Ch. 11 & 12 Physical and Chemical Agents for Microbial Control

early protection attempts

• burning wood, releasing formaldehyde

• herbs, perfume, and vinegar contain mild antimicrobial substances

disinfection

the destruction or removal of vegetative pathogens but not bacterial endospores. usually used on only inanimate objects

sterilization

the complete removal or destruction of all viable microorganisms. used on inanimate objects

antisepsis

chemicals applied to body surfaces to destroy or inhibit vegetative growth

highest resistance of microbes

prions and bacterial endospores

moderate resistance of microbes

• protozoan cysts

• naked viruses

• bacteria with no endospores but resistant walls

least resistance of microbes

• other gram + bacteria

• yeasts

• enveloped viruses

• protozoan trophozoites

microbicidal agents

antimicrobial agent aimed at destroying a certain group of microorganisms

agents that cause microbistasis

antimicrobial agent aimed at temporarily preventing microbes from multiplying

methods that reduce the numbers of microorganisms

sanitization and degermination

determination/degerming

reduction of microbial load from living tissue by mechanical means

microbial death is

hard to detect

microbes die _________

logarithmically

factors that affect microbial death

• number of microbes

• nature of microbes in the population

• temperature and pH of environment

• concentration or dosage of agent

• mode of action of the agent

• presence of solvents, organic matter, or inhibitors

cellular targets of physical and chemical agents

• cell wall

• cell membrane

• protein and nucleic acid synthesis

• protein function

what happens to the cell wall when it is a cellular target?

becomes fragile and cell lyses

what happens to the cell membrane when it is a cellular target?

loses integrity

what happens to protein and nucleic acid synthesis when it is a cellular target?

prevention of replication, transcription, translation, peptide bond formation, and protein synthesis

what happens to protein function when it is a cellular target?

disruption or denaturing of proteins

what are primary targets in microbial control?

microorganisms capable of causing infection or spoilage

physical agents

heat (dry or moist)

types of dry heat

incineration (sterilization) or dry oven (sterilization)

types of moist heat

steam under pressure (sterilization) or boiling water, hot water, pasteurization (disinfection)

examples of ionizing radiation

x-ray, cathode, gamma (sterilization)

examples of non ionizing radiation

UV (disinfection)

what is the mechanical removal method of microbial control?

filtration (air = disinfection, liquids = sterilization)

chemical agents of microbial control

gases, liquids (animate and inanimate), and chemotherapy

moist heat

lower temps and shorter exposure time; coagulation and denaturation of proteins which halts cellular metabolism

dry heat

moderate to high temps; dehydration, alters protein structure; incineration

thermal death time (TDT)

shortest length of time required to kill all test microbes at a specified temp

thermal death point (TDP)

lowest temp required to kill all microbes in a sample in 10 min

decimal reduction time (DRT)

minutes to kill 90% of population at a given temp

methods of moist heat control

1. sterilization with steam under pressure

2. non pressurized steam

3. boiling water

4. pasteurization

autoclave

pressure increases steam temp > produces denaturation of proteins, destruction of membranes and DNA

tyndallization

intermittent sterilization for substances that would be destroyed by autoclaving

• exposed to steam 30-60min, incubated 23-24 hours, steam again

• repeat cycle for 3 days

what can tyndallization be used for?

some canned foods and lab media

pasteurization

heat applied to kill potential agents of infection and spoilage without destroying the food flavor or value

not sterilization pasteurization

kills non-spore-forming pathogens and lower overall microbe count; doesn’t kill endospores or many nonpathogenic microbes

incineration

ignites and reduces microbes and other substances to ashes and gas

examples of incineration

flame, electric heating coil, infrared incinerators

hot air (dry) ovens

heated, circulated air that coagulates proteins

cold

• microbistatic: slows growth of microbes

• used to preserve food, media, and cultures

desiccation

gradual removal of water from leads that leads to metabolic inhibition

why is desiccation not effective microbial control?

