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Disinfectants, Antimicrobial Agent Susceptibility Testing, and Resistance


  1. Bactericidal means with the ability to kill bacteria. Bactericidal substances have the ability to destroy important components of susceptible bacterial cells hence killing them. Bacteriostatic means with the ability to prevent further bacterial multiplication without necessarily showing the effect on the mature cells (Nemeth, 2015). Virucidal means with the ability to deactivate viruses. Fungistatic means with the ability to inhibit the growth of fungi. Fungistatic compounds will prevent the growth of fungi without necessarily destroying the mature fungi.
  2. The control culture includes the test sample without the test chemical. It is used as a marker of whether or not the test chemical achieves the required action. It can also be crucial in understanding the degree of growth inhibition and/or cell death.
  3. The activity of a disinfectant is affected by several factors including: the contact time with the target, the concentration and potency of the disinfectant being used, physical factors associated with the contact e.g the temperature during the disinfection process, the resistance nature of the target organisms, and the number and location of the targeted organisms. It is crucial to understand how a change in any one factor affects the action of the disinfectant used in order to maximize their effects on the target organism.
  4. Resistance to disinfectants is due to the interplay of physical factors and genetic makeup. In some organisms, the resistance is conferred by their being impermeable to the disinfectant. Other organisms are resistant due to the production of substances that neutralize the effect of the disinfectant being used. The ability to resist disinfectants can be conferred by intrinsic genetic makeup or plasmids (Russell, 2002).
  5. Gram-positive organisms are the most susceptible to disinfectants while mycobacteria and endospores are the most resistant forms.

Antimicrobial Agent Susceptibility Testing and Resistance

  1. Resistance is the ability of an organism to prevent an agent such as an antibiotic from harming it. Resistance leads to persistence of the infection. Susceptibility means that the agent meant to stop the infection is effective against the target organism (Richardson, 2017).
  2. Pure cultures involve the use of one strain or clone. Using a pure culture offers more accurate results since different strains show different resistance profiles. Some forms of resistance may be plasmid coded hence transmissible from one strain to the other. It is, therefore, crucial to use pure cultures in antimicrobial susceptibility testing to avoid interference from other strains.
  3. The use of mixed cultures is allowed in some cases. It is crucial to mimic the conditions associated with the organism being tested. If the organism’s pathogenicity is dependent on the existence of other organisms, it is crucial to provide a mixed culture in order to mimic the conditions that confer pathogenicity.
  4. The accuracy of the data obtained can be affected by the alteration in pH of the test medium, variation in the size of inoculum used in the different tests, and variations in the standards applied in the tests. It is crucial to use the same test standards throughout the experiment (Gould, 2000).
  5. A growth control tube helps identify the presence and rate of growth in uninhibited conditions and serves as a comparison to the levels of resistance of the tests. The sterile control is used to make sure that the broth used was not contaminated.


Gould, I. M. (2000). Towards a common susceptibility testing method? Journal of Antimicrobial Chemotherapy, 45(6), 757-762.

Nemeth, J. G. (2015). Bacteriostatic versus bactericidal antibiotics for patients with serious bacterial infections: systematic review and meta-analysis. Journal of Antimicrobial Chemotherapy, 70(2), 382-395.

Richardson, L. A. (2017). Understanding and overcoming antibiotic resistance. PLoS biology, 15(8).

Russell, A. D. (2002). Bacterial resistance to disinfectants. British Journal of Infection Control, 3(3), 22-24.



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