Comparative Effect on Skin Bacterial Counts of Surgical Skin Preparations, After Preoperative Showering with Anionic and Cationic Soaps

Surgical site infections (SSIs) occur in approximately 2%-5% of patients who undergo clean extra abdominal surgeries, such as thoracic and orthopedic surgery, and in up to 20% of patients who undergo “open” intra-abdominal surgery interventions [2,3]. They can lead to increased morbidity and mortality and are associated with prolonged hospital stay and greater hospital costs [2-5]. The significance of skin bacteria around surgical wounds and their direct/ indirect effect as a cause of Surgical site infections (SSI’s) have therefore been well documented and recommendations made by agencies such as the United States Centre for Disease Control (USCDC) and the UK National Institute for Health and Care Excellence (NICE) [6,7].


As one of the skin disinfectants recommended is CHG in 70%
alcohol, and it is well known that this is affected by the ionic charge from soaps [8][9][10][11], it was felt that the effect of the choice of soap in relation to the outcome of the recommended skin disinfectants, plus other commonly used skin disinfectants that were not mentioned in the USCDC documents, should be investigated. CHG came into medical use in the 1950s. It is on the World Health Organization's List of Essential Medicines, the most effective and safe medicines needed in a health system. There is tentative evidence that it is more effective than povidone-iodine. CHG is active against Gram-positive and Gram-negative organisms, facultative anaerobes, aerobes, and yeasts. It is particularly effective against Gram-positive bacteria.
Significantly higher concentrations are required for Gram-negative bacteria and fungi. There are now numerous strains of dangerous pathogens resistant to CHG [12]. Unfortunately, some of these pathogens are also resistant to Colistin (last resort antibiotic) [12]. CHG is deactivated by forming insoluble salts with anionic compounds, including the anionic surfactants commonly used as detergents or soaps. The potential for residual anionic soaps to be left on the skin after preoperative showering was not mentioned in any of the guidance documents.
The authors felt the surgical community needed to understand if the potential combination choice would have any effect on skin CFU counts, and species, therefore potentially on surgical infection rates. In 2009, the WHO produced their guidance on hand hygiene in healthcare [8]. One of the cautions in this document (section 11.4) states that; "because chlorhexidine is a cationic molecule, its activity can be reduced by natural soaps, various inorganic anions, non-ionic surfactants, and hand creams containing anionic emulsifying agents". Additionally, in June 2016, in a paper that reviewed all the available evidence of measured interactions between CHG and anionic chemistries [1], the authors gave the following recommendations; "clinicians should carefully consider the nature of topical agents used if CHG is concurrently applied.
Increased awareness of CHG incompatibility may result in better antibacterial activity thus ensuring optimal patient management".
If these variables are not isolated in studies, the "Evidence" in reference to patient outcome infection control studies, should therefore be considered at best weak [13].  In July 2018, the authors of this study, published the results of a study on the effects over time, of three commonly used surgical skin preparations including CHG [13] on skin CFU counts for patients undergoing abdominal surgery. One of the failings of the paper, was that it did not identify the soap type (anionic or cationic) used for the preoperative showering. Although the patients were randomized into each group by surgical skin prep. All patients were asked to confirm they had preoperatively showered. In order to ensure 100% compliance with the study protocol, all patients who did not confirm this were asked to shower in hospital prior to moving through to surgery. There was no data collection on type of soap used and therefore no data on the Ionic status of the soaps.
The study, therefore, did not control for any potential effect Iodine (PI) [5,16,17]. The earlier study therefore only compared two other commonly used skin disinfectants against CHG and not PI (see graph below) (Graph 1).

Method
As the authors could not find any reference to the type of soap used in any of the other comparative studies, the authors felt the need to include PI in this study. Abdominal surgical patients were the group chosen, and the commonly used area of the longitudinal midline incision of the abdomen as the area to sample. This type and area of incision has been used for many studies looking at levels of bacterial contamination [18][19][20][21][22][23][24]. In addition, the study protocols for this group were already approved. Time periods chosen for the testing were the same as in the first paper, as these were still felt to be the most relevant in the study group. These are, the period between "skin preparation" and "knife to skin" (approximately 5mins), 1 hour, 2 hours and 4 hours post application [24][25][26].
This study includes the use of CHG 2% in 70% alcohol, CHG 0.5% in 70% alcohol, PI 7.5% in 70% alcohol, a 5th generation long acting Silane Quaternary compound (SiQuat) with low concentration of alcohol (aqueous solution) and a 70% ethyl

