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Effects of subinhibitory concentrations of amikacin and ciprofloxacin on the hydrophobicity and adherence to epithelial cells of uropathogenic Escherichia coli strains

https://doi.org/10.1016/j.ijantimicag.2007.01.007Get rights and content

Abstract

The effect of subinhibitory concentrations of amikacin and ciprofloxacin on the hydrophobicity and adherence to uroepithelial cells of Escherichia coli strains was investigated. The hydrophobicity of the tested strains was evaluated by the bacterial adherence to hydrocarbon–xylene test and by the salt aggregation test of ammonium sulphate. The hydrophobic character of strains exposed to 1/2 to 1/8 minimal inhibitory concentration (MIC) of amikacin and 1/2 to 1/16 MIC of ciprofloxacin was altered to a hydrophilic state. Results of the SAT also correlated with these data. Moreover, comparisons were made between the number of bacteria attached to the epithelial cells before and after exposure to 1/2, 1/4 and 1/8 MIC of antibiotics. The greatest loss of adherence capability occurred at 1/2 MIC of ciprofloxacin. In conclusion, antibiotics are often present at sub-MICs and may still be effective in reducing bacterial virulence by interfering with bacterial cell functions.

Introduction

The surface hydrophobicity of bacteria is one of the non-specific factors contributing to their adherence. The hydrophobic properties of bacteria are responsible for formation of biofilm and adhesion to epithelial cells. The hydrophobicity of many Gram-negative rods and Gram-positive bacteria has been described [1], [2], [3], [4], [5], [6].

Bacterial adherence to the epithelial surface is considered as one of the important factors in the infection process. Adherence is an interaction between adhesin molecules on the bacterial cell surface and complementary receptor molecules on the host cell surface. Microorganisms adhere to epithelial cells in a highly selective manner and thus cannot be removed by unspecific defence mechanisms of the urinary, respiratory, gastrointestinal and genital tracts [3], [7].

The effective therapeutic result of antibiotics is best when the concentration is above the minimal inhibitory concentration (MIC) between consecutive doses. Looking at the pharmacokinetic curves of antibiotics, it can be seen that their concentrations exceed the MIC for only a certain period of time, after which they become lower than the MIC and are called subinhibitory concentrations (sub-MICs), especially in tissues, i.e. the sites of infections where antibiotic concentrations are frequently lower than those in blood, e.g. abscesses or chronic osteomyelitis [8]. Sub-MICs of antibiotics cannot kill bacteria but they have been shown in vitro to affect microorganisms in various ways, e.g. through suppression of bacterial growth, induction of morphological changes in bacteria, alteration of the cell surface structure, inhibition of enzyme and toxin production and suppression of bacterial adhesion to host cells [3], [4], [5], [9], [10].

The aim of this study was to evaluate the hydrophobic character of uropathogenic Escherichia coli strains and to study changes in bacterial surface hydrophobicity and adhesion to uroepithelial cells following treatment with amikacin and ciprofloxacin at sub-MICs.

Section snippets

Bacterial strains

Sixty-six E. coli strains isolated from urine of patients with cystitis were studied.

Antibiotics

Amikacin (Biodacyna, Warsaw, Poland) and ciprofloxacin (Proxacin, Warsaw, Poland) were used.

Determination of the MIC

The microdilution broth method using Mueller–Hinton broth enriched with 25 mg/L CaCl2 and 12.5 mg/L MgSO4 (pH 7.4) with serial dilution of the antibiotics was used. MICs of amikacin were in the range 0.5–8.0 μg/mL and those of ciprofloxacin were 0.007–0.031 μg/mL.

Bacterial hydrophobicity

The surface hydrophobicity of bacterial cells treated with

Results

In preliminary studies, we determined the adherence capacity of E. coli strains to epithelial cells. E. coli strains could be divided into four groups (Table 1). In Group I, the mean number of bacteria per epithelial cell was 0–17. There were 38 strains in this group (22 with a hydrophobic surface and 16 with a hydrophilic surface). Group II, in which the mean number of bacteria per epithelial cell was 18–24, included eight strains, only one of which had a hydrophilic surface. All strains

Discussion

The data presented in this paper confirm and extend the previous observation that certain antibiotics at subinhibitory concentrations reduce bacterial hydrophobicity and adhesion. These effects are not limited to E. coli strains, but also occur with many different bacteria [1], [2], [3], [4], [5], [6]. Several researchers have reported that sub-MICs of some antibiotics increase the surface hydrophobicity and the ability of bacteria to adhere to epithelial cells [13], [14].

Because ciprofloxacin

References (17)

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