African Journal of
Microbiology Research

  • Abbreviation: Afr. J. Microbiol. Res.
  • Language: English
  • ISSN: 1996-0808
  • DOI: 10.5897/AJMR
  • Start Year: 2007
  • Published Articles: 4875

Full Length Research Paper

Low accuracy of the McFarland method for estimation of bacterial populations

Lozano Guzmán Eduardo
  • Lozano Guzmán Eduardo
  • Facultad de Ciencias Químicas (Campus Durango), Universidad Juarez del Estado de Durango, Av. Veterinaria s/n, Col. Valle del Sur, Durango, Durango, CP 34120, Mexico.
  • Google Scholar
Beatriz Santos Ramirez
  • Beatriz Santos Ramirez
  • Facultad de Ciencias Químicas (Campus Durango), Universidad Juarez del Estado de Durango, Av. Veterinaria s/n, Col. Valle del Sur, Durango, Durango, CP 34120, Mexico.
  • Google Scholar
Cervantes Flores Maribel
  • Cervantes Flores Maribel
  • Facultad de Ciencias Químicas (Campus Durango), Universidad Juarez del Estado de Durango, Av. Veterinaria s/n, Col. Valle del Sur, Durango, Durango, CP 34120, Mexico.
  • Google Scholar
María Guadalupe Nieto Pescador
  • María Guadalupe Nieto Pescador
  • Facultad de Ciencias Químicas (Campus Durango), Universidad Juarez del Estado de Durango, Av. Veterinaria s/n, Col. Valle del Sur, Durango, Durango, CP 34120, Mexico.
  • Google Scholar
Francisco Javier Moreno Cruz
  • Francisco Javier Moreno Cruz
  • Facultad de Ciencias Químicas (Campus Durango), Universidad Juarez del Estado de Durango, Av. Veterinaria s/n, Col. Valle del Sur, Durango, Durango, CP 34120, Mexico.
  • Google Scholar


  •  Received: 20 May 2018
  •  Accepted: 27 July 2018
  •  Published: 21 August 2018

Abstract

The McFarland method is designed to estimate bacterial concentrations by means of a turbidity scale (absorbance) which consists of a series of tubes previously calibrated, and with an optical density produced by the precipitation of barium sulphate. This absorbance is compared to bacterial populations. The most used absorbance is the one corresponding to 0.5 on that scale, which assumes a population of 1.5×108 cfu/mL (colony forming units per milliliter). In order to verify the accuracy of this scale, 25 different bacterial species were tested, adjusting to 0.5 of the McFarland scale and then an aliquot in plate with agar was cultured in triplicate to account for the population. The results showed very diverse populations, with variations ranging from 30 to 300% of what was expected (0.5×108 to 3×108 cfu/mL). The most important implications of this are in studies of microbial ecology, in clinical microbiology, in studies on sensitivity to antibiotics and in areas of quality control. It is suggested to take special care when it is required to establish, with more accuracy, the population of a crop.

 

Key words: McFarland, accuracy, bacterial population.