Telithromycin microbiology

# Microbiology

## Mechanism of resistance
Staphylococcus aureus and Streptococcus pyogenes with the constitutive macrolide-lincosamide-streptogramin B (cMLSB) phenotype are resistant to telithromycin.
Mutants of Streptococcus pneumoniae derived in the laboratory by serial passage in subinhibitory concentrations of telithromycin have demonstrated resistance based on L22 riboprotein mutations (telithromycin MICs are elevated but still within the susceptible range), one of two reported mutations affecting the L4 riboprotein, and production of K-peptide. The clinical significance of these laboratory mutants is not known.

## Cross resistance
Telithromycin does not induce resistance through methylase gene expression in erythromycin-inducibly resistant bacteria, a function of its 3-keto moiety. Telithromycin has not been shown to induce resistance to itself.
List of Microorganisms
Telithromycin has been shown to be active against most strains of the following microorganisms, both in vitro and in clinical settings as described in the INDICATIONS AND USAGEsection.

## Aerobic gram-positive microorganisms
Streptococcus pneumoniae (including multi-drug resistant isolates )

## Aerobic gram-negative microorganisms
Haemophilus influenzae
Moraxella catarrhalis

## Other microorganisms
Chlamydophila (Chlamydia) pneumoniae
Mycoplasma pneumoniae
The following in vitro data are available, but their clinical significance is unknown.
At least 90% of the following microorganisms exhibit in vitro minimum inhibitory concentrations (MICs) less than or equal to the susceptible breakpoint for telithromycin. However, the safety and efficacy of telithromycin in treating clinical infections due to these microorganisms have not been established in adequate and well-controlled clinical trials.

## Aerobic gram-positive microorganisms
Staphylococcus aureus (methicillin and erythromycin susceptible isolates only)
Streptococcus pyogenes (erythromycin susceptible isolates only)
Streptococci (Lancefield groups C and G)

## Other microorganisms
Legionella pneumophila
MDRSP=Multi-drug resistant Streptococcus pneumoniae includes isolates known as PRSP (penicillin-resistant S. pneumoniae), and are isolates resistant to two or more of the following antimicrobials: penicillin, 2nd generation cephalosporins (e.g., cefuroxime), macrolides, tetracyclines, and trimethoprim/sulfamethoxazole.

## Susceptibility Test Methods
When available, the clinical microbiology laboratory should provide cumulative 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 antibacterial compounds. The MICs should be determined using a standardized procedure. Standardized procedures are based on dilution methods (broth or agar dilution)1,3 or equivalent with standardized inoculum and  concentrations of telithromycin powder. The MIC values should be interpreted according to criteria provided in Table 3.

### Diffusion techniques
Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antibiotics. One such standardized procedure2,3requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 15 g telithromycin to test the susceptibility of microorganisms to telithromycin. Disc diffusion zone sizes should be interpreted according to criteria in Table 3.
A report of "Susceptible" indicates that the antimicrobial is likely to inhibit growth of the pathogen if the antibacterial compound in the blood reaches the concentrations usually achievable. A report of "Intermediate" indicates that the result should be considered equivocal, and, if the microorganism 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 also provides a buffer zone that prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of "Resistant" indicates that the antimicrobial is not likely to inhibit growth of the pathogen if the antimicrobial compound in the blood reaches the concentrations usually achievable; other therapy should be selected.

### Quality control
Standardized susceptibility test procedures require the use of quality control microorganisms to determine the performance of the test procedures1,2,3. Standard telithromycin powder should provide the MIC ranges for the quality control organisms in Table 4. For the disk diffusion technique, the 15-g telithromycin disk should provide the zone diameter ranges for the quality control organisms in Table 4.