulfate• Bordeaux mixture (copper sulfate with

lime)

Chemical Methods of ControlPhenols and detergents

• Phenols• Also known as phenolics• Ortho-phenylphelnol• Hexylresorcinol• Hexachlorophene• Chlorhexidine• Trichlosan

• Detergents• Strong wetting agents• Surface tension reducers• Dissolves microbial cell membranes

Chemical Methods of Control: Phenolics

Chemical Methods of Control

Ethylene oxide• Small molecule• Great penetration capacity (gas)• Sporicidal• Highly toxic• Explosive• Chemical counterpart of autoclave

Antibiotics

Antibiotics

The first antibacterials• Paul Ehrlich

• Magic bullets• Harm bacterial pathogens and not host• Arsphenamine

• Firs syphilis treatment• Contains arsenic

• Gerhard Domagk• Prontosil

• Active ingredient: sulfonilamide

Antibiotics: Sulfonilamide

AntibioticsThe development of penicillin

• Alexander Fleming• Penicillium mold on

Staphylococcus plates• Clearings where mold was

growing

• Howard Florey and Ernst Chain• Industrial production of

penicillin• Helped fight infections during

World War II

© Science Source, photo by Dean Pausett/Photo Researchers, Inc.© National Library of Medicine

AntibioticsPenicillins

• Beta lactam core• Primarily active against Gram-positive bacteria• Block formation of peptidoglycan in cell wall• Penicillinase• Improved penicillins

• Penicillin G• Amoxicillin• Ampicillin• Methicillin• Carbenicillin• Ticarcillin

Antibiotics: Penicillins

AntibioticsCephalosporins and aminoglycosides

• Cephalosporins• Like penicillins, contain beta lactam core• Produced by Cephalosporium• 6-membered ring, as opposed to penicillins’ 5-

membered ring• Cephalexin ( trade name Keflex)• Cephalothin (Keflin)• Cefotaxime (Claforan®)• Ceftriaxone (Rocephin®)• Ceftaxidime (Fortaz®)

AntibioticsCephalosporins and aminoglycosides

• Aminoglycosides• Useful against Gram-negative bacteria• Streptomycin

• Major early weapon against tuberculosis• Now most Mycobacterium tuberculosis is

resistant• Most produced by Streptomyces• Inhibit protein synthesis• Gentamicin• Neomycin

AntibioticsBroad-spectrum antibiotics

• Inhibit or kill many different microbes

• First one discovered: chloramphenicol

• Extremely toxic

• Still used in dire situations

• Tetracyclines

• Minocycline

• Doxycycline

• Used especially for Gram-negative infections

• Few side effects

• Resistance

• Fungal superinfection

• Light sensitivity

• Deposition in teeth

Antibiotics

Broad-spectrum antibiotics• Tetracyclines (cont’d)

• Few side effects• Resistance• Fungal superinfection• Light sensitivity• Deposition in teeth

AntibioticsOther antibiotics

• Macrolides• Inhibit protein synthesis• Erythromycin• Azithromycin (Zithromax ®)• Clarithromycin (Biaxin®)

• Vancomycin• Inhibits cell wall synthesis in Gram-positive bacteria• Severe side effects

• Streptogramins• Quinupristin + dalfopristin (Synercid®)

AntibioticsOther antibiotics

• Rifampin• Inhibits RNA polymerase• Synthetic• First used against M. tuberculosis• Useful against Neisseria, Haemophilus

• Bacillus-produced antibiotics• Only used topically because of toxicity• Bacitracin

• Inhibits cell wall synthesis• Effective against Gram-positive bacteria

Antibiotics

Other antibiotics• Bacillus-produced antibiotics (cont’d)

• Polymyxin B• Inhibits outer membranes• Effective against Gram-negative bacteria

AntibioticsAntiviral and antifungal antibiotics

• Antiviral chemicals• NOT antibiotics• Amantadine• Acyclovir

• Antifungal antibiotics• Nystatin

• Useful against Candida albicans• Reacts with sterols specifically present in

fungal membranes• Griseofulvin

• Ringworm

Antibiotics

Antiviral and antifungal antibiotics• Antifungal antibiotics (cont’d)

• Amphotericin B (Fungizone®)• Fungal infections of internal organs

• Imidazoles• Clotrimazole (Lotrimin®)• Miconazole (Monistat®)

AntibioticsAntibiotic resistance

• Spreading through bacterial populations• Bacterial pneumonia• Streptococcal blood disease• Gonorrhea• Staphylococcal infections• Tuberculosis

• Means of resistance• Destruction of antibiotic• Prevention of uptake• Alteration of metabolic pathway• Mutation that prevents antibiotic binding or efficacy

Antibiotics

Antibiotic resistance• Overuse of antibiotics• Overdose of antibiotics• Abuse in developing countries• Use in animal feeds• Resistance gene transfers from one bacterium to

another• Shigella• Salmonella• Staphylococcus

AntibioticsAntibiotic resistance

• Alternatives to reduce resistance or increase efficacy• New antibiotics• Limited antibiotic use• Phage therapy