Standardized susceptibility test procedures require the use of laboratory control microorganisms to control the technical aspects of the laboratory procedures. Standard sulfamethoxazole/trimethoprim powder should provide the following range of values:
|Escherichia coli||ATCC 25922||≤ 0.5/9.5|
|Haemophilus influenzae *||ATCC 49247||0.03/0.59 – 0.25/4.75|
|Streptococcus pneumoniae †||ATCC 49619||0.12/2.4 – 1/19|
Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure5 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 1.25/23.75 µg of sulfamethoxazole/trimethoprim to test the susceptibility of microorganisms to sulfamethoxazole/trimethoprim.
Reports from the laboratory providing results of the standard single-disk susceptibility test with a 1.25/23.75 µg of sulfamethoxazole/trimethoprim disk should be interpreted according to the following criteria:
|For testing either Enterobacteriaceae or Haemophilus influenzae *:|
|Zone Diameter (mm)||Interpretation|
|≥ 16||Susceptible (S)|
|11 – 15||Intermediate (I)|
|≤ 10||Resistant (R)|
|When testing Streptococcus pneumoniae *:|
|Zone Diameter (mm)||Interpretation|
|≥ 19||Susceptible (S)|
|16 – 18||Intermediate (I)|
|≤ 15||Resistant (R)|
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 sulfamethoxazole/trimethoprim.
As with standardized dilution techniques, diffusion methods require the use of laboratory control microorganisms that are used to control the technical aspects of the laboratory procedures. For the diffusion technique, the 1.25/23.75 µg sulfamethoxazole/trimethoprim disk 1 should provide the following zone diameters in these laboratory test quality control strains:
|Microorganism||Zone Diameter Ranges (mm)|
|Escherichia coli||ATCC 25922||24–32|
|Haemophilus influenzae *||ATCC 49247||24–32|
|Streptococcus pneumoniae †||ATCC 49619||20–28|
- Mueller-Hinton agar should be checked for excessive levels of thymidine or thymine. To determine whether Mueller-Hinton medium has sufficiently low levels of thymidine and thymine, an Enterococcus faecalis (ATCC 29212 or ATCC 33186) may be tested with sulfamethoxazole/trimethoprim disks. A zone of inhibition ≥20 mm that is essentially free of fine colonies indicates a sufficiently low level of thymidine and thymine.
To reduce the development of drug-resistant bacteria and maintain the effectiveness of Bactrim (sulfamethoxazole and trimethoprim) tablets and other antibacterial drugs, Bactrim (sulfamethoxazole and trimethoprim) tablets should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria. When culture and susceptibility information are available, they should be considered in selecting or modifying antibacterial therapy. In the absence of such data, local epidemiology and susceptibility patterns may contribute to empiric selection of therapy.
Urinary Tract Infections: For the treatment of urinary tract infections due to susceptible strains of the following organisms: Escherichia coli , Klebsiella species, Enterobacter species, Morganella morganii , Proteus mirabilis and Proteus vulgaris. It is recommended that initial episodes of uncomplicated urinary tract infections be treated with a single effective antibacterial agent rather than the combination.
Acute Otitis Media: For the treatment of acute otitis media in pediatric patients due to susceptible strains of Streptococcus pneumoniae or Haemophilus influenzae when in the judgment of the physician sulfamethoxazole and trimethoprim offers some advantage over the use of other antimicrobial agents. To date, there are limited data on the safety of repeated use of BACTRIM in pediatric patients under two years of age. BACTRIM is not indicated for prophylactic or prolonged administration in otitis media at any age.
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