Diagnostic update - Rifampicin
Animal Dermatology Clinic Perth Diagnostic Update
Rifampicin mechanism of action: rifampicin inhibits bacterial DNA dependent RNA polymerase, leading to bactericidal activity against staphylococcal spp. and mycobacterial spp. Due to the lipophilic nature of rifampicin, it readily concentrates in the bone, immune cells, CSF, placenta, prostate and abscesses. Previous studies have demonstrated clinical resistance of certain organisms develops more rapidly to rifampicin compared to other drugs.
Study Objective: to determine the MIC of rifampicin in methicillin resistant S.pseudintermedius isolates and to determine in vitro whether the efficacy of rifampicin towards S.pseudintermedius is time dependent or concentration dependent.
Hypothesis: methicillin resistant S.pseudintermedius isolates would be more resistant to rifampicin that methicillin susceptible S.pseudintermedius isolates.
Materials and Methods:
1.) Establishing the MIC of rifampicin: One hundred canine S.pseudintermedius isolates (50 methicillin resistant (MR) and 50 methicillin susceptible (MS)) were obtained from archived isolates collected from skin, joint, blood and urine specimens. Isolates were collected between 2006 and 2016. MR was determined by either oxacillin broth microdilution or oxacillin disk diffusion. The oxacillin breakpoints used to confirm MRSP was MIC >/- 0.5ug/mL and a disk diffusion zone of </- 17mm. The MIC of rifampicin for each isolate was determined by using the ETEST. The MIC was defined as the lowest drug concentration at which the border of the elliptical inhibition zone intercepted the concentration scale on the ETEST strip. Isolates were considered susceptible if MIC was </= to 1ug/mL and resistant if MIC was >/= 4ug/mL.
2.) Time Kill Studies
Two MS isolates and two MR isolates were selected from skin tissue with the lowest and highest rifampicin MIC as determined by the ETEST. Cultures of each S.pseudintermedius isolate was grown in pure culture on Tryptose agar at 37° for 18-24hrs. The growth response was considered concentration dependent if the magnitude of decrease in colony forming units (CFU) was a function of antimicrobial concentration. If not, the response was considered time dependent.
Results:
MIC of rifampicin against S.pseudintermedius isolates: The MIC50 (the MIC required to inhibit 50% of the organisms) was 0.004ug/L. The MIC90 was 0.008ug/L.
Time kill studies: the response of all four S.pseudintermedius isolates to rifampicin was consistent with a time dependent antibiotic because the magnitude of decrease in CFU over 24hrs was unchanged regardless of antimicrobial concentration. The methicillin susceptible isolates exhibited bacteriostatic activity whereas the methicillin resistant isolates exhibited bactericidal activity
Development of rifampicin resistance: Viable bacteria were recovered from all 168 aliquot samples used in the time-kill study. Despite this, only three strains were found to be resistant to rifampicin.
Conclusions:
Rifampicin is a time dependent antibiotic that may be bacteriostatic (for MS) or bactericidal (for MR)
The MIC50 of rifampicin is 0.004ug/mL
The MIC90 of rifampicin is 0.008ug/mL
Both these MIC’s are considered low; and organisms should be considered very susceptible, based on the human breakpoint of 4ug/mL.
Rifampicin should not be used as a sole agent to target Staphylococcal spp. due to the rapid emergence of resistance compared to other drugs. It should only be used when indicated by culture and susceptibility testing and where there is multi drug resistance.
Rifampicin should not be used as a sole antibiotic; it should be used as combination therapy
There is the potential for adverse drug reactions when certain medications are used concurrently because rifampicin is a potent inducer of cytochrome P450.
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