A METHOD FOR DETECTING AND ENUMERATING AIRBORNE VIRULENT BACTERIOPHAGE OF DAIRY STARTER CULTURES

For the assessment of the bacteriophage load in the ambient air of dai ries producing fresh cheese, air samples were taken with an air sampli ng apparatus, and a simple method was elaborated for transportation an d preparation of samples and subsequent determination of phage titers. Sampling was done on gelatin filters which were immersed in a simple buffer system (quarter-strength Ringer's solution supplemented with 10 % (vol/vol) skim milk is recommended). The filters were stored in ste rile plastic bags. Determination of phage titers (plaque-forming units (PFU) per m(3) air) was done with phage-sensitive isolates of the sta rter culture in use (Lactococcus lactis). The collected phage material revealed high stability on the gelatin filters under routine storage and transportation conditions - also after prolonged storage for sever al days at different temperatures and after repeated determination of phage titers. The method is suitable for different phage types known t o be widespread in dairies (in particular: small isometric- and prolat e-headed phages of the 936 and c2 phage species, respectively). The li mit of detection was found to be below 5 PFU per m(3) of ambient air, when sampling was done for 12 min with a flow rate of 100 liters per m in. A phage load of up to 2x10(8) PFU per m(3) ambient air was determi ned in close proximity of the whey separators which are known to be th e main source of phage aerosols. Within the production area, a concent ration gradient down to as low as 2x10(2) PFU per m(3) ambient air (cl ose to the fermentation vessels) was documented.