Rapid detection and respirometric profiling of aerobic bacteria on panels of selective media

Application of tris-(4,7-Diphenyl-1,10 phenanthroline)ruthenium(II) Dichloride to Detection of Microorganisms in Pharmaceutical Products

tris-[(4,7-diphenyl-1,10-phenanthroline)ruthenium(II)] dichloride (Ru(DPP)₃Cl₂), a fluorescent sensor which is sensitive to the amount of oxygen in the sample, was applied using the fluorescent optical respirometry (FOR) technique. The oxygen in the samples quenches the fluorescence. The fluorescence intensity depends on the metabolic rate of the viable microorganisms. The effect of DMSO and plant extracts on bacteria was determined by FOR. It was shown that the MIC values obtained by FOR were consistent with the results of the MIC determinations using the method of serial dilutions; at the same time, the effects of concentrations lower than the growth-inhibitory concentrations on microbial cells were demonstrated. The FOR method enables the detection of multiplying bacteria in sterile and non-sterile pharmaceutical preparations in real time, which significantly shortens the time required to obtain results and allows the introduction of repair processes in the production. This method also allows for quick, unambiguous detection and the counting of the viable cells of aerobic microorganisms in non-sterile pharmaceuticals.

Ru(II) Oxygen Sensors for Co(III) Complexes and Amphotericin B Antifungal Activity Detection by Phosphorescence Optical Respirometry

The measurement of oxygen consumption is an important element in the understanding of an organism's metabolic state. Oxygen is also a phosphorescence quencher, which allows the evaluation of phosphorescence emitted by oxygen sensors. Two Ru(II)-based oxygen-sensitive sensors were used to study the effect of chemical compounds [(1) = [CoCl₂(dap)₂]Cl, and (2) = [CoCl₂(en)₂]Cl (AmB = amphotericin B) against reference and clinical strains of Candida albicans. The tris-[(4,7-diphenyl-1,10-phenanthroline)ruthenium(II)] chloride ([Ru(DPP)₃]Cl₂) (Box) adsorbed onto the Davisil^TM silica gel was embedded in the silicone rubber Lactite NuvaSil^® 5091 and the coating on the bottom of 96-well plates. The water-soluble oxygen sensor (BsOx = tris-[(4,7-diphenyl-1,10-phenanthrolinedisulphonic acid disodium)ruthenium(II)] chloride 'x' hydrate = {Ru[DPP(SO₃Na)₂]₃}Cl₂ = water molecules were omitted in the BsOx formula) was synthesized and characterized by RP-UHPLC, LCMS, MALDI, elemental analysis, ATR, UV-Vis, ¹H NMR, and TG/IR techniques. The microbiological studies were performed in the environment of RPMI broth and blood serum. Both Ru(II)-based sensors turned out to be useful in the study of the activity of Co(III) complexes and the commercial antifungal drug amphotericin B. In addition, a new activity of the oxygen sensor, the soluble Ru(II) complex BsOx, was demonstrated, which is a mixture with amphotericin B that caused a significant increase in its antifungal activity. Thus, it is also possible to demonstrate the synergistic effect of compounds active against the microorganisms under study.

Oxygen Sensor-Based Respirometry and the Landscape of Microbial Testing Methods as Applicable to Food and Beverage Matrices

The current status of microbiological testing methods for the determination of viable bacteria in complex sample matrices, such as food samples, is the focus of this review. Established methods for the enumeration of microorganisms, particularly, the 'gold standard' agar plating method for the determination of total aerobic viable counts (TVC), bioluminescent detection of total ATP, selective molecular methods (immunoassays, DNA/RNA amplification, sequencing) and instrumental methods (flow cytometry, Raman spectroscopy, mass spectrometry, calorimetry), are analyzed and compared with emerging oxygen sensor-based respirometry techniques. The basic principles of optical O₂ sensing and respirometry and the primary materials, detection modes and assay formats employed are described. The existing platforms for bacterial cell respirometry are then described, and examples of particular assays are provided, including the use of rapid TVC tests of food samples and swabs, the toxicological screening and profiling of cells and antimicrobial sterility testing. Overall, O₂ sensor-based respirometry and TVC assays have high application potential in the food industry and related areas. They detect viable bacteria via their growth and respiration; the assay is fast (time to result is 2-8 h and dependent on TVC load), operates with complex samples (crude homogenates of food samples) in a simple mix-and-measure format, has low set-up and instrumentation costs and is inexpensive and portable.

A sensor-based system for rapid on-site testing of microbial contamination in meat samples and carcasses

AIMS

To develop an oxygen sensor-based method for testing total aerobic viable counts (TVC) in raw meat samples and cattle carcass swabs, which is rapid, simple, affordable, provides good sensitivity and analytical performance and allows on-site use.

