Biocides are antimicrobial chemical substances that are widely used in residential, commercial, and healthcare environments to disinfect surfaces. Significant scientific debate about biocides' role in bacterial survival and resistance has been sparked by the increased use of them at various concentration levels(19). Hospitals, healthcare facilities, homes, farms, and the food industry all frequently use biocidal agents like benzalkonium chloride and chlorhexidine digluconate.(9)
The chemical biguanide (two guanidines) is used to make chlorhexidine (CHX). Chlorine can be used to create chlorhexidine when bisbiguanide (two bonded biguanides) is present(20). CHX has a bacteriostatic effect at low concentrations (0.02 to 0.06 percent), where it displaces calcium and magnesium and causes the cell wall to lose potassium. High concentrations of CHX (>0.1 percent) result in the leakage of all the major intracellular components, which has the bactericidal (cell lysis and death) effect(21).
chlorhexidine is a common ingredient in mouthwash solutions, toothpaste and dental gels. In addition to, chlorhexidine-containing solutions are useful for surgical hand antisepsis (5). The effectiveness of a skin preservative is measured as a log reduction in bacterial count, with a 1-log reduction representing a 10-fold reduction in bacterial count (90% bacterial removal) and a 2-log reduction representing a 100 times the number of bacteria reduction (kills 99% of bacteria)(22).Another usage of this disinfectant is for skin preparation prior to venepuncture and vascular access and also disinfecting of central venous catheters(5).
Going back to clinical applications, there are some drawbacks to using CHX as a mouthwash or topical oral gel. The most frequent ones include dry mouth, altered taste perceptions, particularly salt and bitter, and a discolored or coated tongue(23). The more severe Type IV and Type I hypersensitivity reactions that could result in anaphylaxis are more likely to occur as side effects when using CHX orally(21). Additionally, case studies have shown that CHX mouthwash can cause a severe anaphylactic reaction, which can result in respiratory arrest and death(24).
Contrary to antibiotics, little is known about the global distribution of bacterial susceptibility to biocides, including Iran(8) . Numerous laboratory and hospital studies have shown that frequent use of antimicrobial biocides and the subsequent constant exposure of bacteria to them increases bacterial tolerance through selective pressure(8, 25). Their effectiveness as antimicrobials is constrained by this problem. As a result of contaminated biocide solutions, numerous nosocomial infection outbreaks have so far been documented(26).
Unlike most antibiotics, which use clinical outcome data, PK/PD models, and MIC distributions to identify clinical breakpoints (susceptible, intermediate, or resistant) to direct therapy (27). In the absence of a breakpoint-based definition of biocide resistance, In every previous studies on the subject, the MIC (measured using the same techniques as those used for antibiotics) of a wild-type strain is compared to that of an isolate that has a mutation or a gene that is presumed to encode biocide resistance. The second is deemed resistant if it is more resistant than the wild type(28).
Resistance to CHG has been demonstrated by E. coli, S. aureus and CoNS ,Enterobacter spp, Pseudomonas spp, and Enterococcus spp(25). CHG resistance can be found in many resistant isolates such as XDR K. Pneumonia. Isolates with high MICs are often insensitive to CHG for disinfection. Based on our findings in one study, chlorhexidine inhibited 90% of isolates (MIC90) at a concentration of 2µg/mL for MRSA,1µg/mL for MSSA and CoNS, which has shown a pleasing effect of this disinfectant. In mentioned study, from the clinical specimens, which included wounds, blood, sputum, urine, abscesses, synovial fluid, tracheal aspirates, and ascetic fluid, 165 Staphylococcus isolates were collected. Of these, 60 (36.3%) were MRSA, 54 (32.7%) were methicillin-sensitive S. aureus (MSSA), and 51 (31%) were CoNS strains(1).
The lowest effect of chlorhexidine was against Staphylococcus aureus that inhibited 100% of isolates at concentration of 625µg/mL. Another inhibitory effect of chlorhexidine was Enterococcus spp. which prevented 90% of E. faecalis at concentration of 8 µg/mL for clinical isolates and E. faecium at 4 µg/mL and 8 µg/mL for clinicals and Healthy carriers respectively.
As we know Pseudomonas aeruginosa is one of the most important Opportunist pathogens. Three studies with this bacteria have been investigated and the lowest MIC range was between 5-40 µg/mL which used Sub-minimum inhibitory concentrations(sub-MIC) for detection of bacterial resistance to chlorhexidine. Additionally, they demonstrated that Pseudomonas aeruginosa isolates that are cultured in sub-inhibitory chlorhexidine concentrations can produce stronger biofilms when exposed to sub-MIC antibiotic concentrations. The resistance of Pseudomonas aeruginosa to chlorhexidine and many other antiseptics is due to its outer membrane(14).
Finally, comparing estimated MICs revealed that chlorhexidine's antibacterial activity against the most significant nosocomial pathogens was disappointing. However ,maybe by increasing concentration or contact time of our disinfectant we can achieve our desirable result.
There was a drawback to the current study. which was the absence of a common cut-off point for the definition of chlorhexidine susceptibility rate.