# How to Choose the Right ATCC QC Strain for Your Microbiology Test An ATCC QC strain is a permanent, well-characterized reference organism whose known, predictable reaction to a standardized test tells you whether your test system is working, while a clinical isolate only tells you what one patient sample did on one day. The rule of thumb for choosing between them is simple: use the exact strain named in the standard that defines your test method, and only look for a substitute when the standard offers no strain for your organism, your method, or your medium. Almost every wrong strain choice traces back to breaking one half of that rule, either by picking a strain the test was never designed to control or by substituting an unvalidated replacement. This article walks through the main options for choosing ATCC QC strains for microbiology tests, compares them on the criteria that actually drive bench decisions, works through three realistic scenarios, and lists the mix-ups that most often trigger out-of-range results and corrective-action paperwork. ## What an ATCC QC Strain Actually Is ATCC stands for the American Type Culture Collection, a biological resource center that supplies authenticated strains with documented identity, passage history, and storage conditions. A quality control (QC) strain is a reference organism run through a test alongside unknown samples. If the reference strain gives a result inside its published acceptance range, the test system (medium, inoculum, incubation, reader, and reagent lots) is considered in control. If it falls outside the range, the run is rejected and investigated. Two result types dominate microbiology quality control: - **MIC (minimum inhibitory concentration)**: the lowest antimicrobial concentration that visibly inhibits growth in a broth or agar dilution test, reported in µg/ml. - **Zone diameter (ZD)**: the diameter of the zone of growth inhibition around an antimicrobial disk on agar, measured in millimeters in disk diffusion. The acceptance ranges for these results are not guesses. They are established through multilaboratory studies conducted under the Clinical and Laboratory Standards Institute (CLSI) M23 protocol, which defines how QC ranges are developed and how many laboratories must participate [1][2][3][4]. A well-designed study produces ranges that include at least 95 percent of observed results, with MIC ranges spanning no more than about five twofold dilutions and zone ranges kept narrow, often under 12 mm total width [1]. That last point is the heart of the matter. The QC strain is the only instrument in the laboratory with a known answer. Choose the wrong one, and the instrument measures something your test was never built to control. ## Why Strain Choice Changes Your Results Strain choice is not an administrative detail. It determines whether QC can detect the failures that matter. Consider medium composition. A study from Alexandria, Egypt compared Kirby-Bauer disk diffusion performed on the standard Mueller-Hinton agar against nutrient agar, a cheaper substitute, using three reference strains: S. aureus ATCC 25923, E. coli ATCC 25922, and P. aeruginosa ATCC 27853. Every antibiotic-strain combination on nutrient agar was unacceptable, with more than 3 zone diameters falling outside the limits across 30 consecutive testing days, and clinical isolates tested on the substitute medium produced very major error rates as high as 8.98 percent for P. aeruginosa [5]. The reference strains did their job. They exposed a medium that should never have been in service. Consider drug diffusion instead. SCH27899 (Ziracin), a large oligosaccharide antibiotic, showed poor agar diffusion, with disk zone diameters clustering between 11 and 22 mm and poor correlation to reference MICs. The QC study group declined to recommend the disk diffusion method for that compound at all [6]. If a laboratory had improvised its own disk test with a standard QC strain, the narrow, compressed zones would have looked superficially plausible while detecting nothing. Both failure modes are caught only when the QC strain is matched to the method, the medium, and the drug. That matching logic organizes every option below. ## Choosing ATCC QC Strains for Microbiology Tests: The Main Options ### Option 1: CLSI-listed standard strains for routine susceptibility testing For a [clinical