Comparison of the activity of cefixime and activities of other oral antibiotics against adult clinical isolates of Moraxella (Branhamella) catarrhalis containing BRO-1 and BRO-2 and Haemophilus influenzae

Moraxella catarrhalis outer membrane vesicles carry β-lactamase and promote survival of Streptococcus pneumoniae and Haemophilus influenzae by inactivating amoxicillin

Moraxella catarrhalis is a common pathogen found in children with upper respiratory tract infections and in patients with chronic obstructive pulmonary disease during exacerbations. The bacterial species is often isolated together with Streptococcus pneumoniae and Haemophilus influenzae. Outer membrane vesicles (OMVs) are released by M. catarrhalis and contain phospholipids, adhesins, and immunomodulatory compounds such as lipooligosaccharide. We have recently shown that M. catarrhalis OMVs exist in patients upon nasopharyngeal colonization. As virtually all M. catarrhalis isolates are β-lactamase positive, the goal of this study was to investigate whether M. catarrhalis OMVs carry β-lactamase and to analyze if OMV consequently can prevent amoxicillin-induced killing. Recombinant β-lactamase was produced and antibodies were raised in rabbits. Transmission electron microscopy, flow cytometry, and Western blotting verified that OMVs carried β-lactamase. Moreover, enzyme assays revealed that M. catarrhalis OMVs contained active β-lactamase. OMVs (25 μg/ml) incubated with amoxicillin for 1 h completely hydrolyzed amoxicillin at concentrations up to 2.5 μg/ml. In functional experiments, preincubation of amoxicillin (10× MIC) with M. catarrhalis OMVs fully rescued amoxicillin-susceptible M. catarrhalis, S. pneumoniae, and type b or nontypeable H. influenzae from β-lactam-induced killing. Our results suggest that the presence of amoxicillin-resistant M. catarrhalis originating from β-lactamase-containing OMVs may pave the way for respiratory pathogens that by definition are susceptible to β-lactam antibiotics.

Susceptibility patterns for amoxicillin/clavulanate tests mimicking the licensed formulations and pharmacokinetic relationships: do the MIC obtained with 2:1 ratio testing accurately reflect activity against beta-lactamase-producing strains of Haemophilus influenzae and Moraxella catarrhalis?

Amoxicillin/clavulanate has recently undergone formulation changes (XR and ES-600) that represent 14:1 and 16:1 ratios of amoxicillin/clavulanate. These ratios greatly differ from the 2:1 ratio used in initial formulations and in vitro susceptibility testing. The objective of this study was to determine if the reference method using a 2:1 ratio accurately reflects the susceptibility to the various clinically used amoxicillin/clavulanate formulations and their respective serum concentration ratios. A collection of 330 Haemophilus influenzae strains (300 beta-lactamase-positive and 30 beta-lactamase-negative) and 40 Moraxella catarrhalis strains (30 beta-lactamase-positive and 10 beta-lactamase-negative) were tested by the broth microdilution method against eight amoxicillin/clavulanate combinations (4:1, 5:1, 7:1, 9:1, 14:1, and 16:1 ratios; 0.5 and 2 microg/mL fixed clavulanate concentrations) and the minimum inhibitory concentration (MIC) results were compared with those obtained with the reference 2:1 ratio testing. For the beta-lactamase-negative strains of both genera, there was no demonstrable change in the MIC values obtained for all ratios analyzed (2:1 to 16:1). For the beta-lactamase-positive strains of H. influenzae and M. catarrhalis, at ratios >or=4:1 there was a shift in the central tendency of the MIC scatterplot compared with the results of testing 2:1 ratio. As a result, there was a 2-fold dilution increase in the MIC(50) and MIC(90) values, most evident for H. influenzae and BRO-1-producing M. catarrhalis strains. For beta-lactamase-positive strains of H. influenzae, the shift resulted in a change in the interpretive result for 3 isolates (1.0%) from susceptible using the reference method (2:1 ratio) to resistant (8/4 microg/mL; very major error) at the 16:1 ratio. In addition, the number of isolates with MIC values at or 1 dilution lower than the breakpoint (4/2 microg/mL) increased from 5% at 2:1 ratio to 32-33% for ratios 14:1 and 16:1. Our results indicate that, for the beta-lactamase-positive strains of H. influenzae and M. catarrhalis, the results of the amoxicillin/clavulanate reference 2:1 ratio testing do not accurately represent all the currently licensed formulations. Pharmacokinetic/pharmacodynamic (PK/PD) target attainment might be compromised when higher amoxicillin/clavulanate ratios are used clinically. With a better understanding of PK/PD parameters, reevaluation of the amoxicillin/clavulanate in vitro susceptibility testing should be considered by the standardizing authorities to reflect the licensed formulations and accurately predict clinical outcomes.

