Novel mechanisms of diversity generation in Acinetobacter baumannii resistance islands driven by Tn7-like elements.

Mobile genetic elements play an important role in the acquisition of antibiotic and biocide resistance, especially through the formation of resistance islands in bacterial chromosomes. We analyzed the contribution of Tn7-like transposons to island formation and diversification in the nosocomial pathogen Acinetobacter baumannii and identified four separate families that recognize different integration sites. One integration site is within the comM gene and coincides with the previously described Tn6022 elements suggested to account for the AbaR resistance island. We established Tn6022 in a heterologous E. coli host and confirmed basic features of transposition into the comM attachment site and the use of a novel transposition protein. By analyzing population features within Tn6022 elements we identified two potential novel transposon-encoded diversification mechanisms with this dynamic genetic island. The activities of these diversification features were confirmed in E. coli. One was a novel natural gain-of-activity allele that could function to broaden transposition targeting. The second was a transposon-encoded hybrid dif-like site that parasitizes the host dimer chromosome resolution system to function with its own tyrosine recombinase. This work establishes a highly active Tn7-like transposon that harnesses novel features allowing the spread and diversification of genetic islands in pathogenic bacteria.

The effect of liquorice extract-containing starch gel on the amount and microbial composition of plaque.

The aim of this study was to find out whether liquorice-containing starch gel could affect plaque accumulation and its microbial composition. Sixteen healthy volunteers (mean age: 30.4+/-6.9 years) used 6 g of either control [8% acid-hydrolyzed corn starch, 25% maltitol syrup, water (w/w)] or liquorice gel (control + 2.5% liquorice extract), three times a day for 2 weeks. The gels were used in a random order with a 2-week washout period in between. At the end of each fortnight, plaque was allowed to accumulate for 2 days and all available plaque from the right side of the mouth was collected, weighed, and transferred to transport medium. The plaque on the left side was dyed and photographed in a standardized manner. Mutans streptococci, total streptococci, and facultative bacteria were assessed from the plaque using plate culturing. Plaque index (0-5) of incisors and canines on the left side was evaluated from the photographs. The clinical study was preceded by an in vivo acid production test. The acid production from gels containing 2.5-10% liquorice extract was monitored with a microelectrode. The in vivo acid production potential of the maltitol-containing starch gel was about 50% compared to the sucrose control. Liquorice inhibited acid production from the gel. In the clinical study, the weight of plaque after consumption of the liquorice gel did not differ from that of the control gel. No differences were found in the microbial counts nor in the plaque index between the two gels. In addition, the liquorice gel had no effect on the stability of the predominant bacterial populations of the plaque samples of 16 individuals as detected by PCR-denaturing gradient gel electrophoresis. In conclusion, an addition of liquorice extract to starch-containing gel with a low acid production potential had no effect on the plaque formed during a 2-week gel consumption period.