microbiology](/knowledge/diagnostics/microbiology/clinical-microbiology-from-specimen-collection-to-pathogen-identification) laboratory running broth microdilution and disk diffusion daily, the core panel is small and standardized. The workhorse strains are E. coli ATCC 25922, S. aureus ATCC 29213, P. aeruginosa ATCC 27853, S. aureus ATCC 25923, Enterococcus faecalis ATCC 29212, Streptococcus pneumoniae ATCC 49619, and Haemophilus influenzae ATCC 49247. Each carries published MIC and zone diameter ranges in the CLSI M100 tables, and each maps to a specific method. S. aureus ATCC 25923 is a disk diffusion strain. S. aureus ATCC 29213 is a broth microdilution strain. They are not interchangeable. The nafithromycin QC study shows how this mapping works in practice. Across a nine-laboratory, CLSI M23-defined tier 2 study, five QC strains were tested, and each was assigned to the method it controls: S. aureus ATCC 25923 for disk diffusion only, S. aureus ATCC 29213 and E. faecalis ATCC 29212 for broth microdilution only, and S. pneumoniae ATCC 49619 and H. influenzae ATCC 49247 for both disk and broth [4]. The resulting MIC QC ranges, such as 0.06 to 0.25 µg/ml for S. aureus ATCC 29213 and 2 to 8 µg/ml for H. influenzae ATCC 49247, were then approved by the CLSI Subcommittee on Antimicrobial Susceptibility Testing and entered into the M100 document [4]. The zoliflodacin studies extend the same picture with concrete ranges. The agar dilution QC range against Neisseria gonorrhoeae ATCC 49226 was set at 0.06 to 0.5 µg/ml after an eight-laboratory tier 2 study, while separate broth microdilution ranges were approved for S. aureus ATCC 29213 (0.12 to 0.5 µg/ml), E. faecalis ATCC 29212 (0.25 to 2 µg/ml), E. coli ATCC 25922 (1 to 4 µg/ml), S. pneumoniae ATCC 49619 (0.12 to 0.5 µg/ml), and H. influenzae ATCC 49247 (0.12 to 1 µg/ml) [3]. Note the same strain, S. aureus ATCC 29213, appearing across different drugs with different ranges. The strain is the constant. The range is drug-specific and method-specific. **Pros:** published ranges exist for every drug on the routine panel, strains are commercially available from multiple distributors, and the daily and weekly QC logic built into CLSI M100 is written around them. **Cons:** they cover only the organisms and media the standard anticipates, purchase costs and shipping delays can be a real burden for laboratories far from major suppliers [1], and some, such as the fastidious strains, demand more careful handling than the hardy enterics. ### Option 2: Organism- and medium-specific strains for fastidious organisms Fastidious organisms need enriched or defined media, and the QC strain must be grown on exactly the medium named in the method. The clearest example is N. gonorrhoeae ATCC 49226 on GC agar. The gemifloxacin QC study established MIC and disk diffusion ranges for this strain using data from at least seven laboratories, three GC agar medium lots, and two lots each of the 5-µg and 10-µg disks, producing a proposed MIC QC range of 0.002 to 0.016 µg/ml [7]. The zoliflodacin program later returned to the same strain for its agar dilution range [3], which shows the pattern: once a strain becomes the anchor for a fastidious organism and a special medium, subsequent programs reuse it so that ranges accumulate on a single, traceable reference point. The same logic applies to S. pneumoniae ATCC 49619 on Mueller-Hinton agar with 5 percent sheep blood and to H. influenzae ATCC 49247 on Haemophilus Test Medium. The gatifloxacin streptococcal QC work illustrates why organism coverage matters beyond the routine panel, since the program evaluated streptococci across multiple species and validated Etest and disk methods against reference broth microdilution results, with Etest agreement of 95.7 to 100 percent within one log2 dilution [8]. **Pros:** these strains are the only valid controls for their media and organisms, and their ranges are anchored to the exact agar formulation the method requires. **Cons:** fastidious strains are less stable in daily handling, special media add cost and lot-to-lot variability, and a range developed on GC agar cannot be transferred to chocolate agar or any other formulation. ### Option 3: Strains selected during validation of a new agent or new method When a new antimicrobial, antifungal, or biocide enters development, no QC ranges exist yet. The sponsor and the CLSI subcommittee must create them, and the strain selection happens