Moraxella catarrhalis: clinical significance, antimicrobial susceptibility and BRO beta-lactamases

Moraxella catarrhalis is an important pathogen of humans. It is a common cause of respiratory infections, particularly otitis media in children and lower respiratory tract infections in the elderly. Colonisation of the upper respiratory tract appears to be associated with infection in many cases, although this association is not well understood. Nosocomial transmission is being increasingly documented and the emergence of this organism as a cause of bacteremia is of concern. The widespread production of a beta-lactamase enzyme renders Moraxella catarrhalis resistant to the penicillins. Cephalosporins and beta-lactamase inhibitor combinations are effective for treatment of beta-lactamase producers, and the organism remains nearly universally susceptible to the macrolides, fluoroquinolones, tetracyclines and the combination of trimethoprim and sulfamethoxazole. Two major beta-lactamase forms, BRO-1 and BRO-2, have been described on the basis of their isoelectric focusing patterns. The BRO-1 enzyme is found in the majority of beta-lactamase-producing isolates and confers a higher level of resistance to strains than BRO-2. The BRO enzymes are membrane associated and their production appears to be mediated by chromosomal determinants which are transmissible by an unknown mechanism. The origin of these novel proteins is unknown.

In vitro activity of trimethoprim alone compared with trimethoprim-sulfamethoxazole and other antimicrobials against bacterial species associated with upper respiratory tract infections

Trimethoprim-sulfamethoxazole has been used to treat various respiratory tract infections. Nevertheless, for many patients, intolerance of the sulfonamide component precludes use of this combination. This study examined the activity of trimethoprim alone in comparison to that of trimethoprim-sulfamethoxazole and other antimicrobials against bacterial species implicated in respiratory tract infections. For Haemophilus influenzae, minimal inhibitory concentrations of trimethoprim were equal to or one dilution greater than those of trimethoprim-sulfamethoxazole, with 56 of 58 strains inhibited by the former at < or = 0.25 microgram/ml. All oxacillin-susceptible Staphylococcus aureus and 96.7% of Streptococcus pyogenes were inhibited by trimethoprim < or = 2 micrograms/ml. In contrast, only 50% of Streptococcus pneumoniae were inhibited by this concentration of trimethoprim, whereas 93.3% were susceptible to the combination at < or = 2/38 micrograms/ml. All oxacillin-resistant S. aureus and all Moraxella catarrhalis were resistant to trimethoprim, although many of the former and all of the latter were susceptible to trimethoprim-sulfamethoxazole.

Nontypeable Haemophilus influenzae susceptibility: effect of inoculum size and beta-lactamase production

The activities of cefixime, cefpodoxime, cefprozil, cefuroxime, loracarbef, and amoxicillin/clavulanate against 72 clinical isolates of nontypeable Haemophilus influenzae were determined by using an agar dilution method. The effects of beta-lactamase production and bacterial inoculum size were investigated. All antimicrobials exhibited a significant inoculum effect, demonstrating the importance of accurately determining inoculum size in the performance of antimicrobial susceptibility testing of H. influenzae.

Cefuroxime axetil. A review of its antibacterial activity, pharmacokinetic properties and therapeutic efficacy

Cefuroxime axetil is an oral cephalosporin which is rapidly hydrolysed to the active parent compound, cefuroxime. Cefuroxime has a broad spectrum of in vitro antibacterial activity which encompasses methicillin-sensitive staphylococci and the common respiratory pathogens Streptococcus pneumoniae, Haemophilus influenzae, Moraxella (Branhamella) catarrhalis and group A beta-haemolytic streptococci. Cefuroxime has broad spectrum activity against the beta-lactamase positive respiratory pathogens H. influenzae and M. catarrhalis; it is also active against penicillin-susceptible and -intermediate strains of S. pneumoniae. In clinical trials, cefuroxime axetil (administered twice daily) has been evaluated in the treatment of upper and lower respiratory tract infections and has demonstrated similar efficacy to established antibacterial agents, including amoxicillin/clavulanic acid and cefaclor. Five days' treatment with cefuroxime axetil was recently shown to be as effective as 10 days' treatment with either cefuroxime axetil or amoxicillin/clavulanic acid in patients with acute otitis media or acute bronchitis. Cefuroxime axetil was at least as effective as phenoxymethylpenicillin (penicillin V) in the treatment of patients with group A beta-haemolytic streptococcal tonsillopharyngitis. A number of studies have evaluated the efficacy of cefuroxime axetil as the oral component of intravenous to oral sequential therapy in hospitalised patients with lower respiratory tract infection. In each study patients received parenteral cefuroxime for approximately 2 days followed by cefuroxime axetil for 5 to 10 days. In comparative studies, cefuroxime sequential therapy was as effective as amoxicillin/ clavulanic acid sequential therapy and full courses of parenteral cefuroxime, cefotiam or cefoperazone. Adults with urinary tract infections and skin infections were also effectively treated with cefuroxime axetil, as were adults and adolescents with early stage lyme disease. Cefuroxime axetil is associated with a low incidence of adverse events, with gastrointestinal disturbances being the most frequently observed. Thus, cefuroxime axetil is an effective and convenient treatment for a wide range of infections and may be considered a therapeutic option when empirical treatment of community-acquired infections is required. Moreover, given the promising results of several intravenous/oral sequential treatment studies, cefuroxime axetil may also become established as an oral component of sequential treatment regimens.

