SXT Test – Principle, Procedure, Uses and Interpretation

Although bacitracin susceptibility has routinely been used for the presumptive identification of group A streptococci, it has been observed that streptococcal groups vary in their susceptibilities to Taxo A (bacitracin, 0.04 units) and SXT (trimethoprim [1.25 mg] plus sulfamethoxazole [23.75 mg],) disks.

  • Group A beta hemolytic strep is susceptible to the A disc, but resistant to SXT
  • Group B beta hemolytic strep is resistant to both A and SXT discs
  • Other types of beta-hemolytic streps are resistant to the A disc, but sensitive to SXT (example Group C)

This property can thus be effectively used for the identification of the different Streptococcal groups.


To differentiate among Streptococcal groups based on their susceptibility to sulfamethoxazole/ trimethoprim (SXT) antibiotics.


Trimethoprim/sulfamethoxazole also known as co-trimoxazole is an antibiotic which consists of a combination of two antifolate agents: one part trimethoprim to five parts sulfamethoxazole. Trimethoprim and sulfamethoxazole have a greater effect when given together because they inhibit successive steps in the folate synthesis pathway. It disrupts the synthesis of purines, thymidine, and methionine which are needed for the production of DNA and proteins during bacterial replication. Thus the net effect of each of these drugs is a bacteriostatic halt in replication.

A disk impregnated with a definite amount of Trimethoprim/sulfamethoxazole is placed on the agar plate, allowing the antibiotic to diffuse into the medium and inhibit the growth of susceptible organisms. After incubation, the inoculated plates are examined for zones of inhibition surrounding the disks. If the organism grows up to the edge of the disk, it is resistant to the antimicrobial compound infusing the disk.  If there is a zone around the edge of the disk where the organism has not grown, the organism is susceptible to the antimicrobial in the disk.

Testing Discs

A special high quality paper impregnated with a predefined concentration gradient of antibiotic – Trimethoprim/sulfamethoxazole.


  1. Plate the orgnaism onto a 5% defibrinated sheep blood agar plate (5% sheep blood in Trypticase soy agar).
    Note: Streptococci often require several colonies seeded to achieve adequate growth.
  2. Aseptically place the antibiotic disks onto the inoculated surface of the sheep blood agar plates.
  3. Incubate for 18 to 24 h at 370C in an atmosphere of 5% C02 in air.
  4. Examine the plate for a zone of inhibition around the disk.

Expected Results

  • Susceptible: A zone of inhibition around the disc indicates susceptibility to sulfamethoxazole/ trimethoprim.
  • Resistant: The absence of a zone of inhibition around the SXT disk indicates resistance to sulfamethoxazole/ trimethoprim.

SXT Test


  • SXT disc susceptibility test in conjunction with bacitracin is used for the presumptive identification of beta hemolytic streptococci on blood agar (presumptively identifying beta-hemolytic streptococci as either group A, B or not group A and B).
  • The resistance to SXT is used for the primary recovery of groups A and B streptococci from specimens with mixed culture. Their resistance allows them to selectively grow out from contaminating bacteria that are inhibited by this antibiotic.
  • Bacitracin/SXT susceptibility tests are still in use where facilities for serologic group determination are unavailable.


  • Bacitracin test should be performed in conjunction with a SXT susceptibility test as the combined results increase the sensitivity and predictive value.
  • It is recommended that biochemical, immunological, molecular, or mass spectrometry testing be performed on colonies from pure culture for complete identification.


  1. Gunn B. A. (1976). SXT and Taxo A disks for presumptive identification of group A and B streptococci in throat cultures. Journal of clinical microbiology4(2), 192-3.

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