Physical and Chemical Methods of Control

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Physical and Chemical Methods of Control



Millennia before “animalcules” were viewed by van Leuwenhoek (see Chapter 1, Scope of Microbiology), and before the germ theory of disease and Koch’s postulates had been accepted, references to germicidal agents had been made in the ancient writings of the Egyptians. Their embalmers applied spices, vegetable oils, and gums to their dead, using techniques that have kept them in a superior state of preservation even today. Persians, Greeks, and Romans had legal documents requesting the use of bright copper and silver vessels for the storage of public drinking water. The armies of Alexander the Great boiled their drinking water and buried their wastes. Very early on, salting, spicing, and smoking of foods were practiced as means of preservation. Moreover, the use of wine and vinegar in wound dressings dates back to the time of Hippocrates. Iodine was used to treat wounds before pus formation was understood because putrefaction and disease were believed to be associated.

Food preservation techniques have evolved along with culture and technology, with drying and freezing markedly influenced by the climates in which we live. Although humankind invented many methods of food preservation, fermentation was more or less discovered, not invented. Not to be upstaged by past authors who emphasized the impact of religion on science, Benjamin Franklin wrote that “…wine [is] constant proof that God loves us, and loves to see us happy.” Pickling, curing (actually dehydration), the use of honey and sugars to make jams and jellies, and canning have historically impacted food-processing technologies as well.


Ignaz Semmelweis (see Chapter 1), in 1847 at the maternity clinic of Vienna’s General Hospital, requested that interns who had performed autopsies wash their hands with chlorinated lime before examining expectant mothers. After hand-washing procedures had been instituted the incidence of death due to post delivery infection fell to about 1%.

Joseph Lister (see Chapter 1) instituted aseptic surgical techniques by the use of carbolic acid on surgeons’ hands, the operating field, the surgical instruments used, and the hospital environment in general. Protective surgical gloves were developed to protect both patients and surgeons, which greatly reduced infection during and after surgical procedures.

Food preservation by canning procedures was pioneered in the late 1700s by the French and later perfected by the British, who instituted the use of tin cans in 1810. Coupled with Pasteur’s work on the relationship between microbes and food spoilage/sickness, pressure canners led to canning at temperatures in excess of 100° C (212° F) and in the 1920s this canning procedure showed a significant reduction in the survival of the spores of Clostridium botulinum.


Improvements in disinfection techniques along with food preservation processes are essential to the future of physical and chemical methods of control of microorganisms in our environments. For example, better control of biofilms and of new and/or emerging resistant strains of microbes such as methicillin-resistant Staphylococcus aureus (MRSA) will all benefit from renewed attention.

The use of minimal food-processing techniques results in increased susceptibilities to resistant microbial strains. Therefore, in addition to developing better technologies for the improvement of disinfection and antisepsis, much greater shelf life of food is being emphasized because of the movement of population centers from the countryside to the city and the coming of the space age. Current conventional, thermal, and nonthermal methods are being investigated as potential preservation technologies for future space mission products, where minimization of mass and volume, water use, power requirements, crew size, time, and resistance to breakthrough contaminants are only some of the factors essential to the success of mission products and products having greater shelf lives.

Microbial Control

General Considerations in Microbial Control

The control of microorganisms in the environment is a never-ending concern in healthcare, in the laboratory environment (see Chapter 4, Microbiological Laboratory Techniques), as well as in various industries, especially the food industry. Microbial control can be achieved by physical methods, chemical agents, or a combination of both. Under ideal circumstances the methods used for microbial control should be inexpensive and fast-acting. Several factors need to be considered before deciding which method is most appropriate in a given circumstance. These factors include the following:

• Site to be treated: The nature of the site or item to be treated will determine the method of treatment. It needs to be determined whether or not the item is treatable with harsh chemicals, heat, or radiation, or a combination thereof, without undue damage to the item.

• Environmental conditions: Environmental conditions have an impact on the effectiveness (efficacy) of the antimicrobial method. For example, warm disinfectants generally work better than cold ones, because chemical reactions usually occur faster at warm temperatures, therefore reducing the time of exposure. Furthermore, certain chemical agents will perform better under acidic or alkaline conditions. Items contaminated with organic substances such as feces, vomit, blood, and so on need to be cleaned before disinfection or sterilization is performed for optimal results. Biofilms notoriously prevent the penetration of chemicals into all layers of the biofilms, and physical methods may have to be used before chemical methods can be effective.