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Copy@ Andrew Kemp alcohol solution with no other active ingredient. 200 patients were randomized into each disinfectant group, and then randomized again into a liquid cationic soap group and a liquid anionic soap group, for preoperative showering. Therefore, each subgroup has 100 patients in it. All patients showered in hospital prior to surgery, to ensure they used the correct product. As the soaps and the skin disinfectants are easily identified by staff by color and smell, only the patients were blinded to the study groups. Again, as per the previous study, testing of skin bacterial CFU's was done using the Bacteria Specific Rapid Metabolic Assay (BSRMA) techniques.
Although very reliable and accurate, these techniques were almost unknown in 2018, however they have been used in numerous published studies and are well described in a 2019 paper, that reviewed and compared the available bacterial test methods [27][28][29][30][31]. Average CFU counts for each group were then compared, as an indication of any changes in efficacy of the chemical disinfectant after anionic and cationic soap were used.
In addition, samples were also cultured to determine if a change in bacterial species had occurred, as this had been seen in studies using high concentrations of alcohol [27,32,33]. A nonselective media, Nutrient Agar (NA) and Brain Heart Infusion (BHI) agar, were used for both Total Aerobic Counts (TAC) and total anaerobic counts. Inoculated media were incubated at an appropriate temperature between 30-370C for a minimum of 24hours. Indicator species for contaminated/very contaminated samples were outlined as S. aureus/MRSA, S. epidermidis, VRE and Gram-ve bacilli.
All samples were taken from areas of abdominal skin 2cmx1cm along the midline running superior to inferior using a sterile technique. These areas were marked with a surgical skin marker to ensure that further samples were taken from the same areas. A sample from every area was taken prior to any skin prep being used and a surface count recorded using a BSRMA, technique, as well as a sample sent for culture. All areas were prepared for surgery using one of the chosen skin antimicrobial preparations. Due to color variations in the liquid preparations it was impossible to blind the study. A 2nd culture sample was taken at 4 hours post application.

Results
The graphs below show the average skin bacterial counts for the 2 concentrations of CHG when used with Anionic and Cationic soaps (Graph 2).

Graph 2:
The graphs shows the average skin bacterial counts for the 2 concentrations of CHG when used with Anionic and Cationic soaps.
The following table shows the % reduction in efficacy for both CHG groups when an anionic soap is used in comparison to a cationic soap (Table 1). When compared to other 3 other commercially available surgical skin preparations a different picture emerges.
The graphs below show the average skin bacterial counts for the 5 types of surgical skin prep when used with Anionic and Cationic soaps (Graph 3).

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Copy@ Andrew Kemp There appears to be only a small difference in bacterial counts for the PI and alcohol 70% with bacterial counts peaking at 1-hour post application, and then reducing slightly over the next 3 hours.
The 5th generation SiQuat group was not affected by the choice of ionic soap. The pattern of antimicrobial activity observed in previous papers remains essentially unchanged for all disinfecting chemistries. After a large initial reduction in bacterial counts at 5 mins post application, there is a continual increase in bio burden over the 4-hour period in all but the 5th generation SiQuat group.
There are, however, large differences in bacterial counts in both CHG groups, dependent on the ionic content of preoperative shower soap, from 1-hour post application. In the alcohol 70% group, there was also a change in dominant species from S. Epidermidis to Gram-ve Bacilli, this change in species was not observed in any other group, all of which also contain alcohol.
Graph 3: The graphs shows the average skin bacterial counts for the 5 types of surgical skin prep when used with Anionic and Cationic soaps.

Conclusions and Recommendations
These results from this study, confirm the WHO caution for clinicians that states based on the cited papers [9][10][11], anionic soaps may reduce the efficacy of CHG on patients' skin is correct.
There is a clear reduction in antimicrobial efficacy seen over time when an anionic soap is used for preoperative showering. As in the 2018 study [13], the results showed that the long acting SiQuat not only killed more quickly than the CHG, and other skin prep

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Copy@ Andrew Kemp solutions tested, it continued to do so more effectively for 4 hours after application. Due to the species change from S. epidermidis to Gram-ve bacilli, and the overall poor CFU results over time, when using alcohol without a 2nd antimicrobial, the authors would not recommend this as a suitable preoperative skin disinfectant. As there was no opportunity to study the SSI outcomes of the patients, outcomes is essential to improving those outcomes.