Comparison of the inoculum effect of cefoxitin and other cephalosporins and of beta-lactamase inhibitors and their penicillin-derived components on the Bacteroides fragilis group

The impact of inoculum size on the activity of cefoperazone-sulbactam against multidrug resistant organisms

OBJECTIVES

This study aims to assess the in vitro activity of cefoperazone alone and different cefoperazone-sulbactam ratios against different inoculum sizes of multidrug resistant organisms.

METHODS

Minimum inhibitory concentrations (MICs) of cefoperazone, cefoperazone-sulbactam at fixed ratio of 1:1 and 2:1 against a normal inoculum size of 5 × 10⁵ CFU/ml and a high inoculum size of 5 × 10⁷ CFU/ml were measured.

RESULTS

Each 33 isolates of extended-spectrum β-lactamases (ESBL)-producing Escherichia coli, ESBL-producing Klebsiella pneumoniae, carbapenem-resistant E. coli, and carbapenem-resistant Pseudomonas aeruginosa and a total of 122 isolates of carbapenem-resistant Acinetobacter baumannii were collected. After the addition of sulbactam at a 1:1 ratio, most MIC₅₀ and MIC₉₀ values decreased. Cefoperazone-sulbactam at a 1:1 ratio had a higher susceptibility rate against ESBL-producing E. coli, carbapenem-resistant E. coli, and carbapenem-resistant A. baumannii than cefoperazone-sulbactam at a 2:1 ratio (all P < 0.05). For ESBL-producing E. coli, the susceptibility rate of cefoperazone-sulbactam at ratios of (1:1) and (2:1) decreased from 97.0 to 87.9% and 90.9 to 60.6%, for normal to high inoculum, respectively. For ESBL-producing K. pneumoniae, both susceptibility rate of cefoperazone-sulbactam at ratios of (1:1) and (2:1) decreased from 75.8%, and 63.6% at normal inoculum to 51.5% and 42.4% at high inoculum.

CONCLUSIONS

Cefoperazone-sulbactam at a 1:1 ratio has greater in vitro activity against most multidrug resistant organisms than cefoperazone-sulbactam at a 2:1 ratio. Such combinations were not influenced by the inoculum size of ESBL-producing E. coli and K. pneumoniae and could be a therapeutic option for treating severe infections.

In vitro and in vivo activities of piperacillin-tazobactam and meropenem at different inoculum sizes of ESBL-producing Klebsiella pneumoniae

The inoculum effect is a laboratory phenomenon in which the minimal inhibitory concentration (MIC) of an antibiotic is increased when a large number of organisms are exposed. Due to the emergence of extended-spectrum β-lactamase-producing Klebsiella pneumoniae (ESBL-Kpn) infections, the inoculum effect of ESBL-Kpn on β-lactams was studied in vitro and in vivo using an experimental model of pneumonia. The in vitro inoculum effect of 45 clinical ESBL-Kpn isolates on β-lactams was evaluated at standard (10(5) CFU/mL) and high (10(7) CFU/mL) organism concentrations. The MIC50 of piperacillin-tazobactam, cefotaxime and cefepime was increased eight-fold or more and that of meropenem was increased two-fold. The in vivo inoculum effect was evaluated in an ESBL-Kpn pneumonia mouse model treated with bacteriostatic effect-adjusted doses of piperacillin-tazobactam (1000 mg/kg four times daily, %T>MIC; 32.60%) or meropenem (100 mg/kg twice daily, %T>MIC; 28.65%) at low/standard (10(4) CFU/mouse) and high (10(6) CFU/mouse) inocula. In mice administered a low inoculum, no mice died after treatment with piperacillin-tazobactam or meropenem, whereas all the control mice died. In contrast, in the high inoculum model, all mice in the piperacillin-tazobactam-treated group died, whereas all meropenem-treated mice survived and had a decreased bacterial load in the lungs and no invasion into the blood. In conclusion, meropenem was more resistant to the inoculum effect of ESBL-Kpn than piperacillin-tazobactam both in vitro and in vivo. In the management of severe pneumonia caused by ESBL-Kpn, carbapenems may be the drugs of choice to achieve a successful outcome.

Correlation between in vitro and in vivo activities of GM 237354, a new sordarin derivative, against Candida albicans in an in vitro pharmacokinetic-pharmacodynamic model and influence of protein binding

The antifungal effect of GM 237354, a sordarin derivative, was studied in an in vitro pharmacokinetic (PK)-pharmacodynamic dynamic system (bioreactor) which reproduces PK profiles observed in a previously described model of drug efficacy against murine systemic candidiasis. Immunocompetent mice infected intravenously with 10(5) CFU of Candida albicans were treated with GM 237354 at 2.5, 10, and 40 mg/kg of body weight every 8 h subcutaneously for 7 days. Free concentrations in serum were calculated by multiplying total concentrations measured in vivo by 0.05, the free fraction determined in vitro by equilibrium dialysis. In the bioreactor the inoculum was approximately 10(6) CFU/ml; and a one-compartment PK model was used to reproduce the PK profiles of free and total GM 237354 in serum obtained in mice, and clearance of C. albicans was measured over 48 h. A good correlation was observed when the in vivo fungal kidney burden and the area under the survival time curve were compared with the in vitro broth "burden," although only when free in vivo levels in serum were reproduced in vitro. GM 237354 displayed a 3-log decrease effect both in vivo and in vitro. The very few reports available on in vitro-in vivo correlations have been obtained with antibiotics. The good in vitro-in vivo correlation obtained with an antifungal agent shows that the in vitro dynamic system could constitute a powerful investigational tool prior to assessment of the efficacy of an anti-infective agent in animals and humans.

