Quality Control Strains (standard strains) and their Uses

Quality control strains (standard strains) are well-characterized microorganisms with defined susceptibility or resistance profiles to the antimicrobial agent(s) tested. They are routinely used to standardize the antibiotic susceptibility testing (AST) methodologies and to check the efficacy of antibiotic discs before use.


For example, the zone size of ATCC E.coli 25922 against ampicillin is 15-22 mm by disc diffusion. If that strain gives a zone of 17 mm when tested against recently purchased ampicillin discs, we can assume that these antibiotic discs are of correct potency (working well) as the zone size lies within the range set by quality control strain.


QC strains are also used to monitor the test procedure and quality control of culture media & reagents.

How to get Quality Control Strains?


Standard strains are often derived from huge culture collection banks often managed by a region or specific country. ATCC (American type culture collection), NCTC (National Collection of Type Cultures), NCIMB, World data center for Microorganism (WDCM) are some common sources that characterize and store standard strains. There are many more culture collections that can be accessed here.

Quality control strains
Quality control strains (image source: abscientific.com)


They can be procured from various commercial sources. They are available in freeze-dried form in glass ampoules or as sticks or swabs that contain a dried form of the microorganism. The organism can be revived following the manufacturer’s instructions.

How to select the right type of microorganism for QC?


Each organism with a distinguished property is designated a different number. For example, E.coli ATCC 25922 is a non-β-lactamase producing strain whereas E.coli ATCC 35218 is a β-lactamase-producing strain. So, you have to select the right type of QC strains based on your requirements.


Microbiology laboratory requires only few strains for routine QC testing (for example, E.coli ATCC 25922 for enterobacteriaceae, S.aureus ATCC 25923 for Gram positive organisms and Pseudomonas aeruginosa ATCC 27853 for other non-enterobacteriaceae).


Supplemental strains are used to assess a new test, for training new personnel, investigation of special susceptibility or resistance characteristics, etc., and are not included in the routine QC of AST.


Some of the common QC strains along with their property and use are enlisted below.

