Effect of subinhibitory concentrations on formation of biofilm of Pseudomonas aeruginosa

OBJECTIVE To study the effects of the subinhibitory concentrations on the biofilm formation of Pseudomonas aeruginosa(PAE).METHODS We first determined the minimal inhibitory concentration of azithromycin,amikacin,ciprofloxacin and ceftazidime of the PAE cultured in MHB and LB.We made the static biofilm model of P.aeruginosa in 96-well microtiter plates with the four antibiotics and medical silica gel with the azithromycin concentration,the OD value was determined with crystal violet staining microplate reader,and the biofilm was observed with silver staining microscope.RESULTS The MIC of azithromycin of PAE in MHB and LB were 512 mg/L and 16 mg/L,respectively;the MIC of amikacin,ciprofloxacin and ceftazidime of the PAE were 4 mg/L,0.125 mg/L and 4 mg/L,respectively;the results of the static-made biofilm were as follows: in the MHB and LB media,azithromycin could induce the biofilm formation of PAE at the subinhibitory concentration.Ciprofloxacin and ceftazidime could inhibit the biofilm formation at these concentrations.Amikacin could inhibit the biofilm formation at the concentration of 0.25 to 4 mg/L(1/16MIC-MIC),but could induce biofilm formation at the concentration of 0.125 mg/L(1/16 MIC)in MHB;and when PAE was cultured in LB,amikacin could inhibit the biofilm formation at the concentration of 1 to 4 mg/L(1/4MIC-MIC),and induce the biofilm formation at 0.25 mg/L(1/16MIC).All the results above had statistical differences compared with the control group.Silver staining results: when the concentration of azithromycin was 8mg/L,the formation of the biofilm was the most,the second was the blank group and it was the least as the concentration was 256 mg/L.CONCLUSION The sensitivity of PAE planktonic bacteria to azithromycin varies in different culture condition;and the azithromycin may induce the biofilm formation at the subinhibitory concentration;ciprofloxacin and ceftazidime can inhibit the biofilm formation;the inhibitory of amikacin weakens as the subinhibitory concentration decreases and it can induce the biofilm formation at the concentration of 1/32 MIC and 1/16 MIC.