Microbial Quality Control Strains: Introduction, Application and Related Terms

Microbial Quality Control Strains: Introduction, Application and Related Terms

Common Microbial ATCC Strains or Microbial Quality control strains

Microbial quality control strains include common bacterial ATCC (American Type Culture Collection)  strains are E. coli ATCC 25922, S. aureus 25923, P. aeruginosa  ATCC 27853, and Enterococcus faecalis ATCC 29212  as shown above picture. Similarly, other microbial ATCC strains are Candida albicans ATCC 10231,

Cryptococcus neoformans ATCC 320145,

Entamoeba histolytica ATCC 30459,

Human immunodeficiency virus 1 (HIV-1) RNA ATCC VR-3245SD, etc.

Applications of Microbial Quality Control Strains

ATCC provides top-quality microbial strains needed to maintain outstanding Quality Control programs. Commercial firms specify ATCC strains i.e. microbial quality control strains as controls for the following purposes and are-

  • rapid identification,
  • minimum inhibitory concentration (MIC)  of antimicrobial agents,
  • antimicrobial susceptibility panel, etc.

Wild type strains  introduction

Wild type strains are specific reference micro-organisms in microbial genetics e.g. Staphylococcus aureus ATCC 25923,   Escherichia coli ATCC 25922, Paeudomonas aeruginosa ATCC 27853, Enterococcus faecalis ATCC 29212, Candida albicans ATCC 90028 ( fungus)  S. cerevisiae ATCC 9763( fungus), etc, and descendants that have mutations in their genomes are called mutants. Therefore, mutants are characterized by the inherited differences between them and their ancestral wild-type strains.

Features of Wild type strains versus mutants

  • Alleles:Variant forms of a specific genetic determinant
  • Genotypic symbols: They are lower case, italicized abbreviations that specify individual genes, with a (+) superscript indicating the wild type allele.
  • Phenotypic symbols: They are capitalized and not italicized, to distinguish them from genotypic symbols. e.g. the genotypic symbol for the ability to produce β-galactosidase, required to ferment lactose, is lacZ+, and mutants that cannot produce β-galactosidase are lacZ.
  • The lactose-fermenting phenotype: It is designated Lac+, and the inability to ferment lactose is Lac–.
  • Differential media permit wild-type and mutant bacteria to grow and form colonies that differ in appearance.
  • Selective and differential media are helpful for isolating bacterial mutants. Some selective media permit particular mutants to grow, but do not allow the wild-type strains to grow.
  • Consider a wild-type strain of E. coli that is susceptible to the antibiotic streptomycin (phenotype Strs) and can utilize lactose as the sole source of carbon (phenotype Lac+)
  • The above image is showing an organism ( bacterium) that is susceptible to following antimicrobial agents like erythromycin, clindamycin, ciprofloxacin, ofloxacin, gentamycin, chloramphenicol, doxycycline, sulfamethoxazole-trimethoprim, and cefoxitin that reflects the probability of bacterium being wild type strain.

 Some related terminologies-


  • Clone: A strain developed from  a single parent cell
  • Strain: The descendants of single isolation in pure culture
  • Isolation: Separation of particular microorganisms from a mixed population that exists in nature.
  • Pure culture: A culture that contains only one type of microorganisms.


  1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1951032/
  2. https://www.ncbi.nlm.nih.gov/books/NBK7908/
  3. Textbook of practical Microbiology by Subhash Chandra Parija
  4. Jawetz, Melnick and Adelberg’s Medical Microbiology. Editors: Geo. F. Brook, Janet S. Butel & Stephen A. Morse, 21st ed 1998, Publisher Appleton & Lance, Co Stamford Connecticut.
  5. Mackie and Mc Cartney Practical Medical Microbiology. Editors: J.G. Colle, A.G. Fraser, B.P. Marmion, A. Simmous, 4th ed, Publisher Churchill Living Stone, New York, Melborne, Sans Franscisco 1996.
  6.  Textbook of Diagnostic Microbiology. Editors: Connie R. Mahon, Donald G. Lehman & George Manuselis, 3rd edition2007, Publisher Elsevier.
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