Susceptibility testing of Clostridium difficile against metronidazole and vancomycin by disk diffusion and Etest

A prospective study on the susceptibility of Clostridium difficile to metronidazole and vancomycin using the Etest and disk diffusion test was performed over a 6-month period. One hundred strains were tested; one strain was highly resistant to metronidazole (MIC = 64 micrograms/mL). The zone size of inhibition by the disk diffusion test correlated with the MIC as determined by the Etest (regression coefficient = -0.043 for metronidazole and -0.044 for vancomycin, p < 0.001 for both antibiotics). However, the correlation coefficient was low for both metronidazole (r = 0.574) and vancomycin (r = 0.473); hence the zone of inhibition by disk diffusion test could not predict the MIC satisfactorily. Metronidazole is still the first-line antibiotic for the treatment of C. difficile-associated diarrhea because the incidence of metronidazole resistant strains remains very low. However, the efficacy of metronidazole in the treatment of infections attributable to isolates with high-level metronidazole resistance may be compromised because the fecal concentration of metronidazole is relatively low when compared with the MIC values of the less susceptible strains. Oral vancomycin is the drug of choice under such circumstances, as its fecal concentration is much higher than that of metronidazole.

Beta-lactamase inhibitor combinations

The beta-lactamase inhibitor combinations present a novel approach to the problem of beta-lactamase-induced resistance to antibiotics. These agents are derived from the generally safe beta-lactam class of antibiotics. They are all principally excreted through the kidneys and require dosage adjustment in the face of significant renal insufficiency. They show significantly increased activity against methicillin-sensitive S. aureus, H. influenzae, B. catarrhalis, and B. fragilis. The activity against Enterococcus, Pseudomonas, and most gram-negative bacilli, however, remains limited to that of the respective antibiotic component. Although shown to be clinically useful in a variety of clinical situations, they appear to be most useful for skin and soft tissue infections and lower respiratory tract infections as well as intra-abdominal and gynecologic infections caused by susceptible pathogens.

Piperacillin/tazobactam. A review of its antibacterial activity, pharmacokinetic properties and therapeutic potential

Combining tazobactam, a beta-lactamase inhibitor, with the ureidopenicillin, piperacillin, successfully restores the activity of piperacillin against beta-lactamase-producing bacteria. Tazobactam has inhibitory activity, and therefore protects piperacillin against Richmond and Sykes types II, III, IV and V beta-lactamases, staphylococcal penicillinase and extended-spectrum beta-lactamases. However, tazobactam has only species-specific activity against class I chromosomally-mediated enzymes. Resistant organisms include some Citrobacter spp., Enterobacter spp., Serratia spp., Xanthomonas maltophilia and Enterococcus faecium. Consistent with its in vitro activity, preliminary clinical data indicate that the fixed combination of piperacillin/tazobactam (dose ratio 8:1) is effective in the treatment of moderate to severe polymicrobial infections, including intra-abdominal, skin and soft-tissue and lower respiratory tract infections. In limited comparative trials, piperacillin/tazobactam demonstrated equivalent or better efficacy than standard comparator regimens in these infections. Piperacillin/tazobactam in combination with an aminoglycoside was effective in the empirical treatment of fever in patients with neutropenia and compared favourably with ceftazidime in combination with an aminoglycoside, although second-line therapy with a glycopeptide antibiotic may be indicated in unresponsive episodes. Data from phase III trials indicate that piperacillin/tazobactam has a tolerability profile typical of a penicillin agent. Piperacillin/tazobactam provides a broad spectrum of antibacterial activity in a convenient single formulation suitable for use in the treatment of polymicrobial infections. Possible limitations concern its restricted activity against class I beta-lactamases, enzymes that are becoming increasingly important in the nosocomial environment. Combined therapy with an aminoglycoside may be necessary in more serious infections.

In vitro activity of FK037, a new parenteral cephalosporin, against anaerobic bacteria

The activity of FK037, a new parenteral cephalosporin, was compared with those of cefpirome, ceftazidime, and flomoxef against 322 recent clinical isolates of anaerobic bacteria. A fastidious facultative anaerobe, Gardnerella vaginalis, was also studied. FK037 inhibited 90% of isolates of Peptostreptococcus anaerobius, Peptostreptococcus asaccharolyticus, Clostridium perfringens, Mobiluncus spp., G. vaginalis, and Porphyromonas gingivalis at < or = 0.78 micrograms/ml. The MICs of FK037 for 50 and 90% of Bacteroides fragilis isolates were 25 and > 200 micrograms/ml, respectively; the activity of FK037 was comparable to those of cefpirome and ceftazidime but lower than that of flomoxef. The activity of FK037 against Fusobacterium nucleatum, Fusobacterium varium, and Bilophila wadsworthia decreased when inoculum size was increased from 10(6) to 10(8) CFU/ml. Little influence of inoculum size on the activity of FK037 was observed for other isolates tested. Medium pH affected the activity of FK037 against F. varium (MICs at pHs 5 and 7, 3.13 and 100 micrograms/ml, respectively) and Bacteroides gracilis (MICs at pHs 5 and 7, 12.5 and 1.56 micrograms/ml, respectively) but not against other organisms tested. FK037 was less resistant than flomoxef to hydrolysis by beta-lactamase group 2e derived from B. fragilis GAI 0558 and GAI 10150.