In vitro activity of commercially manufactured disinfectants against Pseudomonas aeruginosa

High-level tolerance to triclosan may play a role in Pseudomonas aeruginosa antibiotic resistance in immunocompromised hosts: evidence from outbreak investigation

Background and methods

Pseudomonas aeruginosa is a major infectious threat to immunocompromised patients. We recently reported a fatal epidemic of multidrug-resistant P. aeruginosa in an onchoematology unit, linked to massive contamination of a triclosan-based disinfectant. The aim of this study is to evaluate the antimicrobial activity of triclosan and chlorhexidine digluconate against the epidemic strain of P. aeruginosa, to confirm the hypothesis that the soap dispenser acted as a continuous source of the infection during the outbreak, and to explore the potential role of triclosan in increasing the level of resistance to selected antibiotics.

Susceptibility tests and time-kill assays for disinfectans were performed using two commercial formulations containing triclosan and chlorhexidine digluconate, respectively. Antibiotic susceptibility testing was performed by the broth microdilution method.

Findings

The P. aeruginosa epidemic strain exhibited an extremely high level of triclosan resistance (apparent MIC = 2,125 mg/L), while it was markedly susceptible to chlorhexidine digluconate (apparent MIC = 12.5 mg/L). Upon gradual adaptation to triclosan, the epidemic strain survived for a long period (> 120 h) in the presence of 3,400 mg/L (equivalent to 1.6 × MIC) of triclosan, concomitantly increasing the resistance to six antibiotics that are typical substrates of drug efflux pumps of the resistance nodulation division family. This effect was reversed by efflux pump inhibitors.

Conclusions

The epidemic P. aeruginosa strain was resistant to triclosan and its previous exposure to triclosan increases antibiotic resistance, likely through active efflux mechanisms. Since P. aeruginosa can become tolerant to elevated triclosan concentrations, the use of triclosan-based disinfectants should be avoided in those healthcare settings hosting patients at high risk for P. aeruginosa infection.

Effect of disinfectants on the metabolism of Salmonella enterica serovar enteritidis

Antimicrobial activity of 19 commercially manufactured disinfectant substances on a Salmonella enteritidis strain was determined. The substances represented 8 quaternary ammonium salts (QAS) and 10 QAS combined with other additives. The antimicrobial efficacy was characterized by influencing the growth of bacterial cells expressed as MIC and ED50 values as well as by the inhibition of the incorporation rate of 14C-adenine and 14C-leucine. According to their efficacy the disinfectants were divided into three groups: (1) substances with strong inhibitory effect (MIC 6-45 micrograms/L) such as Diesin forte, Hexaquart plus, Neoquat S, Triquart, Almyrol, Hexaquart S, ID212, ID213 and Microbac forte; (2) substances with good antibacterial efficacy (MIC 90-780 micrograms/L); (3) substances with MIC values > 780 micrograms/L (up to 3120 micrograms/L). Cetrimide had very low activity (MIC 3.12-6.25 mg/L). The effect of disinfectants on the biosynthetic processes was expressed by R values (IC50(Ade):IC50(Leu)); all these values were < 1 except Benzalkonium chloride, FD312, Divoquat forte, 5P plus, Almyrol, Hexaquart S and Hexaquart plus. Low R values suggested interference of these substances with the synthesis of both nucleic acids and proteins. All substances except 5P plus caused an inhibition of endogenous respiration. The most effective were Almyrol, Diesin forte, Microbac forte and Neoquat which completely inhibited respiration at 190 mg/L. Kvart showed the lowest effect on the respiration over the whole concentration range. The disinfectants also suppressed growth of S. enteritidis, probably by interfering with energy-yielding and-requiring processes in the cells.

The effect of new disinfectant substances on the metabolism of Enterobacter cloacae

The antimicrobial mechanism of 16 new commercially manufactured disinfectant substances on an Enterobacter cloacae strain was studied. The substances tested represent 11 quaternary ammonium salts (QAS) and five combinated QAS with other ingredients. The antimicrobial efficacy was characterized by influencing the growth of bacterial cells expressed by MIC and ED50 values as well as by the inhibition of the incorporation rate of [14C] adenine and [14C] leucine. The disinfectants are divided into three groups according to their efficacy. The first group comprised substances with strong inhibitory effect (MIC 0.006-0.048 mg l(-1)) such as triquart, topax 91, benzalkonium chloride, neoquat S, ID 213, and antibacteric P. The second group represented substances with good antibacterial efficacy (MIC 0.048-0.15 mg l(-1)), and the third group were substances with MIC values up to 0.195-0.39 mg l(-1). Cetrimide had low activity (MIC 3.12-6.25 mg l(-1)). The effect of substances studied on the biosynthetic processes expressed by R values (IC50 Ade:IC50 Leu) showed that these values were < 1 except ADL 007. Much lower IC50 Ade and IC50 Leu values of the disinfectant substances studied suggested interference of these substances with nucleic acid synthesis and proteins synthesis which was expressed by inhibition of both precursors. All substances except cetrimide caused an inhibition of the endogenous respiration. The highest inhibition was caused by benzalkonium chloride. This affected the respiration significantly in the presence of intermediators of the Krebs' cycle (glycerol, aspartate). The tested substance suppressed the growth of E. cloacae probably through interference with energy-yielding and energy-requiring processes of the cells.