Differences in biofilm formation and transcription of biofilm-associated genes among Acinetobacter baumannii clinical strains belonging to the international clone II lineage

Acinetobacter baumannii isolates belonging to international clonal lineage (IC) II are often multidrug-resistant and are the predominant cause of nosocomial outbreaks. While many studies have investigated the genetic and functional basis of antimicrobial resistance of these strains, few have examined specific virulence characteristics such as biofilm formation or overall pathogenic potential. Here, we analyzed biofilm formation and the associated mechanisms in A. baumannii clinical isolates from Japan belonging to the IC II lineage. Draft whole-genome sequence data for each of the isolates was analyzed to detect biofilm-associated genes, including csu (pili) and bfmS/R (two-component regulatory system), and transcription of these genes was evaluated using reverse transcription quantitative PCR. Biofilm formation was measured by crystal violet staining assay. csu operon genes showed some variation in prevalence among the isolates, with an overall prevalence of 73.7% (14/19). The biofilms formed by csu operon-positive isolates were significantly more mature than those of csu operon-negative isolates, supporting the importance of the csu operon in biofilm formation by A. baumannii. However, there was substantial variation among the csu operon-positive isolates, indicating the influence of other factors in biofilm formation. Furthermore, transcriptional levels of csu operon genes were highly divergent, with comprehensive analysis indicating that regulatory factors other than bfmS/R were involved. Our findings are a first step towards understanding the mechanisms of biofilm formation by A. baumannii IC II strains.

Reduction in chlorhexidine efficacy against multi-drug-resistant Acinetobacter baumannii international clone II

BACKGROUND

Nosocomial infections caused by Acinetobacter baumannii international clone II (IC II) can cause severe clinical outcomes.

AIM

Differential evaluation of bactericidal efficacy of chlorhexidine gluconate (CHX) and benzethonium chloride (BZT) disinfectants against IC II and non-IC II isolates.

METHODS

Minimum inhibitory concentrations (MICs) of CHX and BZT were determined for 137 A. baumannii IC II, 99 non-IC II and 69 non-baumannii isolates, further classified according to MIC values into disinfectant-reduced susceptible (DRS) and disinfectant-susceptible (DS) groups. Time-kill curves and minimum bactericidal concentrations (MBCs) were evaluated for representative isolates in each group.

RESULTS

CHX and BZT MIC₉₀s for IC II isolates were 100 and 175mg/L, respectively, but those for non-IC II and non-baumannii isolates were <100mg/L. Nevertheless, time-kill curves indicated that CHX and BZT reduced live bacterial cell number by 5 log₁₀ for IC II and non-IC II isolates within 30s when used at 1000mg/L, comparable to practical use concentrations. CHX MBC at 30s was 1000mg/L for IC II and non-IC II isolates, and was not influenced by addition of 3% bovine serum albumin (BSA); BZT MBC at 30s was 100mg/L without BSA and increased up to 500mg/L upon addition of BSA. No significant differences in BSA were found between DRS and DS isolates.

CONCLUSION

CHX and BZT were effective against Acinetobacter spp. including IC II at a concentration of 1000mg/L and exposure for at least 30s, but their concentrations should be considered carefully to ensure sufficient effects in both clinical and healthcare settings.