Rifampin isoniazid pyrazinamide microbiology

# Microbiology

## Mechanism of Action

### Rifampin
Rifampin inhibits DNA-dependent RNA polymerase activity in susceptible Mycobacterium tuberculosis organisms. Specifically, it interacts with bacterial RNA polymerase, but does not inhibit the mammalian enzyme.

### Isoniazid
Isoniazid inhibits the biosynthesis of mycolic acids which are major components of the cell wall of Mycobacterium tuberculosis.

### Pyrazinamide
The exact mechanism of action by which pyrazinamide inhibits the growth of Mycobacterium tuberculosis organisms is unknown.

## Drug Resistance
Organisms resistant to rifampin are likely to be resistant to other rifamycins. -lactamase production should have no effect on rifampin activity.
In the treatment of tuberculosis, the small number of resistant cells present within large populations of susceptible cells can rapidly become predominant. In addition, resistance to rifampin has been determined to occur as single-step mutations of the DNA-dependent RNA polymerase. Since resistance can emerge rapidly, appropriate susceptibility tests should be performed in the event of persistent positive cultures.

## Activity in vitro and in vivo
Rifampin, isoniazid, and pyrazinamide at therapeutic levels have demonstrated bactericidal activity against both intracellular and extracellular Mycobacterium tuberculosis organisms (see Indications And Usage).
Pyrazinamide alone is only active at a slightly acidic pH (pH 5.5) in vitro and in vivo. Isoniazid kills actively growing tubercle bacilli.

## Susceptibility Testing
Prior to initiation of therapy, appropriate specimens should be collected for identification of the infecting organism and in vitro susceptibility tests.
In vitro testing for Mycobacterium tuberculosis isolates:
Two standardized in vitro susceptibility methods are available for testing isoniazid, rifampin, and pyrazinamide against Mycobacterium tuberculosis organisms. The agar proportion method (CDC or CLSI M24-A) utilizes Middlebrook 7H10 medium impregnated with isoniazid at 0.2 and 1.0 mcg/mL and rifampin at 1.0 mcg/mL for the final concentrations of drug. The final concentration for pyrazinamide is 25.0 mcg/mL at pH 5.5. After 3 weeks of incubation MIC99 values are calculated by comparing the quantity of organisms growing in the medium containing drug to the control cultures. Mycobacterial growth in the presence of drug 1% of the control indicates resistance.
The radiometric broth method employs the BACTEC 460 machine to compare the growth index from untreated control cultures to cultures grown in the presence of 0.2 and 1.0 mcg/mL of isoniazid and 2.0 mcg/mL of rifampin. Strict adherence to the manufacturer's instructions for sample processing and data interpretation is required for this assay. The radiometric broth method has not been approved for the testing of pyrazinamide.
Susceptibility test results obtained by the two different methods can only be compared if the appropriate rifampin or isoniazid concentrations are used for each test method as indicated above. Both test procedures require the use of Mycobacterium tuberculosis H37Rv, ATCC 27294, as a control organism.
The clinical relevance of in vitro susceptibility test results for mycobacterial species other than Mycobacterium tuberculosis using either the radiometric broth method or the proportion method has not been determined.