As with other cephalosporins, the bactericidal action of cefixime results from inhibition of cell wall synthesis. Cefixime is stable in the presence of certain beta-lactamase enzymes. As a result, certain organisms resistant to penicillins and some cephalosporins due to the presence of beta-lactamases may be susceptible to cefixime.
Resistance to cefixime in isolates of Haemophilus influenzae and Neisseria gonorrhoeae is most often associated with alterations in penicillin-binding proteins (PBPs). Cefixime may have limited activity against Enterobacteriaceae producing extended spectrum beta-lactamases (ESBLs). Pseudomonas species, Enterococcus species, strains of Group D streptococci, Listeria monocytogenes, most strains of staphylococci (including methicillin-resistant strains), most strains of Enterobacter species, most strains of Bacteroides fragilis, and most strains of Clostridium species are resistant to cefixime.
Cefixime has been shown to be active against most isolates of the following microorganisms, both in vitro and in clinical infections [see INDICATIONS AND USAGE (1)}.
The following in vitro data are available, but their clinical significance is unknown. At least 90 percent of the following bacteria exhibit an in vitro minimum inhibitory concentration (MIC) less than or equal to the susceptible breakpoint for cefixime against isolates of similar genus or organism group. However, the efficacy of cefixime in treating clinical infections due to these bacteria has not been established in adequate and well-controlled clinical trials.
Susceptibility Test Methods
When available, the clinical microbiology laboratory should provide cumulative reports of in vitro susceptibility test results for antimicrobial drugs used in local hospitals and practice areas to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting an antibacterial drug for treatment.
Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized test method1,2 (broth and/or agar). The MIC values should be interpreted according to criteria provided in Table 3.
Quantitative methods that require measurement of zone diameters can also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. The zone size should be determined using a standardized test method.2.3 This procedure uses paper disks impregnated with 5 mcg cefixime to test the susceptibility of bacteria to cefixime. The disc diffusion breakpoints are provided in Table 3.
|Pathogen||Minimum Inhibitory Concentrations ( mcg / mL )||Disk Diffusion Zone Diameter ( mm )|
|Enterobacteriaceae *||≤ 1||2||≥ 4||≥ 19||16 to 18||≤ 15|
|Haemophilus influenzae † . ‡||≤1||NA||NA||≥21||NA||NA|
|Neisseria gonorrhoeae ‡ , §||≤ 0.25||NA||NA||≥ 31||NA||NA|
A report of Susceptible (S) indicates that the antimicrobial drug is likely to inhibit growth of the pathogen if the antimicrobial drug reaches the concentration usually achievable at the site of infection. A report of Intermediate (I) indicates that the result should be considered equivocal, and, if the microorganism is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated or in situations where a high dosage of the drug can be used. This category also provides a buffer zone that prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of Resistant (R) indicates that the antimicrobial drug is not likely to inhibit growth of the pathogen if the antimicrobial drug reaches the concentration usually achievable at the infection site; other therapy should be selected.
Standardized susceptibility test procedures require the use of laboratory controls to monitor and ensure the accuracy and precision of supplies and reagents used in the assay, and the techniques of the individuals performing the test.1,2,3 Standard cefixime powder should provide the following range of MIC values noted in Table 4. For the diffusion technique using the 5 mcg disk, the criteria in Table 4 should be achieved.
ATCC = American Type Culture Collection
|Quality Control Organisms||Minimum Inhibitory Concentrations ( mcg / mL )||Disk Diffusion Zone Diameter ( mm )|
|E . coli ATCC 25922||0.25 to 1||23 to 27|
|H . influenzae ATCC 49247||0.12 to 1||25 to 33|
|N . gonorrhoeae ATCC 49226||0.004 to 0.03||37 to 45|
|S . pneumoniae ATCC 49619||NA||16 to 23|
|S . aureus ATCC 29213||8 to 32||NA|
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