The Effect of Different Antiseptics on the Growth of Bacteria

Author(s): Si Eun Lee

This investigation aims to determine the effectiveness of different antiseptics including hand soap (Dettol™), hand sanitizer and dishwashing detergent on bacterial growth from the human hand. Bacterial samples were collected using sterile swabs and cultured on agar plates following treatment with each antiseptic. A control sample (hands washed with water only) was also included. 

Results were assessed qualitatively by comparing the density and distribution of bacterial colonies on agar plates. 

This type of investigation is commonly used in microbiology to assess antimicrobial effectiveness, where bacterial growth on nutrient agar serves as an indicator of how well a substance inhibits microbial survival and reproduction. Agar provides a nutrient rich medium that supports bacterial growth, allowing visible colonies to form from individual colony forming units (CFUs). 

Introduction:

Bacteria are microscopic organisms found on many different surfaces, including objects we touch everyday and human skin. While many are harmless, some are pathogenic and can cause diseases. Regular hand washing techniques remove or kill bacteria, which reduces the risk of disease and infections. Antiseptics such as soaps and alcohol based sanitizers reduce microbial load through different mechanisms. 

The skin microbiome consists of both resident flora (permanent microorganisms) and transient flora (temporary microbes acquired through contact). Transient microorganisms are more likely to include pathogens and are therefore the primary target of practices in hygiene and reducing this transient microbial load is essential in preventing the transmission of infectious diseases. 

• Soap contains surfactants and antimicrobial agents.  Surfactants disrupt lipid membranes and emulsify oils, physically removing bacteria. Some formulations also include compounds such as chloroxylenol, which is known to disrupt microbial cell membranes and enzyme activity. Surfactants reduce surface tension, allowing water to interact more effectively with oils and debris. This enables the detachment of microorganisms adhered to the skin. Chloroxylenol, a phenolic compound, interferes with microbial enzyme systems and compromises membrane integrity (which refers to the structural stability and impermeability of a biological or synthetic membrane), leading to leakage of cellular contents. 

• Alcohol based sanitizers (60-80% ethanol or isopropanol) act by denaturing proteins and disrupting phospholipid membranes, leading to rapid cell lysis. However, they are less effective in the presence of organic matter and do not remove microbes physically. Protein denaturation alters the tertiary and quaternary structures of enzymes, making them non-functional. This disrupts the essential metabolic pathways within microbial cells. However, alcohol does not effectively penetrate dirt or organic material, which can shield microorganisms from exposure.

• Dishwashing detergents primarily act as surfactants for grease removal and generally lack strong antimicrobial agents, making them less effective against bacteria on skin. Unlike antiseptics, detergents are not specifically formulated to target microbial structures, which means their effect is largely limited to physical removal rather than biochemical inhibition or destruction. 

Studies by the World Health Organization show that proper handwashing with soap significantly reduces microbial transmission, while alcohol based sanitizers are effective but dependent on correct usage and concentration. Public health guidelines emphasise that effective handwashing should last at least 20 seconds to maximize removal of microorganisms and ensure sufficient contact time for antimicrobial action. 

This experiment aims to compare the effectiveness of different washing liquids (Dettol™, hand sanitizer and washing detergent) on the growth of bacteria. 

It was hypothesised that Dettol™ hand soap would be the most effective at inhibiting bacterial growth due to its antimicrobial ingredients. 

The Independent Variable was the different soaps, namely Hand Soap (Dettol™), a hand sanitiser, and washing detergent (in addition to a control group - that is, bacterial cultures that we allowed to grow uninhibited). The latter (detergent) was an interesting option, considering in principle, one doesn't wash hands with detergent. However, it was decided that it would’ve been curious to see the effect of a soap with a different purpose.

The Dependent Variable was Bacterial growth, measured qualitatively by the number of visible colonies and the density/distribution of these bacterial colonies. 

The Control Variables were limited. Evidently, the bacterial environment had to be maintained (at around 30°C in an incubator), as did the apparatus used. And while it proved difficult, we attempted to maintain the amount of soap applied and control the amount of hand microbes (same amount of time swabbed). Additionally, said hand microbes were collected from the same areas (hand rails, tables), and washed in the same way (water alone).

Equipment: 

• 6 sterile swabs 

• 6 agar plates

  • Petri dish

  • Agar jelly

• Different washing liquids 

  • Dettol™

  • hand sanitiser

  • detergent

• Incubator (Around 30ºC) 

• Disposable gloves 

Method: 

1. 3 students touched multiple surfaces commonly used or in frequent contact by other people (e.g. tables, door knobs and stair rails), collecting bacteria. 

2. Each student applied one type of washing treatment: Hand sanitizer, Hand soap (followed by rinsing with water) and Dishwashing detergent (followed by rinsing with water). 

3. Hands were air dried.

4. Sterile swabs were used to collect bacteria from each student’s hands, using a consistent swabbing technique. 

5. Swabs were streaked onto agar plates using a zig zag pattern in two perpendicular directions.

6. Plates were sealed with tape and labeled.

7. Plates were incubated in a dark environment at approximately 30°C for several days.

8. Bacterial growth was observed and recorded qualitatively.

The zig zag streaking technique used increases the distribution of bacteria across the agar surface, promoting isolated colony formation. Incubation in a dark environment reduces the growth of photosynthetic microorganisms and limits external environmental interference. 

