Increasing genetic relatedness of ciprofloxacin-resistant Streptococcus pneumoniae isolated in Canada from 1997 to 2005

The genetic relatedness of ciprofloxacin-resistant Streptococcus pneumoniae isolates collected from 1997 to 2002 (n = 82) and 2003 to 2005 (n = 123) was compared by pulsed-field gel electrophoresis (PFGE). Increased genetic homogeneity among the isolates from 2003 to 2005 (cluster analysis; P < 0.001) appeared to be due to expansion of existing clonal groups and to introduction of new PFGE types.     

Antimicrobial susceptibility of Pseudomonas aeruginosa isolates obtained from patients in Canadian intensive care units as part of the Canadian National Intensive Care Unit study

The antimicrobial susceptibility profile of 419 clinical isolates of Pseudomonas aeruginosa obtained from intensive care unit patients was determined. Amikacin and piperacillin/tazobactam were the most active antimicrobials evaluated. Fifty isolates (11.9%) were resistant to antimicrobials from > or =3 classes. Ninety-six percent of multidrug-resistant (MDR) isolates remained fully susceptible to colistin (polymyxin E).     

Prevalence of Antimicrobial Resistance in Respiratory Tract Isolates of Streptococcus pneumoniae: Results of a Canadian National Surveillance Study

From October 1997 to November 1998, 1,180 respiratory tract isolates of Streptococcus pneumoniae were collected from 18 medical centers in 9 of the 10 Canadian provinces. Penicillin-intermediate and -resistant isolates occurred at rates of 14.8 and 6.4%, respectively, and these rates varied considerably by geographic region. Trimethoprim-sulfamethoxazole, tetracycline, and macrolide rates of nonsusceptibility were 12.2, 10.6, and 8.0 to 9.3%, respectively. The most potent agents studied were newer fluoroquinolones.     

Effect of subinhibitory antimicrobial concentrations (sub-MICs) on in-vitro bacterial adherence to uroepithelial cells.

Subinhibitory antimicrobial concentrations have been reported to alter the adherence of bacteria to uroepithelial cells. Most investigators assessing the influence of subinhibitory antimicrobial concentrations on bacterial adherence in the urinary tract have employed in-vitro techniques using voided uroepithelial cells. These cells are incubated with bacteria previously exposed to antimicrobials and adherence is assessed by light microscopy. Most investigators have studied urinary Escherichia coli isolates. beta-Lactams, tetracyclines, aminoglycosides, nitrofurantoin, chloramphenicol, quinolones, trimethoprim and sulphonamides have been studied in concentrations ranging from 1/32-1/2x MIC. The following effects of subinhibitory antimicrobial concentrations on bacterial adherence have been reported: penicillins consistently reduce bacterial adherence at concentrations 1/4-1/2x MIC; nitrofurantoin and chloramphenicol demonstrate variable effects on bacterial adherence at 1/4x MIC; tetracyclines, but not doxycycline, decrease adherence at high concentrations (1/4-1/2x MIC) and increase it at low concentrations (1/8-1/32x MIC); both trimethoprim and sulphonamides consistently decrease bacterial adherence at concentrations ranging from 1/32-1/2x MIC and 1/4-1/2x MIC, respectively, but it is unclear whether the combination of trimethoprim and a sulphonamide decreases bacterial adherence to a greater extent than either agent alone; aminoglycosides decrease adherence at 1/2x MIC; and quinolones decrease adherence at 1/4x MIC, with variable effects at 1/8x MIC and 1/16x MIC. Subinhibitory antimicrobial concentrations may exert their antiadhesive effects through suppression of formation and/or expression on the surface adhesin, the formation of functionally aberrant adhesins, or a direct effect on the bacterial surface. Presently, the clinical significance of the alterations in bacterial adherence to uroepithelial cells is not fully understood.     

Molecular characterization of fluoroquinolone resistant Streptococcus pneumoniae clinical isolates obtained from across Canada

There is little published data detailing fluoroquinolone resistance in clinical isolates of S. pneumoniae. The purpose of this study was to characterize the resistance mechanisms of 34 fluoroquinolone-resistant S. pneumoniae clinical isolates obtained from medical centers in 8 of 10 Canadian provinces between 1997 and 2000. The quinolone resistance determining regions of gyrA, parC, and parE from the isolates were sequenced. The isolates were evaluated for reserpine-sensitive efflux of ciprofloxacin and the new fluoroquinolones: gatifloxacin, gemifloxacin, levofloxacin and moxifloxacin. The isolates were typed using pulsed field gel electrophoresis. The majority of the isolates were genetically unrelated. Lower level fluoroquinolone resistance (ciprofloxacin MIC 4-8 microg/ml) was associated with amino acid substitutions in ParC, while higher level resistance (ciprofloxacin MIC > or = 16 microg/ml) was associated with amino acid substitutions in both ParC and GyrA. ParE substitutions were not associated with clinical resistance. Twelve of 34 (35%) isolates demonstrated reserpine-sensitive efflux of ciprofloxacin. Efflux alone conferred low level ciprofloxacin resistance in 3 isolates. Significant reserpine-sensitive efflux of the new fluoroquinolones was not observed.     

Human serum enhances the postantibiotic effect of fluoroquinolones against Staphylococcus aureus.

