University of Babylon
College of medicine
Dep. Of microbiology
Asst. Proph Dr. Ilham alsaedi
Control of bacterial growth
Terminology:
1- Sterility: total absence of viable microorganisms as assessed by no growth on any medium.
2- Sterilization: refers to those processes by which all forms of microbial life, vegetative and spores, are destroyed or killed.
3- Disinfection: refers to those processes that result in the destruction of only the vegetative forms of microbial life, and not spores.
4- Disinfectant: a chemical agent that kills pathogenic and nonpathogenic microorganisms but not spores; generally refers to agents applied to inanimate objects.
5- Bacteriostatic: agent inhibiting bacterial multiplication.
6- Bactericidal: a chemical agent that kill bacteria, pathogenic and nonpathogenic, but not necessarily spores, when applied to either living tissues or inanimate objects.
7- Septic: presence of pathogenic microbes in living tissues.
8- A septic: absence of pathogenic microbes.
9- Antisepsis: any procedure that inhibits the growth and multiplication
Methods used for sterilization or disinfection:-
They are either physical or chemical agents:
Physical agent:
A- application of heat
*Moist heat under pressure:
For bacteriological and surgical work boiling is not sufficient because spores survive after boiling. Hence high pressure sterilizer or ((autoclave)) is used.
In this apparatus material for sterilization is exposed to steam pressure increased to 1.1 Kg./cm² 15 Ib/in².Under these conditions the temperature, rises to 121°C.At this temperature, moisture, cell structure is completely disrupted, protein and nucleic acids are denatured and cell membranes are broken. Articles that can be sterilized in the autoclave include most culture media, saline and other solutions not broken down by the high temperature. For culture media and solutions the release of pressure from the chamber must be slow to avoid blowing out the stoppers and causing bubbling and overflow of these liquids from the flasks and tubes in which they were sterilize.
*Moist heat not under pressure:
- Boiling:
Most of vegetative form of bacteria, fungi, and viruses are killed at 50 to 70°C in short time. For needles and instruments boiling in water for 10 to 30 minutes is sufficient to sterilize them.
Addition of little acid, alkali, or washing soda, markedly increases the sterilizing power of boiling water. Spores and hepatitis viruses are not destroyed by such procedure.
Pasteurization:
Is useful in treating some liquids, such as milk. Pasteurization occurs via either of two procedures; the:
a- Holding method: the liquid is heated to 63°C for 30 minutes.
b- Flash method: the liquid is heated to 72°C for 15 to 20 seconds.
Pasteurization dose not sterilize but kill disease-producing bacteria. It also reduces the number of other bacteria in liquid like milk.
*Dry heat
-Hot air oven
Sterilization by hot air oven requires temperature of 180°C for 2 hours to kill bacterial spores. Dry heat is used for sterilizing such items as glass ware, powders and oils.
-Flaming
Direct exposure of instrument or inculcating loops to open flames for brief time.
-Incineration
This is excellent method for rapidly destroying materials and any contaminating microorganisms that might be present.
B-Radiation
Ultraviolet, X-rays and gamma rays are example of different types of radiation.
The ultraviolet rays:
Possess bactericidal or bacteriostatic properties. The maximum killing effect is at 260 nm units of the spectrum (2600 A°).Isolation room or laboratories can be exposed to UV light to prevent the spread of disease-producing microorganisms from such sources. UV light is damaging to human tissues and direct exposure must be avoided UV light causes following changes in cell:
1- denaturation of protein.
2- damage to DNA.
3- Inhibition of DNA replication.
4- Formation of H2O2 and organic peroxide in culture media.
X-rays and other ionizing radiation:
Ionizing radiation have greater capacity to induce lethal changes in DNA of cell by breaks one or both DNA strands and interfere with DNA replication. Ionizing radiation is used to sterilize products like surgical instruments and disposable plastic items as well as heat-processed canned meat.
Chemical agents:
The chemical substances act as bactericidal agent as under:
a- Coagulating of bacterial protoplasm e.g. heavy metals.
b- Disruption of cell membrane by chemical substances. They may alter physical and chemical property of cell membrane thus results in killing or inhibiting the bacterial cell.
c- Oxidation or burning out the bacterial protoplasm e.g. Halogens.
d- By affecting bacterial enzymes or coenzymes system thus causing interference of bacterial metabolism.
Following chemicals are of common use:
1- Alcohol: Ethyl alcohol is most effective in 70%-95% to kill bacteria, but not spores isopropyl alcohol (90%-95%) is the major form in use in hospitals.
2- Phenol: Strong antibacterial agent, at the higher concentration generally employed (1-2% aqueous solution) e.g. chlorhexidine.
3- Halogens: Iodine is used chiefly for skin. Chlorine combine with water to form hypochloric acid which in bactericidal.
4- Heavy metal ions: Like copper and silver salts that use in very low concentration(because it is dangerous to the tissues.
5- Alkylating agents: Like
a- Formaldehyde: is used as disinfectant in aqueous solution(37%).
b- Ethylenoxide: useful for disinfection of surgical instruments and materials, which must be placed in special vacuum chambers for this purpose.
6- Detergents: They are bactericidal and bacteriostatic detergents act by concentrating at cell membrane and thus disrupting its normal function or denatured protein and enzyme.
Antibiotics:
Are natural substances of low molecular weight produced as secondary metabolites by certain groups of microorganisms (especially; Streptomyces, Bacillus, a few molds Pencillium and Cephalosporium that are inhabitants of soil) that react with and inhibit the growth of other microorganisms.
They are not laboratory chemical as are the antiseptics and disinfectants. In some cases antibiotics have been synthesized by chemists, in which case they are known as chemotherapeutic agents.
*Modes of action of antibiotics
1- Are antimetabolites.
2- Inhibit cell wall biosynthesis.
3- Inhibit protein synthesis.
4- Inhibit nucleic acid synthesis.
5- Alter or inhibit cell membrane permeability or transport.
*Antibiotic resistance:
The basis of microbial resistance to antibiotics are two;
1- Nongenetic mechanism;
- may involve loss of specific target structures, such as cell wall by L-form bacteria.
- may result from metabolic inactivity of microorganisms.
2- Genetic mechanism;
- may result from either chromosomal or extra chromosomal resistance.
- may involve a chromosomal mutation that alters the structure of the receptor of the drug or the permeability of the drug.
- may result from the R plasmid that codes for enzymes(beta-lactamase) that degrade the drug.
The medical problems of bacteria drug resistance:
When the pathogenic bacteria developed resistance to an antibiotics that will be useless in the treatment of infectious disease caused by that pathogen, so we must find new antibiotic to fill the place of the old ones e.g. pathogenic Staphylococcus aureus become resistant to penicillin,(because their ability of production of B-lactamase enzymes, so we must replace it by tetracycline.