Detection of bacteriophages in the dairy industry using MD8 air samplers
Despite global efforts to develop phage-resistant starter cultures, bacteriophages remain the main cause of acid disturbances and ultimately the cause of product spoilage in the dairy industry. In our article, we present a simple method for the quantitative detection of viral bacteriophages using MD8 air samplers and gelatin filters.
Despite global efforts to develop phage-resistant starter cultures, bacteriophages remain the main cause of acid disturbances and ultimately the cause of product spoilage in the dairy industry. Complete and detailed phage monitoring is necessary to replace phage sensitive cultures with phage-resistant strains in time and localize the internal source of infection. In this article, we present a simple method for the quantitative detection of viral bacteriophages in dairy air. This method guarantees high reliability in the detection of bacteriophages under normal conditions of storage and transport in practice.
Conditions for the sampling method in dairies
The research conducted in dairies was based on a method tested using bacteriophages of mesophilic lactic acid streptococci (Lactococcus lactis). The important criteria that played a key role in the application of the method are the following:
- use of a simple buffer system to resuspend water-soluble Sartorius gelatin filters;
- simple filter handling properties after sampling;
- high stability of collected phages - even after storage of the filter for several days in different temperature conditions and after repeated determination of phage titer;
- suitability of the method for different strains of bacterial phages commonly found in dairies (Figure 1).
Figure 1. Scanning electron micrographs of two bacteriophage strains of mesophilic lactic acid streptococci (Lactococcus lactis) with round (isodiametric) or oblong (protruding) heads, commonly found in dairies. The band is reduced to 50 nm.
Detection method using gelatin filters
The bacteriophage detection method was performed using an MD8 air sampler, which proved to be a successful device for detecting microorganisms in the air, eliminating their harmful effects in the test area. In the conducted research, an MD8 air sampler with gelatine filters with a diameter of 80 mm and a pore size of 3 μm was used.
Bacteriophages were sampled at a rate of 100 l per minute during a sampling interval of 3 - 12 minutes. After sampling, the filters were placed in sterile 10 x 15 polyethylene bags, suitable for this purpose. 5-10 ml of buffer composed of ¼ Ringer's solution with the addition of 10% skim milk was then added to the bag, after which the stored samples were sent for analysis to the research laboratory.
Research results
By the method presented here, virulent bacteriophases can be detected over a wide concentration gradient. In the immediate vicinity of the phase aerosol source, it is possible to detect a phage load of up to 3 × 108 units that form plaque per m3 of ambient milk air. The lower limit of detection is less than 5 phages per m3 of ambient air. In the case of mesophilic lactic acid streptococci, there are basically two different strains of phage that cause acid interference (phages with round, isodiametric heads and those with oblong, protruding heads; see scanning micrograms in Fig. 1). The presented method is equally suitable for both phage strains.
For more information on the research, see the original Application Note document.