title: Evaluation of qPCR and plate counting for quantifying thermophilic starters in cheese.
abstract: The respective inputs of plate counting and qPCR for the quantification of starters in cheese were evaluated using hard-cooked cheeses made with various starter combinations. Five starter strains were quantified at their different growth phases, from 0.5 h to day 214 of manufacture: one strain of Streptococcus thermophilus (ST) and two strains each of Lactobacillus delbrueckii (LD) and Lactobacillus helveticus (LH). Numbers of colony-forming units (CFU) were obtained by plate counting (PC) and qPCR (GNCFU). The qPCR standard curves require a special attention since GNCFUdepends on the degree of culturability of the standard culture. Discrepancies were evidenced from the vat milk to the end of ripening. During cheese making, GNCFUwere lower than PC at the inoculation for all ST and LD samples and 83% of the LH samples, and during both the exponential and stationary phases for many of the ST and LD samples. During ripening which corresponds to the decline phase, GNCFUwere higher than PC for 90% of the ST, 78% of the LD and 69% of the LH samples. Hypotheses are discussed to explain those discrepancies. The data provided by GNCFUand PC complement each other, providing a better description of starter growth in cheese.
authors.affiliation: URTAL, INRA, 39800, Poligny, France. Electronic address: christine.achilleos@inra.fr.
authors.forename: Christine
authors.initials: C
authors.lastname: Achilleos
authors.affiliation: URTAL, INRA, 39800, Poligny, France. Electronic address: francoise.berthier@inra.fr.
authors.forename: Françoise
authors.initials: F
authors.lastname: Berthier
mesh_terms: D000818:Animals; D058491:Bacterial Load; D002611:Cheese / Q000382:microbiology*; D004269:DNA, Bacterial; D005516:Food Microbiology; D016680:Genome, Bacterial; D007778:Lactobacillus / Q000235:genetics / Q000302:isolation & purification*; D008892:Milk / Q000382:microbiology; D060888:Real-Time Polymerase Chain Reaction; D048249:Streptococcus thermophilus / Q000235:genetics / Q000302:isolation & purification*; D013696:Temperature
chemical_list: D004269:DNA, Bacterial
keywords: Cheese; Lactobacillus delbrueckii; Lactobacillus helveticus; Plate counting; Quantitative PCR; Streptococcus thermophilus