Description and validation of a rapid (1 h) flow cytometry test for enumerating thermophilic bacteria in milk powders

Application of flow cytometry for rapid bacterial enumeration and cells physiological state detection to predict acidification capacity of natural whey starters

Natural whey starter cultures are undefined microbial communities mainly consisting of thermophilic lactic acid bacteria (LAB). The technological pressure that shapes the natural whey starter community before and during the back-slopping procedure can impact the amount and viability of the different thermophilic LAB. Traditional culture-dependent analytical methods are useful for evaluating natural whey cultures based on plate enumeration with various culture media and are commonly used as self-control procedures in dairy items. These methods have high variability and require days to obtain results. As the dairy industry has been searching for a solution to this problem for a long time, researchers must explore alternative methods for the technological evaluation of natural whey and assessment of the health status of the thermophilic acidifying bacteria community in the cheesemaking process. The flow cytometry approach has been considered an alternative to classical methods in this work sector. This study compared bacterial enumeration by plate counting and flow cytometry on natural whey samples. Flow cytometry results showed positive agreement with a tendency to overestimate, linearity, and correlation with plate counting. Other parameters have also been introduced for evaluating a natural whey starter, measuring the physiological state of the cells. Specifically, cell-wall damage and metabolic activity were also evaluated, allowing us to quantify the number of cells even in sub-optimal physiological conditions.

The Prevalence and Control of Bacillus and Related Spore-Forming Bacteria in the Dairy Industry

Milk produced in udder cells is sterile but due to its high nutrient content, it can be a good growth substrate for contaminating bacteria. The quality of milk is monitored via somatic cell counts and total bacterial counts, with prescribed regulatory limits to ensure quality and safety. Bacterial contaminants can cause disease, or spoilage of milk and its secondary products. Aerobic spore-forming bacteria, such as those from the genera Sporosarcina, Paenisporosarcina, Brevibacillus, Paenibacillus, Geobacillus and Bacillus, are a particular concern in this regard as they are able to survive industrial pasteurization and form biofilms within pipes and stainless steel equipment. These single or multiple-species biofilms become a reservoir of spoilage microorganisms and a cycle of contamination can be initiated. Indeed, previous studies have highlighted that these microorganisms are highly prevalent in dead ends, corners, cracks, crevices, gaskets, valves and the joints of stainless steel equipment used in the dairy manufacturing plants. Hence, adequate monitoring and control measures are essential to prevent spoilage and ensure consumer safety. Common controlling approaches include specific cleaning-in-place processes, chemical and biological biocides and other novel methods. In this review, we highlight the problems caused by these microorganisms, and discuss issues relating to their prevalence, monitoring thereof and control with respect to the dairy industry.

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

AIMS

To evaluate high-throughput optical oxygen microrespirometry for selective detection and predictive identification of aerobic bacteria.

METHODS AND RESULTS

Using GreenLight probe, standard 384-well plates and time-resolved fluorescence reader, a representative panel of 16 partially selective media and 9 aerobic bacteria (Escherichia coli, Bacillus cereus, Staphylococcus aureus, Campylobacter jejuni, Yersinia enterocolitica, Pseudomonas aeruginosa, Streptococcus pyogenes, Salmonella typhimurium and Listeria innocua) were analysed. For each medium, bacterial strain and dilution, growth profiles were recorded, from which calibrations, doubling/generation times and growth patterns in different media were determined. Analytical performance, selectivity and general usability of the method were assessed, and mixed bacterial cultures were analysed.

CONCLUSION

The microrespirometry platform facilitates simple, real-time detection and predictive identification of aerobic bacteria by looking at the patterns of their growth and respiration in several media and determining their growth and doubling times.

SIGNIFICANCE AND IMPACT OF THE STUDY

The new screening method can be used for routine microbiological analysis and testing of aerobic bacterial cultures as well as complex food, environmental and clinical samples.