up front. Three features define these studies. First, the number of laboratories. The exebacase program used a multilaboratory CLSI M23 tier 2 design, and the resulting broth microdilution ranges spanned four log2 dilutions and contained 99.2 percent of all MIC results generated for the two reference strains, S. aureus ATCC 29213 and E. faecalis ATCC 29212 [2]. Second, the statistical criteria. The resulting ranges must capture the central mass of the distribution, which is why published studies report coverage percentages such as the 95 percent benchmark in the Korean reference strain work [1]. Third, spectrum matching. The nafithromycin program chose five strains deliberately to mirror the drug's spectrum, including two fastidious species that a staphylococcal-only panel would have missed [4]. **Pros:** the strains and ranges are tailored to the exact test method, often with a purpose-built reference broth, and they carry regulatory weight for clinical trial data. **Cons:** the work is expensive and slow, and a range can fail to materialize for practical reasons. In the biocide study described next, two strain-biocide combinations could not be assigned ranges at all because the biocide was insufficiently soluble at the concentrations the test required [9]. ### Option 4: QC strains beyond antimicrobial susceptibility testing Microbiology quality control extends well past antibiotic MICs, and each application has its own reference strains. **Biocide and disinfectant susceptibility testing.** A European interlaboratory trial with eleven participating laboratories developed QC ranges for biocide susceptibility testing using four established reference strains: S. aureus ATCC 6538, Enterococcus hirae ATCC 10541, E. coli ATCC 10536, and P. aeruginosa ATCC 15442. Ranges were defined for benzalkonium chloride, chlorhexidine, octenidine, and polyhexanide, spanned three to five dilution steps, and were evaluated with the RangeFinder software across three lots of tryptic soy broth [9]. Notably, ranges for P. aeruginosa ATCC 15442 with chlorhexidine and polyhexanide could not be established because of solubility limits in the required test range [9]. These four strains descend from the classic European disinfectant testing tradition and are the correct choice for that application, not the CLSI AST panel. **Antifungal susceptibility testing.** For dermatophyte testing under CLSI M38-A2 methodology, which specifies RPMI 1640 as the test medium, an eight-laboratory evaluation of the topical antifungal ME1111 tested six blinded Trichophyton rubrum and T. mentagrophytes ATCC strains, with four of the six showing interlaboratory agreement above 90 percent, and two strains were subsequently designated as QC isolates [10]. Routine dermatophyte identification by MALDI-TOF MS likewise uses ATCC dermatophyte strains for QC of the identification system [11]. **Molecular and food microbiology methods.** Real-time PCR assays for food pathogens are validated with artificially contaminated samples. In a comparison of PCR kits against ISO culture procedures for Salmonella and Listeria in ready-to-eat foods, samples were spiked with Salmonella enteritidis ATCC and Listeria monocytogenes ATCC strains at concentrations from 10^3 down to 10^-1 cfu/g, and the PCR method reached a detection sensitivity of 1 cfu/g with full agreement between the two methods [12]. **Molecular and genomic workflows.** [Whole genome sequencing](/blog/guides/whole-genome-sequencing) pipelines benefit from reference strains with finished genome assemblies. One evaluation of rapid library preparation protocols selected 12 ATCC strains of human pathogens with G+C content ranging from 27 percent (Fusobacterium nucleatum) to 73 percent (Micrococcus luteus), each with a high-quality complete genome available, so that sequencing data quality could be judged against a known standard [13]. **Pros:** each field has a small set of strains with an established track record for that exact purpose. **Cons:** the ranges and acceptance criteria do not transfer between applications. S. aureus ATCC 6538 is a biocide testing strain, not an AST strain, and running it on your AST bench controls nothing. ### Option 5: Alternatives to ATCC, including NCTC and locally derived strains ATCC is not the only source of authenticated strains. The National Collection of Type Cultures (NCTC) supplies QC strains used in published