Bacterial resistance to antibiotics is increasing at an alarming rate and many commonly used antibiotics are no longer effective.Thus, there is considerable interest in investigating novel antibacterial compounds, such as the plant-derived pentacyclic triterpenoids, including oleanolic acid (OA), ursolic acid (UA) and their derivatives. These compounds can be isolated from many medicinal and crop plants and their antibacterial, antiviral, antiulcer and anti-inflammatory effects are well documented. OA and UA are active against many bacterial species, particularly Gram-positive species, including mycobacteria. They inhibit bacterial growth and survival, and the spectrum of minimal inhibitory concentration (MIC) values is very broad. In addition, OA, UA and their derivatives display potent antimutagenic activity. Studies to identify the cellular targets and molecular mechanisms of OA and UA action were initiated a few years ago and it has already been demonstrated that both acids influence bacterial gene expression, the formation and maintenance of biofilms, cell autolysis and peptidoglycan turnover. Before these compounds can be used clinically as antimicrobial agents, further extensive studies are required to determine their cytotoxicity and the optimum mode of their application.

Studies on new antibacterial therapeutics and strategies are currently being conducted in many microbiological, pharmaceutical and biochemical laboratories. The antibacterial activity of plant-derived compounds as well as silver and gold nanoparticles is the subject of this minireview. The application of photodynamic therapy is also discussed.

Three distinct strains (KL1, KS1, and KS2) of facultatively chemolitho-autotrophic bacteria able to use carbon disulfide or carbonyl sulfide as sole energy substrates were identified as novel strains of Paracoccus denitrificans. Evidence for their identity as biovars of P. denitrificans and as close relatives of Paracoccus versutus is based on their DNA composition, total sequencing of the genes for their 16S rRNA, muropeptide profiles, amino acid composition of peptidoglycan, kinetics of murein degradation by lysozyme, possession of large plasmids (91-98 kb) and megaplasmids (> 450 kb), and plasmid transfer between the strains and with P. denitrificans and P. versutus. No functions have been identified for the 91- to 98-kb plasmids of strains KL1 and KS2, but curing strain KL1 of its plasmid did not affect growth on carbon disulfide, thiosulfate or succinate. Emendation of the formal description of Paracoccus denitrificans is presented. Autotrophic growth on carbon disulfide and thiosulfate was confirmed by 14CO2 fixation. Evidence is presented for initiation of carbon disulfide oxidation by an NADH-dependent oxygenase. Cell-free extracts catalyzed (1) NADH-stimulated uptake of oxygen in the presence of carbon disulfide, and (2) carbon-disulfide-stimulated oxidation of NADH. The activity was not sedimented at 50,000 x g. Intermediates in aerobic carbon disulfide metabolism were shown by GC and GC/MS to include carbonyl sulfide and hydrogen sulfide, but anaerobic production of COS and H2S from carbon disulfide did not occur. SDS-PAGE of cell-free extracts showed polypeptides that were unique to growth on carbon disulfide, common to carbon disulfide and carbonyl sulfide, or found after growth on carbon disulfide, carbonyl sulfide or thiosulfate. The possible identity of these as proteins involved in sulfur compound metabolism is discussed.

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Three R6K-derived gamma ori minireplicons were successfully transferred by conjugation from Escherichia coli to several species of pathogenic bacteria. The pFL129 replicon encodes the wild-type initiation replication protein pi, while plasmids pFL130 and pAG101 encode mutant forms of the pi protein conferring the plasmid copy-up phenotype. Plasmids could be transferred to all recipient species tested, although high efficiency conjugal transfer was only obtained with genera of the Enterobacteriaceae. The efficiency of plasmid transfer to all recipients was lower for the copy-up derivatives, pFL130 and pAG101, than for pFL129. The three gamma ori replicons were stably maintained in all transconjugants except pFL129 in Listeria monocytogenes. The two mutant plasmids retained their copy-up phenotype in the new bacterial hosts.

The effect of DNA mismatched repair on the genetic recombination of a gene adjacent to the mismatch site (MS) was tested by using four mismatch configurations. An MS was constructed in a well-characterized plasmid recombination substrate, and recombination with a resident compatible plasmid was measured after transformation of the mismatched plasmid into Escherichia coli. The mismatched plasmids were constructed such that one of the DNA strands was methylated by the DNA adenine methylase (Dam), while the other strand was unmethylated. The processing of a hemimethylated single-base-pair mismatch had no effect on the recombination of the adjacent gene, suggesting that the most efficient (Dam-instructed) mismatch repair process does not secondarily promote genetic recombination. However, mismatches that could form an ordered secondary structure resembling a cruciform increased the recombination of this adjacent gene at least 20-fold. An identical mismatch that could not form an ordered secondary structure had no effect in this system. The increased frequency of recombination observed was found to require the recB or recC gene product or both. Furthermore, the recombination appeared unidirectional, in that the cruciform-containing plasmid did not produce stable transformants. Our results support a model in which the cruciform-containing plasmid can participate in recombination with the resident plasmid but is unable to produce stable transformant progeny. A proposed role tor the RecBCD enzyme (ExoV) in this process is discussed.General recombination in Escherichia coli has been extensively studied, and at least 28 genetic loci have been found to alter the recovery of recombinants (for a recent review, see reference 19). The recA gene product has been shown to be required for nearly all of the homology-dependent recombination events in E. coli and its central role in the initiation of strand exchange has been detailed biochemically (17). The recB, recC, and recD genes encode subunits of the ATPdependent exonuclease ExoV (1,26,27). Mutations in either the recB or recC gene result in deficiencies in conjugal, transductional, and bacteriophage X (red) recombination (3; D. S. Thaler, E. Sampson, I. Siddiqi, S. M. Rosenberg, L. C. Thomason, F. W. Stahl, and M. M. Stahl, Genome, in press). Plasmid recombination is substantially elevated in bacterial strains carrying recB recC or recD mutations (2, 6, 11).Most of the detailed studies on the mechanism of RecBCD-mediated recombination have been done with the bacteriophage X. On the basis of density-labeled recombinants, Thaler et al., in press) have suggested that the RecBCD protein enters the duplex linear form of the X chromosome at the terminal cos site and travels leftward, enhancing exchange events. The presence of a cis-acting recombination enhancer site (chi) in the appropriate orientation relative to the cos site increases localized exchange to the left (on the standard X map) of this site (5, 21). Although the exact mechanism of RecBCD-dependent exchange is unknown...