Sub-minimum inhibitory concentrations of colistin and polymyxin B promote Acinetobacter baumannii biofilm formation.

Yoshinori Sato,Yuka Unno,Tsuneyuki Ubagai,Yasuo Ono,Hendrik W Van Veen

doi:10.1371/journal.pone.0194556

Abstract

We investigated the numbers of planktonic and biofilm cells and the expression levels of genes encoding efflux pumps and biofilm-related proteins in 10 clinical isolates of multi-drug resistant Acinetobacter baumannii (MDRA) as well as in its standard strain ATCC 19606 in the presence of colistin (CST), polymyxin B (PMB), minomycin (MIN), and tigecycline (TGC) at their respective sub-MICs. The number of planktonic and biofilm cells of ATCC 19606 decreased in the presence of all aforementioned antibiotics in a dose-dependent manner. Cell number also decreased in two representative MDRA strains, R2 and R3, in the presence of MIN and TGC in a dose-dependent manner. In contrast, the number of biofilm cells in these two strains increased in the presence of CST, while they increased significantly in the presence of PMB in R2 only. Pearson correlation analysis revealed that the number of biofilm cells was positively and significantly correlated with the mRNA levels of genes encoding efflux pumps (adeB and adeG) and autoinducer synthase (abaI) in strain R2 and adeB, adeG, adeJ, poly-acetyl-glucosamine-porin (pgaA), and abaI in strain R3 in the presence of CST. It was positively and significantly correlated with the mRNA levels of genes encoding adeB in strain R2 and an outer membrane protein A (ompA) and biofilm-associated protein (bap) in strain R3 in the presence of PMB. These results provide valuable insights into the biofilm formation potency of clinical isolates of MDRA that depends on efflux pumps and biofilm-related genes and its regulation by antibiotics.

Highlights

Acinetobacter baumannii is an important opportunistic pathogen associated with nosocomial infections, such as bacteremia, pneumonia, meningitis, urinary tract infections, and wound infections [1,2,3]
Biofilm formation in A. baumannii is associated with quorum sensing (QS) of its cells [26], these results suggest that antibiotics at their sub-minimum inhibitory concentration (MIC) induce biofilm formation of A. baumannii associated with the mechanisms of QS
A. baumannii has recently emerged as a major nosocomial pathogen [1,2,3] and an increase in the outbreaks of multi-drug resistant A. baumannii (MDRA) worldwide is becoming a cause for concern [8, 9]

Summary

Introduction

Acinetobacter baumannii is an important opportunistic pathogen associated with nosocomial infections, such as bacteremia, pneumonia, meningitis, urinary tract infections, and wound infections [1,2,3]. A. baumannii is regarded as a low-virulence pathogen [4], recent studies have reported that A. baumannii shows several forms of pathogenicity, such as biofilm formation as well as adherence on and invasion of host cells [10,11,12,13], host cell death [14], and iron acquisition [15]. A. baumannii, especially MDRA, has gradually gained importance as a human pathogen, in hospital or clinical environments [8]. For the treatment of MDRA infection, polymyxins and minomycin (MIN) with its derivative, TGC, are used as active antibiotics [5]

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