programs, including E. coli NCTC 13846, an mcr-1 positive colistin resistance control recommended alongside E. coli ATCC 25922 and P. aeruginosa ATCC 27853 in the evaluation of a colistin MIC kit [14], and Acinetobacter baumannii NCTC 13304, used with E. coli ATCC 25922 as the QC organisms for a sulbactam-durlobactam disk evaluation against CLSI M100 ranges [15]. A second alternative is building a local or regional QC strain bank. Korean investigators working under CLSI M23 selected E. coli, P. aeruginosa, and S. aureus strains that produced well-defined results in preliminary testing, established MIC and zone diameter ranges in which all ranges included at least 95 percent of the data, and produced zone ranges narrower than 12 mm and MIC ranges spanning no more than five dilutions, exactly meeting the reference-standard criteria [1]. Their stated motivation was practical. ATCC strains carry purchase costs and shipping delays that burden laboratories far from suppliers, and a domestic bank reduces both [1]. **Pros:** NCTC strains fill gaps where ATCC does not offer the needed phenotype, such as a defined resistance mechanism, and local banks cut cost and turnaround time [1]. **Cons:** any substitute strain must be validated against the published range before use. Genomic sequencing now offers a rigorous way to do this. Complete genome assemblies of vaccine seed strains have been used to confirm at the strain level that manufacturers are using the same seed stock over time and to support consistent production and quality control [16][17]. The same principle applies to a laboratory QC bank. Sequence your in-house strain once, confirm its identity, and freeze the data with the stock. ## Side-by-Side Comparison of QC Strain Options | Option | Example strains | Typical application | Method and medium | Pros | Cons | |--|--|--|--|--|--| | CLSI standard AST panel | E. coli ATCC 25922, S. aureus ATCC 29213, P. aeruginosa ATCC 27853, S. aureus ATCC 25923, E. faecalis ATCC 29212, S. pneumoniae ATCC 49619, H. influenzae ATCC 49247 | Daily and weekly QC of routine antimicrobial susceptibility testing | Broth microdilution and disk diffusion on Mueller-Hinton agar, with blood-supplemented or Haemophilus Test Medium for fastidious strains | Published M100 ranges, broad commercial availability, daily-to-weekly QC logic built around them | No coverage for unusual organisms or nonstandard media, purchase cost and shipping delay can burden distant labs [1] | | Fastidious organism strains | N. gonorrhoeae ATCC 49226 | QC on enriched or defined media, GC agar | Agar dilution and disk diffusion on GC agar, ranges built from at least seven labs and three agar lots [7] | Only valid control for the medium, ranges accumulate across programs [7][3] | Fragile, medium-specific, ranges do not transfer to other agar formulations | | New agent validation strains | S. aureus ATCC 29213 and E. faecalis ATCC 29212 for exebacase [2], five-strain panel for nafithromycin [4] | Establishing first QC ranges for a new drug | CLSI M23 tier 2 multilaboratory design, sometimes with a purpose-built reference broth | Ranges matched precisely to the method, spectrum coverage designed in, CLSI approval | Expensive, slow, ranges occasionally impossible due to solubility or performance limits [9] | | Biocide testing strains | S. aureus ATCC 6538, E. hirae ATCC 10541, E. coli ATCC 10536, P. aeruginosa ATCC 15442 | Disinfectant and antiseptic susceptibility testing | Broth microdilution in tryptic soy broth, ranges of three to five dilution steps across 11 labs [9] | Established track record for efficacy testing | Ranges missing for some strain-biocide pairs [9], not valid for AST | | Antifungal QC strains | Trichophyton rubrum and T. mentagrophytes ATCC strains | Dermatophyte susceptibility testing and MALDI-TOF identification | CLSI M38-A2, RPMI 1640 medium [10], MALDI-TOF QC [11] | Interlaboratory agreement above 90 percent for designated strains [10] | Mycology-specific, requires mold-appropriate handling | | Molecular method controls | Salmonella enteritidis ATCC, Listeria monocytogenes ATCC strains | PCR and culture method validation in food testing | Artificially contaminated matrices, 10^3 to 10^-1 cfu/g [12] | Directly validate recovery and detection limits | Matrix-dependent, requires spiking expertise | | NCTC and mechanism strains | E. coli NCTC 13846 (mcr-1), A. baumannii NCTC 13304 | Colistin