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 antibiotic susceptibility testing (AST) methodologies and check antibiotic discs’ efficacy before use.
For example, the zone size of ATCC E.coli 25922 against ampicillin is 15-22 mm by disc diffusion. Suppose that strain gives a zone of 17 mm when tested against recently purchased ampicillin discs. In that case, we can assume that these antibiotic discs are of correct potency (working well) as the zone size lies within the range set by the 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.
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.
The microbiology laboratory requires only a 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, train new personnel, investigate 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 Strain | Test(s), for which strain is primarily used |
| Escherichia coli ATCC® 25922 | Disk diffusion and MIC of Enterobacteriaceae, Pseudomonas aeruginosa, Acinetobacter spp., Burkholderia cepacia, Stenotrophomonas 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® 35218 | Disk diffusion and MIC for β-lactam/β-lactamase inhibitor combination drugs of Enterobacteriaceae, Pseudomonas aeruginosa, Acinetobacter spp., Burkholderia cepacia, Stenotrophomonas 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® 700603 | Screening and confirmatory tests for ESBLs (positive) |
| Klebsiella pneumoniae ATCC® BAA-1705 | Confirmatory test for suspected carbapenemase production in Enterobacteriaceae (MHT positive) |
| Klebsiella pneumoniae ATCC® BAA-1706 | Confirmatory test for suspected carbapenemase production in Enterobacteriaceae (MHT negative) |
| Pseudomonas aeruginosa ATCC® 27853 | Disk diffusion and MIC of Pseudomonas aeruginosa, Acinetobacter spp., Burkholderia cepacia, Stenotrophomonas maltophilia |
| MIC of other non-Enterobacteriaceae | |
| Staphylococcus aureus ATCC® 25923 | Disk 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® 29213 | MIC 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® 43300 | Screening 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-976 | Screening 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-977 | Screening 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-1708 | Screening test for high-level mupirocin resistance in Staphylococcus aureus group (mupA positive; disk diffusion and MIC resistant) |
| Enterococcus faecalis ATCC® 29212 | MIC 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® 51299 | Screening 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® 49247 | Disk diffusion and MIC of Haemophilus spp. (BLNAR; β-lactamase negative, ampicillin resistant) |
| Haemophilus influenzae ATCC® 49766 | Disk diffusion and MIC of Haemophilus spp. with selected cephalosporins (β-lactamase positive) |
| Haemophilus influenzae ATCC® 10211 | Checking growth capabilities of medium used for disk diffusion and MIC tests for Haemophilus spp. |
| Neisseria gonorrhoeae ATCC® 49226 | Disk diffusion and MIC of Neisseria gonorrhoeae (CMRNG; chromosomally mediated (penicillin) resistant N. gonorrhoeae) |
| Streptococcus pneumoniae ATCC® 49619 | Disk 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® 25285 | MIC of anaerobes |
| Bacteroides thetaiotaomicron ATCC® 29741 | MIC of anaerobes |
| Clostridium difficile ATCC® 700057 | MIC of anaerobes |
References and further readings
- Karatuna Onur, ‘Quality Assurance in Antimicrobial Susceptibility Testing’ Intechopen.
- CLSI. (2021). Performance Standards for Antimicrobial Susceptibility Testing; CLSI document M100-Ed 31. Wayne, PA: Clinical and Laboratory Standards Institute.
- The European Committee on Antimicrobial Susceptibility Testing – EUCAST