The current study was designed to investigate the antimicrobial, antifungal effects of laundry detergents, additives and conditions. Although many studies have investigated the antimicrobial and antifungal effects of laundry detergents [15], there is no available data focusing on the form of laundry detergent. Likewise, the powder-form detergents clear C. acnes during laundry procedures to prevent the acne from recurring and transmission has not been investigated before, although it is known that the dermatologists ask the patients to do so to increase the hygiene performance of laundering. Finally, the COVID-19 pandemic since 2019 has resulted in a rising demand for practical solutions to improve skin protection.
Generally, Gram-negative bacteria have an outer membrane containing lipopolysaccharides and lipids, a thin peptidoglycan layer, and an inner phospholipid membrane, while Gram-positive bacteria are distinguished by a thick peptidoglycan layer and an inner membrane [16]. The difference in structure could explain that C. acnes did not respond to lipase but trypsin, while K. aerogenes could be reduced by both enzymes. Lipase acted against the K. aerogenes better than trypsin supposedly because of the thickness of the cell wall, but the EDTA in Trypsin-EDTA could chelate the iron in K. aerogenes outer membrane.
BPO is mainly used to treat acne, topically, the effects of BPO include sebostatis, comedolytsis, and inhibition of the growth of C. acnes [17]. Okamoto et al. [18] showed that without the protection of the outer membrane, the cell wall of C. acnes was disrupted partly after exposure to BPO, and cytoplasmic materials were released to the extracellular space. In our study, BPO could only get a complete reduction in C. acnes at 60°C, and the C. acnes BPO group had significantly stronger effect than K. aerogenes and T. rubrum. In Gram-negative bacteria, the outer membrane could block the BPO unless EDTA damaged the membrane [18].
In addition to EDTA could chelate the outer membrane of gram-negative bacteria [19], surfactants with a good match alkyl chain length to cell lipid structure in efficacy. In 1997 [16], non-ionic surfactants were found to have an effect against gram-negative bacteria. Non-ionic surfactants with C10 (alkyl chain with 10 carbon atoms) could adsorb on the cell wall, while C12 (alkyl chain with 12 carbon atoms) material entered the membrane bilayer. The viability of gram-negative bacteria could be interfered with by C10 and C12, but not non-ionic surfactants with other alkyl chain length. In our test, Brij C10 as a non-ionic surfactant with C16, neither inhibited colony generation nor interfered with microbial viability.
Unlike non-ionic with C10 or C12 alkyl chain length, zwitterionic surfactant 12-16 alkyl chain length have been shown to have the highest antimicrobial activity [20]. CAPB is a medium-strength zwitterionic surfactant widely used in cosmetics [21], but in the antimicrobial experiments, CAPB lacks the ability to kill in all tests. According to A. Krasowska et al.'s study [20], CAPB and alkyl dimethylamine oxide could achieve synergy in antimicrobial performance but still inferior to benzalkonium chloride ( a cationic surfactant). No cationic surfactant are used in laundry detergents because the anionic surfactants are most commonly used in detergents and would lead to precipitates together with the cationic ones. For that reason, we did not test the antimicrobial activity of benzalkonium chloride.
Similarly, anionic surfactants with 12-16 alkyl chain length could get a good match with the cell wall lipid structure, which could improve the efficacy of antimicrobials. SDBS as the most widely used synthetic detergents, could be found in numerous cleaning supplies [22], with C12 alkyl chain length and strong acids, has a good affinity for microbial proteins [23]. In our test, all the SDBS groups perform well in inhibiting colony generation and viability.
Besides surfactants, bleaches are typical constituents in detergents that could remove color and have broad-spectrum bactericidal properties. Peroxide-based bleaches NaBO3*4H2O and 2NaCO3*3H2O could dissolve in water then generated hydrogen peroxide. Hydrogen peroxide is inefficient when used under 60°C. C. acnes as an aerotolerant anaerobic bacterium, possesses the cytochrome D oxidase gene, enabling it to grow in the limited amounts of oxygen that are present and to tolerate oxygen for a few hours [4]. Moreover, C. acnes is aerotolerant, then detergents could not achieve a complete reduction of C. acnes at RT. The observed increase in the TAED group could be attributed to TAED and hydrogen peroxide synthesize peroxyacetic acid, which has antibacterial, antifungal properties under 60°C [24]. TAED as a commonly used ingredient is widely used in laundry detergents currently, it is stabilized by granulation and not stable in water-based liquid detergents.
As mentioned in the literature [25], T. rubrum could be removed from the fabric at a normal washing process at RT, but later research noted that T. rubrum could be detected in the rinsing wastewater [26]. In our test, all the regular detergents could completely reduce colony generation, and fluorescence showed that the compounds made a different damage level of T. rubrum cells. It is a limitation that we did not check the microbial load in rinsing water. Still, with the development of laundry, all the rinsing water would leave into the wastewater pipe, the microbial go the cloth and human, back in nature is plausible.
Overall, the powder A and D had better antimicrobial performance than the liquid B and C in clearance of microbe. By analysing the ingredients in detergent, a possible explanation for this is that the compounds like bleach activator TAED are only stable in powder form. Besides, Peroxide-based bleach would also be affected by the form [26]. Like NaBO3*4H2O, the different hydrations cause the different PH, and an appropriate PH could significantly impact the antimicrobial of surfactant. For example, the acidic PH could make the zwitterionic surfactant take on a positive charge, the affinity for cell membranes will be stronger if the cell membrane isoelectric point is not achieved. Furthermore, isoelectric points of many proteins are in acidic PH, the PH may cause a switch in cell charge from negative to positive, resulting in increased affinity for anionic surfactants.
There are some limitations in this study. We just picked one kind of gram-positive bacterium, one gram-negative and one fungus. There could be some difference in other types of bacteria, but we do not have to eliminate all the microbes, it is impossible, and lots of the microbes are symbiotic bacteria, many are beneficial. In further research, it might be possible to determine what part of microbial or genes affects the clearance. We provide a preclinic basis for the dermatologist, could help patients with diseases like acne improve quality of life, and give good advice in domestic laundry processes.