MIC QC [14], Acinetobacter disk QC [15] | Broth microdilution and disk diffusion per CLSI methods | Fills phenotype gaps ATCC does not cover | Must be mapped to published ranges before routine use | | Locally derived QC strains | Domestic E. coli, P. aeruginosa, S. aureus strains meeting CLSI M23 criteria [1] | Cost- and time-efficient QC where ATCC access is difficult | Validated per CLSI M23, ranges covering at least 95 percent of data [1] | Lower cost, faster supply [1] | Requires a full multilaboratory or in-house validation program and identity confirmation | ## How to Compare Options: The Criteria That Decide Real Choices ### Method and medium fit The single most decisive criterion. Ask three questions. Does the strain have a published range for this method (broth microdilution, agar dilution, disk diffusion)? Does it have a range for this medium (Mueller-Hinton, GC agar, Haemophilus Test Medium, RPMI 1640, tryptic soy broth)? Is the range written for this drug concentration or disk potency? The nafithromycin study makes the method split explicit, with 25923 restricted to disk diffusion and 29213 restricted to broth, and only the two fastidious strains cleared for both [4]. When a strain fails these questions, the answer is not to stretch the range. The answer is a different strain or a different method, as the Ziracin disk diffusion experience shows [6]. ### Spectrum and organism coverage A QC strain should exercise the antimicrobial class and the organism group your test claims to measure. The zoliflodacin program established ranges across five strains spanning Gram-positive, Gram-negative, respiratory fastidious, and gonococcal coverage, because the drug would be tested against all of them [3]. For a narrow-spectrum agent, fewer strains suffice, as with the two-strain exebacase panel [2]. For an agent aimed at a specific niche, such as a dermatophyte antifungal, the QC strains must be dermatophytes grown under mycology conditions [10], not the bacterial panel. ### Published acceptance ranges and their statistical backing ![Worked sum showing the zoliflodacin gonococcal MIC range of 0.06 to 0.5 µg/ml spans about three twofold dilutions, inside the five-dilution limit.](/article-images/illustrations/choosing-atcc-qc-strains-for-microbiology-tests-qc-range-width.webp) Prefer strains whose ranges were established under the CLSI M23 framework with documented multilaboratory data, coverage of at least 95 percent of results, MIC spans of roughly five dilutions or fewer, and narrow zone ranges [1]. The exebacase study reports what good output looks like, four log2 dilutions containing 99.2 percent of results [2]. When a vendor supplies a "recommended range" without documented provenance, treat it as a starting point for in-house verification, not as an equivalent. ### Availability, cost, and turnaround These practical factors drive real decisions, particularly outside major distribution hubs. The Korean reference strain program was launched precisely because ATCC strains impose purchase costs and shipping delays on laboratories, and the program showed that domestically selected strains can meet the same statistical criteria as commercial reference strains [1]. If supply is your constraint, the ordering of solutions is: first, use the CLSI-listed strain through an authorized distributor, second, use an equivalent collection strain such as NCTC where the published program supports it [14][15], and third, consider a locally validated bank only if you can replicate the M23 validation work [1]. ### Strain identity and traceability A QC strain is only as good as your confidence that it is still the strain on the label. Genomic verification has matured into a practical tool. Complete, high-quality, gap-free genome assemblies generated by hybrid short-read and [long-read sequencing](/knowledge/bioinformatics/long-read-sequencing-technologies-pacbio-and-oxford-nanopore) have been used to verify, at the strain level, that vaccine manufacturers are using the same seed strains over time and to underpin reliable quality control of biological products [16][17]. The same approach applies to a laboratory QC bank. Sequencing a handful of housekeeping and identity markers, or the whole genome, when a stock is first banked gives you an unambiguous record. The WGS method-comparison work shows why finished reference genomes matter, since strains with high-quality complete