BIOL 2421 Microbiology Lecture Notes: Control of Microbial Growth Dr. Weis
Definitions:
a) Sterilization:
destroy all forms of microbial life
aimed at destroying the endospores of Clostridia botulinum
methods: heat and filtration
b) Disinfection
destroying all harmful microorganisms
if used on living tissue à antiseptic
methods: chemical, radiation, steam
c) Degerming
mechanical removal of microbes in a limited area
example: alcholol
d) Sanitation
minimize disease transfer [Public Health]
methods:
steam, chemicals
Word Parts
-cide = kill bacteriocidal
-stasis = stop, steady bacteriostatic
sepsis = decay, putrid asepsis
Rate of Microbial Death
Microbe treatments with antibacterial chemicals die at a constant rate
Effectiveness depends on
1) Number of microbes, increase number of microbes, increase time
2) Environment
organic material pH inhibits chemical
works better with heat
3) Time of exposure
more resistant bacterial require longer time
increase heat ~ decrease time [and vice versa]
4) Microbial character
capsule
endospore
Actions of Agents
I. Alter cell membrane permeability by damaging lipids or proteins causing cell to leak.
II. Damage to Proteins or NA
Protein enzyme denature
DNA and RNA breakdown
Affect Hydrogen bonds of proteins and NA
Affect Covalent bonds (S-S or S-H) of proteins
PHYSICAL METHODS of MICROBIAL CONTROL
A) Early Methods: Drying and Salting
B) Heat à denature enzymes
1) Thermal death point: lowest temperature where microbes are killed
2) Thermal death time: minimal time all microbes killed at a certain temperature
3) Decimal reduction time: time takes for 90% of bacteria to be killed at a given temperature
Types of Heat
* Moist Heat - coagulates / denatures proteins
^ Boiling
approximately 10 minutes minimum
kill vegetative forms of bacteria, most viruses, fungal spores
resistance with some viruses, endospores of bacteria
^ Sterilize with steam
use autoclave and verify with sterilization indicator
high pressure and temperature [if material can withstand]
~ 15 minutes minimum
all organisms killed
* Pasteurization
heat applied for a specific time
if increase temperature, can decrease time
phosphatase test à negative for enzyme if done correctly
* Dry Heat Sterilization – kills by oxidation effects
flame (loop)
incineration
hot air (oven)
* Filtration
screen like materials with pores {.22µm to .45 µm}
use on heat sensistive materials [vaccines]
HEPA
filters
C) Low Temperature
* Refrigeration
decrease metabolic rate, therefore bacteriostatic
except for physchrotrophs and
Listeria
* Freezing
slow freeze and thaw
ice crystals
disrupts structure
* Subfreezing
microbes dormant
D) High Pressure
Alter protein and carbohydrate structure of vegetative bacteria
Endospores resistant
to high pressure alone need heat
E) Dessication
Decreased water content à freeze drying à used to preserve
Resistance of microbes varies
[i.e. hours, months, years, centuries]
F) Osmotic Pressure
Use of high concentration of salts and sugars to create osmotic effects by creating hypertonic environment, therefore the water leaves the microbial cell
* salts à preserve meat
* sugar à preserve fruit
G) Radiation
Depends on wavelength, intensity, and duration
Two types used to kill microbes
1) Ionizing
alpha, X-rays, electrons
short wavelength, more Energy
ionize water à H+ + OH-
radicals react with DNA to create mutants
Use: surgical plastic materials, mail
2) Nonionizing
longer wavelengths [U.V. light]
only at surface, does not penetrate
damages DNA bonds à restructure and inhibit DNA replication
Can also damage human tissues
NOTE: microwaves à heat can kill some vegetative bacteria, but not
Good for killing microbes because of uneven heat if no
Moisture
CHEMICAL METHODS OF MICROBIAL CONTROL
Primarily to decrease numbers
Few chemicals achieve sterility
Evaluation of chemical as a disinfectant
a) Activity
concentration
prep area before use
presence of organic material pH will affect
b) Test using three Bacteria
Salmonella, Staphylococcus, Pseudomonas
Types of Chemicals
I. Phenols / Phenolics
Phenols are no longer used, very irritating
Phenolics: chemically altered phenols
Fxn: injure lipid containing plasma membranes
Remain active in the face of organic material
e.g. Cresols à Lysol
Use: Environmental surfaces,
Skin, MM
II. Bisphenols
2 phenols
e.g. Hexachlorophene [pHisoHex], used for G (+)
Triclosan [ingredient in soap], used for G(+), G(-)
Use: hand soap, skin lotions
III. Biguanides
e.g. Chlorhexidine [surgical scrub]
Use: skin disinfection
IV. Halogens
Iodine
Tincture
Iodophore à Betadine, Isodine
Use: antiseptic
Chlorine
Cl2 gas, HOCL [Cl added to water], NaOCL [Chlorox]
Used to treat water,
glassware, eating utensils
V. Alchols
Kill bacteria, fungi
Protein denatured, coagulated
Types: Ethanol, Isopropyl
If applied to skin, called degerming (wiping away dirt and some microbes)
Use: equipment [thermometers]
VI. Heavy Metals
A. Silver
1% silver nitrate
antiseptic
- eyes of newborn to prevent Gonorrheal infections
- catheters
- dressings
B. Mercury
Bacteriostatic
Toxic
Corrosive
C. Copper
Algicide
Used to treat water
D. Zinc
Mouthwash
Paint
VII. Surfactants
Soaps and Detergents
Emulsify oils [skin] à wash away (degerm)
VIII. Quaternary Ammonium Compounds [NH4+]
e.g. Cepacol
Used primarily for G(+)
Antiseptic for skin,
instruments, rubber gloves, utensils
IX. Chemical Food Perservatives
SO2 [sulfur dioxide]
Sodium nitrate, nitrite
Sorbic acid
Calcium propionate
X. Antibiotics
To control for microbial growth
Some limited to food preservation
Nisin à cheese, to decrease endospores
Natamycin à cheese, to decrease fungal spores
* refer to chemotherapeutic agent chapter
notes for other information
XI. Aldehydes
Inactivate protein crosslinks = antimicrobial
Formaldehyde [Formalin]
Glutaraldehyde [Cidex]
XII. Gaseous Chemosterilizers
Ethylene oxide
Denatures proteins
Penetrates, but requires increased time
Sterilize hospital equipment
? carcinogenic
XIII. Peroxygens
H2O2 [hydrogen peroxide]à disinfect inanimate objects
O3 [ozone] à used to supplement chlorination of water
Benzoyl peroxide à for anaerobic bacteria
Peracecitic acid à sporocide
Resistant Organisms
Most resistant à to least resistant organisms to chemical biocides
Prions à Bacterial endospores à Mycobacteria à Protozoan cysts à vegetative protozoaà
G(-) bacteria à Fungal / fungal spores à viruses without envelopes à G (+) bacteria à
Viruses with envelopes
NOTE:
See charts in book that summarize physical and chemical agents that control microbial growth.