In this study, we tested eight registered disinfectants under label use conditions against
S. aureus and
P. aeruginosa biofilms using EPA methods MB-19 and MB-20. We found statistically significant quantitative differences among disinfectant active ingredients and products against
S. aureus and
P. aeruginosa. Specifically, we found (i) statistically significant differences in disinfectant efficacy among disinfectants, (ii) similar performance of hydrogen peroxide and sodium hypochlorite-based products against
S. aureus and
P. aeruginosa biofilms, and iii) significantly higher bactericidal efficacy of quaternary ammonium-based products against
S. aureus than
P. aeruginosa. Bacterial biofilms are common on a wide range of surfaces made of different materials and have been reported to be present in drains, metal pipes [
10], sanitizing bottles, trolleys and clipboards [
23] thus are potential sources of HAIs.
Disinfectant efficacy varies by active ingredient
We found significant differences among quaternary ammonium compound disinfectants compared to hydrogen peroxide and sodium hypochlorite disinfectants. The quaternary ammonium compounds did not achieve the current EPA regulation minimum stating that the disinfectant must decrease the bacterial load by 10
6 CFU [
24]. The findings in this study underscoring low quaternary ammonium compound efficacy against laboratory-grown biofilms. This raises concerns for healthcare facilities as quaternary ammonium disinfectants are reported to be among the most commonly used disinfectants in healthcare facilities [
25,
26]. Quaternary ammonium compounds are cationic in nature [
27,
28] and their interaction with a negatively charged biofilm matrix could inhibit their bactericidal efficacy [
29]. Tseng et al. found that the efficacy of tobramycin, a positively charged antibiotic, was decreased as it was sequestered at the surface of the negatively charged biofilm matrix thus did not penetrate the matrix to contact underlying viable
P. aeruginosa cells [
29]. In addition, the bactericidal efficacy of quartenary ammonium compounds may fluctuate because they have been shown to be biogradeble under aerobic condictions [
30].
Hydrogen peroxide and sodium hypochlorite disinfectants were effective against
P. aeruginosa and
S. aureus biofilms at the EPA required reduction levels. Hydrogen peroxide and sodium hypochlorite disinfectants have been reported to destroy both the biofilm matrix and the bacteria cells within, making them better anti-biofilm agents [
31,
32]. Specifically, sodium hypochlorite disinfectant products irreversibly kill bacterial cells in biofilms by denaturing proteins in the biofilm matrix and inhibiting major enzymatic functions in bacterial cells. Although sodium hypochlorite disinfectants at concentrations as low as 0.0219% are effective against the formation of
S. aureus biofilms [
33], the use of sub-lethal concentrations of some sodium containing disinfectants could actually promote the formation of biofilms on environmental surfaces [
34]. In a study conducted by West et al. [
22], hydrogen peroxide products and sodium hypochlorite products were more effective against both
S. aureus and
P. aeruginosa planktonic cells compared to quaternary ammonium. On another note, surfaces disinfected with hydrogen peroxide based antimicrobials have demonstrated significantly lower chances of bacterial regrowth than those disinfected with quaternary ammonium compounds [
35]. To this effect, the study by Boyce et al. [
35] concluded that the risk of the incidence of HAIs was lower with hydrogen peroxide disinfectants than with the use of quaternary ammonium compounds. Our data suggest that hydrogen peroxide or sodium hypochlorite products should be used in healthcare facilities for routine use, particularly on surfaces prone to biofilm development. However, hydrogen peroxide disinfectants have also been reported to be corrosive on medical equipment such as flexible endoscopes [
36] and can discolor metal finishes [
37]. Despite these limitations, Alfa et al. [
38] also demonstratated that a 0.5% hydrogen peroxide antimicrobial is highly efficient at disinfecting medical devices. Moreover, hydrogen peroxide disinfectants are neither irritating or malodorous [
37].
The ability of biofilm matrices to prevent contact between disinfectant products and bacterial cells is complex [
39]. Biofilms are characterized by high cell population densities that supply large amounts of polymeric substances, which consequently enables the formation of well-structured, functional matrices [
39]. Moreover, biofilm cells are genetically primed to better tolerate disinfectant products compared to plaktonic cells [
39,
40]. These features prevent the diffusion of disinfectants and limit bactericidal efficacy [
41]. While our study emphasized the efficacy of disinfectants at label concentration and contact time, it did not investigate the efficacy of disinfectants at off label use or with varying environmental effects. Monoculture biofilms will be rare in healthcare environments and soil levels and surface type will vary. Further, this work was conducted on glass coupons per the EPA protocol, which does not necessarily represent how cells will grow on other surfaces (e.g. hard plastics, stainless steel). Recognizing these limitations, more work is needed to investigate other variables that can impact disinfectant efficacy (e.g. dry biofilms) as well as applications in healthcare settings.