Extension of platelet shelf life from 4 to 5 days by implementation of a new screening strategy in Germany

BACKGROUND

The Paul-Ehrlich-Institute analysed all fatalities due to bacterial infections between 1997 and 2007. Thereafter, the platelet shelf life was reduced to a maximum of 4 days after blood donation because the majority of all cases of severe transfusion-transmitted bacterial infections occurred with day 5 platelets. The current study compares the analytical sensitivity and the diagnostic specificity of four rapid bacterial detection procedures.

METHODS

Nine transfusion-relevant bacterial strains were spiked in pooled platelets or apheresis platelets at a low concentration (10 CFU/bag). Samples were collected after day 3, day 4 and day 5 and investigated by four rapid bacterial detection methods (modified BacT/ALERT, Bactiflow, FACS method and 16s DNA PCR methods).

RESULTS

Seven out of nine bacterial strains were adequately detected by BacT/ALERT, Bactiflow and PCR in apheresis platelets and pooled platelets after sample collection at day 3, day 4 and day 5. For three bacterial strains, analytical sensitivity was reduced for the FACS method. Two bacterial strains did not grow under the storage conditions in either pooled or apheresis platelets.

CONCLUSIONS

A late sample collection on day 3, day 4 or day 5 after blood donation in combination with a rapid bacterial detection method offers a new opportunity to improve blood safety and reduce errors due to sampling., BacT/ALERT, Bactiflow or 16s ID-NAT are feasible for late bacterial screening in platelets may provide data which support the extension of platelet shelf life in Germany to 5 days.

Detection of microbial contaminants in mammalian cell cultures using a new fluorescence-based staining method

AIMS

Microbial contamination of cell culture production processes is a current concern for biopharmaceutical industries. Traditional testing methods require several days to detect contamination and may advantageously be replaced by a rapid detection method. We developed a new method combining membrane filtration to microcolonies fluorescence staining method (MFSM) and compared it to epifluorescence microscopy.

METHODS AND RESULTS

Both methods were used to detect bacteria in CHO cells cultures. The epifluorescence microscopy showed to be limited by filterability, media interference and nonrobustness issues, whereas MFSM enabled consistent detection of Bacillus cereus, Staphylococcus epidermidis and Propionibacterium acnes after, respectively, 8, 9 and 48 h of incubation. Thanks to the nondestructive feature of the MFSM, stained membranes could be reincubated on culture media to yield visible colonies used for identification.

CONCLUSIONS

The new method described in this study showed its ability to detect microbial contaminants in cell culture samples with time-to-results from 2-5 times shorter than the traditional testing method.

SIGNIFICANCE AND IMPACT OF THE STUDY

The MFSM can be used as monitoring tool for cell cultures to significantly shorten detection times of microbial contamination, while preserving the ability to identify the contaminants and their viability.

Development of a nondestructive fluorescence-based enzymatic staining of microcolonies for enumerating bacterial contamination in filterable products

AIMS

Develop a nondestructive fluorescence-based staining procedure to rapidly detect and enumerate bacteria in filterable samples.

METHODS AND RESULTS

The study consists in the development of a staining solution and a protocol to fluorescently detect microcolonies on cellulose membranes. After detection, membranes can be re-incubated on media to yield colonies. Carboxyfluorescein diacetate was selected among other carboxyfluorescein derivatives for its staining efficiency and the absence of background. Several permeabilizers were evaluated for their ability to promote dye uptake into cells without affecting viability. We demonstrated that a combination of n-Octyl β-D-glucopyranoside, sodium hexametaphosphate, lithium chloride and rubidium chloride significantly increased the staining efficiency of bacteria without affecting their viability. The method developed allowed the detection in <9 h of all tested aerobic bacteria and in 48 h of the anaerobic slow grower Propionibacterium acnes.

CONCLUSIONS

This method allows the rapid detection of bacteria in filterable samples in at least three to five times faster than traditional microbiological method.

SIGNIFICANCE AND IMPACT OF THE STUDY

The advantage of this nondestructive procedure is to allow contaminants identification after membrane re-incubation. This method could be easily applied in routine in pharmaceutical, clinical and food and beverage industries to monitor contaminations.

