Screening and enumeration of lactic acid bacteria in milk using three different culture media in Petrifilm TM Aerobic Count plates and conventional pour plate methodology

Abstract

This study aimed to compare Petrifilm™ Aerobic Count (AC) plates and the conventional pour plate methodology using de Mann-Rogosa-Sharpe (MRS), Kang-Fung (KF) and Kang-Fung-Sol (KFS) culture media for screening and enumeration of lactic acid bacteria (LAB) in milk. Suspensions of 10 LAB species in reconstituted powder skim milk and 30 raw milk samples, without experimental inoculation, were tested. For selective enumeration, all samples were previously diluted in MRS, KF and KFS broths and then plated in Petrifilm™ AC and conventional pour plate methodology, using the same culture media with added agar. All plates were incubated at 37°C for 48 h in anaerobic conditions. Differences in the counts were observed only for raw milk samples using KFS in conventional methodology, when compared with the counts obtained from MRS and KF (P⩽0·05). The results showed excellent correlation indexes between both methodologies using the three culture media for LAB suspensions (r=0·97 for MRS, KF and KFS). For raw milk samples, the correlation indexes were excellent (r=0·97, for MRS) and good (r=0·84 for KF, and r=0·82 for KFS), showing some interference in Petrifilm™ AC when supplements were added, especially lactic acid. These results indicate the possibility of using Petrifilm™ AC plates for enumeration of LAB in milk, even with the use of selective supplements.