Mechanisms of antimicrobial resistance: their clinical relevance in the new millennium.

Antimicrobials show selective toxicity. Suitable targets for antimicrobials to act at include the bacterial cell wall, bacterial protein and folic acid synthesis, nucleic acid metabolism in bacteria and the bacterial cell membrane. Acquired antimicrobial resistance generally can be ascribed to one of five mechanisms. These are production of drug-inactivating enzymes, modification of an existing target, acquisition of a target by-pass system, reduced cell permeability and drug removal from the cell. Introduction of a new antimicrobial into clinical practice is usually followed by the rapid emergence of resistant strains of bacteria in some species that were initially susceptible. This has reduced the long-term therapeutic value of many antimicrobials. It used to be thought that antibacterial resistance was mainly a hospital problem but now it is also a major problem in the community. Organisms in which resistance is a particular problem in the community include members of the Enterobacteriaceae, including Salmonella spp. and Shigella spp., Mycobacterium tuberculosis, Streptococcus pneumoniae, Haemophilus influenzae and Neisseria gonorrhoeae. Multi-resistant Gram-negative rods, methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci are major causes of concern in the hospital setting. Prevalence of antibacterial resistance depends both on acquisition and spread. Decreasing inappropriate usage of antimicrobials should lessen the rate of acquisition, and spread can be minimised by sensible infection control measures.

Evaluation of azithromycin, trovafloxacin and grepafloxacin as prophylaxis for experimental murine melioidosis.

The efficacies of the azalide azithromycin and the fluoroquinolones trovafloxacin and grepafloxacin for pre- and post-exposure prophylaxis of infection with high or low challenge doses of Burkholderia pseudomallei strain 576 were assessed in an experimental mouse model. Trovafloxacin and grepafloxacin afforded significant levels of protection, whereas azithromycin was ineffective and potentially detrimental. Overall, the data suggest that some fluoroquinolones may have potential utility in prophylaxis of melioidosis and suggest that azithromycin would not be effective in prophylaxis of B. pseudomallei infection.

Evaluation of azithromycin, trovafloxacin and grepafloxacin as prophylaxis against experimental murine Brucella melitensis infection.

The prophylactic potential of the azalide azithromycin as well as the fluoroquinolones trovafloxacin and grepafloxacin was assessed for the control of infection with Brucella melitensis in an experimental mouse model, determined by reduction in splenic bacterial burden. Trovafloxacin showed limited protective efficacy when administered 2h following a low-dose B. melitensis challenge, whereas grepafloxacin was ineffective. In comparison, azithromycin provided significant control of infection both following low- and high-dose challenges. Overall, the data confirm the potential utility of azithromycin in the prophylaxis of brucellosis and suggest that neither trovafloxacin nor grepafloxacin would likely be valuable for post-exposure prophylaxis of Brucella infection.

High throughput identification of clinical isolates of Staphylococcus aureus using MALDI-TOF-MS of intact cells.

Staphylococcus aureus remains an important human pathogen responsible for a high burden of disease in healthcare and community settings. The emergence of multidrug-resistant strains is of increasing concern world-wide. The identification of S. aureus is currently based upon phenotypic and genotypic methods. Here, an alternative approach involving mass spectral analysis of surface-associated proteins of intact bacterial cells by matrix-assisted laser desorption/ionisation time of flight mass spectrometry (MALDI-TOF-MS) was investigated using 95 isolates obtained directly from a clinical laboratory at The Royal London Hospital and 39 isolates from the Staphylococcal Reference Unit, Health Protection Agency, London. Results obtained indicate that clinical isolates share many common mass ions with-type/reference strains which allowed their correct identification when searched against a comprehensive database that has been in the process of development for several years. The existing database contains more than 5000 profiles of various bacterial pathogens, but comprises mainly type or reference strains. The MicrobeLynx software successfully identified all isolates to the correct genus and all but four to the correct species. These were misidentified in the first instance due to contamination or low mass ion intensity but once the cultures were purified and re-analysed they were confirmed as S. aureus by both MALDI-TOF-MS and 16S rRNA sequence analysis. The high percentage of correct identifications coupled with the high speed and the minimal sample preparation required, indicate that MALDI-TOF-MS has the potential to perform high throughput identification of clinical isolates of S. aureus despite the inherent diversity of this species. The method is, however, only reproducible if variable parameters such as sample preparation, media, growth condition, etc. are standardised.

Properties and potential of ertapenem.

Ertapenem is a carbapenem that shares the activity of imipenem and meropenem against most species, but is less active against non-fermenters. Activity is retained against most strains with AmpC and extended-spectrum beta-lactamases, although resistance can arise if these enzymes are combined with extreme impermeability. Resistance can also be caused by IMP, VIM, KPC and NMC carbapenemases, but again, co-requires impermeability. Although the spread of carbapenemases in the future is a concern, they are currently very rare. Given as a 1 g intravenous (iv) infusion once daily, ertapenem has a plasma half-life of approximately 4 h in healthy volunteers, and a Cmax of 155 mg/L and 13 mg/L for total and free drug, respectively. Excretion is largely renal, divided equally between native drug and an open-ring derivative. Trials show equivalence to piperacillin/tazobactam or ceftriaxone in (a) intra-abdominal infections, (b) community-acquired pneumonia, (c) acute pelvic infections, (d) skin and skin structure infections and (e) complicated urinary tract infections. The USA licence grants all these five indications; the EU licence grants the first three. Further potential uses include home iv therapy, directed therapy against Enterobacteriaceae with AmpC or extended-spectrum cephalosporinases, and tentatively, surgical prophylaxis. Widening the usage of carbapenems raises public health concerns, somewhat allayed by the continued rarity of carbapenemases after 17 years of imipenem use, and by the fact that carbapenemases occur mostly in non-fermenters outside the spectrum of ertapenem, and co-require impermeability to confer resistance in Enterobacteriaceae. Nevertheless, if ertapenem is to be used widely, its effects on the resistance ecology need to be monitored carefully.