Mechanisms of quinolone resistance in clinical isolates of Shigella dysenteriae

Resistance to fluoroquinolones by the combination of target site mutations and enhanced expression of genes for efflux pumps in Shigella flexneri and Shigella sonnei strains isolated in Korea

Among 5938 clinical Shigella spp. isolates, two S. flexneri strains were isolated as those resistant to fluoroquinolones based on the MICs of the following antibiotics: ciprofloxacin, norfloxacin, ofloxacin, sparfloxacin and levofloxacin. S. flexneri 021787 had three substitutions, one in GyrA (Ser83Leu) and two in ParC (Ser80Ile and Arg91Gln). S. flexneri 021895 had four substitutions, two in GyrA (Ser83Leu and Asp87Gly) and two in ParC (Ser80Ile and Arg91Gln). The increased susceptibility of S. flexneri 021787 and S. flexneri 021895 to ciprofloxacin, norfloxacin and ofloxacin in the presence of the uncoupler carbonyl cyamide-m-chlorophenyldrazone implied that energy-dependent active efflux pumps contributed to the resistance against fluoroquinolones. Both S. flexneri 021787 and S. flexneri 021895 were also induced to express tolC (encoding a resistance-nodulation-division transporter), mdfA (encoding a major facilitator superfamily transporter), and ydhE (encoding a multidrug and toxic compound extrusion transporter) in the presence of ciprofloxacin. Thus, these results indicated that chromosome-mediated fluoroquinolone resistance of S. flexneri 021787 and S. flexneri 021895 resulted from the combination of target site mutations and enhanced expression of genes encoding efflux pumps.

Molecular characterization of multidrug-resistant Shigella species isolated from epidemic and endemic cases of shigellosis in India

Shigella species represent one of the growing numbers of antimicrobial-resistant bacteria in developing countries. Fluoroquinolone-resistant strains of Shigella dysenteriae type 1 and Shigella flexneri type 2a emerged in India during 2002 and 2003, respectively. Sixty strains of Shigella from different parts of India were analysed for antimicrobial susceptibility, the presence of the qnr plasmid, mutations in the quinolone resistance determining regions (QRDRs), fluoroquinolone accumulation, and the presence of other genes encoding resistance to various antimicrobials. Fluoroquinolone-resistant strains had mutations in gyrA and parC genes and had an active efflux system. They were also resistant to several other antimicrobials but were susceptible to azithromycin and ceftriaxone. The majority of the strains harboured genes encoding resistance to ampicillin (97 %), tetracycline (95 %), streptomycin (95 %) and chloramphenicol (94 %). PFGE analysis revealed clonality among strains of S. dysenteriae types 1 and 5, S. flexneri type 2a and Shigella boydii type 12.

Increasing spectrum in antimicrobial resistance of Shigella isolates in Bangladesh: resistance to azithromycin and ceftriaxone and decreased susceptibility to ciprofloxacin

Antimicrobial resistance of Shigella isolates in Bangladesh, during 2001-2002, was studied and compared with that of 1991-1992 to identify the changes in resistance patterns and trends. A significant increase in resistance to trimethoprim-sulphamethoxazole (from 52% to 72%, p < 0.01) and nalidixic acid (from 19% to 51%, p < 0.01) was detected. High, but unchanged, resistance to tetracycline, ampicillin, and chloramphenicol, low resistance to mecillinam (resistance 3%, intermediate 3%), and to emergence of resistance to azithromycin (resistance 16%, intermediate 62%) and ceftriaxone/cefixime (2%) were detected in 2001-2002. Of 266 recent isolates, 63% were resistant to > or =3 anti-Shigella drugs (multidrug-resistant [MDR]) compared to 52% of 369 strains (p < 0.007) in 1991-1992. Of 154 isolates tested by E-test in 2001-2002, 71% were nalidixic acid-resistant (minimum inhibitory concentration [MIC] > or =32 microg/mL) and had 10-fold higher MIC90 (0.25 microg/mL) to ciprofloxacin than that of nalidixic acid-susceptible strains exhibiting decreased ciprofloxacin susceptibility, which were detected as ciprofloxacin-susceptible and nalidixic acid-resistant by the disc-diffusion method. These strains were frequently associated with MDR traits. High modal MICs were observed to azithromycin (MIC 6 microg/mL) and nalidixic acid (MIC 128 micdrog/mL) and low to ceftriaxone (MIC 0.023 microg/mL). Conjugative R-plasmids-encoded extended-spectrum beta-lactamase was responsible for resistance to ceftriaxone/cefixime. The growing antimicrobial resistance of Shigella is worrying and mandates monitoring of resistance. Pivmecillinam or ciprofloxacin might be considered for treating shigellosis with caution.

Crystal structure of Streptococcus pneumoniae acyl carrier protein synthase: an essential enzyme in bacterial fatty acid biosynthesis

Acyl carrier protein synthase (AcpS) catalyzes the formation of holo-ACP, which mediates the essential transfer of acyl fatty acid intermediates during the biosynthesis of fatty acids and lipids in the cell. Thus, AcpS plays an important role in bacterial fatty acid and lipid biosynthesis, making it an attractive target for therapeutic intervention. We have determined, for the first time, the crystal structure of the Streptococcus pneumoniae AcpS and AcpS complexed with 3'5'-ADP, a product of AcpS, at 2.0 and 1.9 A resolution, respectively. The crystal structure reveals an alpha/beta fold and shows that AcpS assembles as a tightly packed functional trimer, with a non-crystallographic pseudo-symmetric 3-fold axis, which contains three active sites at the interface between protomers. Only two active sites are occupied by the ligand molecules. Although there is virtually no sequence similarity between the S.pneumoniae AcpS and the Bacillus subtilis Sfp transferase, a striking structural similarity between both enzymes was observed. These data provide a starting point for structure-based drug design efforts towards the identification of AcpS inhibitors with potent antibacterial activity.