QC StrainTest(s), for which strain is primarily used
Escherichia coli ATCC® 25922Disk diffusion and MIC of EnterobacteriaceaePseudomonas aeruginosaAcinetobacter spp.Burkholderia cepaciaStenotrophomonas maltophilia
MIC of other non-Enterobacteriaceae
Screening and confirmatory tests for ESBLs (negative)
Disk diffusion and MIC of Neisseria meningitidis (for ciprofloxacin, nalidixic acid, minocycline, and sulfisoxazole)
Escherichia coli ATCC® 35218Disk diffusion and MIC for β-lactam/β-lactamase inhibitor combination drugs of EnterobacteriaceaePseudomonas aeruginosaAcinetobacter spp.Burkholderia cepaciaStenotrophomonas maltophilia, Staphylococcus spp.
MIC for β-lactam/β-lactamase inhibitor combination drugs of other non-Enterobacteriaceae
Testing of amoxicillin-clavulanic acid for Haemophilus spp.
Klebsiella pneumoniae ATCC® 700603Screening and confirmatory tests for ESBLs (positive)
Klebsiella pneumoniae ATCC® BAA-1705Confirmatory test for suspected carbapenemase production in Enterobacteriaceae (MHT positive)
Klebsiella pneumoniae ATCC® BAA-1706Confirmatory test for suspected carbapenemase production in Enterobacteriaceae (MHT negative)
Pseudomonas aeruginosa ATCC® 27853Disk diffusion and MIC of Pseudomonas aeruginosaAcinetobacter spp.Burkholderia cepaciaStenotrophomonas maltophilia
MIC of other non-Enterobacteriaceae
Staphylococcus aureus ATCC® 25923Disk diffusion of Staphylococcus spp. and Enterococcus spp.
Screening test for β-lactamase production of Staphylococcus aureus group and coagulase-negative Staphylococci (negative)
Screening test for mecA-mediated oxacillin resistance using cefoxitin in Staphylococcus aureus group and coagulase-negative Staphylococci (mecA negative; disk diffusion susceptible)
Screening test for inducible clindamycin resistance in Staphylococcus aureus group and coagulase-negative Staphylococci with disk diffusion (D-zone test) (negative)
Screening test for high-level mupirocin resistance in Staphylococcus aureus group (mupA negative; disk diffusion susceptible)
Staphylococcus aureus ATCC® 29213MIC of Staphylococcus spp.
Screening test for β-lactamase production in Staphylococcus aureus group and coagulase-negative Staphylococci (positive)
Screening test for oxacillin resistance in Staphylococcus aureus group (susceptible)
 Screening test for mecA-mediated oxacillin resistance using cefoxitin in Staphylococcus aureus group (mecA negative; MIC susceptible)
Screening test for inducible clindamycin resistance in Staphylococcus aureus group, coagulase-negative Staphylococci, and Streptococcus spp. β-hemolytic group with broth microdilution (no growth)
Screening test for high-level mupirocin resistance in Staphylococcus aureus group (mupA negative; MIC susceptible)
Staphylococcus aureus ATCC® 43300Screening test for oxacillin resistance in Staphylococcus aureus group (resistant)
Screening test for mecA-mediated oxacillin resistance using cefoxitin in Staphylococcus aureus group (disk diffusion and MIC) and coagulase-negative Staphylococci (disk diffusion) (mecA positive)
Staphylococcus aureus ATCC® BAA-976Screening test for inducible clindamycin resistance in Staphylococcus aureus group, coagulase-negative Staphylococci, and Streptococcus spp. β-hemolytic group with broth microdilution (no growth)
Staphylococcus aureus ATCC® BAA-977Screening test for inducible clindamycin resistance in Staphylococcus aureus group, coagulase-negative Staphylococci and Streptococcus spp. β-hemolytic group with broth microdilution (growth)
Staphylococcus aureus ATCC® BAA-1708Screening test for high-level mupirocin resistance in Staphylococcus aureus group (mupA positive; disk diffusion and MIC resistant)
Enterococcus faecalis ATCC® 29212MIC of Enterococcus spp.
 Screening test for vancomycin MIC ≥8 µg/mL in Staphylococcus aureus group (susceptible)
Screening test for high-level aminoglycoside resistance in Enterococcus spp. (disk diffusion, broth microdilution, agar dilution: susceptible)
Screening test for vancomycin resistance in Enterococcus spp. (agar dilution: susceptible) checking that medium is acceptable for testing sulfonamides, trimethoprim, and trimethoprim/sulfamethoxazole
Enterococcus faecalis ATCC® 51299Screening test for vancomycin MIC ≥8 µg/mL for Staphylococcus aureus group (resistant)
Screening test for high-level aminoglycoside resistance in Enterococcus spp. (broth microdilution, agar dilution: resistant)
Screening test for vancomycin resistance in Enterococcus spp. (agar dilution: resistant)
Haemophilus influenzae ATCC® 49247Disk diffusion and MIC of Haemophilus spp. (BLNAR; β-lactamase negative, ampicillin resistant)
Haemophilus influenzae ATCC® 49766Disk diffusion and MIC of Haemophilus spp. with selected cephalosporins (β-lactamase positive)
Haemophilus influenzae ATCC® 10211Checking growth capabilities of medium used for disk diffusion and MIC tests for Haemophilus spp.
Neisseria gonorrhoeae ATCC® 49226Disk diffusion and MIC of Neisseria gonorrhoeae (CMRNG; chromosomally mediated (penicillin) resistant N. gonorrhoeae)
Streptococcus pneumoniae ATCC® 49619Disk diffusion and MIC of Streptococcus pneumoniae (penicillin intermediate), Streptococcus spp. β-hemolytic group Streptococcus spp. viridans group and Neisseria meningitidis
Screening test for inducible clindamycin resistance in Streptococcus spp. β-hemolytic group with disk diffusion (D-zone test) and broth microdilution (negative)
Bacteroides fragilis ATCC® 25285MIC of anaerobes
Bacteroides thetaiotaomicron ATCC® 29741MIC of anaerobes
Clostridium difficile ATCC® 700057MIC of anaerobes

References and further readings

  1. Karatuna Onur, ‘Quality Assurance in Antimicrobial Susceptibility Testing’ Intechopen.
  2. CLSI. (2021). Performance Standards for Antimicrobial Susceptibility Testing; CLSI document M100-Ed 31. Wayne, PA: Clinical and Laboratory Standards Institute.
  3. The European Committee on Antimicrobial Susceptibility Testing – EUCAST

About Nisha Rijal 48 Articles
I am working as Microbiologist in National Public Health Laboratory (NPHL), government national reference laboratory under the Department of health services (DoHS), Nepal. Key areas of my work lies in Bacteriology, especially in Antimicrobial resistance.

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