assemblies allowed direct judgment of sequence data quality across protocols [13]. ### Maintenance burden and passage management QC strains degrade with subculture. Standard practice, spelled out in CLSI susceptibility documents and laboratory quality management textbooks, is to maintain a deep stock (lyophilized or frozen at -70 °C or below), prepare a limited working stock from it, and discard working stocks after about five passages from the authenticated original. Daily-use plates should come from the working stock, never from serially passaged plates. Fastidious strains such as N. gonorrhoeae ATCC 49226 and H. influenzae ATCC 49247 drift faster than enterics and warrant tighter replacement schedules. ## Worked Scenarios ### Scenario 1: Daily QC on a routine clinical AST bench You run Kirby-Bauer disk diffusion and broth microdilution on a mixed bacterial workload. Your daily panel includes S. aureus ATCC 25923 for disk testing, S. aureus ATCC 29213 and E. coli ATCC 25922 for broth panels, and P. aeruginosa ATCC 27853 for both, with S. pneumoniae ATCC 49619 and H. influenzae ATCC 49247 added on the days you test those organism groups. You record zones and MICs against the M100 ranges. After 20 consecutive days of acceptable results with no more than 3 out-of-range events, per standard CLSI practice, you shift to weekly QC for drugs that qualify. When a new cephalosporin lot arrives, the same strains control it, because the ranges are drug-specific and already exist. Nothing in this scenario requires choosing anything novel. The strain choice was made by the standard, and your job is maintenance, passage discipline, and documentation. ### Scenario 2: Bringing a new antimicrobial into a development or research laboratory Your group is testing a new antistaphylococcal compound and needs MIC data that will withstand regulatory review. Following the exebacase precedent, you select the reference strains that match the compound's spectrum and method, in that case S. aureus ATCC 29213 and E. faecalis ATCC 29212 for broth microdilution [2]. If no range exists yet, you either join or replicate a multilaboratory tier 2 study under CLSI M23, collect data across enough laboratories and reagent lots, and propose ranges that capture at least 95 percent of results within about five dilutions [1][2]. The zoliflodacin and nafithromycin programs are the templates, with eight and nine participating laboratories respectively and ranges approved at the CLSI subcommittee level before entering the M100 tables [3][4]. If your compound is highly soluble and diffuse-able, disk diffusion may be viable. If it is a large, poorly diffusing molecule, expect the Ziracin outcome, where disk testing was not recommended at all [6]. ### Scenario 3: Disinfectant efficacy testing in an industrial or public health laboratory You evaluate biocide activity, not antibiotics. The correct panel comes from the biocide tradition: S. aureus ATCC 6538, E. hirae ATCC 10541, E. coli ATCC 10536, and P. aeruginosa ATCC 15442, tested by broth microdilution in tryptic soy broth [9]. You plan to verify performance across at least three medium lots and to analyze reproducibility with a dedicated tool such as RangeFinder [9]. You check the published table before ordering reagents and discover that two strain-biocide combinations, P. aeruginosa ATCC 15442 with chlorhexidine and with polyhexanide, lack ranges because the biocide could not be dissolved at the concentrations the Pseudomonas endpoint required [9]. Your QC plan then routes those two combinations to an alternative validated endpoint rather than inventing an acceptance band. This is the correct response to a missing range. Narrow the claim of the test instead of widening the acceptance criteria. ## Common Mix-Ups **Mixing the two S. aureus strains.** ATCC 25923 belongs to disk diffusion and ATCC 29213 belongs to broth microdilution. The nafithromycin study assigned them exactly this way. Running 25923 on a broth panel or 29213 on a disk plate produces results with no valid range. **Assuming any ATCC number controls any test.** S. aureus ATCC 6538 is a biocide testing strain [9]. S. mutans ATCC 25175 served as a dental research reference strain [18]. Neither has standing on an AST bench, even though all three are S. aureus-family or streptococcal reference organisms in their own domains. **Substituting a cheaper medium without revalidation.