Chemical methods of controlling Microbial growth, Biology tutorial


A large number of chemical compounds encompass the capability to inhibit the metabolism and growth of microorganisms or to kill them. They are termed as antimicrobial agents. These diverse chemicals are available for use as the disinfectants and each consists of its own features and mode of action, merits and demerits.

Definition of Disinfection:

a) Disinfection is the killing, inhibition or elimination of organisms that might be able of causing diseases.

b) This is the procedure of destroying the infectious agents.

c) Disinfectants are antimicrobial agents, generally chemicals, employed to carry out disinfection; they are generally employed on an inanimate object, example: disinfection might not lead to the total elimination of micro-organisms as viable spores and a few microorganisms might remain.

d) Antimicrobial bleach (that is, sodium hypochlorite) solution, for illustration, is a disinfectant employed to clean and disinfects food preparation regions.

Antimicrobial Agents:

An antimicrobial agent is a synthetic or natural chemical which kills or controls the growth of microorganisms. Agents which kill organisms are termed as cidal agents, having a prefix indicating the kind of microorganisms killed. Therefore, they are termed as bactericidal, fungicidal and viricidal agents as they kill bacteria, fungi and viruses, correspondingly. Agents which don't kill however only inhibit growth are termed as static agents. These comprise: bacteriostatic, fungistatic and viristatic compounds.

Features of an Ideal Antimicrobial Agent or Disinfectant:

1) It must contain a broad spectrum of antimicrobial activity that is; it should be efficient against a broad range of infectious agents like gram positive and Gram negative bacteria.

2) It should be active even at small concentration.

3) It should be active in the presence of organic matter.

4) Non-toxicity to human and other animals. It must be toxic to the infectious agent.

5) It should be non-staining and non-corroding.

6) It should be stable on storage.

7) Odorless or having pleasant smell.

8) It should be soluble in lipids and water for proper penetration of micro-organisms.

9) It should be uniform in composition in such a way that active ingredients are present in each and every application.

10) It should encompass a low surface tension in such a way to penetrate the cracks in surfaces.

Factors for the choice of a Chemical Agent:

The main factors which require to be considered in the procedure of choosing the most suitable chemical agent for a particular practical application are as follows:

1) The nature of material to be treated, example: chemical agent employed to disinfect contaminated appliances may be quite unsatisfactory for application to skin.

2) Kinds of microorganisms: Chemical agents are not all uniformly efficient against bacteria, viruses, fungi and other microorganisms. Spores are much resistant than the vegetative cells. Dissimilarity exists between Gram-positive and Gram-negative bacteria.

3) Environmental condition: Like pH, temperature, time, concentration and presence of extraneous organic materials, might all encompass a bearing on the rate and effectiveness of the antimicrobial action.

Main groups of Chemical Antimicrobial Agents:

The main groups of Chemical Antimicrobial agents comprise: Phenol and phenolic compounds, Alcohols, Halogens, Heavy metals and their compounds, Detergents, Dyes, Quaternary ammonium compounds, Gaseous agents and Aldehydes


a) Phenol is as well termed as carbolic acid and is the oldest acknowledged disinfectant.

b) Phenol (that is, Carbolic acid) was the first broadly employed antiseptic and disinfectant. In the year 1867, Joseph Lister employed it to decrease risk of infection throughout surgery.

c) The manner of the Action of phenol is through disrupting plasma membranes; inactivate enzymes and denaturing the proteins of microorganism.

d) They are stable if heated or dried and retain their activity in the presence of organic material. It is employed for disinfection of hospital walls and floors.

e) Illustrations of phenol and Phenolics (that is, phenol derivatives) are cresols, xylenols and orthophel/phenols are employed as disinfectants in hospitals and laboratories.

Merits of Phenol:

They are efficient against microbial agents of the tuberculosis, efficient in the presence of organic material and remain on the surface long after application.

Demerits of Phenol:

They encompass a disagreeable odor and can be responsible for skin irritation.


Alcohols are broadly employed as antiseptic and disinfectants. They are bactericidal and fungicidal however not sporicidal. A few viruses are as well destroyed through alcohols.

