Development of a robust flow cytometric assay for determining numbers of viable bacteria

Several fluorescent probes were evaluated as indicators of bacterial viability by flow cytometry. The probes monitor a number of biological factors that are altered during loss of viability. The factors include alterations in membrane permeability, monitored by using fluorogenic substrates and fluorescent intercalating dyes such as propidium iodide, and changes in membrane potential, monitored by using fluorescent cationic and anionic potential-sensitive probes. Of the fluorescent reagents examined, the fluorescent anionic membrane potential probe bis-(1,3-dibutylbarbituric acid)trimethine oxonol [DiBAC(inf4)(3)] proved the best candidate for use as a general robust viability marker and is a promising choice for use in high-throughput assays. With this probe, live and dead cells within a population can be identified and counted 10 min after sampling. There was a close correlation between viable counts determined by flow cytometry and by standard CFU assays for samples of untreated cells. The results indicate that flow cytometry is a sensitive analytical technique that can rapidly monitor physiological changes of individual microorganisms as a result of external perturbations. The membrane potential probe DiBAC(inf4)(3) provided a robust flow cytometric indicator for bacterial cell viability.

Rapid assessment of antibiotic effects on Escherichia coli by bis-(1,3-dibutylbarbituric acid) trimethine oxonol and flow cytometry

The effects of selected antibiotics on Escherichia coli were studied by flow cytometry with the fluorescent anionic membrane potential probe bis-(1,3-dibutylbarbituric acid) trimethine oxonol [DiBAC4(3)]. The actions of azithromycin, cefuroxime, and ciprofloxacin at five times the MIC on E. coli were compared by the traditional CFU assay and flow cytometry. Changes in viable counts of bacteria determined with DiBAC4(3) and by flow cytometry following treatment with the antibiotics showed trends similar to those found by the CFU assays. However, viable counts determined by flow cytometry following antibiotic treatment were 1 to 2 logs higher than those determined by the corresponding CFU assays. All the results obtained by flow cytometry were provided within 10 min after sampling, whereas the conventional CFU assay results took at least 18 h. The results indicated that flow cytometry is a sensitive analytical technique that can rapidly monitor the physiological changes of individual microorganisms following antibiotic action and can provide information on the mode of action of a drug. The membrane potential probe DiBAC4(3) provides a robust flow cytometric indicator for bacterial cell viability.

Analysis of pH-induced population oscillations of Saccharomyces cerevisiae and Escherichia coli using photon correlation spectroscopy

Following a recent successful application of p.c.s. to liquid chromatography in the biotechnology industry, its usefulness as a contamination monitor in the fermentation industry was assessed. It was found that: (i) the intensity bias of the technique limits its uses to the detection of contaminants when they are larger than the host; (ii) the inherent heterogeneity of microbial cultures prevents the use of multiangle studies, and (iii) the large size of bacteria make the use of p.c.s. in flowing, on-line systems impractical.

Agglutination of Legionella pneumophila by antiserum is accelerated in an ultrasonic standing wave

The agglutination of Legionella pneumophila (LP) by diluted anti-LP whole rabbit serum has been compared in conventional microwell plates and in capillary containers where the suspension was exposed to a 1 MHz ultrasonic standing wave field. A positive reaction in the standing wave field was detected as a series of cell agglutinates, separated by half an acoustic wavelength (0.75 mm), distributed along the length of the capillary. Agglutination occurred in 60 s or less with ultrasound, while the incubation period for a positive microwell test was often of the order of hours. At a given antiserum concentration, ultrasound-induced agglutination occurred at LP concentrations two-fold lower than those giving a positive result in the microwell plate assays. At cell concentrations near the lower limit for detection of a positive result in the microwell plates a positive reaction was detected in the standing wave field at antiserum concentrations up to 500-fold lower than those forming visible precipitates in the conventional assay.

The effect of oxygen-dependent antimicrobial systems on strains of Legionella pneumophila of different virulence

Four strains of Legionella pneumophila of different virulence as identified by ability to produce pneumonia and death in guinea-pigs infected by a fine-particle aerosol were examined for factors which may intracellularly influence virulence. Possible bactericidal mechanisms possessed by alveolar phagocytes were examined. A relationship could be established between resistance to H2O2, catalase activity and virulence amongst the strains. Virulent strains resisted the bactericidal activity generated by the xanthine oxidase system; avirulent strains did not. Incorporation of various specific inhibitors of the xanthine oxidase system indicated that the main bactericidal activities were associated with the production of H2O2 and hydroxyl radicals (.OH). All strains of L. pneumophila were susceptible to the bactericidal activity generated by the myeloperoxidase-H2O2-halide system, confirming earlier observations that polymorphonuclear neutrophil leucocytes (PMNLS) are able to kill both virulent and avirulent strains of L. pneumophila.

A comparison of virulence of two strains of Legionella pneumophila based on experimental aerosol infection of guinea-pigs

Two strains of Legionella pneumophila (LP) serogroup I, of differing virulence, were examined in terms of numbers of viable organisms in tissues, pyrexia and mortality following aerosol infection. The Corby strain was the more virulent, with pyrexia and deaths of guinea-pigs 3 to 6 days after infection. This strain multiplied very rapidly in the lungs to reach a peak of 5 X 10(11) viable organisms/lung. Organisms were present in the blood, liver, spleen and kidney. The Philadelphia-1 strain (NCTC 11192) was unable to replicate in the lung and was cleared between 14 and 21 days after infection. Pyrexia was not observed. No guinea-pigs died and viable LP was not found in any organ other than the lung. Lung lavages on aerosol infected animals were performed and the virulent Corby strain was found to be mainly intracellular. The avirulent Philadelphia-1 strain was found predominantly in the extracellular location. There were approximately 10 times the number of viable virulent LP in the lung macrophage fraction than in the lung PMNL fraction. In comparison, there were approximately equal numbers of the viable avirulent strain in the macrophages and the PMNL. Experimental evidence suggests that the macrophage preferentially supports the growth of the virulent Corby strain compared with the PMNL. The avirulent strain on the other hand appears to be destroyed by both the macrophages and the PMNL.