Fidaxomicin microbiology

# Microbiology

## Spectrum of Activity
Fidaxomicin is a fermentation product obtained from the Actinomycete Dactylosporangium aurantiacum. In vitro, fidaxomicin is active primarily against species of clostridia, including Clostridium difficile.

## Mechanism of Action
Fidaxomicin is bactericidal against C. difficile in vitro, inhibiting RNA synthesis by RNA polymerases.

## Mechanism of Decreased Susceptibility to Fidaxomicin
In vitro studies indicate a low frequency of spontaneous resistance to fidaxomicin in C. difficile (ranging from <1.4  10-9 to 12.8  10-9). A specific mutation (Val-ll43-Gly) in the beta subunit of RNA polymerase is associated with reduced susceptibility to fidaxomicin. This mutation was created in the laboratory and seen during clinical trials in a C. difficile isolate obtained from a subject treated with DIFICID who had recurrence of CDAD. The C. difficile isolate from the treated subject went from a fidaxomicin baseline minimal inhibitory concentration (MIC) of 0.06 g/mL to 16 g/mL.

## Cross-Resistance/Synergy/Post-Antibiotic Effect
Fidaxomicin demonstrates no in vitro cross-resistance with other classes of antibacterial drugs. Fidaxomicin and its main metabolite OP-1118 do not exhibit any antagonistic interaction with other classes of antibacterial drugs. In vitro synergistic interactions of fidaxomicin and OP-1118 have been observed in vitro with rifampin and rifaximin against C. difficile (FIC values 0.5). Fidaxomicin demonstrates a post-antibiotic effect vs. C. difficile of 6-10 hrs.

## Susceptibility Testing
The clinical microbiology laboratory should provide cumulative results of the 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 appropriate antimicrobial drug therapy.

## Dilution Techniques
Quantitative anaerobic in vitro methods can be used to determine the MIC of fidaxomicin needed to inhibit the growth of the C. difficile isolates. The MIC provides an estimate of the susceptibility of C. difficile isolate to fidaxomicin. The MIC should be determined using standardized procedures.1 Standardized methods are based on an agar dilution method or equivalent with standardized inoculum concentrations and standardized concentration of fidaxomicin powder.

## Susceptibility Test Interpretive Criteria
In vitro susceptibility test interpretive criteria for fidaxomicin have not been determined. The relation of the in vitro fidaxomicin MIC to clinical efficacy of fidaxomicin against C. difficile isolates can be monitored using in vitro susceptibility results obtained from standardized anaerobe susceptibility testing methods.

## Quality Control Parameters for Susceptibility Testing
In vitro susceptibility test quality control parameters were developed for fidaxomicin so that laboratories determining the susceptibility of C. difficile isolates to Fidaxomicin can ascertain whether the susceptibility test is performing correctly. Standardized dilution techniques require the use of laboratory control microorganisms to monitor the technical aspects of the laboratory procedures. Standardized fidaxomicin powder should provide the MIC with the indicated quality control strain shown in Table 4.