INTRODUCTION:

Biofilm-associated infections are persistent and difficult to treat due to the multifactorial antimicrobial resistance.¹ Sugar alcohols have a significant inhibitory effect on dental biofilms.^2-4 They can reduce the amount of dental plaque and the extracellular polysaccharide (glucans) and intracellular matrix production.⁵ The interference with the production of glucans could decrease the extracellular matrix component of mixed oral biofilms.⁶

Sorbitol is a sugar alcohol that the human body metabolizes slowly. It is a sugar substitute that prevents the formation of glucan and dental plaque.^7-9 Xylitol is another sugar alcohol that has been shown to suppress the formation of S. aureus by inhibiting the formation of glycocalyx^10,11 and P. aeruginosa biofilm by disrupting the biofilm structure.¹²

Our previous study¹³ showed that sorbitol and xylitol have antibiofilm activities against P. aeruginosa biofilms, so it is important to study the augmentation of antibiotic activities against P. aeruginosa biofilms by xylitol and sorbitol.

The P. aeruginosa strains used in this study were obtained by endotracheal aspiration from intensive care unit patients in Zagazig university Hospitals. Biofilm formation was assessed according to Stepanovic et al¹⁴. The antibiotic susceptibility of the planktonic cells was measured by determination of MICs according to the Clinical and Laboratory Standards Institute Guidelines (CLSI)¹⁵ and that of biofilm cells was determined by measuring minimum regrowth concentrations (MRCs) according to the method of Černohorská and Votava.¹⁶

The four strains were high biofilm forming. The biofilm cells showed a much higher antibiotic resistance than planktonic cells (Table 1).

The effect of xylitol and sorbitol on the biofilm susceptibility to antibiotics was investigated by determination of their minimum biofilm regrowth concentrations in the presence of 50, 100 and 200 mg/ml of the sugar alcohols (Figures 1 and 2).

Xylitol and sorbitol potentiated the antibiotic activity against pre-formed biofilms. The susceptibility of biofilm cells to the tested antibiotics increased by up to 2-8 folds with xylitol and by up to 2-32 folds with sorbitol. Sorbitol exerted higher synergistic effect than xylitol. The interaction of sorbitol with antibiotics was synergistic in all isolates with amikacin, in 3 isolates with ciprofloxacin and tobramycin, in 2 isolates with imipenem and with cefoperazone in one isolate. On the other hand, combining xylitol with antibiotics was synergistic with ciprofloxacin, amikacin and tobramycin in two isolates and with cefoperazone and imipenem in one isolate.

Amikacin, ciprofloxacin and tobramycin were more potentiated than cefoperazone and imipenem when combined with xylitol and sorbitol. Comparing the synergy of antibiotics when combined with sorbitol, amikacin was the most potentiated, followed by ciprofloxacin and tobramycin, while imipenem and cefoperazone were the least potentiated. In the presence of xylitol, amikacin, ciprofloxacin and imipenem were more augmented than cefoperazone and imipenem. The synergistic effect of sorbitol was concentration-dependent in all isolates with the aminoglycosides amikacin and tobramycin and in two isolates with ciprofloxacin, cefoperazone and imipenem. Xylitol synergistic effect, on the other hand, was dose-dependent in all isolates with imipenem, in 3 isolates with cefoperazone, in 2 isolates with ciprofloxacin and amikacin and in one isolate with tobramycin.

These results are in accordance with the study of Dowd et al.¹⁷ who found that xylitol could inhibit P. aeruginosa biofilms in a dose-dependent manner using concentrations of 20 mg/ml, 100 mg/ml and 200 mg/ml. Xylitol at concentration of 200 mg/ml completely abolished biofilm formation by P. aeruginosa.

Table 1. Antibiotic susceptibility against biofilm and planktonic cells of Pseudomonas aeruginosa.

Isolate

MRC/MIC ratio

Amikacin

Tobramycin

Ciprofloxacin

Imipenem

Cefoperazone

256

1024

4096

2048

512

128

256

1024

2048

4096

8192

4098

1024

Xyl, Xylitol

Figure 1. Effect of xylitol on the antibiotic activity against P. aeruginosa biofilms.

Sorb, Sorbitol

Figure 2. Effect of sorbitol on the antibiotic activity against P. aeruginosa biofilms.

These results suggest that combining antibiotics with sorbitol and xylitol may be beneficial for decreasing the resistance of biofilm cells to antibiotics.

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15. Clinical and Laboratory Standards Institute. Methods for dilution antimicrobial susceptibility tests for bacteria that grow aerobically; approved standard. 7th ed. CLSI Document M7-A7.Wayne, PA. 2006.

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Received on 08.10.2012 Accepted on 25.10.2012

Asian J. Pharm. Res. 2(4): Oct. - Dec. 2012; Page 129-131