No abstract

Imipenem resistance in Pseudomonas aeruginosa is considered to be associated with loss of the porin OprD combined with activity of chromosomal beta-lactamase (AmpC), while overexpression of multidrug efflux pumps is considered to confer meropenem resistance. Carbapenem resistance can also result from production of metallo-beta-lactamases. Transcription of oprD and efflux pump genes mexB, mexY and mexF was analysed in 23 clinical isolates of P. aeruginosa by quantitative RT-PCR. oprD was sequenced in all, and mexR, regulator of efflux pump MexAB-OprM, in selected isolates. Four isolates that were imipenem susceptible had significant reduction of oprD mRNA and presence of oprD mutations causing frameshift or translational stop. In strains only resistant to imipenem no significant difference in transcription of oprD was observed between low-level and high-level resistant isolates. The differences could not be explained by either pattern of oprD mutations. Increased transcription of mexB generally correlated well with meropenem resistance. One high-level meropenem-resistant isolate showed no significant change in mexB mRNA, but sequencing confirmed presence of a nalB mutation. Furthermore, one meropenem-susceptible isolate showed significant increase in mexB transcription, but no mexR mutations. In summary, our findings indicate that the resistance patterns observed cannot be fully explained by the currently described carbapenem resistance mechanisms.

Principal mechanisms of bacterial resistance to quinolones are modification of target enzymes, DNA gyrase (gyrA) and topoisomerase IV (parC), or reduction of intracellular concentration due to mutations in the regulatory genes for efflux systems, such as mexR and nfxB. We have examined gyrA, parC, mexR, and nfxB genes from 16 quinolone-resistant clinical isolates of Pseudomonas aeruginosa to determine the relation between mutations in DNA replicating enzymes or regulatory genes for efflux systems and to correlate the mutations with minimal inhibitory concentrations (MICs). The quinolone resistance-determining regions (QRDR) of these genes were amplified by PCR and sequenced by capillary electrophoresis. Fourteen of 16 isolates had mutations in gyrA, and 13/14 strains with MIC to norfloxacin > or = 8 mg/L had threonine at position 83 changed to isoleucine. Seven of 8 strains with MIC > or = 32 mg/L had mutations in parC. One of these strains showed a parC mutation at position 74 without any mutation in gyrA. Four strains had mexR and two strains nfxB mutations. The data indicate that gyrA mutation is the most important component of quinolone resistance, and simultaneous presence of parC mutations is associated with high-level resistance. parC mutation alone may contribute to resistance, and gyrA mutation may not be a prerequisite for parC mutation to express resistance. mexR and nfxB mutations were found mostly in strains with high-level resistance.

We have investigated the occurrence of mutations in topoisomerase II (DNA gyrase) subunit B(gyrB) and topoisomerase IV subunit E(parE) and the hyperexpression of genes for four efflux pump proteins in 20 previously described, fluoroquinolone-resistant clinical strains of Pseudomonas aeruginosa. Amino acid alterations were found in GyrB in five strains and in ParE in three strains with MIC of norfloxacin > or = 8 mg/L, and it is likely that some of the alterations contribute to the quinolone resistance exhibited by these strains. Seventeen of the 20 strains overproduced mRNA for one or more pump proteins (MexB, MexD, MexF, or MexY), which caused multidrug resistance phenotype in more than half of strains. Two strains were hypermutable and one of them was highly resistant, but the other strain was only moderately resistant.

This report describes the development of a real-time LightCycler assay for the detection and identification of Candida and Aspergillus spp., using the MagNa Pure LC Instrument for automated extraction of fungal DNA. The assay takes 5-6 h to perform. The oligonucleotide primers and probes used for species identification were derived from the DNA sequences of the 18S rRNA genes of various fungal pathogens. All samples were screened for Aspergillus and Candida to the genus level in the real-time PCR assay. If a sample was Candida-positive, typing to species level was performed using five species-specific probes. The assay detected and identified most of the clinically relevant Aspergillus and Candida spp. with a sensitivity of 2 CFU/mL blood. Amplification was 100% specific for all Aspergillus and Candida spp. tested. To assess clinical applicability, 1,650 consecutive samples (1,330 blood samples, 295 samples from other body fluids and 25 biopsy samples) from patients with suspected invasive fungal infections were analysed. In total, 114 (6.9%) samples were PCR-positive, 5.3% for Candida and 1.7% for Aspergillus spp. In patients with positive PCR results for Candida and Aspergillus, verification with conventional methods was possible in 83% and 50% of cases, respectively. In conclusion, the real-time PCR assay allows sensitive and specific detection and identification of fungal pathogens in vitro and in vivo.