METHODS AND RESULTS

The test uses the same sample preparation procedure as the established plate counting TVC method for meat samples and carcasses, ISO4833-1:2013. After this liquid samples are transferred into standard 25-ml vials with built-in phosphorescent O₂  sensors and incubated on a block heater with hourly readings of sensor signals with a handheld reader, to determine signal threshold time (TT, hours) for each sample. The method is demonstrated with the quantification of TVC in industrial cuts of raw beef meat (CFU per g) and carcass swabs (CFU per cm² ). Calibration curves were generated, which give the following analytical equations for calculating the TVC load in unknown samples from measured TT values: TVC [Log(CFU per cm² )] = 7.83-0.73*TT(h) and TVC [Log(CFU per g)] = 8.74-0.70*TT(h) for the carcass swabs and meat samples respectively. The new tests show good correlation with the ISO methods, with correlation coefficients 0.85 and 0.83 respectively. The testing requires no dilutions, covers the ranges 2-7 Log(CFU per g) for the meat samples and 1-7 Log(CFU per cm² ) for carcass swabs, and has time to result 1-10 h with faster detection of more contaminated samples.

CONCLUSIONS

The sensor-based testing demonstrates simplicity, high speed, sample throughput and automation. It can provide a straightforward replacement for the conventional TVC tests, which are time consuming, laborious and have time to result of 48-72 h.

SIGNIFICANCE AND IMPACT OF THE STUDY

The method(s) can be adopted by the meat industry and research labs, and used to improve microbial quality and safety of meat products and processes.

Time-Resolved Fluorescence Assay for Measuring Oxygen Consumption Rates in Staphylococcus aureus

Oxygen consumption is a fundamental characteristic of staphylococcal physiology reflecting the energy and metabolic state of the bacterial cell. During aerobic growth, oxygen consumption rates (OCR) depend on nutrient availability and vary at different growth stages. The measurement of oxygen consumption rates provides a versatile tool to characterize the impact of various mutations, environmental cues, and antibiotics on bacterial growth and fitness. In this chapter, we describe a MitoXpress^® Xtra-based oxygen consumption assay for fast and reliable determination of respiration rates in Staphylococcus aureus. This highly reproducible and simple method requires a minimal set of reagents and allows rapid screening of multiple samples through real-time determination of the OCR with an oxygen-sensing probe and fluorescence plate reader.

Bacterial Respiration Used as a Proxy to Evaluate the Bacterial Load in Cooling Towers

Evaporative cooling towers to dissipate excess process heat are essential installations in a variety of industries. The constantly moist environment enables substantial microbial growth, causing both operative challenges (e.g., biocorrosion) as well as health risks due to the potential aerosolization of pathogens. Currently, bacterial levels are monitored using rather slow and infrequent sampling and cultivation approaches. In this study, we describe the use of metabolic activity, namely oxygen respiration, as an alternative measure of bacterial load within cooling tower waters. This method is based on optical oxygen sensors that enable an accurate measurement of oxygen consumption within a closed volume. We show that oxygen consumption correlates with currently used cultivation-based methods (R² = 0.9648). The limit of detection (LOD) for respiration-based bacterial quantification was found to be equal to 1.16 × 10⁴ colony forming units (CFU)/mL. Contrary to the cultivation method, this approach enables faster assessment of the bacterial load with a measurement time of just 30 min compared to 48 h needed for cultivation-based measurements. Furthermore, this approach has the potential to be integrated and automated. Therefore, this method could contribute to more robust and reliable monitoring of bacterial contamination within cooling towers and subsequently increase operational stability and reduce health risks.

Comparison of fluorescence optical respirometry and microbroth dilution methods for testing antimicrobial compounds

An analysis of the usefulness of the fluorescence optical respirometry test method to study several antimicrobials was performed. An oxygen-sensitive sensor: ruthenium-tris(4,7-diphenyl-1,10-phenanthroline) dichloride (Ru(DPP)3Cl2), the phosphorescence of which is quenched by molecular oxygen, was synthesised according to a method modified by us and then applied. A prototype sensitive measurement system was designed and constructed. Analyses of the impact of various antimicrobial chemical factors were performed: ampicillin, co-trimoxazole, nystatin, and newly synthesised compounds. It was shown that optical respirometry allows for analysis of the culture growth kinetics of bacteria and fungi and determination of cell growth parameters. It was shown also that MIC values obtained by fluorescence optical respirometry are consistent with the results of the MIC determinations made by serial dilution method (traditional MIC testing using CLSI). The method allows the time to obtain results to be significantly reduced (from 24-48 h to 5-7 h for bacteria and 24 yeasts) and allows the effect of concentrations below the MIC for the metabolic activity of microorganisms to be monitored. The sensitivity of the method allowed the volume of the tested samples to be lessened from 160 μl to 50 μl. Fluorescence optical respirometry allows for the rapid detection and evaluation of the action of various chemical compounds on the metabolic activity of microorganisms in real-time measurement of fluorescence intensity.