many cells retain ability to grow when water is reintroduced

lyophilization

freeze drying; preservation

radiation

energy emitted from atomic activities and dispersed at high velocity through matter or space

ionizing radiation

deep penetrating power sufficient energy to cause electrons to leave their orbit > breaks DNA

non ionizing radiation

little penetrating power

• UV light creates thymine dimers

• interferes with replication

• induces mutations

desirable qualities of germicides

• rapid action in low concentration

• solubility in water or alcohol, stable

• broad spectrum, low toxicity

• penetrating

• noncorrosive and nonstaining

• affordable and readily available

high-level germicides

kill endospores; may be sterilants. used for devices that are not heat-sterilizable and intended to be used in sterile environments

intermediate-level germicides

kill fungal spores, tubercle bacillus, and viruses. used to disinfect devices that will come in contact with mucous membranes but are not invasive

low-level germicides

eliminate only vegetative bacteria, vegetative fungal cells, and some viruses. cleans surfaces that touch skin but not mucous membranes

factors that affect germicidal activity of chemicals

• nature of material being treated

• degree of contamination

• time of exposure required

• concentration of chemical agent

• strength and chemical action of germicide expressed in various ways

Chlorine

• denature proteins by disrupting disulfide bonds

• intermediate level

• unstable in sunlight, inactivated by organic matter

examples of chlorine halogens

Cl2, hypochlorites, chloramines

examples of iodine halogens

I2, iodophors (betadine)

iodine

• interferes with disulfide bonds of proteins

• intermediate level

phenol

disrupts cell walls and membranes and denatures proteins

chlorhexidine

a surfactant and protein denaturant with broad microbicidal properties

what is chlorhexidine used for?

skin degerming agents for preoperative scrubs, skin cleaning, and burns (ex. hibiclens and habitane)

what are the only alcohols suitable for microbial control?

ethyl and isopropyl

hydrogen peroxide

produce highly reactive hydroxyl-free radicals that damage protein and DNA while also decomposing to O2 gas

aldehydes

kill by alkylating protein and DNA

glutaraldehyde

• in 2% solution

• high level

• used as sterilant for heat sensitive instruments

formaldehyde

• intermediate to high level

• disinfectant, preservative, toxicity limits use

strong alkylating agents

ethylene oxide (ETO), propylene oxide (PO), and chlorine dioxide

gases and aerosols

• high level

• sterilize and disinfect plastics and prepackaged devices and food

detergents

• polar molecules, surfactants

• very low level

soaps

• alkaline compounds

• mechanically remove soil and grease containing microbes

• weak microbicides, destroy only highly sensitive forms

heavy metals

• kill vegetative cells in low concentrations by inactivating proteins

• low level

• oligodynamic actions

oligodynamic actions

having antimicrobial effects in exceedingly small amounts

dyes as antimicrobial agents

• Aniline dyes are very active against gram-positive species of bacteria and various fungi

• Sometimes used for antisepsis and wound treatment

• Low level, narrow spectrum of activity

acids and alkalis

• Organic acids prevent spore germination and bacterial and fungal growth

• Acetic acid inhibits bacterial growth

• Propionic acid retards molds

• Lactic acid prevents anaerobic bacterial growth

• Benzoic and sorbic acid inhibit yeast

goal of antimicrobial chemotherapy

administer a drug to an infected person that destroys the infective agent without harming the host’s cells

antimicrobial drugs are produced _________ or ________

naturally or synthetically

ideal antimicrobial drug characteristics

• Selectively toxic to the microbe but nontoxic to host cells

• Microbicidal rather than microbistastic

• Remains potent long enough to act and is not broken down or excreted prematurely