Results and analysis

Type of Washing Liquid and Qualitative analysis	Photographic evidence
Hand Sanitizer  The agar plate appears to be partially clear, but some patches of fuzziness and bacterial growth is visible. The growth is less than dish soap and water but the distribution of bacteria is denser and wider than hand soap.  It is usually thought that hand sanitizer containing alcohol kills all bacteria by denaturing their proteins and dissolving their membranes. However, from this experiment we can see that there are bacteria that are possibly alcohol-resistant, leading to moderate growth of bacteria.  This may also be due to incomplete coverage during application or insufficient contact time. Additionally, some microorganisms are inherently resistant to alcohol based disinfectants.
Hand soap (Dettol™) The agar plate shows a few, circular colonies that are mostly evenly distributed across the agar. There is the least amount of bacterial colonies from all of the tested liquids. This result suggests limited bacterial growth, but shows that it does not sterilize completely.  This may be because soaps remove bacteria to kill them, but may not stop the regrowth of bacteria after some time has passed.  Soap primarily removes rather than kills bacteria, meaning that some microorganisms may be reintroduced after washing. Complete sterilization is very hard to achieve if not controlled under laboratory conditions.
Dish soap (Detergent) The agar plate shows a dense network of bacterial colonies that cover the whole agar plate. The agar plate is largely opaque, showing that the bacterial colonies were distributed throughout the agar plate.  This indicates heavy microbial and even fungal growth, suggesting that dish soap is least effective among the liquids killing bacteria.  This result is expected because dish detergent is mainly used to remove grease, food particles and oily residues through emulsification, not removal of bacteria as disinfectants would do. The presence of fungal growth suggests environmental contamination, reinforcing the idea that the detergent did not significantly inhibit microbial survival.
Water (control) The agar plate clearly shows large bacterial colonies completely covering the surface. Maggots were seen growing inside them, possibly because the surface that the students touched contained fly eggs as well.  The state of the agar plate shows the highest microbial growth, clearly indicating the consequences of not washing your hands after touching surfaces.  The presence of maggots indicates macroscopic contamination, highlighting a breakdown in aseptic technique and suggesting that external organisms were introduced during handling or incubation.

Evaluation (Limitations and Errors) 

The lack of replication significantly reduces reliability, as single trials cannot account for randomness. Furthermore, the absence of statistical analysis prevents objective comparison between treatments. There were additional significant limitations: 

1. Poor control of variables 

Bacterial samples were inconsistent. For example, one sample involved soil contamination from grass. Also, different individuals may have had different skin microbiomes. 

2. Lack of Quantitative Data

Results were solely based on subjective visual estimation. No colony counting or measurement of zones of inhibition was performed. 

3. Methodological errors 

One plate was invalid due to antiseptic being poured directly onto the agar, preventing bacterial growth measurement. Also, opening agar plates frequently increased contamination risks. 

Conclusion 

Hand soap (Dettol™) appeared to be the most effective antiseptic, while dishwashing detergent was the least effective. 

However, due to methodological weaknesses, these findings are not conclusive.

Overall, the results align with established microbiological principles regarding the mechanisms of antimicrobial agents, particularly the combined mechanical and chemical action of soap. 

Real World Application:

The findings of this investigation have direct relevance to the transmission of infectious diseases and the functioning of the immune system. Human skin acts as a physical barrier, forming part of the innate immune system, but it is constantly exposed to microorganisms. While pathogens are transferred from the hands to entry points such as the mouth, nose or eyes, they can bypass this barrier and initiate infection. Effective hand hygiene therefore reduces the pathogen load, lowering the probability of infection before the immune system is required to respond. 

The innate immune system includes physical barriers, phagocytes cells and antimicrobial proteins. If pathogens surpass this first line of defense, the adaptive immune system is activated, involving specific recognition by lymphocytes and the production of antibodies.

For example, diseases such as COVID-19, Influenza and Salmonella infection are commonly transmitted via contaminated surfaces and hand to face contact. 

Soap is particularly effective as supported by the experiment because it disrupts the lipid envelopes of viruses (such as SARS-CoV-2), leading to viral inactivation, while also physically removing other microbes from the skin. 

Alcohol based hand sanitizers are widely used in both healthcare and community settings due to their rapid antimicrobial action and convenience. They contain ethanol or isopropanol which denature microbial proteins and disrupt phospholipid membranes, leading to cell lysis. They are effective against enveloped viruses but are less effective against non enveloped viruses such as Norovirus infection, which lack a lipid membrane. These viruses rely on a protein cap side that is more resistant to alcohol denaturation. As a result, outbreaks of norovirus are often better controlled through handwashing with soap and water rather than sanitizer alone. Moreover, it is important to note that alcohol based hand sanitizers do not remove organic matter like grease which can shield pathogens and frequent use can disrupt the normal skin microbiota, which reduces beneficial microbial competitions. 

From an immunological standpoint, the use of hand sanitizers reduces the initial infectious dose of pathogens. This is important because probability often depends on whether the number of invading microorganisms exceed the threshold that the innate immune system can control. By lowering pathogen load, sanitizers reduce the likelihood that the adaptive immune response needs to be activated. This concept is known as the infectious dose, and reducing it significantly lowers the probability of successful pathogen colonization and disease progression.