Mercury tolerance of thermophilic Bacillus sp. and Ureibacillus sp.

Although resistance of microorganisms to Hg(II) salts has been widely investigated and resistant strains have been reported from many eubacterial genera, there are few reports of mercuric ion resistance in extremophilic microorganisms. Moderately thermophilic mercury resistant bacteria were selected by growth at 62 degrees C on Luria agar containing HgCl(2). Sequence analysis of 16S rRNA genes of two isolates showed the closest matches to be with Bacillus pallidus and Ureibacillus thermosphaericus. Minimum inhibitory concentration (MIC) values for HgCl(2) were 80 microg/ml and 30 microg/ml for these isolates, respectively, compared to 10 microg/ml for B. pallidus H12 DSM3670, a mercury-sensitive control. The best-characterised mercury-resistant Bacillus strain, B. cereus RC607, had an MIC of 60 microg/ml. The new isolates had negligible mercuric reductase activity but removed Hg from the medium by the formation of a black precipitate, identified as HgS by X-ray powder diffraction analysis. No volatile H(2)S was detected in the headspace of cultures in the absence or presence of Hg(2+), and it is suggested that a new mechanism of Hg tolerance, based on the production of non-volatile thiol species, may have potential for decontamination of solutions containing Hg(2+) without production of toxic volatile H(2)S.

Mercury transport and resistance.

Resistance to mercuric ions in bacteria is conferred by mercuric reductase, which reduces Hg(II) to Hg(0) in the cytoplasmic compartment. Specific mercuric ion transport systems exist to take up Hg(II) salts and deliver them to the active site of the reductase. This short review discusses the role of transport proteins in resistance and the mechanism of transfer of Hg(II) between the mercury-resistance proteins.

A putative type III secretion gene cluster is widely distributed in the Burkholderia cepacia complex but absent from genomovar I.

Using probes constructed from Ralstonia solanacearum and Burkholderia pseudomallei, putative type III secretion (TTS) genes were identified in Burkholderia cepacia J2315 (genomovar III). A cosmid clone containing DNA with homology to five TTS genes was sub-cloned and regions were sequenced in order to design oligonucleotides for polymerase chain reaction assays. These indicated that two putative TTS genes (bcscQ and bcscV) were present in all members of the B. cepacia complex with the exception of strains from genomovar I. Southern blot assays confirmed this observation, suggesting that the lack of a TTS gene cluster may define a major difference between B. cepacia genomovar I and other members of the B. cepacia complex, including genomovar III. In contrast to TTS gene clusters in other bacteria, a putative gene homologous to the virB1 gene of Brucella suis was located directly downstream of bcscQR.