Quinupristin dalfopristin microbiology

# Microbiology
The streptogramin components of Synercid, quinupristin and dalfopristin, are present in a ratio of 30 parts quinupristin to 70 parts dalfopristin. These two components act synergistically so that Synercid's microbiologic in vitro activity is greater than that of the components individually. Quinupristin's and dalfopristin's metabolites also contribute to the antimicrobial activity of Synercid. In vitro synergism of the major metabolites with the complementary parent compound has been demonstrated.

## Mechanism of Action
The site of action of quinupristin and dalfopristin is the bacterial ribosome. Dalfopristin has been shown to inhibit the early phase of protein synthesis while quinupristin inhibits the late phase of protein synthesis. Synercid is bactericidal against isolates of methicillin-susceptible and methicillin-resistant staphylococci. The mode of action of Synercid differs from that of other classes of antibacterial agents such as -lactams, aminoglycosides, glycopeptides, quinolones, macrolides, lincosamides and tetracyclines. Therefore, there is no cross resistance between Synercid and these agents when tested by the minimum inhibitory concentration (MIC) method.

## Mechanism of Resistance
Resistance to Synercid is associated with resistance to both components (i.e., quinupristin and dalfopristin). In non-comparative studies, emerging resistance to Synercid has occurred.

## Interaction with other Antibacterials
In vitro combination testing of Synercid with aztreonam, cefotaxime, ciprofloxacin, and gentamicin against Enterobacteriaceae and Pseudomonas aeruginosa did not show antagonism.
In vitro combination testing of Synercid with prototype drugs of the following classes: aminoglycosides (gentamicin), -lactams (cefepime, ampicillin, and amoxicillin), glycopeptides (vancomycin), quinolones (ciprofloxacin), tetracyclines (doxycycline) and also chloramphenicol against enterococci and staphylococci did not show antagonism.
Synercid has been shown to be active against most isolates of the following bacteria, both in vitro and in clinical infections, as described in the Indications And Usage section.

### Gram-positive bacteria
- Staphylococcus aureus (methicillin-susceptible isolates only)
- Streptococcus pyogenes
The following in vitro data are available, but their clinical significance is unknown.
The combination of quinupristin and dalfopristin (Synercid) exhibits in vitro minimum inhibitory concentrations (MIC's) of 1.0 g/mL against most (90%) isolates of the following microorganisms; however, the safety and effectiveness of Synercid in treating clinical infections due to these bacteria have not been established in adequate and well-controlled clinical trials.

### Gram-positive bacteria
- Corynebacterium jeikeium
- Staphylococcus aureus (methicillin-resistant isolates)
- Staphylococcus epidermidis (including methicillin-resistant isolates)
- Streptococcus agalactiae

## Susceptibility Test Methods

### 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. Standardized procedures are based on a dilution1 method (broth or agar) or equivalent using standardized inoculum concentrations, and standardized concentrations of quinupristin/dalfopristin (Synercid) in a 30:70 ratio made from powder of known potency. The MIC values are interpreted according to the following criteria:
A report of "Susceptible" indicates that the pathogen is likely to be inhibited if the concentration of the antimicrobial compound in the blood reaches usually achievable levels. A report of "Intermediate" indicates that the result should be considered equivocal, and if the bacterium 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 high dosage of drug can be used. This category provides a buffer zone which prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of "Resistant" indicates that the pathogen is not likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable; other therapy should be selected.

## Quality Control
A standardized susceptibility test procedure requires the use of laboratory control bacteria to control the technical aspects of the laboratory procedures. Standard quinupristin/dalfopristin powder in a 30:70 ratio should provide the following MIC values with the indicated quality control strains:

## 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 procedure1 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 15 g quinupristin/dalfopristin in a ratio of 30:70 (Synercid) to test the susceptibility of bacteria to quinupristin/dalfopristin. Reports from the laboratory providing results of the standard single-disk susceptibility test with a 15 g quinupristin/dalfopristin disk are interpreted according to the following criteria:
Interpretation should be as stated above for results using dilution techniques. Interpretation involves correlation of the diameter obtained in the disk test with the MIC for quinupristin/dalfopristin.

## Quality Control
As with standardized dilution techniques, diffusion methods require the use of laboratory control bacteria that are used to control the technical aspects of the laboratory procedures. For the diffusion technique, the 15 g quinupristin/dalfopristin (30:70 ratio) disk should provide the following zone diameter with the indicated quality control strain.