Low accuracy of the McFarland method for estimation of bacterial populations

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 1.5×10 8 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 (0.5×10 8 to 3×10 8 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.

Rapid detection of Staphylococcus aureus in blood culture samples using human IgG-based lateral flow assay

“…The numbers are valid only for those microorganisms similar to E. coli ( 19 ). The McFarland method has low accuracy for the estimation of the bacterial populations ( 20 ). In this study, the limit of the LFIA for detecting S. aureus from bacterial colonies was 10 7 CFU/mL (10 6 CFU/reaction).…”

Green synthesis of Silver Nanoparticles Using Garlic Extract with Enhanced Antibacterial Activities Against Lactobacillus Acidophilus

“…MRS (Man, Rogosa, Sharpe) medium was used as a growth medium to determine the sensitivity of Lactobacillus acidophilus. A standard microbial suspension with a concentration equivalent to 0.5 McFarland's of Lactobacillus acidophilus was prepared to perform Susceptibility tests [23].…”

A Mild catalyzed imino Diels-Alder reaction for the synthesis of N-(2-(o-tolyl)-1,2,3,4-tetrahydroquinoline-4-yl)formamide derivatives as regulators of Quorum Sensing in Pseudomona Aeruginosa

“…The inoculum was standardized with a 0.5 McFarland pattern, which was prepared by adding 0.5 mL BaCl2 0.048 M with 99.5 mL H2SO4 0.18 M and stirring for 2 minutes. This solution was read in a spectrophotometer at a wavelength of 690 nm, obtaining a turbidity (absorbance) of 0.082 corresponding to 0.5 on the McFarland scale, comparable to a bacterial suspension containing 1.5 x 108 CFU/mL [70]. After 12 hours, the absorbance of the inoculum was measured to the 0.5 McFarland scale (absorbance approximately 0.082).…”

“…Pyocyanin Production. It was measured from the bacterial culture supernatant following the protocol reported by O'Loughlin et al 2013 [70] with some modifications. 1 mL of a 1:1000 subculture of P. aeruginosa was seeded in 9 mL LB broth and incubated at 37°C.…”