[beta-Lactamases of Branhamella catarrhalis and their phenotypic implications]

Of the 50 strains of beta-lactamase-producing Branhamella catarrhalis isolated at Saint Joseph's Hospital (Paris) that were studied, 94% produced BRO-1 type beta-lactamase and 6% produced the BRO-2 type. We examined the transfer of BRO-1 and BRO-2 genes and found that, among 7 donor strains producing BRO-1, all were able to transfer the gene for BRO-1 production by conjugation. Of the 4 donor strains producing BRO-2, 2 were able to transfer the gene for BRO-2 production by conjugation. Three BRO-1 beta-lactamase-producing transformants were obtained from total DNA extracted from 3 strains producing BRO-1. Plasmid bands were demonstrated in strains of B. catarrhalis, but no change in plasmid profiles was seen in beta-lactamase-positive recombinants, supporting previous studies that suggested the beta-lactamases are chromosomal. In vitro activity of oral beta-lactams was tested for 67 strains of B. catarrhalis (56 beta-lactamase-producing strains). Cefixime, cefpodoxime and the combination ampicillin-clavulanic acid were very active against the beta-lactamase-producing strains. BRO-1 beta-lactamase appears to affect the activity of cefaclor, cefuroxime and loracarbef. BRO-2 beta-lactamases have no effect on the activity of these cephalosporins. Cefixime and cefpodoxime seemed the least affected by beta-lactamase production.

Cefixime. A review of its therapeutic efficacy in lower respiratory tract infections

Cefixime is an orally active third generation cephalosporin with in vitro antibacterial activity against most important lower respiratory pathogens. The drug is active against Haemophilus influenzae, Moraxella catarrhalis and penicillin-susceptible Streptococcus pneumoniae but not Staphylococcus aureus. Cefixime has a long elimination half-life (3 hours compared with 0.5 hours for cefaclor and 1.5 hours for cefalexin), which allows once daily administration. Several trials have established the clinical efficacy of the drug in patients with lower respiratory tract infection (LRTI). In comparative studies cefixime had similar efficacy to amoxicillin +/- clavulanic acid, cefaclor, cefalexin, cefuroxime axetil and clarithromycin. Trials evaluating the efficacy of cefixime as the oral component of intravenous to oral switch therapy have produced promising preliminary results although further carefully designed trials are needed in this area. As with certain other drugs of its class, gastrointestinal disturbances are the most frequently reported adverse events in patients taking cefixime and cases of pseudomembranous colitis have been reported. Thus, cefixime is an effective treatment for mild to moderate LRTI and may have a role as the oral component of intravenous to oral switch therapy although further well designed studies are needed to confirm initial favourable results in this important emerging area of antibacterial therapy.

Current trends in bacterial respiratory pathogens

The relationship of virulence and antimicrobial susceptibility with morbidity due to bacterial respiratory pathogens is complex and evolving. Ultimately, decreasing the incidence of pneumonia due to bacterial pathogens will be dependent on successful preparation and distribution of effective vaccines. Until effective vaccines are widely available, control of a majority of respiratory infections will depend on promotion of rational therapeutic strategies. Though limited to a few specific serotypes and strains, changes in virulence of bacterial respiratory pathogens have been noted. Co-infections due to multiple respiratory pathogens may increase morbidity; however, the epidemiology of co-infections is not clear. Relationships between respiratory viruses and bacteria may exist that increase virulence of both agents, but information regarding these relationships awaits further investigation. Resistance of respiratory pathogens to the more commonly used antimicrobials, such as penicillin, erythromycin, chloramphenicol, and cotrimoxazole, is being documented globally with increasing frequency. The evolution of antimicrobic resistance, especially among strains of Streptococcus pneumoniae, the most common and deadly agent of lower respiratory tract infections, provides impetus to develop and promote effective pneumococcal vaccines and to search for new and effective antimicrobials.