** The nutrient agar study showed that every reference strain combination failed on the substitute medium, with error rates for clinical isolates reaching 8.98 percent very major errors for P. aeruginosa [5]. The QC strains caught it. But a laboratory that ignores the QC failures ships bad results. **Using the fastidious strain on the wrong agar.** The N. gonorrhoeae ATCC 49226 ranges were built on GC agar across multiple agar lots [7]. Transfer the strain to a different base medium and the range is void. **Testing a drug against a QC strain outside its spectrum.** SCH27899 showed no measurable activity against Gram-negative bacteria except M. catarrhalis [6]. A Gram-negative QC strain tested against such a compound produces uniformly large zone diameters and tells you nothing about test performance. **Treating NCTC and ATCC numbers as interchangeable without checking.** Some programs used NCTC strains with CLSI M100 ranges, such as A. baumannii NCTC 13304 [15], and some used NCTC strains precisely because ATCC lacked the phenotype, such as the mcr-1 positive E. coli NCTC 13846 for colistin QC [14]. Substitution works when a published program established it. Improvised substitution is uncontrolled. **Ignoring the identity question over years of storage.** The genomic verification work on vaccine seed strains demonstrates the modern standard for confirming that a banked strain remains the named strain [16][17]. A QC strain that has silently acquired a mutation no longer measures your test. It measures the mutation. ## Common Mistakes and Limitations - **Passage creep.** Serial subculture is the most common silent failure. Keep a frozen or lyophilized master, restrict working stocks to roughly five passages, and date every retrieval. Replace fastidious strains on a shorter schedule than hardy ones. - **Lot changes without a QC bridge.** Medium, disk, and panel lots all drift. When you change a lot, run the QC strain against both lots in parallel for at least the standard consecutive-day evaluation before retiring the old lot. - **Stretching a range to save a run.** An out-of-range result means the run is rejected and investigated, not averaged in. Corrective action starts with inoculum turbidity, medium lot, incubation conditions, and strain identity, in that order. - **Confusing QC with calibration.** A QC strain confirms that a standardized system performs as published. It does not make a nonstandardized system valid. The nutrient agar experience is the proof, since well-run QC on the wrong medium still failed everything [5]. - **Building a local strain bank without the statistics behind it.** The Korean program succeeded because it applied the CLSI M23 criteria, multilaboratory data, at least 95 percent coverage, and narrow ranges, rather than simply freezing convenient isolates [1]. - **Skipping genomic identity checks on long-held stocks.** Hybrid short-read plus long-read sequencing now produces complete, gap-free assemblies cheaply enough to bank alongside every QC stock [16][17][13]. - **A limitation on scope.** Strain selection and QC ranges govern the test system, not the interpretation of any single patient or sample result. Individual diagnostic decisions belong with the treating veterinarian or physician who has the full clinical picture. ## Frequently Asked Questions ### What is the difference between an ATCC QC strain and an ATCC type strain? A QC strain is selected and validated for a specific test because it gives stable, well-distributed results with a published acceptance range. A type strain is the nomenclature reference specimen for a species. Some strains serve both roles, but the roles are different, and a type strain without a published QC range cannot control a standardized test. ### How many QC strains does a routine susceptibility testing bench actually need? Most clinical benches need four to seven strains, covering Gram-negative, Gram-positive, and the fastidious groups they test. The exact set follows from the organism groups on your menu and the methods you use, with disk and broth strains tracked separately. ### Can I substitute an NCTC strain for an ATCC strain? Yes, when a published validation program supports the substitution, as with the Acinetobacter and colistin QC strains used alongside ATCC organisms. Do not improvise a substitution without mapping the NCTC strain to a published range or validating it yourself. ### How