title: Development of an alternative culture medium for the selective enumeration of Lactobacillus casei in fermented milk.
abstract: Monitoring the populations of probiotic strains of the species Lactobacillus casei in food is required by food industries in order to assure that a minimum concentration of these organisms will be ingested by consumers. In this context, Petrifilm™ AC plates can be used along with selective culture media to allow the enumeration of specific groups of lactic acid bacteria. The present study aimed to assess chemical substances as selective agents for Lb. casei in order to propose a selective culture medium to be used with Petrifilm™ AC plates as an alternative protocol for the enumeration of probiotic strains of this species in fermented milk. Twenty-six probiotic and starter cultures (including six strains of Lb. casei) were plated on de Man Rogosa and Sharpe (MRS) agar with distinct concentrations of nalidixic acid, bile, lithium chloride, metronidazole, sodium propionate, and vancomycin. Vancomycin at 10 mg/L demonstrated selective activity for Lb. casei. In addition, 2,3,5-triphenyltetrazolium chlorine was identified as a compound that did not inhibit Lb. casei, and Petrifilm™ AC plates used with MRS and vancomycin at 10 mg/L (MRS-V) demonstrated more colonies of this organism when incubated under anaerobic conditions than aerobic conditions. Acidophilus milk and yoghurt were prepared, added to Lb. casei strains, and stored at 4 °C. Lb. casei populations were monitored using MRS-V and MRTLV by conventional plating and associated with Petrifilm™ AC plates. All correlation indices between counts obtained by conventional plating and Petrifilm™ AC were significant (p < 0.05), but the best performance was observed for growth on MRS-V. The obtained data indicate the efficiency of using MRS-V associated with Petrifilm™ AC plates for the enumeration of Lb. casei strains in fermented milk. However, the selective potential of this culture medium must be evaluated considering the specific strains of Lb. casei and the starter cultures inoculated in the fermented milk that requires monitoring.
authors.affiliation: Departamento de Veterinária, Universidade Federal de Viçosa, Campus UFV, 36570 000 Viçosa, MG, Brazil.
authors.forename: Monique
authors.initials: M
authors.lastname: Colombo
authors.affiliation: Departamento de Veterinária, Universidade Federal de Viçosa, Campus UFV, 36570 000 Viçosa, MG, Brazil.
authors.forename: Aline Evelyn Zimmermann
authors.initials: AE
authors.lastname: de Oliveira
authors.affiliation: Departamento de Tecnologia de Alimentos, Universidade Federal de Viçosa, Campus UFV, 36570 000 Viçosa, MG, Brazil.
authors.forename: Antonio Fernandes
authors.initials: AF
authors.lastname: de Carvalho
authors.affiliation: Departamento de Veterinária, Universidade Federal de Viçosa, Campus UFV, 36570 000 Viçosa, MG, Brazil. Electronic address: nero@ufv.br.
authors.forename: Luís Augusto
authors.initials: LA
authors.lastname: Nero
mesh_terms: D000818:Animals; D015169:Colony Count, Microbial; D003470:Culture Media / Q000737:chemistry* / Q000378:metabolism; D043302:Cultured Milk Products / Q000382:microbiology*; D005285:Fermentation; D005516:Food Microbiology / Q000295:instrumentation / Q000379:methods*; D007780:Lacticaseibacillus casei / Q000254:growth & development* / Q000378:metabolism; D019936:Probiotics / Q000032:analysis
chemical_list: D003470:Culture Media
keywords: Fermented milk; Lactobacillus casei; Petrifilm; Selective enumeration