Two most popular alcohols germicides are isopropanol and ethanol and generally employed at 70 to 80% concentration. Isopropanol consists of the highest bactericidal action and is the most broadly employed. They act by denaturing the proteins and by dissolving membrane lipids and acting as a dehydrating agent. 10 to 15 minutes soaking a thermometer in alcohol is adequate to disinfect the thermometer


The halogen is any of the 5 elements in group VIIA of the periodic table. They are fluorine, chlorine, bromine, iodine and astatine and are efficient microbial elements broadly employed as disinfectants. The halogens iodine and chlorine are vital antimicrobial agents.


a) Chlorine is the common disinfectant for swimming pools and municipal water supplies. It is as well employed in diary and food industries. Different forms of chlorine are employed for disinfection. It might be exerted as chlorine gas, sodium hypochlorite (bleach) or calcium hypochlorite. All give in hypochlorous acid (HCLO) followed through atomic oxygen. It causes oxidation of the cellular materials, destruction of vegetative bacteria and fungi through disrupting membranes and inactivating enzymes however doesn't demolish spores.

b) Chlorine is as well a superb disinfectant for individual use. Small quantities of drinking water can be disinfected by halozone tablets. It is often employed by campers lacking access to sterilized drinking water.

c) Chlorine is an efficient disinfectant as it is cheap, efficient and simple to utilize.

d) It is generally employed in disinfecting and deodorizing most of the houses.

e) Chlorine is condensed into liquids and is broadly employed for disinfection and is the standard treatment for disinfecting drinking water in numerous communities.

Heavy Metals and their Compounds:

Heavy metals like silver, zinc, mercury, copper and arsenic are employed as germicides. For illustration, copper sulphate is an efficient algicide in swimming pools and lakes.

They act by joining with proteins and inactivating them. They might as well precipitate cell proteins.

1 percent of Silver nitrate is a solution added to the eyes of infants to prevent the ophthalmic gonorrhea.

Quaternary Ammonium Compounds (Detergents):

a) These are detergents that encompass antimicrobial activity.

b) Detergents are organic cleaning agents which are amphipathic; contain both polar hydrophilic and non-polar hydrophobic components.

c) They act by disturbing microbial membrane and by denaturing proteins.

d) When the detergents are electrically charged, they are known ionic.

e) Anionic (that is, negatively charged) detergents are simply mildly bactericidal and are employed as laundry detergents to get rid of soil and debris. They as well decrease number of microorganisms related with the item being washed.

f) Cationic (that is, positively charged) detergents are highly bactericidal, that is, they kill bacteria. They are efficient against Staphylococcus and different viruses.


a) Formaldehyde and glutaraldehyde are helpful for disinfection. They are extremely reactive molecules which join by nucleic acid and proteins and inactive them. They are sporicidal and can be employed as chemical sterilants.

b) Formaldehyde is generally dissolved in alcohol or water before use. A 2 percent buffered solution of glutaraldehyde is an efficient disinfectant. It is less irritating than formaldehyde and is employed to disinfect hospitals and laboratory equipment. Glutaraldehyde generally disinfects objects in minutes however might need as long as 12 hours to demolish all spores.

Sterilizing Gases:

Gases like ethylene oxide gas are employed to sterilize heat sensitive items like disposable plastic Petri dishes, syringes, heart lung machines components, sutures and catheters.

Ethylene Oxide (EtO):

This is both sporicidal and microbicidal and kills by joining with cell proteins. Sterilization is taken out in a special ethylene oxide sterilizer, very much resembling an autoclave in appearance, which control the EtO concentration, temperature and humidity. Since pure EtO is explosive, it is generally mixed by either CO2 or dichlorodifluoromethane. The ethylene oxide concentration, temperature, humidity and influence the rate of sterilization. A clean object can be sterilized when treated for 5 to 8 hours at 40 to 50 percent and the EtO concentration at 700mg/litre.

Betapropiolactone (BPL):

This is occasionally used as a sterilizing gas. In the liquid form it has been employed to sterilize vaccines and sera. BPL decomposes to an inactive form after some hours and is thus not as hard to remove as EtO. It as well destroys microorganisms more readily than an ethylene oxide however doesn't penetrate materials well. It might be carcinogenic. For such reasons, BPL has not been employed as extensively as EtO.

Vaporized Hydrogen Peroxide:

This can be employed to decontaminate biological safety cabinets, operating rooms and other big facilities. Such systems introduce vaporized hydrogen peroxide (H2O2) into the enclosure for some time, based on the size of the enclosure and material within. Hydrogen peroxide is toxic and kills a broad diversity of microorganisms. Though, during the course of the sanitization process, it breaks down to H2O and oxygen, both of which are harmless. Other merits of these systems are that they can be employed at a broad range of temperatures (4 to 80oC) and they don't spoil most materials.

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