The MexZ-MexX-MexY multidrug efflux system in Pseudomonas aeruginosa was studied to determine its contribution to aminoglycoside resistance. Amikacin-resistant (AR) mutants were generated from P. aeruginosa strain PAO1, and clinical isolates of P. aeruginosa were collected from cystic fibrosis patients. The regulatory gene mexZ and the intergenic region (mexOZ) between mexZ and mexX were investigated for mutation by PCR and DNA sequence analysis. The results showed that 14 of 15 AR clinical isolates and one of ten laboratory mutants had at least one mutation in mexZ and/or mexOZ. To study the effect of mexZ and mexOZ mutations, the production of MexY mRNA was investigated quantitatively by real-time PCR. Seven of ten AR mutants (MIC 4-8 mg/L) produced 8-21-fold more MexY mRNA than PAO1. These isolates were sensitive to fluoroquinolones, carbapenems and ceftazidime. One AR mutant (MIC 64 mg/L) that produced > 200-fold more MexY mRNA than PAO1 was also resistant to fluoroquinolones, carbapenems and ceftazidime. Thirteen of 15 AR clinical isolates produced 3.4-727-fold more MexY mRNA. No evidence was found for the aminoglycoside-modifying enzymes 6'-N-acetyltransferase type Ib, 4'-O-nucleotidyltransferase type IIb or aminoglycoside 3'-phosphotransferase IIps in these strains. Nine AR mutants overproduced MexY without mutations in mexZ or mexOZ, suggesting that MexXY efflux is also regulated by gene(s) other than mexZ.

Twenty P. aeruginosa isolates were collected from six cystic fibrosis (CF) patients, aged 27 to 33, in 1994 (9 isolates) and 1997 (11 isolates) at the CF Center, Copenhagen, Denmark, and were typed by pulse-field gel electrophoresis (PFGE) or ribotyping. Five of the patients had isolates with the same PFGE or ribotyping patterns in 1997 as in 1994, and ciprofloxacin had a two-to fourfold higher MIC for the isolates collected in 1997 than those from 1994. Genomic DNA was amplified for gyrA, parC, mexR, and nfxB by PCR and sequenced. Eleven isolates had mutations in gyrA, seven isolates had mutations at codon 83 (Thr to Ile), and four isolates had mutations at codon 87 (Asp to Asn or Tyr). Sixteen isolates had mutations in nfxB at codon 82 (Arg to Leu). Increased amounts of OprN were found in six isolates and OprJ in eight isolates as determined by immunoblotting. No isolates had mutations in parC or mexR. Six isolates had mutations in efflux pumps without gyrA mutations. The average number of mutations was higher in isolates from 1997 than in those from 1994. The results also suggested that efflux resistance mechanisms are more common in isolates from CF patients than in strains from urine and wounds from non-CF patients, in which mutations in gyrA and parC dominate (S. Jalal and B. Wretlind, Microb. Drug Resist. 4:257-261, 1998).Fluoroquinolones (FQ) are the only available antibiotics for oral treatment of Pseudomonas aeruginosa infections in most countries. However, P. aeruginosa easily becomes resistant to these drugs, which severely limits their usefulness. The main mechanisms of resistance are mutations in the target genes, those encoding DNA gyrase (gyrA) and topoisomerase IV (parC) (3, 9), and mutations in regulatory genes for drug efflux pumps. Three different multidrug efflux-pump systems have been identified, MexAB-OprM, MexCD-OprJ, and MexEFOprN, which are regulated by mexR (nalB), nfxB, and mexT (nfxC), respectively (4,8,17).In this study, 20 quinolone-resistant P. aeruginosa isolates from cystic fibrosis (CF) lungs have been analyzed for gyrA, parC, nfxB, and mexR genes and the expression of membrane proteins mexT (nfxC)-related OprN and nfxB-related OprJ associated with FQ resistance. Our aim was to determine molecular mechanisms for quinolone resistance of repeated isolates of P. aeruginosa from CF patients. MATERIALS AND METHODSBacterial strains. Twenty P. aeruginosa isolates were collected from six CF patients (ages, 27 to 33) with chronic P. aeruginosa infections (duration, 18 to 26 years) in 1994 (9 isolates) and 1997 (11 isolates) at the CF Center, Copenhagen, Denmark, and were typed by pulsed-field gel electrophoresis (PFGE) ( Table 1). However, four isolates (B1, B2, B3, and D1) could not be typed by PFGE, probably because of high DNase activities, and were subjected to ribotyping using Riboprinter (Qualicon, Wilmington, Del.). All patients had been exposed to repeated treatment periods with ciprofloxacin since 1986, and all of them had received at least 15 treatment courses (14 days/course)...

Mutations in the genes for the subunits GyrA and ParC of the target enzymes DNA gyrase and topoisomerase IV are important mechanisms of resistance in quinolone-resistant bacteria, including Neisseria gonorrhoeae. The target enzymes also consist of the subunits GyrB and ParE, respectively, though their role in quinolone-resistance has not been fully investigated. We sequenced the quinolone-resistance-determining regions (QRDR) of gyrA, gyrB, parC, and parE in 25 ciprofloxacin-resistant strains from Bangladesh (MIC 4-->32 mg/l) and 5 susceptible strains of N. gonorrhoeae. All the resistant strains had three or four mutations. Two of these were at positions 91 and 95 of gyrA. Fourteen strains had an additional mutation in parC at position 91, and 17 strains had an additional mutation in parE in position 439. No alterations were found in gyrB. The five susceptible strains had identical DNA sequences. Data indicate that the mutations detected in the QRDR of gyrA and parC may be important in the development of quinolone resistance. According to transformation experiments we assume that the alteration in parE is not related to a high degree of quinolone resistance. There was no correlation between ciprofloxacin MICs and pattern or number of mutations in the target genes.