A new method for the detection of microorganisms in blood cultures: Part I. Theoretical analysis and simulation of blood culture processes

Abstract

The physical and chemical changes occurring in blood as a consequence of microbial activity can be used as quantitative indicators of the presence of microorganisms in blood cultures. This paper reports on the theoretical analysis and computer simulation of the changes in the physical and chemical properties of blood expected as a result of the presence of microorganisms and explores the possibilities of spectrophotometric systems for the early detection of pathogens. It is concluded that multi-wavelength reflectance methods are suitable and that the approach reported herein can lead to considerable simplifications and cost reduction of blood culture systems.Les changements physiques et chimiques survenant dans le sang suiteà une activité microbienne peuventêtre utilisés comme indicateurs quantitatifs de la présence de microorganismes dans les cultures sanguines. On présente dans cet article l'analyse théorique et la simulation par ordinateur des changements dans les propriétés physiques et chimiques du sang issus de la présence de microorganismes et on explore les possibilités des systèmes spectrophotométriques pour la détection précoce des pathogènes. On conclut que les méthodes de réflectanceà longueurs d'onde multiple conviennent et que l'approche mentionnée ici peut conduireà des simplifications considérables et une réduction des coûts des systèmes de culture sanguine.

Kinetically Limited Differential Centrifugation as an Inexpensive and Readily Available Alternative to Centrifugal Elutriation

“…If this is taken into account using Equation 12, then the predicted settling velocities for RBCs in Mediums 1 and 2 are 307 microns/second and 62 microns/second respectively. Also, at the time blood cultures turn positive, they contain 10 8 CFU (colony forming units) (cells) of bacteria per mL of suspension (24). However, because of their small size (radius 1 micron), their volume fraction remains less than 0.001, and the hindered settling effect can be neglected.…”

Novel Electrical Method for Early Detection of Viable Bacteria in Blood Cultures

“…A significant increase in CO 2 is taken to indicate the presence of viable bacteria in the suspension and hence in blood. Due to inherent limitations imposed by the metabolic rate of individual bacterial cells (e.g., one Escherichia coli bacterium consumes only ϳ2 ϫ 10 Ϫ14 moles of O 2 per hour [10]), the concentrations of bacteria in the suspension typically have to rise to ϳ10 8 CFU/ml before they can be detected (12). Given the low initial loads, this implies that 20 to 30 doubling times must elapse before the bacteria can be detected, resulting in the long TTPs (12 to 72 h) typically observed.…”

Multi-frequency electrical impedance method for detection of viable micro-organisms, their quantification and their characterization

“…For a well-oxygenated medium (with a dissolved oxygen concentration of concentration ~ 2x10 -4 M), this represents only a 0.00001% change in the O 2 level in the medium as a whole. Such a change is too small to be discerned by virtually any sensor, and hence one has to wait for the cells to proliferate to ~10 8 CFU/ml (Smith et al 2008) before a discernable change in the physical properties of the medium can be observed. This also implies that attempts to improve performance by building better sensors for metabolic products such as CO 2 are only likely to produce marginal improvements at best.…”

“…For instance, Garcia Rubio and coworkers have developed a technique based on multi-wavelength reflectance spectra that detects changes to the relative amounts of O 2 /CO 2 based on optical properties of hemoglobin present in the sample (decrease of O 2 leads to the conversion of oxyhemoglobin to hemoglobin). By essentially having the "sensors" (hemoglobin) dispersed in the media, this method reduces the TTP of cultures by 20%, compared to BACTEC (Smith et al 2008).…”

“…For instance, even a relatively fast growing/metabolizing bacterium like E coli consumes only 2 x 10 -14 moles of O 2 per hour (Sengupta et al 2006a), whereas a typical well-oxygenated suspension has a dissolved O 2 concentration ~ 10 -6 moles/ml (Körner and Zumft 1989). Hence, with the current systems, one has to wait for the concentration of living microorganisms to reach a "threshold" of ~10 6 to 10 8 /ml before their metabolism can change the medium properties (O 2 /CO 2 concentration, pH, conductivity etc) to a measurable degree (Smith et al 2008). The time that elapses before a change in the medium properties is measured (and the presence of live microorganisms in the original culture inferred), is referred to as the Time to Detection (TTD).…”