Application of quantitative reverse-transcription PCR (qRT-PCR) for the determination of the total viable count (TVC) on meat samples

AIM

To apply a quantitative reverse-transcription PCR (qRT-PCR) method to determine the total viable count (TVC) on meat samples.

METHODS AND RESULTS

Using two sets of primers to target the ribonuclease-P (RNase P) RNA transcripts of Gram-positive and Gram-negative bacteria, standard curves were generated using the LightCycler 2.0 instrument (Roche Diagnostics). RNA standards were extracted from known cell numbers and subsequently converted to cDNA for the construction of standard curves for quantification of the TVC of beef carcass swabs (n = 60) and beef (n = 30), chicken (n = 50) and pork (n = 49) pieces. A high correlation between the standard plate count method and the qRT-PCR was observed for beef swabs (R(2) = 0·93) and beef pieces (R(2) = 0·82). The correlation coefficient for chicken pieces and pork pieces were R(2) = 0·34 and 0·55, respectively. Using beef pieces (n = 13), an interlaboratory study was conducted and each participating laboratory (n = 3) found a reasonable degree of agreement between the cultural method and the PCR method.

CONCLUSIONS

The qRT-PCR assay used in this study can enumerate the total bacteria on beef samples with a high degree of accuracy.

SIGNIFICANCE AND IMPACT OF THE STUDY

The qRT-PCR method may have the potential to be applied to various sample types as an alternative rapid method for determining TVCs; however, further validation would be required.

Novel flow cytometry-based screening for bacterial contamination of donor platelet preparations compared with other rapid screening methods

BACKGROUND

Bacterial contamination is the major infectious hazard associated with transfusion of platelet preparations (PLTs). Routine testing for bacterial contamination in PLTs has become common, but transfusion-transmitted bacterial sepsis has not been eliminated. Here, we describe a novel flow cytometry-based method for point-of-issue screening of PLTs for bacterial contamination.

METHODS

We used the BactiFlow flow cytometer to detect and count bacteria based on esterase activity in viable cells. We compared the assay to incubation (BacT/Alert culture system) and rapid nucleic acid-based or immunoassay (reverse transcription PCR, Pan Genera Detection) methods.

RESULTS

We established a protocol for bacterial screening of PLTs consisting of enzymatic digestion and centrifugal filtration for the elimination of viable platelets and selective labeling of bacteria with fluorescent esterase substrate (ChemChrome V23). Results from the BactiFlow showed an excellent correlation (r = 0.9923 E. coli, r = 0.9736 S. epidermidis) to traditional plate count results. The lower detection limit of the assay was determined to be 150 CFU/mL, and the time to result was <1 h.

CONCLUSIONS

Our study demonstrates that BactiFlow flow cytometry is suitable for rapid screening of PLTs for bacterial contamination and fulfils the requirements for a point-of-issue testing of PLTs with acceptable time to result, specificity, sensitivity, and cost.

Rapid methods for diagnosis of bloodstream infections

Direct detection technologies for pathogenic microorganisms are emerging to be applied in the diagnosis of serious bloodstream infections and infections at sterile body sites, as well as for quality control measures prior to the release of sterile blood products and to ascertain microbial safety of food. Standard blood cultures as the current gold standard for detection of bacteraemia/sepsis and other culture-based microbiological identification procedures are comparatively slow and have limited sensitivity for fastidious or slow-growing microorganisms. Rapid nucleic acid-based technologies with PCR amplification or hybridisation probes for specific pathogens, broad-range bacterial or fungal assays, flow cytometry, as well as protein-based characterisation by mass spectrometry, aim at identification of pathogenic microorganisms within minutes to hours. Interpretation of direct detection of panbacterial or panfungal nucleic acids instead of living microorganisms in blood is complex, given the risk of contamination, the ubiquitous presence of bacterial and fungal DNA, and the lack of a gold standard. Since many of the infections at sterile sites, particularly sepsis, are medical emergencies requiring immediate therapeutic responses, rapid technologies could contribute to reduction of morbidity, mortality, and of the economic burden. This review summarises the currently available data on rapid non-culture-based technologies and outlines the potential clinical usefulness in infectious disease diagnosis.