Nafcillin sodium microbiology
Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Mohamed Moubarak, M.D. [2]
Microbiology
Penicillinase-resistant penicillins exert a bactericidal action against penicillin-susceptible microorganisms during the state of active multiplication. All penicillins inhibit the biosynthesis of the bacterial cell wall. The drugs in this class are highly resistant to inactivation by staphylococcal penicillinase and are active against penicillinase producing strains of Staphylococcus aureus. The penicillinase-resistant penicillins are active in vitro against a variety of other bacteria.
Susceptibility Test Methods
When available, the clinical microbiology laboratory should provide the results 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 the most effective antimicrobial.
Dilution Techniques
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 procedure based on dilution method1,2 (broth, agar or microdilution) or equivalent. It has been reported that determination of susceptibility or resistance of a microorganism to all penicillinase-resistant penicillins, including nafcillin, may be deduced by testing microorganisms against either oxacillin or cefoxitin.2 For this reason, routine dilution testing of nafcillin is not advised and susceptibility to nafcillin should be determined by dilution using standardized inoculum and concentrations of oxacillin according to the criteria in table 1.
Diffusion Techniques
Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure2,3 requires the use of standardized inoculum concentrations. It has been reported that determination of susceptibility or resistance of a microorganism to all penicillinase-resistant penicillins, including nafcillin, may be deduced by testing microorganisms against either oxacillin or cefoxitin.2 Disk diffusion results using cefoxitin have been shown to be more reproducible than those obtained with oxacillin,4,5 therefore cefoxitin is the preferred reagent for testing nafcillin susceptibility by diffusion. Reports from the laboratory providing results of the standard single-disk susceptibility test with a 30 microgram cefoxitin disk should be interpreted according to the following criteria in Table 2.[1]
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References
Adapted from the FDA Package Insert.