• Susceptibility: Susceptibility of the microorganisms needs to be considered for any of the treatments (see later).

The methods used in the control of microbes belong to the general measures of decontamination. In the world of microbiology, contaminants are microorganisms present at a place and a time when they’re undesirable for human health. Decontamination is defined as the reduction or removal of chemical or biological agents so that they no longer present a hazard. The prerequisite for any decontamination procedure is adequate precleaning of the item to be decontaminated. Organic material such as feces, blood, and soil may inactivate disinfectants and protect microorganisms from the decontamination process; therefore, the actual physical removal of microorganisms by scrubbing is as important as the antimicrobial effect of disinfectants.

In addition to everyday management of microbial control, emergency departments and emergency medical services are responsible for the management of potential biological and chemical disasters caused by accident or by terrorist activities. Details on this topic are provided in the section First Responders to Natural Disasters and Bioterrorism in Chapter 24 (Microorganisms in the Environment and Environmental Safety).

Resistance of Microbes

Contamination with microbes is a concern in many environments; industrial, home, and healthcare, to name a few. The adequate control of these contaminants is necessary to avoid infections and disease. Although physical and chemical methods of control are available, the resistance of microbes to these methods of control varies greatly, depending on the type of microbe as well as the life stage the microorganism is in. Resistance ranges from the least resistant organisms to those with highest resistance (Box 19.1).


Bacterial endospores are dormant, highly resistant structures formed by vegetative bacterial cells when unfavorable environments are present. Endospore-forming bacteria have a two-phase life cycle—a vegetative state and an endospores state. In the vegetative state, the cell is metabolically active, replicates, and grows. As endospores, they are a dormant alternate life form that allows the organism to withstand extremely hostile conditions and increase the chances of survival. Endospores are rather resistant to high temperatures (including boiling), most disinfectants, antibiotics, and even some radiation. It is possible for endospores to survive thousands of years until environmental stimuli trigger germination. Bacillus, Clostridium, Desulfotomaculum, Sporosarcina, Sporolactobacillus, Oscillospira, and Thermoactinomyces are some of the genera that are capable of endospore formation.

Endospores can threaten human health by causing infections such as tetanus, anthrax, and botulism. Bacterial endospores can enter the body by a break in the skin, inhalation, and/or ingestion. Once the bacterial spores are in the human body and they find favorable conditions, the bacteria may germinate and cause infection and disease.

Terminology for Microbial Control

All healthcare professionals, scientists, and government workers need to be familiar with, and use the precise terminology pertaining to, the control of microbial growth (Table 19.1). Some of this terminology is also explained in Chapter 4 (Microbiological Laboratory Techniques).

TABLE 19.1

Terminology in the Control of Microbial Growth

Term Definition
Asepsis Technique to prevent the entry of microorganisms into sterile tissues
Antisepsis Destruction of pathogens on living tissue
Commercial sterilization Sufficient treatment with heat to kill Clostridium botulinum endospores; used in the food industry
Decontamination Destruction, removal, or reduction of the number of undesirable microbes
Degermination Removal of microbes from a limited area (i.e., area of skin being prepared for injection)
Disinfection Destruction of vegetative pathogens
Sanitization Treatment to reduce microbial counts on eating and drinking utensils to achieve safe public health levels
Sterilization The complete destruction of all forms of microbial life to include endospores and prions


Pasteurization was introduced by Louis Pasteur in 1857 (see Chapter 1, Scope of Microbiology), using heat to kill vegetative bacteria and therefore reducing the number of microorganisms that have the potential to spoil food. To this day, pasteurization is used in the food industry in the preparation of milk, fruit juices, wine, and beer. Pasteurization does not kill all the microorganisms but reduces their numbers, or eliminates dangerous pathogens enough to prevent foodborne diseases and food spoilage.

Microbial Death

Microbial death is the permanent loss of the reproductive ability of microbes, as well as the permanent loss of all other vital activities. Lethal agents do not always alter the appearance of a microbial cell, even if the motility of the organism is gone. Therefore, the term microbial death can be used only if reproduction cannot occur even if the organism is given the optimal growth conditions.