long can I keep a QC strain in use before replacing it? Maintain a frozen or lyophilized master stock and discard working stocks after about five passages from the authenticated original. Fastidious strains need more frequent replacement than hardy enterics because they drift faster with subculture. ### What should I do when a QC result falls outside the accepted range? Reject the run, repeat the test, and investigate the usual suspects in order, including inoculum density, medium and disk lots, incubation conditions, reader calibration, and strain identity or passage history. Document the corrective action under your [laboratory quality management system](/knowledge/diagnostics/emerging-tech/laboratory-quality-management-systems-a-practical-implementation-framework-for-diagnostic-labora) before reporting patient results from that run. ### Do I need separate QC strains for disk diffusion and broth microdilution? Yes for some organisms. S. aureus ATCC 25923 controls disk diffusion and S. aureus ATCC 29213 controls broth microdilution, and other strains may be cleared for both methods. Check the method assignment for every drug, not just for every organism. ## Sources 1. [Establishing quality control ranges for antimicrobial susceptibility testing of Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus: a cornerstone to develop reference strains for Korean clinical microbiology laboratories.](https://pubmed.ncbi.nlm.nih.gov/26354353/) 2. [Determination of MIC Quality Control Parameters for Exebacase, a Novel Lysin with Antistaphylococcal Activity.](https://pubmed.ncbi.nlm.nih.gov/33910968/) 3. [Determination of MIC Quality Control Ranges for the Novel Gyrase Inhibitor Zoliflodacin.](https://pubmed.ncbi.nlm.nih.gov/31315953/) 4. [Determination of Disk Diffusion and MIC Quality Control Ranges for Nafithromycin (WCK 4873), a New Lactone-Ketolide.](https://pubmed.ncbi.nlm.nih.gov/28747372/) 5. [Evaluation of antibiotic susceptibility test results: how guilty a laboratory could be?](https://pubmed.ncbi.nlm.nih.gov/30686832/) 6. [Antimicrobial activity and spectrum of SCH27899 (Ziracin) tested against gram-positive species including recommendations for routine susceptibility testing methods and quality control. Quality Control Study Group.](https://pubmed.ncbi.nlm.nih.gov/10354859/) 7. [Development of gemifloxacin in vitro susceptibility test methods for gonococci including quality control guidelines. The Quality Control Study Group.](https://pubmed.ncbi.nlm.nih.gov/10904193/) 8. [Comparative antimicrobial activity of gatifloxacin tested against Streptococcus spp. including quality control guidelines and etest method validation. Quality Control Study Group.](https://pubmed.ncbi.nlm.nih.gov/10354857/) 9. [Development of Quality Control Ranges for Biocide Susceptibility Testing.](https://pubmed.ncbi.nlm.nih.gov/35215165/) 10. [Multilaboratory Evaluation of In Vitro Antifungal Susceptibility Testing of Dermatophytes for ME1111.](https://pubmed.ncbi.nlm.nih.gov/26719434/) 11. [A Comparative Evaluation of Matrix-Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry (MALDI-TOF MS) and Conventional Methods for the Diagnosis of Dermatophytes](https://pubmed.ncbi.nlm.nih.gov/40034625/) 12. [Real-time PCR detection of Salmonella spp. and Listeria monocytogenes in ready-to-eat foods: a comparison between the biomolecolar method and traditional microbiology](https://doi.org/10.2427/5900) 13. [Evaluation of Rapid Library Preparation Protocols for Whole Genome Sequencing Based Outbreak Investigation](https://pubmed.ncbi.nlm.nih.gov/31508405/) 14. [Comparative evaluation of the UMIC Colistine kit to assess MIC of colistin of gram-negative rods](https://pubmed.ncbi.nlm.nih.gov/30885126/) 15. [2515. Evaluation of the Equivalency of the Oxoid Sulbactam-Durlobactam (10/10µg) Antimicrobial Disc to the CLSI M07 Broth Microdilution Frozen Reference Method](https://doi.org/10.1093/ofid/ofad500.2133) 16. [High-quality genome assembly and annotation of live animal vaccine bacteria strains in South Korea.](https://pubmed.ncbi.nlm.nih.gov/40898044/) 17. [High-quality genome assembly and annotation of inactivated animal vaccine bacteria strains in South Korea.](https://pubmed.ncbi.nlm.nih.gov/42576204/) 18. [Comparative Evaluation of the Antimicrobial Effects of Different Mouthrinses against Streptococcus Mutans: An in Vitro Study.](https://pubmed.ncbi.nlm.nih.gov/31657984/)