Antibacterial properties of Ethanol and Isopropanol [closed]

Question

Ethyl alcohol is widely used in beverages, while isopropyl alcohol is commonly sold as an antiseptic. However, some use both ethyl alcohol and isopropyl alcohol as antibacterial protection.

I am curious to know which of the two alcohols will provide better antibacterial protection? Does the chemical structure of the alcohol affect the antibacterial properties?

Answer 1

This question is really a broad one. For example, see Karl's comment elsewhere. Yet, according to a review on the subject (Ref.1):

Although several alcohols have been shown to be effective antimicrobials, ethyl alcohol (ethanol, alcohol), isopropyl alcohol (isopropanol, propan-2-ol) and n-propanol (in particular in Europe) are the most widely used (Ref.2). Alcohols exhibit rapid broad-spectrum antimicrobial activity against vegetative bacteria (including mycobacteria), viruses, and fungi but are not sporicidal. They are, however, known to inhibit sporulation and spore germination (Ref.3), but this effect is reversible (Ref.4). Because of the lack of sporicidal activity, alcohols are not recommended for sterilization but are widely used for both hard-surface disinfection and skin antisepsis.

Also, Ref.1 instates this interesting fact:

In general, isopropyl alcohol is considered slightly more efficacious against bacteria (Ref.5) and ethyl alcohol is more potent against viruses (Ref.6); however, this is dependent on the concentrations of both the active agent and the test microorganism. For example, isopropyl alcohol has greater lipophilic properties than ethyl alcohol and is less active against hydrophilic viruses (e.g., poliovirus) (Ref.6). Generally, the antimicrobial activity of alcohols is significantly lower at concentrations below 50% and is optimal in the 60 to 90% range.

The the optimal bactericidal nature of 70% ethanol is well documented in Ref. 7:

On the basis of these experimental findings, a routine method of preoperative preparation of the hands has been evolved which, while simple, is a great deal more effective than technics in use at present in many leading hospitals. This procedure has been employed with satisfaction in my operating room for the last four years and, having passed that clinical test, is now recommended to others.

Seventy per cent by weight alcohol is recommended also for use in preparing the field of operation and in disinfecting contaminated hands.

Why exactly 70 per cent should be more toxic for bacteria than any other concentration of alcohol remains an interesting biochemical problem.

Also, the Ref.7 would answer your question about whether the chemical structure of the alcohol affects the antibacterial properties or not, which states:

The activity of the alcohols in inhibiting spore germination increases as their molecular weights and chain lengths increase (Fig. 3). This same relationship is observed for the action of alcohols as bacterial disinfectants. It is of interest that primary alcohols are more effective bacterial disinfectants than are secondary alcohols, and in this study a primary alcohol, n-propanol, was more effective than a secondary alcohol, 2-propanol, in inhibiting the spore germination process.

References:

1. Gerald McDonnell, A. Denver Russell, “Antiseptics and Disinfectants: Activity, Action, and Resistance,” Clinical Microbiology Reviews 1999, 12(1), 147–179 (DOI: 10.1128/CMR.12.1.147).
2. S. E. Morton, "Alcohols" in Disinfection, sterilization and preservation; Seymour S Block, Ed.; 3rd ed.; Lea & Febiger: Philadelphia, PA, 1983, pp. 229–235.
3. Y. Yasuda-Yasuki, S. Namiki-Kanie, Y. Hachisaka, "Inhibition of germination of Bacillus subtilis spores by alcohols," In Spores VII; eds G. Chambliss, J. C. Vary, Eds.; American Society for Microbiology: Washington, DC, 1978, pp. 113–116.
4. Ralph Trujillo, Nancy Laible, "Reversible inhibition of spore germination by alcohols," Applied & Environmental Microbiology 1970, 20(4), 620–623.
5. C. E. Coulthard, G. Skyes, "Germicidal effect of alcohol," Pharm. J. 1936, 137, 79–81.
6. M. Klein, A. Deforest, "Principles of viral inactivation" in Disinfection, sterilization and preservation; Seymour S Block, Ed.; 3rd ed.; Lea & Febiger: Philadelphia, PA, 1983, pp. 422–434.
7. Philip B. Price, “Ethyl alcohol as a germicide,” Archives of Surgery 1939, 38(3), 528–542 (doi:10.1001/archsurg.1939.01200090135010).

Answer 2

Thanks for asking this question because after reading this source by the CDC (Center for Disease Control) on 'Guideline for Disinfection and Sterilization in Healthcare Facilities' (2008), to quote:

In the healthcare setting, “alcohol” refers to two water-soluble chemical compounds—ethyl alcohol and isopropyl alcohol—that have generally underrated germicidal characteristics 482. FDA has not cleared any liquid chemical sterilant or high-level disinfectant with alcohol as the main active ingredient. These alcohols are rapidly bactericidal rather than bacteriostatic against vegetative forms of bacteria; they also are tuberculocidal, fungicidal, and virucidal but do not destroy bacterial spores. Their cidal activity drops sharply when diluted below 50% concentration, and the optimum bactericidal concentration is 60%–90% solutions in water (volume/volume)..

An important point per above relative to employing alcohol as a disinfectant, in my opinion, is that they are 'generally underrated' with respect to their germicidal abilities. Also, interestingly, ethyl and isopropyl alcohol are each somewhat selective within the broad characterization given above. To quote further:

Ethyl alcohol, at concentrations of 60%–80%, is a potent virucidal agent inactivating all of the lipophilic viruses (e.g., herpes, vaccinia, and influenza virus) and many hydrophilic viruses (e.g., adenovirus, enterovirus, rhinovirus, and rotaviruses but not hepatitis A virus (HAV) 58 or poliovirus) 49. Isopropyl alcohol is not active against the nonlipid enteroviruses but is fully active against the lipid viruses 72. Studies also have demonstrated the ability of ethyl and isopropyl alcohol to inactivate the hepatitis B virus(HBV) 224, 225 and the herpes virus, 490 and ethyl alcohol to inactivate human immunodeficiency virus (HIV) 227, rotavirus, echovirus, and astrovirus 491.

In tests of the effect of ethyl alcohol against M. tuberculosis, 95% ethanol killed the tubercle bacilli in sputum or water suspension within 15 seconds 492. In 1964, Spaulding stated that alcohols were the germicide of choice for tuberculocidal activity,...

So, if one is confronted, for example, by a flu outbreak in your educational institute, perhaps one should lean in the direction of ethyl alcohol as it is effective against the influenza virus. However, always applying both separately remains an albeit inconvenient option, and in either choice, bacterial spores are still a potential issue.