The destructive forces of chemical or physical agents act on the individual cells and if exposed intensively and long enough, the cell structures become dysfunctional and the cells show irreversible damage. In general, cells in a given culture/environment vary in their susceptibility to antimicrobial agents depending on their level of metabolic activity. Young, rapidly dividing cells have the tendency to die more rapidly than older, less active cells. When microorganisms are killed by any method of control, they have the tendency to display exponential death curves. In other words, they die at some fractional rate per unit time. If 50% of microorganisms in a population die every minute, after 2 minutes 25% will still be alive, after 3 minutes 12.5% are still alive, and so on (Figure 19.1). It is therefore safe to say that the total number of organisms present when the disinfection process began determines the time required to eliminate all microbes. The effectiveness of an agent is influenced by other factors besides time, such as:

• The number of microorganisms, which dictates the amount of time required for the destruction of all contaminants.

• The nature of the organism(s) to be destroyed. Biofilms, for example, include a number of different organisms and species. Other contaminants can include vegetative organisms as well as spores. Any target population that contains more than one organism can present a broad spectrum of resistance.

• The temperature and pH of the environment have an influence on the effectiveness of the antimicrobial agent.

• The overall concentration of the microbial control agent: Most are more active at higher concentrations.

• The presence of other materials, such as organic matter, solvents, and other substances, can inhibit or interfere with the actions of antimicrobial agents.

Microbial control requires many considerations to find the most appropriate method to achieve the desired control in a given environment. Several questions need to be answered before a given method and/or agent is selected for microbial control. Some of these questions are as follows:

Physical Control

The optimal growth of most microorganisms depends on the most favorable environmental conditions for each species; most grow best in a narrow range of temperature, pH, osmotic pressure, and atmospheric conditions (Table 19.2). For example, obligate anaerobic bacteria will be killed instantly by exposure to oxygen. If the nature of the contaminant is known, the best agent of control can be determined rather easily and selectively. However, under most circumstances the exact nature of the microbe is unknown and rather stringent methods of decontamination will have to be employed.

TABLE 19.2

Optimal pH for the Growth of Some Bacteria

Bacteria Minimal pH Optimal pH Maximal pH
Thiobacillus 1.0 2–2.8 4–6
Escherichia coli 4.4 6–7 9.0
Clostridium sporogenes 5.4 6–7.6 9.0
Pseudomonas aeruginosa 5.6 6.6–7 8.0
Nitrobacter 6.6 6.6–8.6 10.0


The nature of the contaminated item(s) or areas needs to be considered to efficiently and effectively remove the unwanted organisms without destroying the item. In the case of the food industry, nutrients might be inactivated by improper treatment (see Food Preservation, later in this chapter). A summary of physical methods for the control of microbial growth is provided in Table 19.3.

TABLE 19.3

Summary of Physical Methods Available for the Control of Microbial Growth

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Method Mechanism Comments Use
Dry Heat
Flaming Burning to ashes Sterilization Inoculating loops
Incineration Burning to ashes Sterilization Dressings, wipes, animal carcasses
Hot air sterilization Oxidation 170° C/2 h Glassware, needles, glass syringes
Moist Heat
Boiling Denaturation Kills vegetative cells but not spores and prions Equipment, dishes
Autoclaving Denaturation Sterilization Media, linens, equipment
Tyndallization Denaturation Intermittent sterilization; free-flowing steam (30–60 min) for 3 d in a row Substances that cannot withhold the high temperature of an autoclave
LTLT (low temperature, long time) Denaturation 63° C for 30 min Milk, milk products
UHT (ultrahigh temperature) Denaturation 138° C for a fraction of a second Milk, juices
Low Temperatures
Refrigeration Slows down microbial growth Bacteriostatic for most bacteria Foods, drugs
Deep freezing Stops most microbial growth Preservation Foods, drugs, cultures
Desiccation, Lyophilization, Osmotic Pressure
Desiccation Inhibits microbial growth Loss of water; preservation Foods
Lyophilization Inhibits growth Loss of water; preservation; combination of drying and freezing Storage of cultures, vaccines, cells
Osmotic pressure Plasmolysis Loss of water Food preservation