• Is not subject to the development of antimicrobial resistance

• Complements or assists the activities of the host's defenses

• Remains active even when diluted in body fluids and tissues

• Readily delivered to the site of infection

• Reasonably priced

• Doesn't disrupt the host's health by causing allergies or predisposing the host to other infections

antibiotics

are common metabolic products of aerobic bacteria and fungi

antimicrobial drugs should be ________

selectively toxic

5 major components that are useful drug targets in an actively dividing cell

• Inhibition of cell wall synthesis

• Breakdown of the cell membrane structure or function

• Interference with functions of DNA and RNA

• Inhibition of protein synthesis

• Blockage of key metabolic pathways

spectrum

range of activity of a drug

narrow-spectrum drugs

effective on small range of microbes. target a specific cell component found only in certain microbes

medium or broad-spectrum drugs

greatest range of activity. target cell components common to most pathogens

antimicrobial drugs that affect the bacterial cell walls

• Most bacterial cell walls contain peptidoglycan

• Penicillins and cephalosporins block synthesis of peptidogylcan, causing the cell wall to lyse

• Active on young, growing cells

• Penicillins that do not penetrate the outer membrane and are less effective against gram-negative bacteria

• Broad spectrum penicillins and cephalosporins can cross the cell walls of gram-negative bacteria

antimicrobial drugs that disrupt cell membrane function

• A cell with a damaged membrane dies from disruption in metabolism or lysis

• These drugs have specificity for a particular microbial group, based on differences in types of lipids in their cell membranes

• Polymyxins interact with phospholipids and cause leakage, particularly in gram-negative bacteria

• Amphotericin B and nystatin form complexes with sterols on fungal membranes which causes leakage

drugs that affect nucleic acid synthesis

• May block synthesis of nucleotides, inhibit replication, or stop transcription

• Chloroquine binds and cross-links the double helix; quinolones inhibit DNA helicase

• Antiviral drugs are analogs of purines and pyrimidines insert in viral nucleic acid, preventing replication

drugs that block protein synthesis

• Ribosomes of eukaryotes differ in size and structure from prokaryotes; antimicrobics usually have a selective action against prokaryotes; can also damage the eukaryotic mitochondria

• Aminoglycosides (streptomycin, gentamycin) insert on sites on the 30S subunit and cause misreading of mRNA

• Tetracyclines block attachment of tRNA on the A acceptor site and stop further synthesis

drugs that affect metabolic pathways

• Competitive inhibition

  • Metabolic analog drugs are "dead-end" and cannot function as required

  • As the enzyme is no longer able to produce a needed product, cellular metabolism slows or stops

  • Ex. Sulfonamides, trimethoprim, retrovir

• Synergistic effect

competitive inhibition

drug competes with normal substrate for enzyme’s active site

synergistic effect

the effects of a combination of antibiotics are greater than the sum of the effects of the individual antibiotics

drug-resistance

is an adaptive response in which microorganisms begin to tolerate an amount of drug that would ordinarily be inhibitory

what is drug-resistance a result of?

genetic versatility and adaptability of microbial populations

what is the main problem for microbial chemotherapy?

acquisition of drug resistance

newly acquired drug resistance

• Spontaneous mutations in critical chromosomal genes

• Acquisition of new genes or sets of genes via transfer from another species

drug resistance through antimicrobial transfer

• Transfer of resistance (R) factors (plasmids) encoded with drug resistance

• Transposons duplicated and inserted from one plasmid to another or from a plasmid to the chromosome

mechanisms of acquired drug resistance

1. drug inactivation

2. decreased permeability

3. activation of drug pumps

4. change in drug binding site

5. use of alternate metabolic pathway

drug inactivation

an enzyme that cleaves a portion of the molecule and renders it inactive

decreased permeability

the receptor that transports the drug is altered so that the drug cannot enter the cell

activation of drug pumps

specialized membrane proteins are activated and continually pump the drug out of the cell

change in drug binding site

binding on target (ribosome) is altered so drug has no effect

use of alternate metabolic pathway

the drug has blocked the usual metabolic pathway so the microbe circumvents it by using an alternate, unblocked pathway that achieves the required outcome