The postantibiotic effect (PAE) of fluoroquinolones against Staphylococcus aureus was determined in Mueller-Hinton broth and normal human serum. At both 4X and 10X the MIC, serum significantly increased the duration of the PAE in all strains tested (P less than 0.05). Reducing the pH of the serum from 7.9 to 7.2 had no effect on the PAE. Heat treating the serum (56 degrees C, 30 min) reduced the PAE of ciprofloxacin at 10X the MIC approximately 25% (P less than 0.05). The PAE of cloxacillin was reduced approximately 80% in serum, and PAE experiments with gentamicin and cephalexin produced findings similar to those obtained with the fluoroquinolones. Serum increased the MICs of ciprofloxacin and norfloxacin less than twofold and increased the MIC of pefloxacin approximately fourfold. We conclude that normal human serum considerably increases the PAE of fluoroquinolones against S. aureus.     

Development of multiple-antibiotic-resistant (Mar) mutants of Pseudomonas aeruginosa after serial exposure to fluoroquinolones.

Laboratory-derived fluoroquinolone-resistant mutants were created by serially passaging wild-type Pseudomonas aeruginosa on fluoroquinolone-containing agar to obtain high-level fluoroquinolone resistance (e.g., ciprofloxacin MIC of 1,024 micrograms/ml). With increases of 4- to 32-fold in MICs of fluoroquinolones, these organisms demonstrated (relative to wild-type) normal morphology, resistance to fluoroquinolones only, no change in fluoroquinolone uptake, and no change in lipopolysaccharide profiles or outer membrane protein profiles. Complementation with wild-type Escherichia coli gyrA restored fluoroquinolone susceptibility, suggesting that these were gyrA mutants. After 4- to 32-fold increases in fluoroquinolone MICs (with continued passage on fluoroquinolone-containing agar) isolates demonstrated altered morphology, a multiple-antibiotic-resistant (Mar) phenotype (including cross-resistance to beta-lactams, chloramphenicol, and tetracycline), reduced fluoroquinolone uptake and altered outer membrane proteins (reductions in the 25- and 38-kDa bands as well as several bands in the 43- to 66-kDa region). Complementation with wild-type E. coli gyrA partially reduced the level of fluoroquinolone resistance by approximately 8- to 32-fold, suggesting that these mutants displayed both gyrA and non-gyrA mutations.     

Antimicrobial resistance in respiratory tract Streptococcus pneumoniae isolates: results of the Canadian Respiratory Organism Susceptibility Study, 1997 to 2002

A total of 6,991 unique patient isolates of Streptococcus pneumoniae were collected from October 1997 to June 2002 from 25 medical centers in 9 of the 10 Canadian provinces. Among these isolates, 20.2% were penicillin nonsusceptible, with 14.6% being penicillin intermediate (MIC, 0.12 to 1 microg/ml) and 5.6% being penicillin resistant (MIC, > or =2 microg/ml). The proportion of high-level penicillin-resistant S. pneumoniae isolates increased from 2.4 to 13.8% over the last 3 years of the study, and the proportion of multidrug-resistant S. pneumoniae isolates increased from 2.7 to 8.8% over the 5-year period. Resistant rates (intermediate and resistant) among non-beta-lactam agents were as follows: macrolides, 9.6 to 9.9%; clindamycin, 3.8%; doxycycline, 5.5%; chloramphenicol, 3.9%; and trimethoprim-sulfamethoxazole, 19.0%. Rates of resistance to non-beta-lactam agents were higher among penicillin-resistant strains than among penicillin-susceptible strains. No resistance to vancomycin or linezolid was observed; however, 0.1% intermediate resistance to quinupristin-dalfopristin was observed. The rate of macrolide resistance (intermediate and resistant) increased from 7.9 to 11.1% over the 5 years. For the fluoroquinolones, the order of activity based on the MICs at which 50% of isolates are inhibited (MIC(50)s) and the MIC(90)s was gemifloxacin > clinafloxacin > trovafloxacin > moxifloxacin > grepafloxacin > gatifloxacin > levofloxacin > ciprofloxacin. The investigational compounds ABT-773 (MIC(90), 0.008 microg/ml), ABT-492 (MIC(90), 0.015 microg/ml), GAR-936 (tigecycline; MIC(90), 0.06 microg/ml), and BMS284756 (garenoxacin; MIC(90), 0.06 micro g/ml) displayed excellent activities. Despite decreases in the rates of antibiotic consumption in Canada over the 5-year period, the rates of both high-level penicillin-resistant and multidrug-resistant S. pneumoniae isolates are increasing in Canada.     

Chicken as reservoir for extraintestinal pathogenic Escherichia coli in humans, Canada

We previously described how retail meat, particularly chicken, might be a reservoir for extraintestinal pathogenic Escherichia coli (ExPEC) causing urinary tract infections (UTIs) in humans. To rule out retail beef and pork as potential reservoirs, we tested 320 additional E. coli isolates from these meats. Isolates from beef and pork were significantly less likely than those from chicken to be genetically related to isolates from humans with UTIs. We then tested whether the reservoir for ExPEC in humans could be food animals themselves by comparing geographically and temporally matched E. coli isolates from 475 humans with UTIs and from cecal contents of 349 slaughtered animals. We found genetic similarities between E. coli from animals in abattoirs, principally chickens, and ExPEC causing UTIs in humans. ExPEC transmission from food animals could be responsible for human infections, and chickens are the most probable reservoir.