PROCESS OF KILLING OR INHIBITING MICROBIAL GROWTH

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Several techniques abound for the killing and/or inhibition of the growth of microorganisms. These courses of action are usually undertaken in vitro or in vivo to impede the untoward activities of microbes when they are perceived to cause apparent damage to man, plants, animals and even the environment. Microorganisms are ubiquitous, and they can cause a variety of health problems as well as the spoilage of food, pharmaceuticals and other products if their growth is not critically controlled or inhibited. Microorganisms inclusive of bacteria, viruses, protozoa and fungi can cause colossal damage in the hospitals, industries, the laboratories, our homes and even in the environment if there population or proliferation is not brought under control using some specific antimicrobial agents. The control of microbes is critical because such measures will help to prevent infection outbreak and the spread of infectious diseases especially in the hospital environments where such disease outburst could occur.

Detrimental microbial growth can be controlled by a number of physical and chemical methods. The physical methods of controlling the growth of microorganisms usually involve the use of heat, radiation and filtration techniques while the chemical method employs a number of chemicals with potent antimicrobial action including phenols, alcohols, mercury and chlorine to restrain the proliferation of microorganisms capable of causing harm to man, animals and the environment. Any of these methods (whether physical or chemical) could be effectively used for the control of microbial proliferation in any environment depending on the situation. Disinfectants, antiseptics, germicides, preservatives and a number of inhibitory (i.e. static) or cidal (killing) antimicrobial agents are some of the chemical agents involved or used in the control of microbial growth. Preservation, sanitization, disinfection, antisepsis, filtration, decontamination and sterilization are some of the techniques employed for the physical and chemical control of microorganisms in any given environment.

In the hospital environment for example, when medical instrument (inclusive of surgery tools) and other invasive materials used for the treatment of patients are not properly decontaminated (i.e. cleaned up and made microbial-free) prior to their regular usage, they could serve as route via which infectious diseases and disease-causing agents spread to patients. Even in the industry, food stuffs and pharmaceutical products can easily become contaminated during storage and processing, and they too could serve as means of food spoilage and disease transmission when the processes that led to their production are devoid of basic microbial (aseptic) control procedures. Microbial contamination can also occur at home when we fail to observe some basic personal hygienic practices such as washing the hands after visiting the toilet or washing the hands before eating and especially after shaking the hands of friends, colleagues or visitors during the day.

Hand washing alone significantly reduces the number of microbial loads on our hands that is occasionally inundated with different microbial species. It is therefore vital for health scientists (in particular Microbiologists) to acquaint themselves with the requisite knowledge of some of the processes involved or used to kill and inhibit the growth of microorganisms. This shall enable microbiologists to effectively pin down the growth of microorganisms within their vicinity. The physical and chemical processes used for the control of microbial growth in any environment usually exhibit two modes of antimicrobial action which may be cidal (killing) or inhibitory (static). Cidal agents generally kill microbial cells while inhibitory agents hinder or stop the growth and proliferation of harmful microorganisms. Antimicrobial agents used for the control of microbial growth target some specific sites on the organisms. The inhibition of key metabolic processes in the organism (for example, inhibition of cell wall synthesis in bacteria) ultimately leads to their death or inhibition of growth (Figure 1).    

Figure 1. Site of antimicrobial action on microorganisms and possible modes of resistance. Photo courtesy: https://www.labome.com/method/Antibiotic-Discovery.html

Generally, antimicrobial agents work in three different ways as follows:

  1. They slow down the growth of microorganisms.
  2. They inhibit or stop the growth of microorganisms.
  3. They kill microorganisms.

“A microbial cell is considered to be dead when it fails to grow or give rise to a colony on a growth medium that normally supports its growth.” While some antimicrobial agents may only slow down the growth of microorganisms or inhibit (i.e. stop) their growth to levels that are safe for the populace, other agents out-rightly kill the microbial population so that a sterile condition can be obtained. Based on the target organisms and its mode of action, antimicrobial agents can be classified as:

  • Bacteriostatic agents: These are agents that inhibit the growth of bacteria. The multiplication and/or proliferation of bacterial cells resumes again upon the removal of the bacteriostatic agent; and this is also applicable to other inhibitory (static) antimicrobial agents used to control the growth of other forms of microorganisms. 
  • Bactericidal agents: Bactericidal agents kill bacterial cells. While the action of a bacteriostatic agent can be reversed, the activity of a bactericidal agent is irreversible because the killed bacteria cannot reproduce again (i.e. grow and multiply) even after the removal of the cidal agent. This also applies to other antimicrobial agents that control microbial growth by killing.
  • Fungistatic agents: Fungistatic agents inhibit or stop the growth of fungi.
  • Fungicidal agents: These are agents that kill fungi.
  • Sporostatic agents: Sporostatic agents inhibit spores or spore formation. 
  • Sporicidal agents: Sporicidal agents destroy the spores of microorganisms and kill spore forming microbes.  
  • Virucidal agents: Virucidal agents are used to kill viruses.   
  • Viristatic agents: Antimicrobial agents that inhibit the growth of viruses are known as viristatic agents.
  • Algicide: Agents that kill algae are called algicides.

FURTHER READING

Ashutosh Kar (2008). Pharmaceutical Microbiology, 1st edition. New Age International Publishers: New Delhi, India. 

Block S.S (2001). Disinfection, sterilization and preservation. 5th edition. Lippincott Williams & Wilkins, Philadelphia and London.

Courvalin P, Leclercq R and Rice L.B (2010). Antibiogram. ESKA Publishing, ASM Press, Canada.

Denyer S.P., Hodges N.A and Gorman S.P (2004). Hugo & Russell’s Pharmaceutical Microbiology. 7th ed. Blackwell Publishing Company, USA. Pp.152-172.

Finch R.G, Greenwood D, Norrby R and Whitley R (2002). Antibiotic and chemotherapy, 8th edition. Churchill Livingstone, London and Edinburg.

Russell A.D and Chopra I (1996). Understanding antibacterial action and resistance. 2nd edition. Ellis Horwood Publishers, New York, USA.

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