The Mesorhizobium loti strain R7A symbiosis island is a 502-kb chromosomally integrated element which transfers to nonsymbiotic mesorhizobia in the environment, converting them to Lotus symbionts. It integrates into a phenylalanine tRNA gene in a process mediated by a P4-type integrase encoded at the left end of the element. We have determined the nucleotide sequence of the island and compared its deduced genetic complement with that reported for the 611-kb putative symbiosis island of M. loti strain MAFF303099. The two islands share 248 kb of DNA, with multiple deletions and insertions of up to 168 kb interrupting highly conserved colinear DNA regions in the two strains. The shared DNA regions contain all the genes likely to be required for Nod factor synthesis, nitrogen fixation, and island transfer. Transfer genes include a trb operon and a cluster of potential tra genes which are also present on the strain MAFF303099 plasmid pMLb. The island lacks plasmid replication genes, suggesting that it is a site-specific conjugative transposon. The R7A island encodes a type IV secretion system with strong similarity to the vir pilus from Agrobacterium tumefaciens that is deleted from MAFF303099, which in turn encodes a type III secretion system not found on the R7A island. The 414 genes on the R7A island also include putative regulatory genes, transport genes, and an array of metabolic genes. Most of the unique hypothetical genes on the R7A island are strain-specific and clustered, suggesting that they may represent other acquired genetic elements rather than symbiotically relevant DNA.

Comparative Sequence Analysis of the Symbiosis Island of Mesorhizobium loti Strain R7A

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Extraction of DNA from soil

Screening of Plant Growth-Promoting Rhizobacteria

Objective: The objective of this study was to evaluate the extent of Pseudomonas aeruginosa contamination of Dental Unit Water (DUW) at a Dental Teaching Center in Jordan. Methods: Water samples were collected from 30 dental units, 10 from each of three teaching clinics, namely conservative dentistry, periodontology, and prosthodontics. Samples were collected from the outlet of the air/water syringe, high-speed handpiece and water cup filler, at the beginning of the working day (before use), after 2 min flushing, and at midday. Results: P. aeruginosa was detected in 86.7% (26/30) of the dental units at the beginning of the working day, and in 73.3% (22/30) after 2 min of flushing and at midday. Conservative dentistry units had the highest counts, followed by periodontology and prosthodontics (P< 0.05). Overall, the highest counts (log 10 count CFU ml -1 ) were at the beginning of the working day (1.38 ± 1.05), and the lowest counts after flushing for 2 min (1.10 ± 1.03), and higher numbers were seen again at midday (1.15 ± 1.04) (P < 0.05). Conclusions: 86.7% of the dental units were contaminated with P. aeruginosa, the conservative dentistry units had the highest amount of contamination. Flushing the DUW for 2 min significantly reduced the counts of P. aeruginosa.

The presence of Pseudomonas aeruginosa in the dental unit waterline systems of teaching clinics.

A study was carried out to evaluate the extent of the colonization of dental water systems by Comamonas acidovorans and to investigate how the occurrence of these bacteria is related to certain water characteristics. The 152 water samples were collected from the oral rinsing cup, air-water syringe, turbine and supply lines to dental units. Comamonas acidovorans was found most frequently and in greatest quantities in samples taken from water entering the units and in samples with a lower total bacterial count at 22 degrees C, higher temperature, lower content of organic matter and, in general, higher concentrations of residual chlorine.

Comamonas acidovorans contamination of dental unit waters.

Pseudomonas aeruginosa is an opportunistic bacterial pathogen frequently found in nosocomial infections and is a major cause of morbidity and mortality in patients with cystic fibrosis. To facilitate molecular studies of this organism, we have generated a bacterial artificial chromosome (BAC) library. Genomic DNA was isolated from the prototype strain PAOl, partially digested with HindIII, size selected after pulsed-field gel electrophoresis, and used to construct a BAC library using the pBeloBACll vector. DNAs from approximately 850 clones, representing more than 9.5-fold physical coverage of the 5.9-Mb PAOl genome, were analyzed after Spel and HindIII digestions and agarose gel electrophoresis. The BAC library had clones with insert fragments ranging from 20 to more than 290 kb. A subset of 264 BACs having inserts >80 kb, representing >4 genome equivalents, were rearrayed into 96-well plates, and a clone pooling and PCR screening strategy was developed. The PCR library screening enabled the identificati...

Laurent Sabbagh

Papers

Pseudomonas aeruginosa PAO1 bacterial artificial chromosomes: strategies for mapping, screening, and sequencing 100 kb loci of the 5.9 Mb genome.