The spot test method for the in-plant enumeration of bacteriophages with paired cultures of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus

Abstract

Abstract Using M17 and acidified MRS media for the selective development of Streptococcus salivarius subsp. thermophilus (S. thermophilus) and Lactobacillus delbrueckii subsp. bulgaricus (L. bulgaricus), it was possible to determine phage populations in plates seeded with the paired cultures using the spot test or the standard double layer plaque (DLP) method. There were no significant differences in the enumeration of L. bulgaricus phage 448-c5 on acidified MRS agar when a pure culture of L. bulgaricus 448 or a paired culture of L. bulgaricus 448 and S. thermophilus 1151 were inoculated in the top agar layer. Population readings of S. thermophilus phage 1151-B7 on M17 agar were 0.2 log lower with paired cultures compared to the pure S. thermophilus 1151 strain. Nevertheless, a good correlation (R2 = 0.92) was achieved between phage counts obtained between the two host inoculation procedures. The spot test was compared to the standard DLP method for the enumeration of phages of the two thermophilic cultures. There were no significant differences in the enumeration of L. bulgaricus phage 448-c5 on acidified MRS agar when spot or standard DLP methods were used to determine the phage titre. Population readings of S. thermophilus phage 1151-B7 on M17 agar were slightly lower using the spot test, but correlation between counts obtained between the spot and standard DLP methods was acceptable (R2 = 0.80). Petri plates preinoculated with the host organism could be stored at 4 °C for 18 days and used for a spot test determination of phage titres. The spot test is proposed as a time-saving and easy method for the in-plant enumeration of phages of paired cultures of S. thermophilus and L. bulgaricus.